arXiv subjects

Eiji J. Takahashi

Publications and source records attributed to Eiji J. Takahashi.

At least 19 recordsLinked to original sources

arXiv2026

Energy-scalable single-burst attosecond emission at kiloelectronvolt photon energies

Attosecond soft-X-ray pulses at kiloelectronvolt photon energies would provide direct access to the L-edges of transition metals, enabling element-specific studies of charge, spin, and orbital dynamics on their intrinsic timescales. However, attosecond emission in this spectral region has remained elusive because the only phase-matched keV high-harmonic source demonstrated to date employed multicycle driving pulses without carrier-envelope-phase (CEP) control. A long-standing gap has therefore persisted between keV photon energies and CEP-controlled attosecond emission. Here, we bridge this gap using a CEP-stabilized mid-infrared laser system with pulse durations tunable to the sub-cycle regime. Through phase-matched highharmonic generation in low-pressure neutral helium under meter-scale loose-focusing conditions, we generate coherent soft-X-ray continua reaching 1.2 keV. Crucially, we observe CEP-dependent spectral modulation that reaches into the transition-metal L-edge region, providing experimental evidence consistent with the generation of isolated attosecond pulses. To demonstrate the capabilities of our source, we perform broadband soft-X-ray absorption spectroscopy spanning the Ti, Fe, Co, and Ni L-edges, as well as the O K-edge, and resolve near-edge fine structures. By combining meter-scale loose focusing with low-pressure phase matching, our approach addresses key energy-scaling limitations of previous keV high-harmonic sources and establishes a phase-matched, CEP-controlled keV soft-X-ray platform for attosecond spectroscopy of magnetic, quantum, and strongly correlated materials.

physics.optics↗

arXiv2026

Complex-gate all-optical frequency-resolved optical gating for ultrabroadband isolated attosecond pulse characterization

We experimentally demonstrate all-optical frequency-resolved optical gating (AO-FROG) for the characterization of ultrabroadband isolated attosecond pulses (IAPs) generated by a mid-infrared sub-cycle laser field. By extending the AO-FROG framework beyond the conventional phase-only modulation approximation, we develop a strong-field approximation (SFA)-based theoretical framework and show that the weak perturbing field induces both phase and amplitude modulations during high-order harmonic generation. A complex-valued gate function is therefore required for accurate pulse reconstruction. Using a 2.26-$μ$m sub-cycle driving laser, we characterize IAPs spanning 100-180 eV in argon. The measured AO-FROG traces exhibit delay-dependent spectral modulations arising from perturbation-induced modifications of the electron trajectories and ionization probability. SFA simulations reproduce the experimental observations and confirm the importance of including ionization-induced amplitude modulation. The reconstructed temporal and spectral properties reveal an IAP duration of approximately 300 as and its spectral phase, providing access to the attosecond chirp of the generated pulses. Our results establish AO-FROG as a promising approach for temporal characterization of ultrabroadband attosecond sources driven by long-wavelength infrared fields.

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arXiv2026

Broadband gain characterization of Co:MgF$_2$ for mid-infrared femtosecond pulse amplification

The broadband gain characteristics of Co:MgF$_2$ were investigated to assess its potential as a gain medium for ultrashort-pulse amplification around the 2 $μ$m spectral region. Single-pass gain measurements performed using femtosecond seed pulses revealed broadband amplification in the 1.5-2.4 $μ$m region. The temporal dynamics and spectra of the gain were experimentally characterized and utilized for numerical simulations to assess the feasibility of Co:MgF$_2$ as a broadband gain medium for ultrashort pulses. The results indicate its potential for broadband amplification in future short-wave infrared to mid-infrared ultrafast laser systems, particularly when combined with coherent waveform synthesis or post-compression techniques.

physics.optics↗

arXiv2026

40% boost in extreme ultraviolet conversion efficiency via simultaneous dual-beam 2-μm laser irradiation

Scaling extreme ultraviolet (EUV) source power for next-generation lithography demands higher conversion efficiency (CE) at reduced per-pulse energies. We demonstrated a 40% CE enhancement by simultaneous dual-beam irradiation of a planar Sn target with a 2090-nm, 20-ns Ho:YAG laser. Single-beam irradiation at 40 mJ yielded an EUV CE of 2.6%; splitting the same total energy equally into two beams of 20 mJ each - at identical peak intensity - raised the EUV CE to 3.6%, which was the highest reported for 2-μm-driven laser-produced plasma sources. The EUV source size (60-70 μm) and energetic-ion spectra were nearly identical across both configurations, confirming comparable plasma conditions. Because the scheme requires only passive beam splitting and scales readily to three or more beams, it offers a practical route toward multi-kW-class, energy-efficient EUV sources for high-NA and hyper-NA lithography.

physics.optics↗

arXiv2026

Submicrometer focusing of isolated attosecond XUV pulses approaching 10$^{16}$ W/cm$^2$

We demonstrate submicrometer focusing of isolated attosecond pulses (IAPs) in the extreme ultraviolet (XUV) region using a custom ellipsoidal mirror. The obtained focal spot sizes were verified using knife-edge measurements with a sharp silicon edge, confirming reproducible dimensions down to 0.46 $μ$m $\times$ 0.36 $μ$m (FWHM), approaching the diffraction limit. Focusing a 1.1-GW tabletop IAP source yields a peak intensity of 3 $\times$ 10$^{15}$ W/cm$^2$, and a realistic pathway toward 10$^{16}$ W/cm$^2$ is obtained by optimizing the beamline throughput. These results establish a practical route toward attosecond nonlinear optics in both gas and solid phases, driven by intense XUV fields.

physics.optics↗

arXiv2025

Sub-cycle pulse control of holographic and non-holographic electron interferences

We investigate the influence of sub-cycle laser pulses on holographic and non-holographic intracycle interferences by analyzing the photoelectron momentum distributions of helium using TDSE simulations supported by classical trajectory calculations. The results show that the forward-scattering holographic (FSH), backward-scattering holographic (BSH), and time double-slit (TDS) structures are found to be highly sensitive to the pulse duration, carrier-envelope phase (CEP), and temporal envelope in the sub-cycle regime. Sub-cycle pulses with CEP values of $0^\circ$ and $90^\circ$ selectively enhance or suppress distinct features, isolating holographic patterns and enhancing BSH fringes. Classical analysis reveals that the intrinsic chirp inherent to sub-cycle fields shortens the recollision time for scattering trajectories, thereby increasing the fringe spacing in FSH and BSH patterns, while simultaneously enlarging the ATI peak spacing associated with TDS interference. Pulse envelope variations, even at fixed FWHM duration, further reshape the fringe spacings by modifying the instantaneous frequency and vector potential slope near ionization times. These results demonstrate that sub-cycle pulses enable precise temporal control of holographic interference, offering new opportunities for probing and manipulating attosecond electron dynamics.

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arXiv2025

Advances in dual-chirped optical parametric amplification

High-energy infrared lasers have enabled the generation of strong field phenomena, and among such phenomena, high-order harmonic generation (HHG) from gases has enabled attosecond-scale observations in atoms or molecules. Lasers with longer wavelengths and shorter pulse widths are advantageous for generating higher photon energy and shorter attosecond pulses via HHG. Thus, the development of ultrashort mid-infrared (MIR) lasers has progressed. This paper reviews research on developing high-energy MIR lasers using the dual-chirped optical parametric amplification (DC-OPA) method. We developed TW-class multi-cycle lasers in the MIR region, which was previously difficult. The advanced DC-OPA method, an extension of the conventional DC-OPA method, enables one-octave amplification of the wavelength, and a TW-class single-cycle laser was developed. These lasers were utilized for HHG, enabling single-shot absorption spectroscopy, and one-octave supercontinuum soft X-ray generation for single-cycle isolated attosecond pulse. We also show the development of multi-TW sub-cycle DC-OPA pumped by Ti:sapphire laser and high average power MIR single-cycle DC-OPA using thin-disk laser technology.

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arXiv2024

Perturbed three-channel waveform synthesizer for efficient isolated attosecond pulse generation and characterization

The generation of gigawatt-class isolated attosecond pulses (IAPs) is vital for attosecond pump-probe experiments. In such experiments, the temporal duration of IAPs must be determined quickly and accurately. In this study, we developed a perturbed three-channel waveform synthesizer for efficient IAPs generation and characterization at low repetition rates ( 10 Hz). Intense IAPs centered at photon energies of 60 eV (227 as duration) in Ar and 107 eV (128 as duration) in Ne were generated by the driving field from a three-channel waveform synthesizer and characterized using all-optical frequencyresolved optical gating (AO-FROG), which accelerated the measurement time to several minutes, providing fast feedback for the tunability of the IAP source. The peak power of the IAPs is higher than that reported in the literature.

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arXiv2024

Systematic analysis of an attosecond pulse generation by a sub-cycle laser field

We investigated the influence of sub-cycle driving fields on high-order harmonic generation (HHG), with a focus on intrinsic chirp, carrier-envelope phase (CEP), and number of laser cycles. Our findings reveals that the center frequency of a laser pulse scales as $τ^{-5/4}$ with pulse duration $τ$, and that attochirp exhibits a similar dependence on pulse duration. Additionally, we identified CEP-specific trends in harmonic yield: it increases as $τ^{5/4}$ for $ϕ_0=0^\circ$ and decreases as $τ^{-4.1}$ for $ϕ_0= -90^\circ$. Although sub-cycle pulses can generate intense isolated attosecond pulses (IAPs), they also tend to produce higher attochirp and reduced cutoff energies. However, effective compensation for attochirp can mitigate these drawbacks, thereby increasing the capability of sub-cycle pulses to generate short-duration, high-intensity IAPs. These results offer valuable insights into HHG using sub-cycle pulses and have important implications for the advancement of ultrafast light sources and the understanding of ultrafast phenomena at the attosecond timescale.

physics.optics↗

arXiv2024

Octave-spanning supercontinuum coherent soft X-ray for producing a single-cycle soft X-ray pulse

This study demonstrates the potential to generate a soft X-ray single-cycle attosecond pulse using a single-cycle mid-infrared pulse from the advanced dual-chirped optical parametric amplification. Supercontinuum high harmonic (HH) spectrum was generated in argon (80 eV - 160 eV) and neon (150 eV - 270 eV). The experimental spectra reasonably agree with those calculated by the strong-field approximation model and Maxwell's equations. In addition, simulation results indicate that the dispersion of HHs in argon can be compensated using a 207-nm Zr filter to obtain 40 as pulses (1.1 cycles at 118 eV). For neon, a 278-nm Sn filter can compensate for dispersion of HH and create 23 as pulses (1.1 cycles at 206 eV). This soft X-ray single-cycle attosecond pulse is expected to be highly valuable for ultrafast science and applications in quantum information science.

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arXiv2023

Quantitative diffraction imaging using attosecond pulses

We have proposed and developed a method to utilize attosecond pulses in diffraction imaging techniques applied to complex samples. In this study, the effects of the broadband properties of the wavefield owing to attosecond pulses are considered in the reconstruction of images through the decomposition of the broad spectrum into multi-spectral components. This method successfully reconstructs the multi-spectral information of complex samples, probes, and spectral bandwidths using broadband diffraction intensities generated from computational scanning experiments. The results obtained in this research open the opportunities to perform quantitative ultrafast imaging using the attosecond pulses.

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arXiv2023

Efficient and scalable scheme for overcoming the pulse energy bottleneck of single-cycle laser sources

We propose a novel scheme called advanced dual-chirped optical parametric amplification (DC-OPA) that employs two kinds of nonlinear crystals (BiB$_3$O$_6$ and MgO-doped lithium niobate) to overcome the bottleneck of pulse energy scalability for single-cycle mid-infrared (MIR) laser pulses. In experiments, the advanced DC-OPA scheme achieved carrier-to-envelope phase-stable MIR laser pulses for a bandwidth of over one octave (1.4-3.1 $μ$m) with an output pulse energy of 53 mJ. The pulse duration was compressed to 8.58 fs, which corresponds to 1.05 cycles with a central wavelength of 2.44 $μ$m and peak power of 6 TW. To our knowledge, the obtained values for the pulse energy and peak power are the highest achieved for optical parametric amplification of single-cycle MIR laser pulses. Thanks to the energy scalability of the advanced DC-OPA scheme, it is potentially applicable to multi-TW sub-cycle laser pulses.

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arXiv2022

100-mJ class, sub-two-cycle, carrier-envelope phase-stable dual-chirped optical parametric amplification

Based on the dual-chirped optical parametric amplification and type-I BiB$_3$O$_6$(BiBO) crystals, the generation of $>$100 mJ, 10.4 fs, 10 Hz, carrier-to-envelope phase (CEP)-stable laser pulses, which are centered at 1.7 $μ$m, is demonstrated; it produces a peak power of 10 TW. CEP-dependent high harmonic generation is implemented to confirm the sub-two-cycle pulse duration and CEP stabilization of infrared (IR) laser pulses. As far as we know, the obtained pulse energy and peak power represent the highest values for sub-two-cycle CEP-stable IR optical parametric amplification. Additionally, the prospects of achieving high-energy water window isolated attosecond pulses via our developed laser source are discussed.

physics.optics↗

arXiv2021

Advent of a gigawatt-class, tabletop, isolated-attosecond-pulse light source

We have produced a soft x-ray supercontinuum with a pulse energy of 0.24-$μ$J using high-order harmonics from a multiterawatt, 10-Hz, stable, three-channel waveform synthesizer [Sci. Adv. eaay2802 (2020)]. We report here an attosecond streaking scheme, which is designed for measuring low-repetition-rate, high-energy, isolated attosecond pulses. The retrieved pulse duration is 226 as, which conclusively demonstrates the realization of 1.1-gigawatt isolated attosecond pulses at the generation point. The gigawatt-level peak power of this tabletop, isolated-attosecond-pulse light source is equivalent to that of an x-ray free-electron laser in a large facility.

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arXiv2021

A custom-tailored multi-TW optical electric field for gigawatt soft-x-ray isolated attosecond pulses

The bottleneck for an attosecond science experiment is concluded to be the lack of a high-peak-power isolated attosecond pulse source. Therefore, currently, generating an intense attosecond pulse would be one of the highest priority goals. In this paper, we review a TW-class parallel three-channel waveform synthesizer for generating a gigawatt-scale soft-x-ray isolated attosecond pulse (IAP) using high-order harmonics generation (HHG). Simultaneously, using several stabilization methods, namely, the low-repetition-rate laser carrier-envelope phase stabilization, Mach-Zehnder interferometer, balanced optical cross-correlator, and beam-pointing stabilizer, we demonstrate a stable 50-mJ three-channel optical-waveform synthesizer with a peak power at the multi-TW level. This optical-waveform synthesizer is capable of creating a stable intense optical field for generating an intense continuum harmonic beam thanks to the successful stabilization of all the parameters. Furthermore, the precision control of shot-to-shot reproducible synthesized waveforms is achieved. Through the HHG process employing a loose-focusing geometry, an intense shot-to-shot stable supercontinuum (50-70 eV) is generated in an argon gas cell. This continuum spectrum supports an IAP with a transform-limited duration of 170 as and a submicrojoule pulse energy, which allows the generation of a GW-scale IAP. Another supercontinuum in the soft-x-ray region with higher photon energy of approximately 100-130 eV is also generated in neon gas from the synthesizer. The transform-limited pulse duration is 106 as. According to this work, the enhancement of HHG output through optimized waveform synthesis is experimentally proved. The high-energy multicycle pulse with 10-Hz repetition rate is proved to have the same controllability for optimized waveform synthesis for HHG as few- or subcycle pulses from a 1-kHz laser.

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arXiv2021

Nonlinear terahertz-wave radiation generated by mid-infrared dual-chirped optical parametric amplification femtosecond pulse laser excitation

We demonstrate an efficient procedure for terahertz-wave radiation generation by applying a nonlinear wavelength conversion approach using a mid-infrared pump source. We used a 3.3 μm dual-chirped optical parametric amplification source and a 1.5 μm fiber laser for a comparison of energy conversion efficiencies. Nonlinear inorganic and organic crystals were used, and for the periodically-poled lithium niobate crystal, the efficiencies were 1.3*10^-4 and 5.6*10^-12 for the 3.3 μm source and the 1.5 μm source, respectively. We confirmed that nonlinear crystals could be pumped with tera-watt / cm^2 class by using an mid-infrared source, which reduces several undesirable nonlinear optical effects.

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arXiv2020

Apparatus for generation of nanojoule-class water-window high-order harmonics

In our recent study [Commun. Phys. 3, 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in water-window region under neutral-medium condition. More specifically, we obtained nanojoule-class water-window soft x-ray harmonic beam under phase match condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: terawatt mid-infrared femtosecond driving laser [ Sci. Rep. 8, 7692 (2018)] and specially designed gas cell. Despite the dramatic drop in the optimal gas pressure due to loose focusing geometry, it still reached 1 bar level for helium. Moreover, faster leaking speed caused by larger pinhole size of the gas cell made the use of a normal gas cell impossible. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. Robust energy-scalable apparatus for high-order harmonic generation developed in in this study will enable the generation of over tens nanojoule water-window attosecond pulses in the nearest future.

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arXiv2020

Energy scaling of water window high-order harmonic generation for single-shot soft X-ray spectroscopy and live-cell imaging

Full coherent soft X-ray attosecond pulses are now available through high-order harmonic generation (HHG); however, its insufficient output energy hinders various applications, such as attosecond-scale soft X-ray nonlinear experiments, the seeding of soft X-ray free-electron lasers, attosecond-pump-attosecond-probe spectroscopies, and single-shot imaging. In this paper, towards the implementation of these exciting studies, we demonstrate a soft X-ray harmonic beam that is more than two orders of magnitudes stronger up to the water window region compared to previous works. This was achieved by combining a newly developed TW class mid-infrared femtosecond laser and a loosely focusing geometry for HHG in the mid-infrared region for the first time. Thanks to a loosely focusing geometry with a neutral medium target, we achieve a high conversion efficiency, a low beam divergence, and a significantly reduced medium gas pressure. As the first application of our nano-joule level water window soft X-ray harmonic source, we demonstrate near edge X-ray absorption fine structure (NEXAFS) experiments with clear fine absorption spectra near the K- and L-edges observed in various samples. The systematic study of a robust energy scaling method on HHG opens the door for demonstrating single-shot absorption spectrum and live cell imaging with a femtosecond time resolution.

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