金島 圭佑

Abstract Observing ultrafast pulse-to-pulse dynamics of highly photoexcited materials could foster a comprehensive understanding of the initial stage of irreversible photoinduced events, such as phase change, structural deformation, and laser ablation. In this study, using high-repetition-rate single-shot spectroscopy and a laser microscope, the pulse-to-pulse ultrafast dynamics of energy relaxation in Ge₂Sb₂Te₅ thin films are revealed under high-density photoexcitation that induces sequential events involving the crystalline-to-amorphous phase transition, melt and quench processes, and formation of laser-induced periodic surface structures (LIPSSs). Above the threshold excitation density for LIPSS formation, the first excitation pulse induces the transient transmittance change of the crystalline phase in a picosecond timescale, and subsequent pulses provoke the amorphous phase energy relaxation with a long decay time of hundreds of picoseconds. We observed that the subsequent pulses gradually reduce the amplitude and decay time of the transient transmittance, leading to efficient energy relaxation and LIPSS formation in the photoinduced amorphous phase.

Objective lenses are frequently employed in state-of-the-art ultrafast time-resolved microscopy techniques. While it enables tight spatial focusing, its dispersion causes a longer optical pulse duration. Since an objective lens is a combination of different lens materials, finding its correct dispersion can be challenging. In this paper, we propose a dispersion measurement method for an objective lens using white light interferometry. Our proposed method enables the experimental determination of the lens dispersion and improves the temporal resolution of ultrafast time-resolved microscopy techniques.

Chirality of molecules play an important role in biological activity. Chiral molecules are not superimposable on their own mirror images, called enantiomers, and do not have a symmetry plane nor a center of inversion symmetry. The chemical reactivity of the enantiomers sometimes depends on the chirality. Hence the distinguishing the enantiomers is important and circularly polarized light has been used to discern the enantiomers for a long time. The development of ultrashort pulse technology enables us to investigate nonlinear interactions of chiral molecules with an intense light field. Among various nonlinear optical interactions, we focus on high harmonic generation as the probe into molecular chirality. Here, we describe two our recent results: one is circularly polarized high harmonic generation (CP-HHG) from a chiral molecule, limonene, driven by a CP counter-rotating two-color laser field. The other is the development of elliptically polarized single-order high harmonic light source for the application to photoelectron spectroscopy of chiral molecules.

A scanning probe microscope coupled with either femtosecond laser pulses or terahertz pulses holds great promise not only for observing ultrafast phenomena but also for fabricating desirable structures at the nanoscale. In this study, we demonstrate that a few-nanometer-scale phase change can be non-thermally stored on the Ge2Sb2Te5 surface by a laser-driven scanning tunneling microscope (STM). An atomically flat Ge2Sb2Te5 surface was irradiated with the optical near-field generated by introducing femtosecond laser pulses to the STM tip-sample junction. The STM topographic images showed that few-nanometer-scale mounds appeared after irradiation. In addition, tunneling conductance spectra showed that the bandgap increased by 0.2eV in the area of 5x5nm(2). These indicate that the nanoscale crystal-to-amorphous phase change was induced by the STM-tip-induced near field. Our approach presented here offers an unprecedented increase in the recording density of optical storage devices and is, therefore, expected to facilitate the development of next-generation information technology.

We studied the phase properties of circularly polarized high harmonics (CP-HHs) via interferometric measurements using a mixed gas. The results showed that the phase properties of CP-HHs are well described by a semiclassical model. (C) 2020 The Author(s)

We demonstrate the simultaneous observation of the electronic and vibrational dynamics of the photo-isomerizing 1,3-cyclohexadiene via time-resolved high-harmonic spectroscopy. The observed attosecond high-harmonic interference reveals how the excited-state ionization potential evolves along the reaction coordinate. (C) 2019 The Author(s)

The formation of nitrous acid (HONO) through photolysis of o-nitrophenol was captured in real time by time-resolved photoelectron spectroscopy using single-order high harmonic probe pulses. HONO is released 250 fs after photoexcitation.

We report, to the best of our knowledge, the first time-resolved high-harmonic spectroscopy (TR-HHS) study of a chemical bond rearrangement. We investigate the transient change of the high-harmonic signal from 1,3-cyclohexadiene (CHD), which undergoes ring-opening and isomerizes to 1,3,5-hexatriene (HT) upon photoexcitation. By associating the variation in the harmonic yield to the changes in the electronic state and vibrational frequencies of the molecule due to isomerization, we find that the CHD excited via two-photon absorption of 3.1 eV photons isomerizes to HT, i.e., ring-opening occurs, around 400 fs after the excitation. The present results demonstrate that TR-HHS, which can track both electronic and nuclear dynamics, is a powerful tool for studying ultrafast photochemical reactions.

Photoexcitation effects on ferroelectricity were investigated in a hydrogen-bonded supramolecular cocrystal by using nonlinear femtosecond spectroscopy. Irradiation of a terahertz pulse that caused proton vibrations modulated the second harmonic (SH) signal in proportion to the applied electric field of the pulse and the magnitude of the change in the SH signal reached about 10%. By contrast, a mid-infrared pulse exciting the molecular vibration nonlinearly enhanced the SH intensity by about 15%. The proton dynamics were analyzed on the basis of the nonlinear equation of motion of the molecules. The calculated results indicate that the resultant dynamical repositioning of the protons caused the averaged increase in SH intensity. Thus, photoexcitations can be viewed as a novel technique for controlling the ferroelectric cocrystal at room temperature.

Circularly polarized high harmonic generation from a chiral molecule was found to depend both on the chirality and on the rotating direction of the circularly polarized counterrotating two-color driving laser fields.

Time-resolved high-harmonic spectroscopy (TR-HHS) of the photo-isomerization dynamics of 1,3-cyclohexadiene is demonstrated. The present results show that TR-HHS can track both the electronic and the nuclear dynamics, and is useful for unveiling ultrafast photo-chemical reactions.

We observed propagating-surface plasmon-assisted photoemission from a grating irradiated by mid-IR laser pulses by measuring the incidence-angle dependence of photoelectron spectra. The results reveal that emitted electrons are further accelerated by locally enhanced electric fields.

Circularly polarized high harmonic generation from a chiral molecule was found to significantly depend both on the chirality and on the rotating direction of the circularly polarized counter-rotating two-color driving laser fields.

We have developed an ab-initio simulation for high-order-harmonic generation from bulk GaSe. The simulation shows even-order harmonics are exclusively generated from the interband current while odd-order ones are dominated by intraband current below the band-gap.

An optical parametric chirped-pulse amplifier (OPCPA) based on bismuth triborate (BiB3O6, BIBO) crystals has been developed to deliver 1.5 mJ, 10.1 fs optical pulses around 1.6 mu m with a repetition rate of 1 kHz and a stable carrier-envelope phase. The seed and pump pulses of the BIBO-based OPCPA are provided from two Ti: sapphire chirped-pulse amplification (CPA) systems. In both CPA systems, transmission gratings are used in the stretchers and compressors that result in a high throughput and robust operation without causing any thermal problem and optical damage. The seed pulses of the OPCPA are generated by intrapulse frequency mixing of a spectrally broadened continuum, temporally stretched to approximately 5 ps then, and amplified to more than 1.5 mJ. The amplified pulses are compressed in a fused silica block down to 10.1 fs. This BIBO-based OPCPA has been applied to high-flux high harmonic generation beyond the carbon K edge at 284 eV. The high-flux soft-x-ray continuum allows measuring the x-ray absorption near-edge structure of the carbon K edge within 2 min, which is shorter than a typical measurement time using synchrotron-based light sources. This laser-based table-top softx- ray source is a promising candidate for ultrafast soft x-ray spectroscopy with femtosecond to attosecond time resolution.

<p>近年の光パラメトリック増幅手法の進展によって，中赤外域での高強度極短パルス発生が可能となった．例えば，マイクロジュールクラスの中赤外フェムト秒パルスを集光することにより100MV/cmを超える光電場の発生が可能である．また，光子エネルギーが典型的な物質のバンドギャップと比べて十分小さいことから，多光子イオン化が起きにくく，固体中に非破壊的に強い光電場を印加してコヒーレントな電荷の運動を誘起したり，ナノ構造体における電場増強を利用して光電子を放出させたりすることが可能である．本稿では，我々の研究室で開発された高強度中赤外光源と，それを用いた固体における強光子場現象への応用を概説する．</p>

Intense mid-infrared pulses from a dual-wavelength optical parametric amplifier are used to investigate polarization rotation of high harmonics generated from gallium selenide. Simulations reveal the polarization rotation originated in the gradient of an energy band.

We present laser-induced electron diffraction measurements of elastic differential scattering cross sections (DCSs) of a photoelectron on the parent ion for argon, krypton, and xenon, using waveform-controlled few-cycle pulses. Considering only cutoff electrons and employing the adiabatic theory for the analysis enables us to eliminate ambiguities in extracting the DCSs from experimental spectra. Contrary to previous works, which mainly focused on the angular dependence of the DCS, our method allows us to extract also its dependence on the scattering momentum. In the case of xenon, we demonstrate how this method can be used to obtain the complete angular and momentum dependence of the DCS in a range of these variables determined by the pulse. The obtained results are compared to theoretical calculations based on the single-active-electron approximation, which shows a high level of agreement. Further investigations may provide opportunities to study multielectron effects when more advanced theoretical models become available.

We produced 5-μJ CEP-stable mid-infrared pulses via BiB3O6-based dual-wavelength OPA. The field amplitude exceeds 20 MV/cm. The mid-infrared waveform was directly sampled by synchronized 6.5-fs visible pulses. High harmonic generation in GaSe was also observed.

We produced 5-mu J, 6.5-fs visible pulses at a repetition rate of 1 kHz using filamentation in a gas cell filled with krypton followed by spectral selection and phase compensation by a combination of dielectric mirrors. The visible pulses have a smooth spectrum from 520 to 650 nm with a shot-to-shot stability in each spectral component of better than 2% (standard deviation). This pulse compression scheme is simple and robust, and can be easily integrated into intense ultrashort-pulse laser systems.

Carrier-envelope phase-stable mid-infrared pulses spanning from 5 mu m to 11 mu m with a pulse energy of 5 mu J were produced by difference frequency generation of two-color near-infrared pulses that were produced in a novel inline optical parametric amplifier. The mid-infrared electric waveform was characterized by electro-optic sampling using 6.5-fs pulses at 620 nm. (C) 2016 Optical Society of America

We demonstrate the use of the carrier-envelope phase of few-cycle pulses for laser-induced electron diffraction. The phase serves to tune the rescattering energy, which enables a sensitive measurement of differential scattering cross sections.

We produced , 6.5-fs visible pulses at a repetition rate of 1 kHz using filamentation in a gas cell filled with krypton followed by spectral selection and phase compensation by a combination of dielectric mirrors. The visible pulses have a smooth spectrum from 520 to 650 nm with a shot-To-shot stability in each spectral component of better than 2% (standard deviation). This pulse compression scheme is simple and robust, and can be easily integrated into intense ultrashort-pulse laser systems.

We report a novel method to produce two-color near-infrared pulses in an optical parametric amplifier. Five-mu J, carrier-envelop phase-stable mid-infrared pulses are produced by intra-pulse difference frequency generation, and characterized by electro-optic sampling.

We report on the generation of 10.1-fs, 1.5-mJ pulses at 1 kHz from a BiB3O6 OPCPA with passive CEP stabilization. This light source is promising for the generation of high-flux isolated soft x-ray pulses. © OSA 2015.

We fabricated and characterized a pair of ultrabroadband chirped mirrors (CMs) in the infrared. These mirrors provide smooth control of the spectral phases in the wavelength range 1200-2200 nm, nearly one octave of a bandwidth. A scanning-type white-light Michelson interferometer was developed to measure spectral dispersion. We confirmed that the dispersion of the CMs well reproduced the designed dispersion. Furthermore, the CMs' damage threshold was measured to be &gt;390 mJ/cm(2) for 10-fs pulses, which corresponds to &gt;37 TW/cm(2).

We observe photoelectron spectra from above-threshold ionization of xenon using carrier-envelope phase-stabilized few-cycle pulses at 1.6 mu m. The signature of elastic scattering is imprinted on the spectra, and through careful analysis we successfully retrieve the electron-ion backscattering cross section in the energy range from 10 to 30 eV, where we find a resonance-like structure at 17.5 eV.

A high-flux, soft x-ray light source, based on high harmonic generation, using 1.5-mJ, 10.1-fs IR pulses with CEP stabilization, has been constructed and used to resolve the 1s-pi* and - sigma* transitions in absorption spectra at the carbon K edge.

We generate a 75-eV-wide continuum in the water window via the HHG process using CEP-stabilized, few-cycle IR pulses. A pressure dependence of harmonic spectra indicates sub-cycle deformation of the IR pulses in the process.

We present long-term stability of the carrier-envelope phase of intense infrared few-cycle pulses at 1.6 mu m, generated by optical parametric chirped-pulse amplification. The employed system is based on a passive stabilization scheme that obviates the need for active feedback loops, and requires only control of environmental parameters. The stabilization is demonstrated to last for up to 45 h, during which the carrier-envelope phase drift is measured to be less than . This marks, to the best of our knowledge, the longest demonstrated phase-stable operation of a passively stabilized, amplified few-cycle source to date.

We developed a few-cycle waveform-controlled light source for infrared pulses at 1.6 mu m that is based on optical chirped-pulse amplification in BiB3O6 (BIBO) crystals pumped by Ti:sapphire lasers. Using this source, we observe soft x-ray high harmonics that extend up to a photon energy of approximate to 320 eV, as well as high-energy photoelectrons up to approximate to 1 keV. The spectra of the high harmonics and photoelectrons have clear signatures of half-cycle cutoffs that can be used to extract electronic and molecular dynamics on an attosecond time scale.

High harmonic generation (HHG) using waveform-controlled, few-cycle pulses from Ti:sapphire lasers has opened emerging researches in strong-field and attosecond physics. However, the maximum photon energy of attosecond pulses via HHG remains limited to the extreme ultraviolet region. Long-wavelength light sources with carrier-envelope phase stabilization are promising to extend the photon energy of attosecond pulses into the soft X-ray region. Here we demonstrate carrier-envelope phase-dependent HHG in the water window using sub-two-cycle optical pulses at 1,600 nm. Experimental and simulated results indicate the confinement of soft X-ray emission in a single recombination event with a bandwidth of 75 eV around the carbon K edge. Control of high harmonics by the waveform of few-cycle infrared pulses is a key milestone to generate soft X-ray attosecond pulses. We measure a dependence of half-cycle bursts on the gas pressure, which indicates subcycle deformation of the waveform of the infrared drive pulses in the HHG process.

We observe half-cycle cutoffs in above-threshold ionization spectra using carrier-envelope phase-stabilized, sub-two-cycle infrared pulses. The waveform-dependent photoelectron spectra allow the retrieval of a resonance-like structure in the cross section for elastic backscattering from Xe+. (C) 2014 Optical Society of America

A few-cycle infrared light source is applied to generate isolated soft-x-ray continua in the water window via high harmonic generation. The current status of the light source and some strong-field experiments will be discussed.

Most of strong-field experiments such as high harmonic generation (HHG), attosecond pulse generation, and their applications rely on femtosecond Ti:sapphire lasers around 800 nm. Currently the photon energy range in HHG is limited to 200 eV or less for spectroscopic applications. To overcome this barrier, an IR source is promising because the ponderomotive potential is proportional to the square of a laser's wavelength, therefore, we expect the extension of an electron kinetic energy toward ∼keV [1]. However an infrared (IR) light source must meets all requisites for attosecond pulse generation in soft X-rays such as few-cycle pulse duration, carrier-envelope phase (CEP) stabilization, mJ output energy, and high repetition rate. Although many IR systems have been constructed so far, CEP-sensitive strong field phenomena have not been exploited so far because of difficulty in meeting all the requisites. We have constructed a novel optical parametric chirped pulse amplifier (OPCPA) by use of BiB3O6 crystals and Ti:sapphire lasers. The OPCPA produces 9.0-fs, 550-μJ, CEP-stabilized optical pulses around 1.6 μm at 1 kHz [2]. In this presentation, using this light source, we report on the generation of an attosecond continuum in the water window and their CEP dependences, which can be understood by half-cycle cutoffs [3]. © 2013 IEEE.