地方国立大学の定員増における新教育組織の設置に伴う入口戦略としての入試制度設計と実施――徳島大学 MPEプログラムを例として――

The Journal of University Admissions Research


Spiral interferometry acquires the topography of the sample and determines the elevation or depression of the sample structure by a single measurement. The method has advantages in simple measurements and stable optical setup due to the coaxial interferometer. However, the measurable axial range was limited by the wavelength of the light. Here, we demonstrate dual-wavelength spiral interferometry without a significant modification to the original setup of the spiral interferometry. The retrieved phase profile using a synthesized wavelength of 3.33 µm enlarges the measurable axial range with a magnification factor of 6.2 compared with that measured by the light at 532 nm.


Reduction of temperature drift in refractive-index-sensing optical frequency comb by active-dummy compensation of dual-comb configuration

IEEE/OSA Journal of Lightwave Technology


Raman Spectroscopic Evaluation of Composition of Matrix Synthesized by Osteoblasts under Microvibration Stimulation


Wireless data transmission in the 560-GHz band utilizing terahertz wave generated through photomixing of a pair of distributed feedback lasers injection-locking to a Kerr micro-resonator soliton comb


Quantification of collagen fiber properties in alcoholic liver fibrosis using polarization-resolved second harmonic generation microscopy.

Liver fibrosis is assessed mainly by conventional staining or second harmonic generation (SHG) microscopy, which can only provide collagen content in fibrotic area. We propose to use polarization-resolved SHG (PR-SHG) microscopy to quantify liver fibrosis in terms of collagen fiber orientation and crystallization. Liver samples obtained from autopsy cases with fibrosis stage of F0-F4 were evaluated with an SHG microscope, and 12 consecutive PR-SHG images were acquired while changing the polarization azimuth angle of the irradiated laser from 0° to 165° in 15° increments using polarizer. The fiber orientation angle (φ) and degree (ρ) of collagen were estimated from the images. The SHG-positive area increased as the fibrosis stage progressed, which was well consistent with Sirius Red staining. The value of φ was random regardless of fibrosis stage. The mean value of ρ (ρ-mean), which represents collagen fiber crystallinity, varied more as fibrosis progressed to stage F3, and converged to a significantly higher value in F4 than in other stages. Spatial dispersion of ρ (ρ-entropy) also showed increased variation in the stage F3 and decreased variation in the stage F4. It was shown that PR-SHG could provide new information on the properties of collagen fibers in human liver fibrosis.


Rapid, high-sensitivity detection of biomolecules using dual-comb biosensing

Rapid, sensitive detection of biomolecules is important for biosensing of infectious pathogens as well as biomarkers and pollutants. For example, biosensing of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is still strongly required for the fight against coronavirus disease 2019 (COVID-19) pandemic. Here, we aim to achieve the rapid and sensitive detection of SARS-CoV-2 nucleocapsid protein antigen by enhancing the performance of optical biosensing based on optical frequency combs (OFC). The virus-concentration-dependent optical spectrum shift produced by antigen-antibody interactions is transformed into a photonic radio-frequency (RF) shift by a frequency conversion between the optical and RF regions in the OFC, facilitating rapid and sensitive detection with well-established electrical frequency measurements. Furthermore, active-dummy temperature-drift compensation with a dual-comb configuration enables the very small change in the virus-concentration-dependent signal to be extracted from the large, variable background signal caused by temperature disturbance. The achieved performance of dual-comb biosensing will greatly enhance the applicability of biosensors to viruses, biomarkers, environmental hormones, and so on.


Real-time hybrid angular-interrogation surface plasmon resonance sensor in the near-infrared region for wide dynamic range refractive index sensing


Carrier conversion from terahertz wave to dual-wavelength near-infrared light for photonic terahertz detection in wireless communication


All-polarization-maintaining dual-comb fiber laser with mechanically shared cavity configuration and micro-optic component


Nanometer-precision surface metrology of millimeter-sized stepped objects using full-cascade-linked synthetic-wavelength digital holography using a line-by-line full-mode-extracted optical frequency comb

Digital holography (DH) is a powerful tool for the surface profilometry of objects with sub-wavelength precision. In this article, we demonstrate full-cascade-linked synthetic-wavelength DH for nanometer-precision surface metrology of millimeter-sized stepped objects. 300 modes of optical frequency comb (OFC) with different wavelengths are sequentially extracted at a step of mode spacing from a 10GHz-spacing, 3.72THz-spanning electro-optic modulator OFC. The resulting 299 synthetic wavelengths and a single optical wavelength are used to generate a fine-step wide-range cascade link covering within a wavelength range of 1.54 µm to 29.7 mm. We determine the sub-millimeter and millimeter step differences with axial uncertainty of 6.1 nm within the maximum axial range of 14.85 mm.


Terahertz wireless communication in a 560-GHz band using a Kerr micro-resonator soliton comb


Magnetic strong coupling between rectangular hole metamaterials and Fabry-Pérot microcavity in THz region


Low phase noise THz generation from a ber-referenced Kerr microresonator soliton comb


Highly sensitive detection of nucleocapsid protein from SARS-CoV-2 using a near-infrared surface plasmon resonance sensing system


Major target for UV-induced complete loss of HIV-1 infectivity: A model study of single-stranded RNA enveloped viruses


Assessment of Ultra-Early-Stage Liver Fibrosis in Human Non-Alcoholic Fatty Liver Disease by Second-Harmonic Generation Microscopy.

Non-alcoholic fatty liver disease (NAFLD) is associated with the chronic progression of fibrosis. In general, the progression of liver fibrosis is determined by a histopathological assessment with a collagen-stained section; however, the ultra-early stage of liver fibrosis is challenging to identify because of the low sensitivity in the collagen-selective staining method. In the present study, we demonstrate the feasibility of second-harmonic generation (SHG) microscopy in the histopathological diagnosis of the liver of NAFLD patients for the quantitative assessment of the ultra-early stage of fibrosis. We investigated four representative NAFLD patients with early stages of fibrosis. SHG microscopy visualised well-matured fibrotic structures and early fibrosis diffusely involving liver tissues, whereas early fibrosis is challenging to be identified by conventional histopathological methods. Furthermore, the SHG emission directionality analysis revealed the maturation of each collagen fibre of each patient. As a result, SHG microscopy is feasible for assessing liver fibrosis on NAFLD patients, including the ultra-early stage of liver fibrosis that is difficult to diagnose by the conventional histopathological method. The assessment method of the ultra-early fibrosis by using SHG microscopy may serve as a crucial means for pathological, clinical, and prognostic diagnosis of NAFLD patients.

International Journal of Molecular Sciences


Beam-angle-scanning surface plasmon resonance sensor for rapid, high-precision sensing of refractive index and bio-molecules


Deep-Learning-Assisted Single-Pixel Imaging for Gesture Recognition in Consideration of Privacy


Ultralow-frequency ultranarrow-bandwidth coherent terahertz imaging for nondestructive testing of mortar material

Nondestructive testing of concrete materials is essential in civil engineering to maintain social infrastructure such as buildings or bridges. In this study, we constructed an ultralow-frequency, ultranarrow-bandwidth, coherent terahertz (THz) imaging system based on THz time-domain spectroscopy (THz-TDS). Based on its ultralow-frequency-localized THz wave and coherent detection, the present system achieved a wide dynamic range of THz power over 100 dB at 0.046 THz, which is appropriate to measure the mortar material. The achieved dynamic range of the THz power was 59 dB larger than that of a commercialized THz-TDS system and 49 dB larger than that of an ultralow-frequency noncoherent THz imaging system equipped with a high-power electric THz source. Ultimately, the proposed system could visualize the inner structure of a mortar sample with a thickness of 10 mm, and the present system can investigate a mortar sample with a thickness of over 130 mm. The proposed method is an attractive tool for non-destructive testing of thick concrete structures characterized by non-invasiveness and non-contact remoteness.


Ultrasensitive detection of SARS-CoV-2 nucleocapsid protein using large gold nanoparticle-enhanced surface plasmon resonance.

The COVID-19 pandemic has created urgent demand for rapid detection of the SARS-CoV-2 coronavirus. Herein, we report highly sensitive detection of SARS-CoV-2 nucleocapsid protein (N protein) using nanoparticle-enhanced surface plasmon resonance (SPR) techniques. A crucial plasmonic role in significantly enhancing the limit of detection (LOD) is revealed for exceptionally large gold nanoparticles (AuNPs) with diameters of hundreds of nm. SPR enhanced by these large nanoparticles lowered the LOD of SARS-CoV-2 N protein to 85 fM, resulting in the highest SPR detection sensitivity ever obtained for SARS-CoV-2 N protein.


Thermal control of a Kerr microresonator soliton comb via an optical sideband

We report the thermal control of a dissipative Kerr microresonator soliton comb via an optical sideband generated from an electro-optic modulator. Same as the previous reports using an independent auxiliary laser, our sideband-based (S-B) auxiliary light also enables access to a stable soliton comb and reduces the phase noise of the soliton comb, greatly simplifying the set-up with an auxiliary laser. More importantly, because of the intrinsically high frequency/phase correlation between the pump and S-B auxiliary light, the detuning between the pump and resonance frequency is automatically almost fixed, which allows an 18 times larger "effective" soliton existence range than the conventional method using an independent auxiliary laser, as well as a scanning of the soliton comb of more than 10 GHz without using microheaters.


Characteristics of nonlinear terahertz-wave radiation generated by mid-infrared femtosecond pulse laser excitation


Synthesis of D-π-A type benzothiazolepyridinium salt composite and its application as photo-degradation agent for amyloid fibrils

We have synthesized a cyan fluorescent benzothiazole-pyridinium salt composite based on D-p-A architecture. This salt was found to work as not only a two- and three-photon excitable fluorophore but also a degradation agent against amyloid fibrils under LED irradiation conditions.

Bioorganic & Medicinal Chemistry Letters


Two-photon excitable boron complex based on tridentate imidazo[1,5-a]pyridine ligand for heavy- atom-free mitochondria-targeted photodynamic therapy


Frequency-scanned microresonator soliton comb with tracking of the frequency of all comb modes

Rapid and large scanning of a dissipative Kerr-microresonator soliton comb with characterization of all comb modes along with the separation of the comb modes is imperative for the emerging applications of frequency-scanned soliton combs. However, the scan speed is limited by the gain of feedback systems, and measurement of the frequency shift of all comb modes has not been demonstrated. To overcome the limitation of the feedback, we incorporate feedback with feedforward. With an additional gain of >40 by a feedforward signal, a dissipative Kerr-microresonator soliton comb is scanned by 70 GHz in 500µ, 50 GHz in 125µ, and 25 GHz in 50µ (= 500 THz/s). Furthermore, we propose and demonstrate a method to measure the frequency shift of all comb modes, in which an imbalanced Mach-Zehnder interferometer with two outputs with different wavelengths is used. Because of the two degrees of freedom of optical frequency combs, the measurement at two different wavelengths enables estimation of the frequency shift of all comb modes.


Multicascade-linked synthetic-wavelength digital holography using a line-by-line spectral-shaped optical frequency comb


Hybrid optical imaging with near-infrared, mid-infrared, and terahertz wavelengths for nondestructive inspection [Invited]

Optical imaging is a powerful tool for nondestructive inspection, with high spatial resolution and low invasiveness. As lightmaterial interactions vary a great deal depending on the wavelength, it is difficult to select the best imag- ing wavelength without prior knowledge of the optical properties of the material. To overcome this difficulty, we constructed a hybrid optical imaging system using three different wavelengths: near-infrared (NIR), mid-infrared (MIR), and terahertz (THz) regions. The same imaging optics were integrated with different light sources and detectors. Depending on the lightmaterial interaction and detection sensitivity, NIR and THz imaging indicated some potential for nondestructive inspection, but MIR imaging showed difficulty. A combination of NIR and THz imaging will be a powerful tool for optical nondestructive inspection.


Quantitative evaluation of SARS-CoV-2 inactivation using a deep ultraviolet light-emitting diode.

for 300 nm are required to inactivate 99.9% of SARS-CoV-2. Our results provide quantitative antiviral effects of DUV irradiation on SARS-CoV-2, serving as basic knowledge of inactivation technologies against SARS-CoV-2.


Quantitative evaluation of both histological and mechanical recovery in injured tendons using Fourier-transform second-harmonic-generation microscopy

IEEE Journal of Selected Topics in Quantum Electronics


Computationally image-corrected dual-comb microscopy with a free-running single-cavity dual-comb fiber laser

Dual-comb microscopy (DCM), an interesting imaging modality based on the optical-frequency-comb (OFC) mode and image pixel one-to-one correspondence, benefits from scan-less full-field imaging and simultaneous confocal amplitude and phase imaging. However, the two fully frequency-stabilized OFC sources requirement hampers DCM practicality due to the complexity and costs. Here, a bidirectional single-cavity dual-comb fiber laser (SCDCFL) is adopted as a DCM low-complexity OFC source. Although the residual timing jitter in the SCDCFL blurs the image of a static object acquired by DCM, computational image correction significantly suppresses the image blur. Nanometer-order step surface profilometry with a 14.0 nm uncertainty highlights the computationally image-corrected DCM effectiveness. We further discuss a possibility to expand the computational image correction to a dynamic object and demonstrate its preliminary experiment. The proposed method enhances the DCM generality and practicality due to low-complexity OFC source.


Full-field fluorescence lifetime dual-comb microscopy using spectral mapping and frequency multiplexing of dual-comb optical beats

Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool for quantitative fluorescence imaging because fluorescence lifetime is independent of concentration of fluorescent molecules or excitation/detection efficiency and is robust to photobleaching. However, since most FLIMs are based on point-to-point measurements, mechanical scanning of a focal spot is needed for forming an image, which hampers rapid imaging. Here, we demonstrate scan-less full-field FLIM based on a one-to-one correspondence between two-dimensional (2D) image pixels and frequency-multiplexed radio frequency (RF) signals. A vast number of dual-comb optical beats between dual optical frequency combs are effectively adopted for 2D spectral mapping and high-density frequency multiplexing in the RF region. Bimodal images of fluorescence amplitude and lifetime are obtained with high quantitativeness from amplitude and phase spectra of fluorescence RF comb modes without the need for mechanical scanning. The parallelized FLIM will be useful for rapid quantitative fluorescence imaging in life science.


Molecular imaging analysis of microvesicular and macrovesicular lipid droplets in non-alcoholic fatty liver disease by Raman microscopy.

Predominant evidence of non-alcoholic fatty liver disease (NAFLD) is the accumulation of excess lipids in the liver. A small group with NAFLD may have a more serious condition named non-alcoholic steatohepatitis (NASH). However, there is a lack of investigation of the accumulated lipids with spatial and molecular information. Raman microscopy has the potential to characterise molecular species and structures of lipids based on molecular vibration and can achieve high spatial resolution at the organelle level. In this study, we aim to demonstrate the feasibility of Raman microscopy for the investigation of NAFLD based on the molecular features of accumulated lipids. By applying the Raman microscopy to the liver of the NASH model mice, we succeeded in visualising the distribution of lipid droplets (LDs) in hepatocytes. The detailed analysis of Raman spectra revealed the difference of molecular structural features of the LDs, such as the degree of saturation of lipids in the LDs. We also found that the inhomogeneous distribution of cholesterol in the LDs depending on the histology of lipid accumulation. We visualised and characterised the lipids of NASH model mice by Raman microscopy at organelle level. Our findings demonstrated that the Raman imaging analysis was feasible to characterise the NAFLD in terms of the molecular species and structures of lipids.


Synthesis and Optical Property of Quadrupolar Pyridinium Salt and Its Application as Bioimaging Agent


Two- and three-photon excitable quaternized imidazo[1,2-a]pyridines for mitochondrial imaging and potent cancer therapy agent


Dynamic characterization of polarization property in liquid-crystal-on-silicon spatial light modulator using dual-comb spectroscopic polarimetry

Spectroscopic polarimetry (SP) is a powerful tool for characterization of thin film, polarization optics, semiconductor, and others. However, mechanical polarization modulation of broadband light hampers its application for dynamic monitoring of a sample. In this article, we demonstrate the dynamic SP with features of polarization-modulation-free polarimetry and spectrometer-free spectroscopy benefiting from dual-comb spectroscopy (DCS) using a pair of optical frequency combs (OFCs). DCS enables the direct determination of polarization without the need for polarization modulation by using mode-resolved OFC spectra of amplitude and phase for two orthogonally linear-polarized lights while securing rapid, high-precision, broadband spectroscopy without the need for spectrometer. Effectiveness of the proposed system is highlighted by visualizing the hysteresis property of dynamic response in a liquid-crystal-on-silicon spatial light modulator at a sampling rate of 105 Hz.


Investigation of the phase noise of a microresonator soliton comb

Optical frequency combs generated from microresonators (especially microresonator soliton combs) have been attracting significant attentions because of the potential to be fully chip-scale. Among various promising applications of soliton combs, coherent optical communications and mm/THz wireless communications require low phase noise of the comb modes and low relative phase noise between the comb modes, respectively. Here, we measure the phase noise of a soliton comb, investigating how the thermorefractive noise of a microresonator influences on the phase noise. We observe the quadratic increase of the phase noise of the comb modes, as the comb mode number, counted from the wavelength of a pump cw laser, increases. In addition, we measure the relative phase noise between the comb modes, showing less influence of the phase noise of pump cw lasers by comparing soliton combs generated from pump cw lasers with low and large phase noise.


Adaptive-sampling near-Doppler-limited terahertz dual-comb spectroscopy with a free-running single-cavity fiber laser

Dual-comb spectroscopy (DCS) is an emerging spectroscopic tool with the potential to simultaneously achieve a broad spectral coverage and ultrahigh spectral resolution with rapid data acquisition. However, the need for two independently stabilized ultrafast lasers significantly hampers the potential application of DCS. We demonstrate mode-resolved DCS in the THz region based on a free-running single-cavity dual-comb fiber laser with the adaptive sampling method. While the use of a free-running single-cavity dual-comb fiber laser eliminates the need for two mode-locked lasers and their frequency control, the adaptive sampling method strongly prevents the degradation of spectroscopic performance caused by the residual timing jitter in the free-running dual-comb laser. Doppler-limit-approaching absorption features with linewidths down to 25 MHz are investigated for low-pressure acetonitrile/air mixed gas by comb-mode-resolved THz spectroscopy. The successful demonstration clearly indicates its great potential for the realization of low-complexity, Doppler-limited THz spectroscopy instrumentation.


Optical image amplification in dual-comb microscopy

Dual-comb microscopy (DCM), based on a combination of dual-comb spectroscopy (DCS) with two-dimensional spectral encoding (2D-SE), is a promising method for scan-less confocal laser microscopy giving an amplitude and phase image contrast with the confocality. However, signal loss in a 2D-SE optical system hampers increase in image acquisition rate due to decreased signal-to-noise ratio. In this article, we demonstrated optical image amplification in DCM with an erbium-doped fiber amplifier (EDFA). Combined use of the image-encoded DCS interferogram and the EDFA benefits from not only the batch amplification of amplitude and phase images but also significant rejection of amplified spontaneous emission (ASE) background. Effectiveness of the optical-image-amplified DCM is highlighted in the single-shot quantitative nanometer-order surface topography and the real-time movie of polystyrene beads dynamics under water convection. The proposed method will be a powerful tool for real-time observation of surface topography and fast dynamic phenomena.


Generation of a microresonator soliton comb via current modulation of a DFB laser


Visualization of internal structure and internal stress in visibly opaque objects using full-field phase-shifting terahertz digital holography

We construct a full-field phase-shifting terahertz digital holography (PS-THz-DH) system by use of a THz quantum cascade laser and an uncooled, 2D micro-bolometer array. The PS-THz-DH enables us to separate the necessary diffraction-order image from unnecessary diffraction-order images without the need for spatial Fourier filtering, leading to suppress the decrease of spatial resolution. 3D shape of a visibly opaque object is visualized with a sub-millimeter lateral resolution and a sub-µm axial resolution. Also, the digital focusing of amplitude image enables the visualization of internal structure with the millimeter-order axial selectivity. Furthermore, the internal stress distribution of an externally compressed object is visualized from the phase image. The demonstrated results imply a possibility for non-destructive inspection of visibly opaque non-metal materials.


One-drop self-assembly of ultra-fine second-order organic nonlinear optical crystal nanowires

In this study, we propose a one-drop self-assembly method, which proved capable of successfully preparing 4-N, N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (DAST) single-crystalline nanowires (NWs). The apparent roughness of the DAST NWs was determined to be less than 100 pm by using a high-resolution atomic force microscope, indicating their ultrafine quality. The DAST NWs also exhibited excellent nonlinear optical properties, including two-photon excited fluorescence and second harmonic generation, which could enable the production of low-cost, low-power-consumption wideband wavelength conversion devices. Thus, the described method may provide a new avenue for organic NW fabrication.


Refractive index sensing with temperature compensation by a multimode-interference fiber-based optical frequency comb sensing cavity

We proposed a refractive index (RI) sensing method with temperature compensation by using an optical frequency comb (OFC) sensing cavity including a multimode-interference (MMI) fiber, namely, the MMI-OFC sensing cavity. The MMI-OFC sensing cavity enables simultaneous measurement of material-dependent RI and sample temperature by decoding from the comb spacing frequency shift and the wavelength shift of the OFC. We realized the simultaneous and continuous measurement of RI-related concentration of a liquid sample and its temperature with precisions of 1.6 × 104 RIU and 0.08 °C. The proposed method would be a useful means for the various applications based on RI sensing.


Dual-comb spectroscopy (DCS) is useful for gas spectroscopy due to the high potential of optical frequency comb (OFC). However, fast Fourier transform (FFT) calculation of a huge amount of temporal data spends significantly longer time than the acquisition time of an interferogram. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection, namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation. Usefulness of LID-DCS is demonstrated in the rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas.


Improvement of dynamic range and repeatability in a refractive-index- sensing optical comb by combining saturable-absorber-mirror mode- locking with an intracavity multimode interference fiber sensor

A mode-locked fiber comb equipped with a multimode interference fiber sensor functions as a high-precision refractive-index (RI) sensor benefitting from precise RF measurement. However, its dynamic range and repeatability are hampered by the inherent characteristics of nonlinear- polarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with a wide dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness against cavity disturbance, the self-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10 × 108 for each mode-locking activation. The improved dynamic range and repeatability will be useful for enhancing the performance of RI sensing.


Lens-less fiber coupling of a 1550-nm mode-locked fiber laser light on a low-temperature-grown GaAs photoconductive antenna


Ultrasonic wave sensing using an optical-frequency-comb sensing cavity for photoacoustic imaging

We propose a novel ultrasonic wave sensing method employing an optical-frequency-comb (OFC) cavity, namely, an OFC sensing cavity-based ultrasonic wave sensor. We confirm the fundamental characteristics of the OFC sensing cavity-based ultrasonic wave sensor by using an ultrasonic wave transducer and realize the frequency response of up to 13 MHz with a linear response. We also confirm the potential applicability in photoacoustic imaging employing an 8-ns pulsed laser. Our demonstrations confirm the capability of the concept of the OFC sensing cavity in ultrasonic wave sensing and will also expand the potential of OFC technology.


Quantitative in situ time-series evaluation of osteoblastic collagen synthesis under cyclic strain using second-harmonic-generation microscopy

The aim of this study is to provide an in situ method to non-invasively monitor osteoblastic collagen synthesis under mechanical stimulation. We applied second-harmonic-generation (SHG) microscopy to monitor the collagen fibers produced by osteoblast-like cells. To evaluate the influence of mechanical stimulation on collagen synthesis and maturation, we compared SHG images of osteoblast-produced collagen fibers with and without a cyclic stretch stimulus. Image analysis of the average SHG intensity indicated that the amount of osteoblastic collagen synthesis was significantly enhanced by the cyclic stretch. Furthermore, the maturity of the collagen fibers was not affected in the early stage of bone formation by the mechanical stimulus.© (2018) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.


Scan-Less, Kilo-Pixel, Line-Field Confocal Phase Imaging with Spectrally Encoded Dual-Comb Microscopy

Confocal laser microscopy (CLM) is a powerful tool in life science research and industrial inspection, and its image acquisition rate is boosted by scan-less imaging techniques. However, the optical-intensity-based image contrast in CLM makes it difficult to visualize transparent non-fluorescent objects or reflective objects with nanometer unevenness. In this paper, we introduce an optical frequency comb (OFC) to scan-less CLM to give the optical-phase-based image contrast. One-dimensional (1D) image pixels of a sample are separately encoded onto OFC modes via 1D spectral encoding by using OFC as an optical carrier of amplitude and phase with a vast number of discrete frequency channels. Then, line-field confocal information of amplitude and phase are decoded from a mode-resolved OFC amplitude and phase spectra obtained by dual-comb spectroscopy without the need for mechanical scanning. The proposed confocal phase imaging will further expand the application fields of CLM.

IEEE Journal of Selected Topics in Quantum Electronics


Multicascade-linked synthetic wavelength digital holography using an optical-comb-referenced frequency synthesizer

Digital holography (DH) is a promising method for non-contact surface topography because the reconstructed phase image can visualize the nanometer unevenness in a sample. However, the axial range of this method is limited to the range of the optical wavelength due to the phase wrapping ambiguity. Although the use of two different wavelengths of light and the resulting synthetic wavelength, i.e., synthetic wavelength DH, can expand the axial range up to several hundreds of millimeters, its axial precision does not reach sub-micrometer. In this article, we constructed a tunable external cavity laser diode phase-locked to an optical frequency comb, namely, an optical-comb-referenced frequency synthesizer, enabling us to generate multiple synthetic wavelengths within the range of 32 µm to 1.20 m. A multiple cascade link of the phase images among an optical wavelength ( = 1.520 µm) and 5 different synthetic wavelengths ( = 32.39 µm, 99.98 µm, 400.0 µm, 1003 µm, and 4021 µm) enables the shape measurement of a reflective millimeter-sized stepped surface with the axial resolution of 34 nm. The axial dynamic range, defined as the ratio of the axial range ( = 2.0 mm) to the axial resolution ( = 34 nm), achieves 5.9 × 10, which is larger than that of previous synthetic wavelength DH. Such a wide axial dynamic range capability will further expand the application field of DH for large objects with meter dimensions.


Polarization-resolved second-harmonic-generation imaging of dermal collagen fiber in prewrinkled and wrinkled skins of ultraviolet-B-exposed mouse

Skin wrinkling is a typical symptom of cutaneous photoaging; however, the skin wrinkling depends on not only the actual age but also exposure history to ultraviolet B (UVB) rays in individuals. Therefore, there is considerable need for its assessment technique in vivo in skin cosmetics and antiaging dermatology. Wrinkles always appear as linear grooves in the skin, and dermal collagen fibers play an important role to determine the morphology and mechanical properties of the skin. Therefore, an optical probe sensitive to dermal collagen fiber and its orientation will be useful. Polarization-resolved second-harmonic-generation (SHG) microscopy is a promising approach for in vivo evaluation of collagen fiber orientation because the efficiency of SHG light is sensitive to collagen fiber orientation when the incident light is linearly polarized. We investigate orientation change of dermal collagen fiber in prewrinkled and wrinkled skins of the UVB-exposed mouse model using polarization-resolved SHG microscopy. A polarization anisotropic image of the SHG light indicates that the change of collagen fiber orientation starts in the prewrinkled skin of UVB-exposed mice, then the wrinkle appears. Furthermore, the dominant direction of collagen fiber orientation in the prewrinkled skin is significantly parallel to the wrinkle direction in the wrinkled skin. This result implies that the change of collagen fiber orientation is a trigger of wrinkling in cutaneous photoaging.


Photonic-Crystal-Fiber-Coupled, Hand-Held, Polarization-Resolved Second-Harmonic-Generation Microscope for In Vivo Visualization of Dermal Collagen Fibers in Human Skin

Second-harmonic-generation (SHG) microscopy is a powerful tool for in vivo monitoring of collagen fibers in human skin. However, its practical use in the dermatological field is still limited due to the bulky and complicated setup. In this paper, we constructed a photonic-crystal-fiber-coupled, hand-held polarization-resolved SHG microscope for in vivo monitoring of collagen fibers in human skin. Fiber delivery of ultrashort pulse light was achieved by a large-mode-area photonic-crystal-fiber whereas the SHG microscopy setup was enclosed into a hand-held probe head. The combination of PCF with the hand-held probe head largely enhances the flexibility of measurement sites in the human skin.


Dual terahertz comb spectroscopy with a single free-running fibre laser

Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications.


Refractive-index-sensing optical comb based on photonic radio-frequency conversion with intracavity multi-mode interference fiber sensor

Optical frequency combs (OFCs) have attracted attention as optical frequency rulers due to their tooth-like discrete spectra together with their inherent mode-locking nature and phase-locking control to a frequency standard. Based on this concept, their applications until now have been demonstrated in the fields of optical frequency metrology. However, if the utility of OFCs can be further expanded beyond their application by exploiting new aspects of OFCs, this will lead to new developments in optical metrology and instrumentation. Here, we report a fiber sensing application of OFCs based on a coherent link between the optical and radio frequencies, enabling high-precision refractive index measurement based on frequency measurement in radio-frequency (RF) region. Our technique encodes a refractive index change of a liquid sample into a repetition frequency of OFC by a combination of an intracavity multi-mode-interference fiber sensor and wavelength dispersion of a cavity fiber. Then, the change in refractive index is read out by measuring the repetition frequency in RF region based on a frequency standard. Use of an OFC as a photonic RF converter will lead to the development of new applications in high-precision fiber sensing with the help of functional fiber sensors and precise RF measurement.


Scan-less confocal phase imaging based on dual-comb microscopy

Confocal laser microscopy (CLM) is a powerful tool in life science research and industrial inspection because it offers two-dimensional optical sectioning or three-dimensional imaging capability with micrometer depth selectivity. Furthermore, scan-less imaging modality enables rapid image acquisition and high robustness against surrounding external disturbances in CLM. However, the objects to be measured must be reflective, absorptive, scattering, or fluorescent because the image contrast is given by the optical intensity. If a new image contrast can be provided by the optical phase, scan-less CLM can be further applied for transparent non-fluorescent objects or reflective objects with nanometer unevenness by providing information on refractive index, optical thickness, or geometrical shape. Here, we report scan-less confocal dual-comb microscopy offering a phase image in addition to an amplitude image with depth selectivity by using an optical frequency comb as an optical carrier of amplitude and phase with discrete ultra-multichannels. Our technique encodes confocal amplitude and phase images of a sample onto a series of discrete modes in the optical frequency comb with well-defined amplitude and phase to establish a one-to-one correspondence between image pixels and comb modes. The technique then decodes these images from comb modes with amplitude and phase. We demonstrate confocal phase imaging with milliradian phase resolution under micrometer depth selectivity on the millisecond timescale. As a proof of concept, we demonstrate the quantitative phase imaging of standing culture fixed cells and the surface topography of nanometer-scale step structures. Our technique for confocal phase imaging will find applications in three-dimensional visualization of stacked living cells in culture and nanometer surface topography of semiconductor objects.


Application of Scan-less Two-Dimensional Confocal Microscopy Based on a Combination of Confocal Slit With Wavelength/Space Conversion

Confocal laser microscope (CLM) has been widely used in the fields of the non-contact surface topography, biomedical imaging, and other applications, because the confocality gives two-dimensional (2D) optical-sectioning or three-dimensional (3D) imaging capability with the depth selectivity. Combination of line-focused CLM with one-dimensional (1D) spectral encoding CLM enables us to obtain the 2D confocal image without the need for the mechanical scanning. So-called scan-less 2D CLM is a unique imaging modality, however, there are no attempts to apply for practical application. In this paper, we constructed scan-less 2D CLM with the image acquisition time of 0.23 ms, the lateral resolution of 1.2 μm, the depth resolution of 2.4 μm, and apply it for different kinds of application to evaluate its practical potential.


Strain sensing based on strain to radio-frequency conversion of optical frequency comb

We propose an optical frequency comb (OFC)-based strain sensing method, namely OFC sensing cavity, which is capable of radio-frequency (RF)-based strain measurement. We developed a null-method-based strain sensing system with a comb-spacing-stabilized OFC generator. We realized strain measurement from 1.83 µ to 1800 µ with a sensing fiber length of 20 mm. The measurable strain frequency range of the developed strain sensing system was from 0 to 310 Hz. Owing to the use of RF-based strain measurement, our approach would be a useful and powerful tool for sensing of strain or other physical quantities, and the concept of the OFC sensing cavity is a new aspect of OFC technology.


Texture analysis of secondharmonicgeneration images for quantitative analysis of reticular dermal collagen fibre in vivo in human facial cheek skin

Secondharmonicgeneration (SHG) microscopy is a powerful tool for in vivo visualisation of collagen fibres in human skin because of its specific collagen selectivity without the need for staining, noninvasiveness and highresolution threedimensional imaging. Although texture analysis of SHG images is a promising method for the quantitative analysis of wellorientated collagen fibre structure in the tendon and cornea, there are few attempts to assess cutaneous ageing. In this study, we applied two texture analysis techniques, namely autocorrelation (2DAC) analysis and twodimensional Fourier transform (2DFT), to evaluate the agedependent changes in reticular dermal collagen fibres in in vivo human cheek skin. Agedependent changes in the reticular dermal collagen fibres of female subjects in their 20s, 40s and 60s clearly appeared in these texture analyses. Furthermore, the parameter from 2DAC analysis showed a significantly higher correlation with skin elasticity measured by a Cutometer®. These results clearly indicate that 2DAC analysis of SHG images is highly promising for the quantitative evaluation of agedependent change in facial collagen fibres as well as skin elasticity. An appropriate texture analysis will help to provide quantitative insight into collagen fibre structure and will be useful for the diagnosis of pathological conditions as well as cutaneous ageing in skin.


Real-Time Amplitude and Phase Imaging of Optically Opaque Objects by Combining Full-Field Off-Axis Terahertz Digital Holography with Angular Spectrum Reconstruction

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials.

Journal of Infrared, Millimeter and Terahertz Waves


Analysis of the influence of collagen fibres in the dermis on skin optical reflectance by Monte Carlo simulation in a nine- layered skin model

Collagen fibres in the dermis play an important structural role in the skin. Age-related changes to these fibres cause wrinkles and slackness of facial skin. However, it is not clear how dermal collagen fibres affect skin colour. The purpose of this study was to clarify the influence of altered collagen fibres on skin colour, using both experimental measurement of fibre density and Monte Carlo simulations in an optical model of skin. Reflection spectra were measured from the cheeks of 12 Japanese women (22-65 years old) by spectral colorimeter. Two-dimensional autocorrelation functions were calculated from second harmonics generation (SHG) images acquired from the same locations and used to calculate collagen density indices. Monte Carlo simulations of light reflectance by skin were performed using a nine-layered model that precisely imitates skin structure. The relationship between dermal collagen fibre density and skin reflection spectra was analysed. A positive correlation was found between collagen density and skin brightness, as measured by the colour value, L* (using the L*a*b* colour space). In addition, collagen density showed a strong inverse correlation with age and with the optical absorption of dermis. The Monte Carlo simulations showed that the reflection spectrum of skin changes when the scattering coefficient of the dermis is altered. These changes were the same for simulated and experimentally measured reflection spectra. When collagen fibre density in the upper dermis is decreased with age, skin colour becomes less bright because light scattering in the skin is decreased.


Dead-band-free, high-resolution microwave frequency measurement using a free-running triple-comb fiber laser

IEEE Journal of Selected Topics in Quantum Electronics


Direct wavefront measurement of terahertz pulses using two-dimensional electro-optic imaging

IEEE Transactions on Terahertz Science and Technology


Spectroscopic ellipsometry is a means of investigating optical and dielectric material responses. Conventional spectroscopic ellipsometry is subject to trade-offs between spectral accuracy, resolution, and measurement time. Polarization modulation has afforded poor performance because of its sensitivity to mechanical vibrational noise, thermal instability, and polarization-wavelength dependency. We combine spectroscopic ellipsometry with dual-comb spectroscopy, namely, dual-comb spectroscopic ellipsometry. Dual-comb spectroscopic ellipsometry (DCSE). DCSE directly and simultaneously obtains the ellipsometric parameters of the amplitude ratio and phase difference between s-polarized and p-polarized light signals with ultra-high spectral resolution and no polarization modulation, beyond the conventional limit. Ellipsometric evaluation without polarization modulation also enhances the stability and robustness of the system. In this study, we construct a polarization-modulation-free DCSE system with a spectral resolution of up to 1.2 × 10(-5) nm throughout the spectral range of 1514-1595 nm and achieved an accuracy of 38.4 nm and a precision of 3.3 nm in the measurement of thin-film samples.Spectroscopic ellipsometry is an established technique to characterize the optical properties of a material. Here, Minamikawa et al. combine the method with dual-comb spectroscopy, which allows them to obtain ellipsometric parameters including the phase difference between s-polarized and p-polarized light.


Scan-less hyperspectral dual-comb single-pixel-imaging in both amplitude and phase.

We have developed a hyperspectral imaging scheme that involves a combination of dual-comb spectroscopy and Hadamard-transform-based single-pixel imaging. The scheme enables us to obtain 12,000 hyperspectral images of amplitude and phase at a spatial resolution of 46 µm without mechanical scanning. The spectral resolution given by the data point interval in the frequency domain is 20 MHz and the comb mode interval is 100 MHz over a spectral range of 1.2 THz centered at 191.5 THz. As an initial demonstration of our scheme, we obtained spectroscopic images of a standard test chart through an etalon plate. The thickness of an absorptive chromium-coated layer on a float-glass substrate was determined to be 70 nm from the hyperspectral phase images in the near-infrared wavelength region.


Interferometric terahertz wavefront analysis


Quantitative Evaluation of Collagen Fiber Structure in Human Dermis Based on Two-Dimensional Auto-Correlation Analysis of SHG (Second Harmonic Generation) Image

SHG(第二高調波発生光)イメージの2次元自己相関解析に基づいたヒト真皮コラーゲン線維構造の定量化

Collagen fibers in the dermis are closely related to skin aging because collagen is one of structural proteins and hence plays an important role to determine the morphology and mechanical properties of the skin. Although histological and/or biochemical studies have been used to reveal the age-related decrease and disorganization of dermal collagen fibers, such invasive methods cannot be used for simple assessment or serial monitoring of skin aging. Therefore, there is a considerable need for non-invasive assessment method of skin aging. Recently, second-harmonic-generation (SHG) microscopy has attracted attention for in situ visualization of human dermal collagen fibers. Due to its specific collagen selectivity without the need for staining, non-invasiveness, and high-resolution 3D imaging capability, collagen fiber structure can be visualized as high-contrast SHG image in situ. However, extraction of quantitative parameters reflecting the collagen fiber structure from SHG image presents difficulties. In this article, we applied two-dimensional auto-correlation analysis to SHG images of dermal collagen fibers in human cheek skin. The collagen fiber structure index obtained from the analysis decreased significantly with aging. Furthermore, we confirmed a moderate correlation between the collagen fiber structure index and skin elasticity measured by a Cutometer®.This correlation indicates that the dermal collagen fiber structure plays an important role to determine skin elasticity. In other words, the auto-correlation analysis of SHG image can be used for evaluation of skin elasticity from the viewpoint of dermal collagen. The proposed method will be a powerful tool in the fields of skin cosmetics and anti-aging dermatology.

Transactions of the Japanese Society for Medical and Biological Engineering


Hand-held Second-harmonic-generation (SHG) Microscope for Dermatological Application

第2高調波発生顕微鏡の小型化

Second harmonic generation (SHG) microscopy allows in situ visualization of dermal collagen fibers in human skin due to its selectivity for collagen fiber. It is an attractive tool because it does not require staining, is non-invasiveness, and has high-resolution 3D imaging capability. However, its practical use in the dermatological field is still limited due to the bulky and complicated setup. Incorporation of fiber transmission of the laser light and a compact probe head into the microscope would enhance its compactness, robustness, flexibility, and hence convenience. In this study, we constructed a photonic-crystal-fiber (PCF) -coupled, hand-held SHG microscope for in situ monitoring of collagen fibers in human skin. By transmitting pulse light from a mode-locked Cr:Forsterite laser source via a large mode area PCF, it was possible to deliver pulse light with median wavelength of 1250nm and pulse width of 80 fs to the SHG microscope, without the need for external dispersion compensation. Components of a SHG microscope;galvano mirrors, relay lenses, objective lens, dichroic mirror, optical filters, and photon-counting photomultiplier, were mounted into a hand-held probe head (width=310mm, height=150mm, and depth=50mm), which is only 2% of the size of a conventional SHG microscope. We compared the imaging performance between the conventional SHG microscope and the hand-held SHG microscope, and confirmed comparable performance of the two. Finally, we demonstrated in situ visualization of collagen fibers in human skin using the hand-held SHG microscope with a compact probe head coupled to PCF. The hand-held SHG microscope greatly enhances the operability of in situ measurement of the human skin. The novel system will be a powerful tool for various dermatological applications.


High brightness, low coherence, digital holographic microscopy for 3D visualization of an in-vitro sandwiched biological sample

We achieve practically a bright-field digital holographic microscopy (DHM) configuration free from coherent noise for three-dimensional (3D) visualization of an in-vitro sandwiched sarcomere sample. Visualization of such sandwiched samples by conventional atomic force microscope (AFM) is impossible, while visualization using DHM with long coherent lengths is challenging. The proposed configuration is comprised of an ultrashort pulse laser source and a Mach-Zehnder interferometer in transmission. Periodically poled lithium niobate (PPLN) crystal was used to convert the fundamental beam by second harmonic generation (SHG) to the generated beam fit to the CCD camera used. The experimental results show that the contrast of the reconstructed phase image is improved to a higher degree compared to a He-Ne laser based result. We attribute this improvement to two things: the feature of the femtosecond pulse light, which acts as a chopper for coherent noise suppression, and the fact that the variance of a coherent mode can be reduced by a factor of 9 due to low loss through a nonlinear medium.


Measurement of absolute frequency of continuous-wave terahertz radiation in real time using a free-running, dual-wavelength mode-locked, erbium-doped fibre laser.

A single, free-running, dual-wavelength mode-locked, erbium-doped fibre laser was exploited to measure the absolute frequency of continuous-wave terahertz (CW-THz) radiation in real time using dual THz combs of photo-carriers (dual PC-THz combs). Two independent mode-locked laser beams with different wavelengths and different repetition frequencies were generated from this laser and were used to generate dual PC-THz combs having different frequency spacings in photoconductive antennae. Based on the dual PC-THz combs, the absolute frequency of CW-THz radiation was determined with a relative precision of 1.2 × 10(-9) and a relative accuracy of 1.4 × 10(-9) at a sampling rate of 100 Hz. Real-time determination of the absolute frequency of CW-THz radiation varying over a few tens of GHz was also demonstrated. Use of a single dual-wavelength mode-locked fibre laser, in place of dual mode-locked lasers, greatly reduced the size, complexity, and cost of the measurement system while maintaining the real-time capability and high measurement precision.


Observation of Tendon Healing with Second-harmonic-generation (SHG) Microscopy

第2高調波発生光(SHG)顕微鏡を用いた腱修復の観察

Tendon rupture is a traumatic injury that is difficult to recover to the condition before injury. In previous studies, histological staining and tensile testing have been widely used to evaluate histological and mechanical healing. However, since both methods are destructive and invasive, it is difficult to apply these methods to clinical diagnosis. If the degree of healing can be visualized nondestructively and noninvasively, new findings may be obtained regarding mechanisms of the tendon healing process. In this study, we used second-harmonic-generation (SHG) microscopy to evaluate the degree of healing of ruptured tendon in a rabbit model. SHG microscopy has high selectivity and high image contrast with respect to the structural maturity, density, and aggregates of collagen molecule, without the need for histological sectioning and staining. Furthermore, since SHG light intensity sensitively reflects the structural maturity of collagen molecule and its aggregates, it has the potential to be a good indicator for the degree of healing of the injured tendon. By comparing the SHG images between 4-week-healing tendons and normal tendons in the animal model, we confirmed that SHG light intensity of the healing tendon was significantly lower than that of the normal tendon, indicating that the collagen structure in the healing tendon is still immature. Furthermore, we performed image analysis based on 2D Fourier transform of the acquired SHG images, and confirmed a significant difference in collagen distribution depending on the sample. These results indicate that SHG microscopy has the unique potential as an indicator of tendon healing.


This study aimed to evaluate the histological and mechanical features of tendon healing in a rabbit model with second-harmonic-generation (SHG) imaging and tensile testing. A total of eight male Japanese white rabbits were used for this study. The flexor digitorum tendons in their right leg were sharply transected, and then were repaired by intratendinous stitching. At four weeks post-operatively, the rabbits were killed and the flexor digitorum tendons in both right and left legs were excised and used as specimens for tendon healing (n = 8) and control (n = 8), respectively. Each specimen was examined by SHG imaging, followed by tensile testing, and the results of the two testing modalities were assessed for correlation. While the SHG light intensity of the healing tendon samples was significantly lower than that of the uninjured tendon samples, 2D Fourier transform SHG images showed a clear difference in collagen fibre structure between the uninjured and the healing samples, and among the healing samples. The mean intensity of the SHG image showed a moderate correlation (R(2) = 0.37) with Young's modulus obtained from the tensile testing. Our results indicate that SHG microscopy may be a potential indicator of tendon healing.Cite this article: E. Hase, K. Sato, D. Yonekura, T. Minamikawa, M. Takahashi, T. Yasui. Evaluation of the histological and mechanical features of tendon healing in a rabbit model with the use of second-harmonic-generation imaging and tensile testing. Bone Joint Res 2016;5:577-585. DOI: 10.1302/2046-3758.511.BJR-2016-0162.R1.


Picometer-resolution dual-comb spectroscopy with a free-running fiber laser

Dual-comb spectroscopy holds the promise as real-time, high-resolution spectroscopy tools. However, in its conventional schemes, the stringent requirement on the coherence between two lasers requires sophisticated control systems. By replacing control electronics with an all-optical dual-comb lasing scheme, a simplified dual-comb spectroscopy scheme is demonstrated using one dual-wavelength, passively mode-locked fiber laser. Pulses with a intracavity-dispersion-determined repetition-frequency difference are shown to have good mutual coherence and stability. Capability to resolve the comb teeth and a picometer-wide optical spectral resolution are demonstrated using a simple data acquisition system. Energy-efficient, free-running fiber lasers with a small comb-tooth-spacing could enable low-cost dual-comb systems.


Scan-Less Full-Field Confocal Microscopy by a Combination of Confocal Slit with Wavelength/Space Conversion

スリット共焦点と波長/空間変換を用いたスキャンレス・フルフィールド共焦点顕微鏡

Journal of the Japan Society for Precision Engineering


Dynamic terahertz spectroscopy of gas molecules mixed with unwanted aerosol under atmospheric pressure using fibre-based asynchronous-optical-sampling terahertz time-domain spectroscopy.

Terahertz (THz) spectroscopy is a promising method for analysing polar gas molecules mixed with unwanted aerosols due to its ability to obtain spectral fingerprints of rotational transition and immunity to aerosol scattering. In this article, dynamic THz spectroscopy of acetonitrile (CH3CN) gas was performed in the presence of smoke under the atmospheric pressure using a fibre-based, asynchronous-optical-sampling THz time-domain spectrometer. To match THz spectral signatures of gas molecules at atmospheric pressure, the spectral resolution was optimized to 1 GHz with a measurement rate of 1 Hz. The spectral overlapping of closely packed absorption lines significantly boosted the detection limit to 200 ppm when considering all the spectral contributions of the numerous absorption lines from 0.2 THz to 1 THz. Temporal changes of the CH3CN gas concentration were monitored under the smoky condition at the atmospheric pressure during volatilization of CH3CN droplets and the following diffusion of the volatilized CH3CN gas without the influence of scattering or absorption by the smoke. This system will be a powerful tool for real-time monitoring of target gases in practical applications of gas analysis in the atmospheric pressure, such as combustion processes or fire accident.


In situ time-series monitoring of collagen fibers produced by standing-cultured osteoblasts using a second-harmonic-generation microscope.

In bone tissue engineering and regeneration, there is a considerable need for an unstained method of monitoring collagen fibers produced by osteoblasts. This is because collagen fibers play an important role as a bone matrix and continuous monitoring of their temporal dynamics is important in clarifying the organization process toward forming bone tissue. In the work described here, using a second-harmonic-generation (SHG) microscope, we performed in situ time-series monitoring of collagen fibers produced by cultured osteoblasts without the need for staining. Use of the 19 fs near-infrared pulsed light enables us to visualize the temporal dynamics in a thin layer of collagen fibers produced by a single layer of osteoblasts in high-contrast SHG images. While the collagen fibers were produced and stored inside the osteoblasts at an early stage of culturing, the network structure of collagen fibers was formed and locally condensed at a late stage. Furthermore, we extracted a quantitative parameter of collagen maturity degree in the cultured sample by use of image analysis based on a two-dimensional Fourier transform of the SHG image. The proposed method will be useful for in situ quality and quantity control of collagen fibers in bone tissue engineering and regeneration.


Terahertz frequency-domain spectroscopy of low-pressure acetonitrile gas by a Photomixing terahertz synthesizer referenced to dual optical frequency combs

Journal of Infrared, Millimeter and Terahertz Waves


Development of a wavefront sensor for terahertz pulses

Wavefront characterization of terahertz pulses is essential to optimize far-field intensity distribution of time-domain (imaging) spectrometers or increase the peak power of intense terahertz sources. In this paper, we report on the wavefront measurement of terahertz pulses using a Hartmann sensor associated with a 2D electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. We quantitatively determined the deformations of planar and converging spherical wavefronts using the modal Zernike reconstruction least-squares method. Associated with deformable mirrors, the sensor will also open the route to terahertz adaptive optics.


Real-time determination of absolute frequency in continuous-wave terahertz radiation with a photocarrier terahertz frequency comb induced by an unstabilized femtosecond laser


Near-infrared broadband dual-frequency-comb spectroscopy with a resolution beyond the Fourier limit determined by the observation time window

We performed broadband dual-frequency-comb spectroscopy in the near-infrared region with a much higher resolution than the Fourier limit by using discrete Fourier transforms and spectral interleaving. We observed the resonant spectrum of a Fabry-Perot cavity over a spectral range of 187 to 218 THz using this technique, and measured its free spectral ranges and finesses. The recorded spectrum includes cavity resonance lines with widths of about 2 MHz, which is much narrower than the resolution of 48 MHz determined by the observation time window.


Adaptive sampling dual terahertz comb spectroscopy using dual free-running femtosecond lasers

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy because the mode-resolved THz comb spectrum includes both broadband THz radiation and narrow-line CW-THz radiation characteristics. In addition, all frequency modes of a THz comb can be phase-locked to a microwave frequency standard, providing excellent traceability. However, the need for stabilization of dual femtosecond lasers has often hindered its wide use. To overcome this limitation, here we have demonstrated adaptive-sampling THz-DCS, allowing the use of free-running femtosecond lasers. To correct the fluctuation of the time and frequency scales caused by the laser timing jitter, an adaptive sampling clock is generated by dual THz-comb-referenced spectrum analysers and is used for a timing clock signal in a data acquisition board. The results not only indicated the successful implementation of THz-DCS with free-running lasers but also showed that this configuration outperforms standard THz-DCS with stabilized lasers due to the slight jitter remained in the stabilized lasers.


Super-resolution discrete Fourier transform spectroscopy beyond time-window size limitation using precisely periodic pulsed radiation

Fourier transform spectroscopy (FTS) has been widely used in a variety of fields due to its high signal-to-noise ratio, simultaneous acquisition of a broad spectrum, and versatility for different radiation sources. Further improvement of the spectroscopic performance will widen its scope of applications. Here, we demonstrate improved spectral resolution by overcoming the time-window size limitation using a mode-locked terahertz (THz) pulse train as precisely periodic pulsed radiation in discrete Fourier transform spectroscopy (dFTS). Since infinitesimal resolution can be achieved at harmonic components of its repetition frequency 1/ when the time-window size is exactly matched to the repetition period , a combination of dFTS with a spectral interleaving technique achieves a spectral resolution limited only by the spectral interleaving interval. Linewidths narrower than 1/(50) are fully resolved by THz-dFTS, allowing rotational-transition absorption lines of low-pressure molecular gases to be attributed within a 1.25 MHz band.


Real-time absolute frequency measurement of continuous-wave terahertz radiation based on dual terahertz combs of photocarriers with different frequency spacings

Real-time measurement of the absolute frequency of continuous-wave terahertz (CW-THz) radiation is required for characterization and frequency calibration of practical CW-THz sources. We proposed a method for real-time monitoring of the absolute frequency of CW-THz radiation involving temporally parallel, i.e., simultaneous, measurement of two pairs of beat frequencies and laser repetition frequencies based on dual THz combs of photocarriers (PC-THz combs) with different frequency spacings. To demonstrate the method, THz-comb-referenced spectrum analyzers were constructed with a dual configuration based on dual femtosecond lasers. Regardless of the presence or absence of frequency control in the PC-THz combs, a frequency precision of 1011 was achieved at a measurement rate of 100 Hz. Furthermore, large fluctuation of the CW-THz frequencies, crossing several modes of the PC-THz combs, was correctly monitored in real time. The proposed method will be a powerful tool for the research and development of practical CW-THz sources, and other applications.


Double-modulation reflection-type terahertz ellipsometer for measuring the thickness of a thin paint coating

We constructed a double-modulation, reflection-type terahertz (THz) ellipsometer for precise measurement of the thickness of a paint film which is coated on a metal surface and which is not transparent to visible or mid-infrared light. The double-modulation technique enabled us to directly obtain two ellipsometric parameters, () and (), as a function of angular frequency, , with a single measurement while reducing flicker noise due to a pump laser. The bias voltage of a photoconductive antenna (PCA) used as a THz pulse emitter was modulated at 100 kHz, and a first lock-in amplifier (LA1) was connected to the output of an electro-optic (EO) signal-sampling unit. In addition, a wire-grid polarizer (WGP) was rotated at 100 Hz to conduct polarization modulation with a frequency of 200 Hz. The output signal from LA1 was fed into a second lock-in amplifier (LA2) that worked in synchronization with the rotating WGP (RWGP). By operating LA2 in a quadrature phase-detection mode, we were able to obtain in-phase and out-of-phase signals simultaneously, from which the two ellipsometric parameters for an isotropic sample could be derived at the same time while cancelling common-mode noise. The lower detection limit of the thickness measurement and the relative standard deviation (RSD) of a black paint film coated on an aluminum substrate were 4.3 µm and 1.4%, respectively. The possibility of determining all elements of the Jones matrix for an anisotropic material is also discussed.


Phase-slope and phase measurements of tunable CW-THz radiation with terahertz comb for wide-dynamic-range, high-resolution, distance measurement of optically rough object

Phase measurement of continuous-wave terahertz (CW-THz) radiation is a potential tool for direct distance and imaging measurement of optically rough objects due to its high robustness to optical rough surfaces. However, the 2 phase ambiguity in the phase measurement of single-frequency CW-THz radiation limits the dynamic range of the measured distance to the order of the wavelength used. In this article, phase-slope measurement of tunable CW-THz radiation with a THz frequency comb was effectively used to extend the dynamic range up to 1.834 m while maintaining an error of a few tens µm in the distance measurement of an optically rough object. Furthermore, a combination of phase-slope measurement of tunable CW-THz radiation and phase measurement of single-frequency CW-THz radiation enhanced the distance error to a few µm within the dynamic range of 1.834 m without any influence from the 2 phase ambiguity. The proposed method will be a powerful tool for the construction and maintenance of large-scale structures covered with optically rough surfaces.


Scleral birefringence as measured by polarization-sensitive optical coherence tomography and ocular biometric parameters of human eyes in vivo

The relationship between scleral birefringence and biometric parameters of human eyes in vivo is investigated. Scleral birefringence near the limbus of 21 healthy human eyes was measured using polarization-sensitive optical coherence tomography. Spherical equivalent refractive error, axial eye length, and intraocular pressure (IOP) were measured in all subjects. IOP and scleral birefringence of human eyes in vivo was found to have statistically significant correlations (r = 0.63, P = 0.002). The slope of linear regression was 2.4 × 102 deg/m/mmHg. Neither spherical equivalent refractive error nor axial eye length had significant correlations with scleral birefringence. To evaluate the direct influence of IOP to scleral birefringence, scleral birefringence of 16 ex vivo porcine eyes was measured under controlled IOP of 560 mmHg. In these ex vivo porcine eyes, the mean linear regression slope between controlled IOP and scleral birefringence was 9.9 × 104 deg/m/mmHg. In addition, porcine scleral collagen fibers were observed with second-harmonic-generation (SHG) microscopy. SHG images of porcine sclera, measured on the external surface at the superior side to the cornea, showed highly aligned collagen fibers parallel to the limbus. In conclusion, scleral birefringence of healthy human eyes was correlated with IOP, indicating that the ultrastructure of scleral collagen was correlated with IOP. It remains to show whether scleral collagen ultrastructure of human eyes is affected by IOP as a long-term effect.


Motion-artifact-robust, polarization-resolved second-harmonic-generation microscopy based on rapid polarization switching with electro-optic Pockells cell and its application to in vivo visualization of collagen fiber orientation in human facial skin

Polarization-resolved second-harmonic-generation (PR-SHG) microscopy is a powerful tool for investigating collagen fiber orientation quantitatively with low invasiveness. However, the waiting time for the mechanical polarization rotation makes it too sensitive to motion artifacts and hence has hampered its use in various applications in vivo. In the work described in this article, we constructed a motion-artifact-robust, PR-SHG microscope based on rapid polarization switching at every pixel with an electro-optic Pockells cell (PC) in synchronization with step-wise raster scanning of the focus spot and alternate data acquisition of a vertical-polarization-resolved SHG signal and a horizontal-polarization-resolved one. The constructed PC-based PR-SHG microscope enabled us to visualize orientation mapping of dermal collagen fiber in human facial skin in vivo without the influence of motion artifacts. Furthermore, it implied the location and/or age dependence of the collagen fiber orientation in human facial skin. The robustness to motion artifacts in the collagen orientation measurement will expand the application scope of SHG microscopy in dermatology and collagen-related fields.


Spectrally interleaved, comb-mode-resolved spectroscopy using swept dual terahertz combs


Prediction of the Thickness of a Thin Paint Film by Applying a Modified Partial-Least-Squares-1 Method to Data Obtained in Terahertz Reflectometry

Journal of Infrared, Millimeter and Terahertz Waves


In vivo visualization of dermal collagen fiber in skin burn by collagen-sensitive second-harmonic-generation microscopy


Terahertz Comb Spectroscopy Traceable Microwave Frequency Standard

IEEE Transactions on Terahertz Science and Technology


PurposeTo investigate the relationship between scleral mechanical properties, its birefringence, and the anisotropy of birefringence alteration in respect of the direction of the strain by using PS-OCT.MethodsThe scleral birefringence of thirty-nine porcine eyes was measured with a prototype PS-OCT. A rectangle strip of sclera with a width of 4 mm was dissected at the temporal region 5 mm apart from the optic nerve head. The strain and force were measured with a uniaxial tension tester as the sample was stretched with a speed of 1.8 mm/min after preconditioning. The birefringence of the sample was measured by PS-OCT at the center of the sample before applying, denoted as inherent birefringence, and after applying stretching of 6.5% strain. The birefringence alteration was obtained by these two measurements and correlations between birefringence and elastic parameters, tangent modulus, and structural stiffness were examined. Twenty and 19 porcine eyes were stretched in meridional or equatorial directions, respectively.ResultsA moderate positive correlation was found between the inherent birefringence and the structural stiffness. A moderate positive correlation was also found between the inherent birefringence and the tangent modulus. The birefringence increased by strains. Marginal significance was found in the birefringence alteration between meridional and equatorial strains, where the mean birefringence elevation by meridional strain was higher than that by equatorial strain.ConclusionsThe birefringence was found to be altered by applying strain and also be related with inherent birefringence. This implies the birefringence of the sclera of the in vivo eye also could be affected by its mechanical property.Citation: Nagase S, Yamanari M, Tanaka R, Yasui T, Miura M, et al. (2013) Anisotropic Alteration of Scleral Birefringence to Uniaxial Mechanical Strain. PLoS ONE 8(3): e58716. doi:10.1371/journal.pone.0058716Editor: Laurent Kreplak, Dalhousie University, CanadaReceived: October 30, 2012; Accepted: February 5, 2013; Published: March 11, 2013Copyright: © 2013 Nagase et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Funding: Financial support for this study was provided in part by Japan Science Technology Agency. YY had financial interest in Tomey Corp. MY is an employee of Tomey Corp. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Competing interests: The authors have read the journal's policy and have the following conflicts: Yasuno and Yamanari are supported by a research grant from Tomey Corp. through their institution. Yamanari is now employee of Tomey Corporation. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.


Noninvasive in situ Assessment of Structural Alternation of Human Dermis Caused by Photoaging Using a Novel Collagen-Specific Imaging Technique


In vivo Imaging of Collagen Fiber Orientation with Rapid, Orthogonally Polarization-resolved SHG Microscopy

高速・直交偏光分解第2高調波発生光顕微鏡を用いたコラーゲン配向のin vivoイメージング

Transactions of the Japanese Society for Medical and Biological Engineering


Fast three-dimensional terahertz computed tomography using real-time line projection of intense terahertz pulse

We demonstrated fast three-dimensional transmission terahertz computed tomography by using real-time line projection of intense terahertz beam generated by optical rectification in lithium niobate crystal. After emphasizing the advantage of intense terahertz pulse generation for two-dimensional spatio-temporal terahertz imaging, peak-to-peak amplitudes of pulsed terahertz electric field have been used to obtain a series of projection images at different rotation angles. Then a standard reconstruction algorithm has been employed to perform final three-dimensional reconstruction. Test samples including a medicine capsule have been investigated with a total acquisition time to only 6 minutes.


In vivo observation of age-related structural changes of dermal collagen in human facial skin using collagen-sensitive second harmonic generation microscope equipped with 1250-nm mode-locked Cr:Forsterite laser


テラヘルツ周波数コムの観測と分光計測への応用

テラヘルツ周波数コムの観測と分光計測への応用


Enhancement of spectral resolution and accuracy in asynchronous-optical-sampling terahertz time-domain spectroscopy for low-pressure gas-phase analysis

The spectral resolution and accuracy of asynchronous-optical-sampling terahertz time-domain spectroscopy (ASOPS-THz-TDS) were evaluated by examining low-pressure gas-phase samples. Use of dual 56-MHz, erbium (Er)-doped, mode-locked femtosecond fiber lasers enhanced the spectral resolution to as low as 50.5 MHz and the spectral accuracy to as low as 6.2 × 10(-6). The results indicate that ASOPS-THz-TDS has the potential to achieve high spectral resolution, high spectral accuracy, and wide spectral coverage at the same time. ASOPS-THz-TDS will open a new door to gas-phase spectroscopy of multiple chemical species in the field of atmospheric gas analysis.


Widely and continuously tunable terahertz synthesizer traceable to a microwave frequency standard

We constructed a widely and continuously tunable terahertz frequency synthesizer traceable to a hydrogen maser linked to coordinated universal time. Photomixing of two optical frequency synthesizers, linked to the hydrogen maser via dual optical frequency combs, gave this THz synthesizer frequency uncertainty of 10 ¹². To demonstrate the potential of wide and continuous tunability in the THz synthesizer, we tuned its output frequency up to 50 GHz discretely and 1.26 GHz continuously in the F-band while maintaining the unprecedented frequency uncertainty by using a uni-traveling-carrier photodiode as a photomixer. This THz synthesizer will be a powerful tool for broadband, high-precision THz spectroscopy and THz frequency metrology.


Absolute distance measurement of optically rough objects using asynchronous-optical-sampling terahertz impulse ranging

We report on a real-time terahertz (THz) impulse ranging (IPR) system based on a combination of time-of-flight measurement of pulsed THz radiation and the asynchronous-optical-sampling (ASOPS) technique. The insensitivity of THz radiation to optical scattering enables the detection of various objects having optically rough surfaces. The temporal magnification capability unique to ASOPS achieves precise distance measurements of a stationary target at an accuracy of -551 μm and a resolution of 113 μm. Furthermore, ASOPS THz IPR is effectively applied to real-time distance measurements of a moving target at a scan rate of 10 Hz. Finally, we demonstrate the application of ASOPS THz IPR to a shape measurement of an optically rough surface and a thickness measurement of a paint film, showing the promise of further expanding the application scope of ASOPS THz IPR. The reported method will become a powerful tool for nondestructive inspection of large-scale structures.


Biomedical applications of a real-time terahertz color scanner

A real-time THz color scanner has the potential to further expand the application scope of THz spectral imaging based on its rapid image acquisition rate. We demonstrated three possible applications of a THz color scanner in the biomedical field: imaging of pharmaceutical tablets, human teeth, and human hair. The first application showed the scanner's potential in total inspection for rapid quality control of pharmaceutical tablets moving on a conveyor belt. The second application demonstrated that the scanner can be used to identify a potential indicator for crystallinity of dental tissue. In the third application, the scanner was successfully used to visualize the drying process of wet hairs. These demonstrations indicated the high potential of the THz color scanner for practical applications in the biomedical field.


Continuously tunable, phase-locked, continuous-wave terahertz generator based on photomixing of two continuous-wave lasers locked to two independent optical combs


A distance meter using a terahertz intermode beat in an optical frequency comb

We propose a distance meter that utilizes an intermode beat of terahertz frequency in an optical frequency comb to perform high resolution and high dynamic range absolute distance measurements. The proposed system is based on a novel method, called multiheterodyne cross-correlation detection, in which intermode beat frequencies are scaled down to radio frequencies by optical mixing of two detuned optical frequency combs with a nonlinear optical crystal. Using this method, we obtained a 1.056 THz intermode beat and achieved a distance resolution of 0.820 microm from its phase measurement. Absolute distance measurement using 1.056 THz and 8.187 GHz intermode beats was also demonstrated in the range of 10 mm, resulting in a precision of 0.688 microm.


Real-time monitoring of continuous-wave terahertz radiation using a fiber-based, terahertz-comb-referenced spectrum analyzer

We propose a fiber-based, terahertz-comb-referenced spectrum analyzer which has the advantages of being a portable, alignment-free, robust, and flexible apparatus suitable for practical use. To this end, we constructed a 1550-nm mode-locked Er-doped fiber laser whose mode-locked frequency was stabilized precisely by referring to a rubidium frequency standard, and used it to generate a highly stable terahertz (THz) frequency comb in a photoconductive antenna or an electro-optic crystal. By standardizing the THz comb, we determined the frequency accuracy of an active-frequency-multiplier-chain (AFMC) source to be 2.4 x 10(-11). Furthermore, the potential of the THz spectrum analyzer was effectively demonstrated by real-time monitoring of the spectral behavior of the AFMC source and a photomixing source of two free-running CW lasers at adjacent wavelengths.


Ex vivo and in vivo second-harmonic-generation imaging of dermal collagen fiber in skin: comparison of imaging characteristics between mode-locked Cr:Forsterite and Ti:Sapphire lasers

Second-harmonic-generation (SHG) microscopy is an interesting new tool for observing dermal collagen fiber in skin. However, conventional SHG microscopy using a mode-locked Ti:sapphire laser suffers from low penetration depth and a slow image acquisition rate caused by scattering and absorption in tissue, making it difficult to use for in vivo applications on human skin. We develop an SHG microscope equipped with a mode-locked Cr:forsterite laser with a long wavelength and compare its imaging characteristics with that of a Ti:sapphire-laser-based SHG microscope for the measurement of dermal collagen fiber in animal and human skins. The results indicate the suitability of the Cr:forsterite laser-based SHG microscope for in vivo imaging of human skin.


Observation of dermal collagen fiber in wrinkled skin using polarization-resolved secondharmonic-generation microscopy

Optical probe methods for in vivo assessments of cutaneous photoaging are necessary in fields such as anti-aging dermatology and skin cosmetic development. We investigated the relation between wrinkle direction and collagen orientation in ultraviolet-B-exposed (UVB-exposed) skin using polarization-resolved second-harmonic-generation (SHG) microscopy. A polarization anisotropic image of the SHG light indicated that wrinkle direction in UVB-exposed skin is predominantly parallel to the orientation of dermal collagen fibers. Furthermore, collagen orientation in post-UVB-exposed skin with few wrinkles changed from that of UVB-exposed wrinkled skin to that of no-UVB-exposed skin. The method proposed has the potential to become a powerful non-invasive tool for assessment of cutaneous photoaging.


Terahertz spectrum analyzer based on a terahertz frequency comb

Precision frequency measurements of terahertz (THz) waves are required to establish metrology in the THz spectral region. However, frequency measurement techniques in this region are immature. We propose a THz spectrum analyzer to measure the absolute frequency and spectral shape of continuous-wave THz waves. Based on a stable frequency comb generated in a photoconductive antenna, the absolute frequency of a sub- THz test source was determined at a precision of 2.8 x 10(-11). Furthermore, the spectral bandwidth of the THz spectrum analyzer can be extended over 1 THz, as demonstrated by measurement of a THz test source. This spectrum analyzer has the potential to become a powerful tool for THz frequency metrology.


Real-time terahertz color scanner for moving objects

Terahertz time-domain spectroscopic (THz-TDS) imaging is an interesting new tool for nondestructive testing and other applications. However, the current speed of image acquisition is relatively low, making it difficult to use for moving objects. In this paper, we propose a real-time THz-TDS line scanner based on electro-optical time-to-space conversion and line focusing of a THz beam. The proposed system functions as a color scanner in the terahertz spectral region with fast line-scanning and has been successfully used to image objects, which are moved on a translation stage. The achieved THz-TDS imaging rate is 23,200 pixels per second. This proposed THz-TDS line scanner has the potential to become a powerful tool for monitoring moving objects in various real-world applications.


Improvement of minimum paint film thickness for THz paint meters by multiple regression analysis

We propose a numerical parameter fitting method to determine the time delay between two temporally overlapped echo pulses in terahertz (THz) tomography measurements. The method is based on multiple-regression analysis with the least-squares method and is applied to decrease the minimum paint film thickness for THz paint meters. Applying multiple-regression analysis to paint thickness measurements is five times more sensitive with regard to the minimum thickness than numerical Fourier deconvolution. We apply the proposed method to determine the optical thickness, geometrical thickness, and group refractive index of dry paint film and wet paint film. The proposed method is useful for decreasing the minimum thickness for a THz paint meter and other THz tomography measurements.


超格子薄膜における異常弾性軟化現象のメカニズム:ピコ秒レーザー超音波による弾性定数測定とマイクロメカニックス解析

In this study, we observed anomalous softening in elasticity of Co/Pt superlattice thin films by measuring the out-of-plane elastic constant using the picosecond laser ultrasounds. It showed correlation with the strain energy of Co and Pt layers; as the strain energy increased, the elastic constant was lowered. In Co/Pt superlattice, the strain of each layer becomes more than 0.01 because of 10.7% lattice misfit at the interfaces, which will be partially released by occurrence of noncohesive-bonding regions. We consider that as the strain energy increases, the volume fraction of the noncohesive-bonding region increases, and the elastic constant is lowered. This view is supported by the micromechanics calculation.

Journal of the Society of Materials Science, Japan


Observation of Dermal Collagen Fiber Distribution by Second-Harmonic-Generation Microscopy

SHG(第2高調波発生光)顕微鏡を用いた真皮コラーゲン線維分布の観察


レーザー誘起ナノ秒蛍光寿命マッピングによる歯の形成と老化の可視化

レーザー誘起ナノ秒蛍光寿命マッピングによる歯の形成と老化の可視化

The auto-fluorescence characteristics of human teeth change with age. AGEs (advanced glycation endproducts), which are a crosslink of collagen, are considered to be involved in the mechanisms that cause the fluorescence characteristics to change. Although fluorescence intensity is a well-examined index of aging, it has inherent limitations. In contrast, fluorescence lifetime is useful for the evaluation because it can be reviewed quantitatively. Therefore, we mapped the distribution of fluorescence lifetime of teeth in relation to the formation and aging of teeth. The fluorescence decay profile of teeth was measured by time-correlated single photon counting, and lifetime was calculated from the fitting curve of the decay profile in a two-component exponential function. As a result, the distribution of fluorescent lifetime in dentin corresponded to tooth development; fluorescent lifetime of the crown dentin was shorter than that of the root dentin. Furthermore, the fluorescence lifetime decreased as human age increased. Because these changes in fluorescent lifetime are attributed to the accumulation of AGEs, whose fluorescence lifetime is short, our mapping of fluorescent life visualizes the formation and aging of teeth.

Transactions of the Japanese Society for Medical and Biological Engineering


Real-time two-dimensional terahertz tomography of moving objects


Study of elastic anisotropy of Cu thin films by resonant-ultrasound spectroscopy coupled with laser-doppler interferometry and pump-probe photoacoustics

Understating elastic properties of thin films is a matter of deep interest both in scientific and industrial fields. In this paper, we propose a combination of resonant-ultrasound spectroscopy coupled with the laser-Doppler interferometry and pump-probe photoacoustics for measuring anisotropic elastic constants of deposited thin films. Among the five independent elastic constants, the resonant-ultrasound spectroscopy is sensitive to the in-plane elastic constants, $C_{11}$, $C_{13}$, and $C_{66}$, and pump-probe photoacoustics to the out-of-plane elastic constant, $C_{33}$. We apply this to Cu thin films deposited on monocrystalline Si substrates by the magnetron-sputtering method. Cu thin films show elastic anisotropy, $C_{33}>C_{11}$. The cause of this elastic anisotropy is attributed to the textured structure and the columnar structure. These effects are estimated by X-ray diffraction measurements and micromechanics calculations.


Automatic characterization and segmentation of human skin using three-dimensional optical coherence tomography

A set of fully automated algorithms that is specialized for analyzing a three-dimensional optical coherence tomography (OCT) volume of human skin is reported. The algorithm set first determines the skin surface of the OCT volume, and a depth-oriented algorithm provides the mean epidermal thickness, distribution map of the epidermis, and a segmented volume of the epidermis. Subsequently, an en face shadowgram is produced by an algorithm to visualize the infundibula in the skin with high contrast. The population and occupation ratio of the infundibula are provided by a histogram-based thresholding algorithm and a distance mapping algorithm. En face OCT slices at constant depths from the sample surface are extracted, and the histogram-based thresholding algorithm is again applied to these slices, yielding a three-dimensional segmented volume of the infundibula. The dermal attenuation coefficient is also calculated from the OCT volume in order to evaluate the skin texture. The algorithm set examines swept-source OCT volumes of the skins of several volunteers, and the results show the high stability, portability and reproducibility of the algorithm.


Tomographic imaging of collagen fiber orientation in human tissue using depth-resolved polarimetry of second-harmonic-generation light


Optical glucose monitoring based on femtosecond two-color pulse interferometry


A terahertz paintmeter for non-contact monitoring of thickness and drying progress in paint film

We propose a paintmeter for noncontact and remote monitoring of the thickness and drying progress of a paint film based on the time-of-flight measurement of the echo signal of a terahertz (THz) electromagnetic pulse. The proposed method is effectively applied to two-dimensional mapping of the painting thickness distribution for single-layer and multilayer paint films. Furthermore, adequate parameters for the drying progress are extracted from the THz pulse-echo signal and effectively applied to monitor the wet-to-dry transformation. The THz paintmeter can be a powerful tool for quality control of the paint film on the in-process monitoring of car body painting.


Co/Pt超格子薄膜の弾性定数のひずみ依存性と磁気異方性

Co/Pt超格子薄膜の弾性定数のひずみ依存性と磁気異方性

In this paper, the contribution of magnetoelastic-anisotropy effect on perpendicular-magnetic anisotropy of Co/Pt superlattice thin films is studied considering the strain-dependent elastic constants. We measured the effective-magnetic-anisotropy energy changing the Pt-layer thickness (2 to 20Å) keeping the Co-layer thickness unchanged (4Å). The effective-magnetic-anisotropy energy changed depending on the Pt-layer thickness and showed a maximum near d_<pt>=12Å. This result is explained by calculating the magnetoelastic-anisotropy energy considering the change of the elastic constants caused by the lattice misfit. We measured anisotropic elastic constants of Co/Pt superlattice thin films by resonance-ultrasound-spectroscopy method coupled with laser-Doppler interferometry and the picosecond laser-ultrasound method. The measured elastic constants showed elastic anisotropy between the in-plane and out-of-plane directions. They agreed with those predicted from the strain-dependent elastic-constant model. The elastic constants show correlations with the perpendicular-magnetic anisotropy.


Multiple-scattering-free optical glucose monitoring based on femtosecond pulse interferometry


Dependence of terahertz electric field on electric bias and modulation frequency in pulsed terahertz emission from electrically-modulated photoconductive antenna detected with free-space electro-optic sampling

Japanese Journal of Applied Physics, Part 1 (Regular Papers & Short Notes)


Non-contact Measurement of Local Water Content in Skin Using a Terahertz Electromagnetic Pulse

テラヘルツ電磁波パルスを用いた非接触・局所皮膚水分量測定の開発

We propose the application of a terahertz electromagnetic pulse (THz pulse) for non-contact measurement of local water content in the skin. The free-space propagating THz pulse is generated as dipole radiation from a photoconductive antenna triggered by a femtosecond pulsed light from a mode-locked Ti : Sapphire laser. Strong interaction of the electromagnetic wave with water in the THz region enables the measurement of water content in the skin using the THz pulse. The ultrashort temporal width of the THz pulse can probe depth information on the skin in the same manner as the ultrasound pulse echo method. In this paper, we demonstrate measurements of water content and thickness of the analytical region in a human skin sample based on the time-of-flight method for THz pulse echo. A relative change of water content in the sample was determined by the change in the peak height of the THz pulse echoed from the skin surface. With respect to tomographic information on the skin, we observed THz pulse echo from an air-corneum interface and that from an epidermis-dermis interface, successively. Simultaneous quantification of the water content and thickness will be a powerful tool for monitoring the hydration level with respect to morphology of the skin.


Determination of collagen fiber orientation in human tissue by polarization measurement of molecular second-harmonic-generation light

Based on the reflection-type polarization measurement of second-harmonic-generation (SHG) light induced by collagen molecules, we are able to determine the collagen fiber orientation in human tissues taken from a cadaver. The resulting SHG radar graph shows the direction of the absolute orientation and the degree of organization of collagen fibers. To evaluate the probing sensitivity to the collagen orientation, we compared the proposed method with other polarimetric methods. Use of the proposed method revealed characteristic orientation differences among collagen fibers and demonstrated significant inhomogeneity with respect to the distribution of collagen orientation in human dentin. The proposed method provides a powerful research and diagnostic tool for examining the collagen orientation in human tissues.