異なる波長の電磁波によるモルタル中の水分と塩分の検出

異なる波長の電磁波によるモルタル中の水分と塩分の検出

コンクリート工学年次論文集


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


Incident-Angle-Dependent Extraordinary Transmission of the Terahertz Bulls-Eye Structure


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


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.


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.


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.


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.


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.


Phase and Direction Control of a Terahertz Wave Propagating in a Waveguide Coupled With a Bulls-Eye Structure


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.


Tunable Backward Terahertz-wave Parametric Oscillation

Backward optical parametric oscillation has attracted attention for cavityless spectral narrowband generation based on perfect photon conversion. Few demonstrations have shown its potential from the aspect of nonlinear photonics, therefore, the mechanisms of momentum conservation among interacting light waves have been concealed by the restricted configuration under the phase-matching condition of periodically poled structures. Here, we unveil a tunable mechanism in the terahertz region by active control of the phase-matching condition. The tunability of backward terahertz-wave parametric oscillation is investigated using a quasi-collinear phase-matching model and its frequency range from the sub-terahertz to terahertz region is identified. Transform-limited terahertz-wave pulse is achieved simply by installing a device on the pump propagating line with no cavity. Moreover, the cascading terahertz-wave generation enhances the photon conversion efficiency, thus making nonlinear optics and its applications more promising. The results highlight new capabilities for using modern ferroelectric materials and encourage further research on nonlinear optics.


Split Resonant Peaks Observed in a Bull's Eye Structure Using Oblique Incidence of Terahertz Waves

斜入射テラヘルツ波を用いたブルズアイ構造共鳴ピークの角度依存分裂の観測

Angular dependence of extraordinary terahertz (THz)-wave transmission is investigated in a bull's eye structure : a subwavelength aperture with concentric grooves. Split resonant peaks in transmission spectra are observed using high-resolution THz spectroscopy with an injection-seeded THz-wave parametric generator used as a bright, tunable THz-wave source. The resonant frequency at normal incidence agrees well with finite-difference time-domain calculation. With the incident angle varied from 0-5 deg, the resonant frequencies are tuned in the frequency range of 180 GHz with the rate of 18 GHz/deg. The behavior is well explained by the folded dispersion curve of surface plasmons at THz frequencies. The measured Q values are 27 at normal incidence, 11 and 14 at 5 deg.


Thin terahertz-wave phase shifter by flexible film metamaterial with high transmission

Thin terahertz (THz)-wave optical components are fundamentally important for integrated THz-wave spectroscopy and imaging systems, especially for phase manipulation devices. As described herein, a thin THz-wave phase shifter was developed using a flexible film metamaterial with high transmission and polarization independent properties. The metamaterial unit structure employs double-layer un-split ring resonators (USRRs) with a designed distance between the two layers to obtain phase retardance of ϖ/2, thus constituting a THz-wave phase shifter. The metamaterial design keeps the transmission coefficient as high as 0.91. The phase shifter also has polarization independence due to the four-fold symmetry of the USRR structure. Because of the subwavelength feature size of the USRR, this shifter can offer benefits for manipulating the spatial profile for the THz-wave phase through design of a binary optics phase plate by arranging a USRR array. The thickness of 48 μm has benefits for developing integrated THz optics and other applications that demand compactness and flexibility. The developed film size of 5 cm × 5 cm from the device fabrication process is suitable for THz lenses or gratings of large optical components.


Fabrication of MEMS-driven Wire Grid Polarizers in THz Region and Evaluation of the Optical Characteristics

THz波用MEMS駆動ワイヤグリッド偏光子の製作と光学特性評価

Micro-electromechanical-systems driven wire grid polarizers (MEMS-WGPs) were proposed and fabricated. MEMS-WGPs mainly consisted of an Au cantilever array and two kinds of electrical leads. One lead was connected with cantilevers and the other lead was located under the cantilevers. Because the cantilevers were electrostatically actuated downward by applying voltage between the two leads and made contact with the next cantilever, transmittance could be changed. Static wire grid polarizers were also fabricated and measured their optical characteristics in THz region, as preliminary step toward MEMS-WGPs. MEMS-WGP was successfully fabricated and the cantilever array was electrostatically actuated. In THz region, transmittance of the fabricated MEMS-WGP was changed from 14.7% to 10.6% on average by applied voltage of 200V.

IEEJ Transactions on Sensors and Micromachines

電気学会論文誌E (センサ・マイクロマシン部門誌)


Tunable terahertz waves from 4-dimethylamino-N'-methyl-4-stibazolium tosylate pumped with dual-wavelength injection-seeded optical parametric generation

We developed a widely tunable terahertz (THz)-wave source covering the sub-THz frequency by difference frequency generation using a 4-dimethylamino-N'-methyl-4'-stibazolium tosylate (DAST) crystal. Near-infrared waves generated by dual-wavelength injection-seeded β-BaB2O4 optical parametric generation (is-BBO-OPG) were used for pumping the DAST crystal, which had separated wavelengths in the spectrum with a difference frequency of sub-THz. Furthermore, the non-collinear phase-matching condition was designed to compensate the walk-off effect of the BBO crystal. Consequently, tunable THz-waves from 0.3 to 4 THz were generated by tuning the wavelength of one of the seeding beams. The generated sub-THz-waves were monochromatic (dν < 33 GHz) with a maximum energy of 80 pJ at 0.65 THz.


High-Brightness Continuously Tunable Sub-Terahertz Wave Generation

We report on the generation of high-brightness (high peak power and narrow linewidth) continuously tunable subterahertz waves by optical parametric wavelength conversion using lithium niobate as a nonlinear optical crystal. The high-brightness subterahertz waves were achieved by expanding the interaction volume of the pumping beam and the idler wave, as well as the subterahertz wave, during wavelength conversion and by injecting the seeding beam into the idler wave. We could continuously tune the wavelengths of the generated subterahertz waves by controlling the angle and wavelength difference between the pumping and injection seeding beams to satisfy the noncollinear phase-matching conditions. The high-brightness (maximum peak output power, linewidth) and tunable range were about 10 MW/sr· cm 2 (>7 kW, < 5 GHz), and 760-150 μm (0.39-2 THz), respectively.

IEEE Transactions on Terahertz Science and Technology


THz-wave parametric gain of stimulated polariton scattering

We have experimentally determined the terahertz- (THz-) wave parametric gain of stimulated Raman scattering (SRS) by phonon-polaritons in LiNbO3. Our approach is based on ultrabright THz-wave generation from SRS under stimulated Brillouin scattering suppression with subnanosecond pump pulses. To obtain the frequency dependence of the parametric gain, we measured the crystal-length dependence of the THz-wave output directly using a surface-coupling configuration. We found that the product of the parametric gain and the threshold crystal length is constant throughout the tuning range. Our result provides a physical basis for the design and performance enhancement of SRS-based ultrabright tabletop THz-wave sources for various applications.


Handedness control in a 2-μm optical vortex parametric oscillator

We present the first handedness control of an optical vortex output from a vortex-pumped optical parametric oscillator. The handedness of the optical vortex was identical to that of the pump vortex beam. Over 2 mJ, 2-μm optical vortex with a topological charge of ± 1 was achieved. We found that the handedness of a fractional vortex with a half integer topological charge can also be selectively controlled.


Efficient high-quality picoseconde Nd:YVO4 bounce laser system

A high-average-power, high-beam-quality picosecond laser is presented that is based on CW diode side-pumped Nd:YVO4 thin-slab amplifiers having a bounce geometry. More than 65 W of output power with a beam quality of 𝑀2≈1.3 is obtained at an optical–optical efficiency of 46%.

Journal of the Optical Society of America. B, Optical Physics


Tunable 2-μm optical vortex parametric oscillator

We generated tunable 2-μm optical vortex pulses with a topological charge of 1 or 2 in the wavelength range 1.953–2.158 μm by realizing anisotropic transfer of the topological charge from the pump beam to the signal output in a vortex-pumped half-symmetric optical parametric oscillator. A maximum vortex output energy of 2.1 mJ was obtained at a pump energy of 22.8 mJ, which corresponds to a slope efficiency of 15%. The topological charges of the signal and idler output were investigated using a shearing interferometric technique employing a low-spatial-frequency transmission grating.


Global evaluation of closed-loop electron dynamics in quasi-one-dimensional conductors using polarization vortices

We evaluate the quasi-one-dimensional (1D) electron dynamics in a NbSe3 ring crystal using polarization vortex pulses with various azimuthal distributions. The single particle relaxation component reveals a large anisotropy on the crystal, indicating that the electrons in the ring maintain their 1D character. The results also suggest that the polarization vortex evaluates the global polarization property of the closed-loop electron that plays an important role in the quantum correlation phenomena such as the Aharonov-Bohm effect.


Supercontinuum optical vortex pulse generation without spatial or topological-charge dispersion

A new achromatic method to generate the optical vortex was proposed and supercontinuum optical vortex generation ranging ~500 to ~800 nm was experimentally demonstrated without spatial nor topological-charge dispersions. In addition, polarization evolution in our system using Jones vectors and matrices was discussed and the condition of the polarizer to transfer polarizations was elucidated. This method is useful for the application to time-resolved nonlinear spectroscopy utilizing ultrabroadband optical vortex pulses in topological materials such as ring-shaped crystals or annular materials.


Frequency-agile injection-seeded terahertz-wave parametric generation

An injection-seeded terahertz (THz)-wave parametric generator (is-TPG) enables access to low-frequency fingerprints of molecules in the THz frequency region. However, its conventional scan repetition rate (SRR) is limited below 1 Hz. Thus, in this Letter, we propose an electrically controlled tuning system for the is-TPG, which provides high-speed scanning and random hopping agility. We achieved rapid THz frequency scanning on a pulse-by-pulse basis by employing a gain-switched laser diode and a micro-electromechanical system tunable vertical-cavity surface-emitting laser as the pump and seed lasers. A THz spectrum was acquired with a 10 times higher SRR of 10 Hz for the 1.6 3 THz range with a sampling resolution of 4.6 GHz.


High-average and high-peak output-power terahertz-wave generation by optical parametric down-conversion in MgO:LiNbO3

A widely tunable terahertz (THz)-wave generation that has a high repetition rate and a narrow line width is demonstrated in this paper by injection-seeded THz-wave parametric generation (is-TPG) in a MgO:LiNbO3 crystal. By pumping the crystal using a passively Q-switched neodymium-doped yttrium vanadate microchip laser with a time duration of 140 ps and an average power of up to 5 W, a THz-wave output with an average output power of 30 μW, a peak power of 4 W, a pulse duration of 73 ps, and a pulse-repetition frequency of 100 kHz is obtained. To prevent laser damage and photorefractive damage to the crystal, the constraints on the pumping condition of the MgO:LiNbO3 crystal are experimentally studied by changing the pumping parameters. As a result, we achieved the stable generation of the THz-wave signal in the 100 kHz regime. Moreover, we performed THz-wave imaging by using the developed is-TPG source. The obtained THz image indicated that the developed system has a good stability over long periods of time.


Off-resonance and in-resonance metamaterial design for a high-transmission terahertz-wave quarter-wave plate

This Letter describes a novel metamaterial design by employing off-resonance and in-resonance excitation for a high-transmission terahertz-wave quarter-wave plate (QWP). The device is demonstrated with a thin film metamaterial with double-layer split ring resonators (SRRs). Different from a usual resonant metamaterial device, here we design the work frequency off from the inductor-capacitor (LC) resonance for the TE mode, while in a dipole resonance for the TM mode to obtain the artificial birefringence. Rectangular SRRs in this Letter provide a choice to optimize the off-resonance and in-resonance excitation, to assist the double-layer design for high transmission. Converting a linearly polarized wave to circular polarization with our QWP, the experiment confirms a transmittance of 0.8 and an ellipticity of 0.99 at 0.98 THz. The developed thin film device is flexible and has a thickness of 48 μm (sub-wavelength). This is an advantage for potential integration in systems where overall device compactness is required.


Transfer of Light Helicity to Nanostructures

We discovered that chiral nanoneedles fabricated by vortex laser ablation can be used to visualize the helicity of an optical vortex. The orbital angular momentum of light determines the chirality of the nanoneedles, since it is transferred from the optical vortex to the metal. Only the spin angular momentum of the optical vortex can reinforce the helical structure of the created chiral nanoneedles. We also found that optical vortices with the same total angular momentum (defined as the sum of the orbital and spin angular momenta) are degenerate, and they generate nanoneedles with the same chirality and spiral frequency.