arXiv subjects

Kaito Nishimiya

Publications and source records attributed to Kaito Nishimiya.

7 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.

physics.optics↗

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↗

arXiv2025

Towards intense single-digit attosecond pulses with a 100-mJ-class mid-infrared sub-cycle laser

The duration of isolated attosecond pulses created via high-order harmonic generation is determined by the number of optical cycles in the driving laser. Achieving shorter attosecond soft X-ray pulses requires minimizing the number of cycles while maintaining a high pulse energy. Here, we demonstrate a carrier-envelope-phase-stable, 100-mJ-class sub-cycle mid-infrared laser that produces a supercontinuum coherent soft X-ray with unprecedented bandwidth. The system delivers 50-mJ, 6.7-fs (0.88-cycle) pulses at a center wavelength of 2.26 $μ$m - over two orders of magnitude more energetic than any previous sub-cycle laser. We applied the system to high-order harmonic generation and compared the results to simulations based on the three-dimensional time-dependent Schrödinger equation to identify unique features of sub-cycle lasers. This work represents a decisive step toward high-energy half-cycle lasers and high-energy single-digit attosecond soft X-ray pulses that can be used to probe matter and light-matter interactions at previously inaccessible temporal resolutions.

physics.optics↗

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.

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.

physics.optics↗