Boron ion implantation on femtosecond-laser-irradiated diamond surface


Characterization of Femtosecond Laser-induced Structural Changes in CVD Diamond by Raman Spectroscopy


Raman Studies of Structural Change in Diamond-like Carbon Film on Si Induced by Ultrafast Laser Ablation


Femtosecond laser-assisted thermal annealing of Ni electrode on SiC substrate


Femtosecond-laser-induced modifications on the surface of a single-crystalline diamond


Femtosecond-Laser-Induced Defects on Silicon Carbide Probed by Electrical Conductivity


Formation of x-ray Newton's rings from nano-scale spallation shells of metals in laser ablation


Femtosecond laser-induced modification at aluminum/diamond interface


The Shape of The Exfoliated Surface during Femtosecond Laser Ablation


Low-Temperature Diffusion at Ni/SiC Interface with the Aid of Femtosecond Laser-Induced Strain


Self-organization of highly ordered honeycomb buckling patterns in crystalline thin films


Instability of Critical Electric Field in Gate Oxide Film of Heavy Ion Irradiated SiC MOSFETs


Hydrodynamics driven by ultrashort laser pulse: simulations and the optical pump X ray probe experiment


Submicron scale image observation with a grazing incidence reflection-type single-shot soft X-ray microscope

A grazing incidence reflection-type soft X-ray microscope, using a Fresnel zone plate and a soft X-ray laser with wavelength 13.9 nm and pulse width 7 ps, was developed. Submicron size groove structures made on a Pt film were clearly captured at a single shot exposure, with spatial resolution of about 360 nm. A wide field view of 100 µm square was secured under the Kohler illumination. This microscope also had a large depth of focus of more than 100 µm and was proven to have a sufficient performance for observing surface morphological changes.


Time-resolved soft x-ray imaging of femtosecond laser ablation processes on metals


Single Event Gate Rupture in SiC MOS Capacitors with Different Gate Oxide Thicknesses


Temperature Dependence of Electric Conductivities in Femtosecond Laser Modified Areas in Silicon Carbide


Linear energy transfer dependence of single event gate rupture in SiC MOS capacitors

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms


Raman intensity profiles of zone-folded modes in SiC: Identification of stacking sequence of 10H-SiC


Laser-Scanning Direction Effect in Femtosecond Laser-Assisted Etching

In this study we examined how the laser-scanning direction during the inscription of modified lines affects to the etching rate along the lines in the femtosecond laser-assisted etching. An isotropic material (silica) was used as the sample. Circular polarization was used for inscribing modified lines, in order to avoid a direction effect arising from polarization. Scanning direction dependence was observed in the etching rate along the modified lines. The relation between the occurrence of a scanning direction effect and the other inscribing parameters (pitch and pulse energy) was examined.


Experimental verification of femtosecond laser ablation schemes by time-resolved soft x-ray reflective imaging

Pump and probe reflective imaging using a soft x-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. In addition, the point where the reflectivity dip was observed at t = 10 ps and t = 40 ps, coincides with the position of the edge of reflectivity drop at t = 160 ps. These results give the critical information about the femtosecond laser ablation.


Formation of Nanovoids in Femtosecond Laser-Irradiated Single Crystals of Silicon Carbide


Microscopic Raman mapping of epitaxial graphene on 4H-SiC (0001)

We propose a quality control method for wafer-scale epitaxial graphene grown on SiC substrates. The peak position of Raman spectra of epitaxial graphene is an excellent indicator of film quality and reveals irregularities, such as graphene thickness inhomogeneity and SiC substrate defects. A comparison of microscopic Raman maps and scanning probe microscopy images of the same position of the sample revealed that wave numbers of Raman peaks (G and 2D band peaks) were strongly correlated with the strain in the graphene film. The increase in number of graphene layers (2 to 3--4 layers) induced phonon softening ({im}6 cm-1) and broadening ({im}6 cm-1) of the 2D band peak. Significant phonon softening and abnormal broadening of the Raman peaks were observed at residual scratches on the SiC substrate. The quantitative layer number distribution of graphene on SiC is successfully estimated from the wave number distribution of the 2D band peak.

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


Reduction of Pulse-to-Pulse Fluctuation in Laser Pulse Energy using the Optical Kerr Effect

An all-optical stabilization method of laser pulse energy is proposed using the optical Kerr effect (OKE). The method uses the OKE induced by a portion of the laser pulse as a power controller. The decrease (increase) in the throughput of the optical setup for OKE compensates for the increase (decrease) in pulse energy, thereby stabilizing the pulse-to-pulse fluctuation in pulse energy. The validity of this principle was proven by experiments with a femtosecond laser.


Electrical conduction properties of SiC modified by femtosecond laser


Picosecond Soft-X-ray Laser Interferometer for Probing Nanometer Surface Structure


Femtosecond Laser-Assisted Etching of Fluoride Crystals


Surface roughness assisted 100 kHz femtosecond laser induced nanostructure formation on silicon surface

Applied Physics. A, Materials Science & Processing


Enhancement of local electrical conductivities in SiC by femtosecond laser modification


Single-shot picosecond interferometry with one-nanometer resolution for dynamical surface morphology using a soft X-ray laser

Using highly coherent radiation at a wavelength of 13.9 nm from a Ag-plasma soft X-ray laser, we constructed a pump-and-probe interferometer based on a double Lloyd's mirror system. The spatial resolutions are evaluated with a test pattern, showing 1.8-mum lateral resolution, and 1-nm depth sensitivity. This instrument enables a single-shot observation of the surface morphology with a 7-ps time-resolution. We succeeded in observing a nanometer scale surface dilation of Pt films at the early stage of the ablation process initiated by a 70 fs near infrared pump pulse.


Microscopic analysis of carbon phases induced by femtosecond laser irradiation on single-crystal SiC


Micro Three-dimensional Removal Processing inside Sapphire Substrate


Raman spectroscopic study of femtosecond laser-induced phase transformation associated with ripple formation on single-crystal SiC


Study of indentation damage in single crystal silicon carbide by using micro Raman spectroscopy


Electronic properties of femtosecond laser induced modified spots on single crystal silicon carbide


Raman Spectroscopic Stress Evaluation of Femtosecond-Laser-Modified Region Inside 4H-SiC

A femtosecond (fs)-laser-modified region inside single-crystal silicon carbide was studied by micro-Raman spectroscopy. Higher and lower peak energy shifts of the transverse optical (TO) phonon mode, which correspond to compressive and tensile stresses, were observed. Mappings of peak energies and spectral widths of the TO phonon mode showed a clear correspondence with the distributions of strained layers observed by transmission electron microscopy. The maximum compressive and tensile stresses were estimated to be 1.4 and 0.4 GPa, respectively. This result indicates that the periodic strained layers contain many nano-voids which are formed by nano-explosions induced by fs laser irradiation.


Fourier-transform Infrared Spectroscopy of Femtosecond Laser-modified SiC


Femtosecond laser-assisted etching of Pyrex glass with aqueous solution of KOH


Formation of periodic strained layers associated with nanovoids inside a silicon carbide single crystal induced by femtosecond laser irradiation


Cross-sectional morphological profiles of ripples on Si, SiC, and HOPG


Laser Fabrication and Manipulation of an Optical Rotator Embedded inside a Transparent Solid Material


Femtosecond Laser-Induced Surface Patterning on 4H-SiC


Preparation of large-scale 2D zeolite crystal array structures to achieve optical functionality --- 基板上にマイクロメートルスケールの円柱型ゼオライトL結晶を組織的に配列する方法について検討を行った．まず，アスペクト比1程度の粒子を細密構造で縦配列する方法を検討した．次にグルーブ構造の基板に対してアスペクト比の大きい結晶をグルーブに沿って横はい列する方法を検討した．後者は蛍光色素を担持した粒子を用いた場合，励起光の偏光方向に応答することを見出した．ゼオライト結晶の配列による材料作製の方向性を示すことができた．

We examined a method to refine a technique to prepare close-packed monolayers of cylindrical zeolite L crystals tethered on a substrate through covalent linkages, which was developed recently for popular use. We attempted to fabricate both vertical and horizontal-unidirectional alignments of the zeolite particles. The vertical alignment for centimeter scale large areas was achieved on a flat glass substrate by using flat-based zeolite crystals with an aspect ratio (length divided by width) of 1.1 while the crystals with an aspect ratio of 1.7 failed to align vertically. Thus the parameters important for successful alignment were determined. On the other hand, the unidirectional alignment in the horizontal packing structures was achieved by employing pregrooved substrates. We found that the relationship between the sizes of zeolite crystals and grooves is a crucial factor for the preparation of a close-packed structure. In this respect, our present preparation requires further improvements because it resulted in structures with many void spaces. Furthermore, we prepared the zeolite films loaded with unidirectionally aligned dyes to achieve optical functionalities.


In-situ spectroscopic measurements of laser ablation-induced splitting and agglomeration of metal nanoparticles in solution --- 金フレークに対して溶液中でパルスレーザー照射することにより金ナノ粒子を作製し，その様子を分光法によりその場観察した．溶媒としてドデカンチオールを用いた場合，生成する金ナノ粒子を非常に小さくすることが可能となった．すなわち，プラズモンバンドが消失する2-3nm以下にすることができた．また，水中においてレーザーアブレーションによる金ナノ粒子のサイズ増大と凝集沈殿も観測した．この現象は照射雰囲気に強く依存し，酸素雰囲気下で増強され，アルゴン雰囲気化で抑制された．サイズ増大はレーザー照射中に生成するカチオン種とアニオン種の結合によることが示唆された．

This report describes the pulsed-laser induced events of noble metal flakes and nanoparticles (NPs) in liquids studied by an in situ extinction spectroscopy which allowed the detection of time-dependent events such as the pulverization of flakes concomitantly with the growth of nanoparticles (NPs) and the splitting of NPs into smaller particles. Two key observations were made in the present study. First, we observed a remarkable spectral modification ascribable to a size reduction during the 532 and 1064 nm laser ablation of the flakes from NPs with a characteristic surface plasmon (SP) band to much smaller NPs (< 2-3 nm diameter) devoid of the SP band in neat and highly concentrated dodecanethiol solutions at moderate laser fluences of 0.7-1.3 J/(cm2 pulse). Previously the production of such small particles was not feasible by the 1064 nm laser light. Only in solvents that thermally decompose by the laser irradiation to give graphite shells to cover NPs or by two-step size reduction procedures, i.e. the initial formation of NPs followed by further splitting due to the absorption of the 532 nm laser light, such observations have been made. Thus we demonstrated a superior nature of dodecanethiol as a solvent to protect the NPs from growing over other solvents such as acetone and water. Second, we observed the laser ablation-induced agglomeration of Au NPs leading to immediate flocculation in pure water. The observation taking place in the time scale of minutes is conceptually different from the previously observed photo-induced coagulation (Kimura, J. Phys. Chem.1994, 98, 2143-2147 & J. Phys. Chem., 1994, 98, 11997-12002), in which a photo-enhanced van der Waals attractive force acting on excitation of the SP band is responsible for the coagulation phenomena taking place over many hours. In the present case, small positively-charged particles produced by the laser ablation of Au NPs by a 532 nm nanosecond pulsed-laser irradiation are responsible for immediate combination reactions with intact negatively-charged species giving rise to unstable bigger particles susceptible to developing flocculations. This deduction is based on the retardation effect of the agglomeration observed in various irradiation atmosphere such as Ar, and O2. Thus the present study shed a light on another aspect of the laser ablation in liquids relevant to the time evolution of NP formation and its growth dynamics in seconds and minutes, which was paid less attention before.


Three-Dimensional Residue-Free Volume Removal inside Sapphire by High-Temperature Etching after Irradiation of Femtosecond Laser Pulses

We applied the femtosecond laser-assisted etching technique, that is, irradiation of focused femtosecond laser pulses followed by selective chemical etching, to volume removal inside sapphire. At room temperature, volume etching only slightly advanced while residue remained inside the volume. By increasing the etching temperature, complete volume etching without residue was achieved. Complete etching was, however, accompanied by undesirable phenomena of surface pits or cracks, which are expected to be excluded through further improvement of processing.


Laser microfabrication and rotation of ship-in-a-bottle optical rotators


Cross-sectional TEM analysis of laser-induced ripple structures on the 4H-SiC single-crystal surface

Applied Physics. A, Materials Science & Processing


Examination of silver nanoparticle fabrication by pulsed-laser ablation of flakes in primary alcohols --- アルコール中でマイクロメートルサイズの銀フレークのナノ秒レーザーを用いたレーザーアブレーションによる銀ナノ粒子の作製を行った．アルコールの炭素鎖が短い場合，生成するナノ粒子は不安定で凝集したが，プロパノール，ブタノール，ペンタノールでは粒子サイズが小さくかつ安定にナノ粒子が形成された．また，炭素鎖がさらに長い場合はやはり凝集体を形成した．レーザー照射雰囲気として，空気雰囲気とアルゴン雰囲気について検討したところ，アルゴン雰囲気のほうが安定にナノ粒子を作製できることが分かった．レーザーアブレーション法により銀ナノ粒子を作製する場合の実験条件の詳細を明らかにすることができた．

This paper describes the formation and subsequent behavior of silver nanoparticles (NPs) observed by optical spectra, AFM (atomic force microscopy), and TEM (transmission electron microscopy) in primary alcohols by nanosecond pulsed-laser irradiation (1064 and 532 nm, typically at 1 J/(cm2 pulse)) of silver flakes. Effects of the carbon chain-length of the solvents and the irradiation atmosphere of the solutions on the Ag NP formation were investigated. The effect of alcohol chain length in aerated solutions can be described as follows: (1) in short-chain alcohols such as methanol and ethanol, the NPs are extremely unstable and easily settled down to form precipitates by centrifugation treatment; (2) very stable NPs are formed with an appreciably smaller particle size distribution in alcohols with chain lengths from C-3 to C-5 than in alcohols with longer chain length than C-5; (3) the yield of NPs are dependent on the alcohol chain length. On the other hand, the yield of NPs is greater in Ar- and N2-saturated solutions than that in aerated solutions. Besides, the yield is similar regardless of the chain length with smaller size distributions than in air-equilibrated solutions. Oxygen molecules dissolved in the solvents are responsible for these observations. The oxygen effect consists of two parts: (1) the scavenging of electrons generated by the plasma formation and thermoionic emission due to extremely high temperature under the ablation condition; (2) the formation of oxide layer on the surface of particles that hamper further growth processes to form NPs. Furthermore, we observed the formation of string segments in evacuated ethanol due to coagulation and coalescence of bare metal particles, giving rise to the splitting of the plasmon band. Thus we demonstrated that the systematic change in the solvent and irradiation atmosphere can control the particle size and size distribution. The present findings may add a new aspect to better manipulate NP fabrication based upon laser ablation method.


Observation of laser-induced surface waves on flat silicon surface


Applications of microlens array and photomask to the laser microfabrication of periodic photopolymer rod array

We report the simultaneous multipoint fabrication of polymer rods by the femtosecond laser processing of a negative photoresist using a microlens array (MLA). The rods were periodically arranged in the form of an array corresponding to the MLA and free-standing on a glass substrate. The use of a photomask enabled us to define the contour of the rod array. Furthermore, sample translation techniques were demonstrated for the effective fabrication of large-area structures.


Femtosecond Laser-Induced Ripple Structures in Semiconductor Materials


Laser Fabrication of Ship-in-a-bottle Microstructures in Sapphire


Effect of surface roughening on femtosecond laser-induced ripple structures


Observation of surface polarity dependent phonons in SiC by deep ultraviolet Raman spectroscopy

Physical Review B, Condensed Matter and Materials Physics


Source of Surface Morphological Defects Formed on 4H-SiC Homoepitaxial Films

Plan-view transmission electron microscopy (TEM) was carried out to investigate the source of morphological defects formed on the surface of 4H-SiC homoepitaxial films. The source at the substrate/epifilm interface consisted of an inclusion and partial dislocations emerging from it. Selected-area diffraction pattern analysis, energy dispersive X-ray spectroscopy and micro-Raman spectroscopy revealed that the chemical composition of the inclusion was ZrO2. From the comparison between TEM images and calculated images, it was suggested that the partial dislocations were sheared Frank dislocations with the Burgers vector of the (1/12)$angle 4ar{4}03angle$ type.

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


Structures of Comets in a Homoepitaxially Grown 4H-SiC Film Studied by DUV Micro-Raman Spectroscopy


Distinct Fine and Coarse Ripples on 4H-SiC Single Crystal Induced by Femtosecond Laser Irradiation

Upon femtosecond pulsed-laser irradiation, periodic structures referred to as ``ripples'' were fabricated on the surface of a 4H-SiC single crystal. The periodic structures consisted of two concentric regions in the irradiated spots which were clearly distinguished by the period. Surface morphologies were characterized as a function of energy, accumulation number, and interval of pulses. The difference in the threshold of fine and coarse ripples was identified for the first time. The possible formation mechanisms of these structures were discussed.

Japanese Journal of Applied Physics, Part 2 (Letters)


Deep-ultraviolet micro-Raman investigation of surface defects in a 4H-SiC homoepitaxially grown film


Pump-Probe Spectroscopy of Exciton Spin Injection Process in Diluted Magnetic Quantum Wells

Journal of Superconductivity: Incorporating Novel Magnetism


Spin Injection Processes in ZnSe-Based Double Quantum Dots of Diluted Magnetic Semiconductors


Structure dependent ultrafast relaxation time of photo-excited carriers in SiC

The ultrafast inter-conduction band carrier dynamics in 6H-SiC and 4H-SiC was observed by using the pump and probe transient absorption technique. The observed decay times of transmission changes are 1.25ps for 6H-SiC and 630 fs for 4H-SiC, respectively. Dependence of the transmission change on probe wavelength and polarization were analyzed by using the steady-state absorption profiles. From this analysis, the transmission change was ascribed to the decrease of electron population in the lowest conduction band. The experimental results show that the decay of the transmission change was dominated by the inter-band electron phonon scattering. The inter-band deformation potential in 4H-SiC, deduced from the observed decay time, was 1.25 times larger than that in 6H-SiC. This polytype dependence of inter-band deformation potential is discussed in terms of the hexagonality and band-gap energies.

Journal of the Physical Society of Japan


Interface magnetic properties of MnSe digitally doped in ZnSe-based quantum wells


Subpicosecond dynamical renormalization of spin-polarized electron-hole plasma in Cd1-xMnxTe

Physica E: Low-dimensional Systems and Nanostructures


Dynamics in magneto-optical properties of digital magnetic Zn1-xCdxSe quantum wells


Exciton dynamics in quantum nano-structures of II-VI diluted magnetic semiconductors fabricated by electron-beam lithography

Physica Status Solidi (C) Current Topics in Solid State Physics


Magnetic-field-induced switching of spin injection in Zn1-xMnxTe/ZnTe double quantum wells


Ultrafast exciton spin dynamics in Cd1-xMnxTe quantum wells studied by transient pump-probe spectroscopy


Spin injection and spin switching of excitons in coupled quantum wells of diluted magnetic semiconductors


Spin dynamics of excitons and magnetic ions in Cd1-xMnxTe/Cd1-yMgyTe quantum wells


Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1-xMnxTe/ZnTe Quantum Wells


Effects of Digital Doping of Mn on Giant Magneto-Optical Properties of Zn1-xMnxSe Quantum Wells

Journal of Superconductivity: Incorporating Novel Magnetism


Excitonic Propeties and Dyanamics in Diluted Magnetic Semiconductor Quantum Nanostructures


Inter-conduction band electron relaxation dynaimcs in 6H-SiC


Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg,Zn)O multiple quantum wells


Raman intensity profiles of folded longitudinal phonon modes in SiC polytypes

Physical Review B, Condensed Matter and Materials Physics


Selective resonance effect of the folded longitudinal phonon modes in the Raman spectra of SiC


Raman Spectral Profiles of Folded Longitudinal Modes in SiC under Off-resonant Condition


Selectively Resonant Raman Spectra of Folded Phonon Modes in SiC


Dynamical study of optical spin injection in ZnMnSe/ZnCdSe double quantum wells

Extended Abstracts of The 10th Symposium on the Physics and Application of Spin-Related Phenomena in Semiconductors


Magnetic-field-induced switching of spin injection in a Zn1-xMnxTe/ZnTe double quantum well

Extended Abstracts of The 9th Symposium on Physics and Application of Spin-related Phenomena in Semiconductors


Transient photoluminescence spectroscopy of spin injection in CdTe-Cd1-xMnxTe double quantum wells


Spin dynamics of excitons in Cd1-xMnxTe / Cd1-yMgyTe quantum wells studied by transient pump-probe spectroscopy


Ultrafast dynamics of spin-polarized exciton formation and relaxation in Cd1-xMnxTe epilayers

Extended Abstracts of The 8th Symposium on The Physics and Application of Spin-related Phenomena in Semiconductors


Spin injection and spin-dependent level crossing of excitons in double quantum wells of II-VI diluted magnetic semiconductors


Low-temperature diffusion assisted by femtosecond laser-induced modifications at Ni/SiC interface


Application of femtosecond laser irradiation to low-temperature diffusion at the Ni/SiC interface