自由電子レーザーで駆動する高繰り返しアト秒光源の研究: 2025

Institutional Repositories DataBase (IRDB) · 2026-02-01

We have launched a research program on attosecond X-ray generation utilizing the extreme nonlinear process of high harmonic generation driven by mid-infrared free-electron lasers (FEL). The team has achieved FEL intensities sufficient for tunnel ionization of electrons in noble gases and have successfully generated high harmonics from solid and gas media at the free-electron laser facilities at Kyoto University and Nihon University. In parallel, the team has been developing mid-infrared lasers with several-cycle pulse duration and carrier-envelope-phase stabilization for enhanced attosecond control. In this presentation, we will report the recent advances of our research thrust.

Attosecond Rabi oscillations in high harmonic generation resonantly driven by extreme ultraviolet laser fields

Physical Review Research · 2025-06-16 · 5 citations

High-order harmonic generation driven by intense extreme ultraviolet (EUV) fields merges quantum optics and attosecond science, giving rise to an appealing route for the generation of coherent EUV and soft x-ray light for high-resolution imaging and spectroscopies. We theoretically investigate ultrafast resonant dynamics during the interaction of He atoms with strong extreme ultraviolet pulses. At high driving intensities, we identify record fast attosecond Rabi oscillations imprinting observable signatures in the high harmonic spectrum. At field strengths suppressing the Coulomb potential barrier for all the bound states, we demonstrate the survival of the attosecond two-level dynamics for several Rabi cycles. Consequently, this intense EUV laser-atom interaction reveals a strong-field scenario in which resonant coupling of two-level bound-bound transitions prevails, in contrast to the bound-continuum transitions typically dominant in conventional infrared strong-field regimes. Our findings of a high-frequency up-conversion regime combining two coexisting pathways—a two-level coupling and a bound-continuum coupling—set an interesting perspective for extreme attosecond nonlinear optics with intense short-wavelength laser fields.

Net gain bandwidth broadening in Yb:CaAlYO4 amplifiers: a prospect for 20-100 fs multi-mJ laser pulses

2024-01-01 · 1 citations

Implementing net gain bandwidth broadening effect in a Yb:CaYAlO 4 CPA-laser, we demonstrate 97 fs pulses with 2.7 mJ energy, 1 kHz at 1038 nm, and a robust, scalable nonlinear post-compression to 58 fs pulse durations.

Attosecond Rabi Oscillations in High Harmonic Generation Resonantly Driven by Extreme Ultraviolet Laser Fields

arXiv (Cornell University) · 2024-04-05

High-order harmonic generation driven by intense extreme ultraviolet (EUV) fields merges quantum optics and attosecond science, giving rise to an appealing route for the generation of coherent EUV and soft X-ray light for high-resolution imaging and spectroscopies. We theoretically investigate ultrafast resonant dynamics during the interaction of He atoms with strong extreme ultraviolet pulses. At high driving intensities, we identify record fast attosecond Rabi oscillations imprinting observable signatures in the high harmonic spectrum. At field strengths suppressing the Coulomb potential barrier for all the bounded states, we demonstrate the survival of the attosecond two-level dynamics for several Rabi cycles. Consequently, this intense EUV laser-atom interaction reveals a new strong-field scenario where the resonant coupling of two-level bound-bound transitions prevails, contrasting with the dominance of bound-continuum transitions in the conventional strong-field infrared regimes. These findings set an interesting perspective for extreme attosecond nonlinear optics with intense short-wavelength fields.

Spatially Chirped Pulses for Multipass Spectral Broadening

2024-01-01

Gas-filled Multi-Pass Cells are commonly used for pulse spectral broadening via SPM, but pulse energy scaling requires larger setups. We propose and verify experimentally the use of spatially chirped pulses to reduce the setup footprint.

Net Gain Bandwidth Broadening in Yb:CALYO Amplifiers: Prospect for 30-100 fs multi-mJ Near Infrared Pulses

2024-01-01 · 1 citations

Implementing a net gain bandwidth broadening in a Yb:CaYAlO 4 CPA-laser, we demonstrate 97 fs pulses with 2.7 mJ energy at 1038 nm, 1 kHz, and a robust, scalable nonlinear post-compression to 58 fs pulse durations.

Highly efficient and aberration-free off-plane grating spectrometer and monochromator for EUV—soft X-ray applications

Light Science & Applications · 2024-01-07 · 31 citations

We demonstrate a novel flat-field, dual-optic imaging EUV-soft X-ray spectrometer and monochromator that attains an unprecedented throughput efficiency exceeding 60% by design, along with a superb spectral resolution of λ/Δλ > 200 accomplished without employing variable line spacing gratings. Exploiting the benefits of the conical diffraction geometry, the optical system is globally optimized in multidimensional parameter space to guarantee optimal imaging performance over a broad spectral range while maintaining circular and elliptical polarization states at the first, second, and third diffraction orders. Moreover, our analysis indicates minimal temporal dispersion, with pulse broadening confined within 80 fs tail-to-tail and an FWHM value of 29 fs, which enables ultrafast spectroscopic and pump-probe studies with femtosecond accuracy. Furthermore, the spectrometer can be effortlessly transformed into a monochromator spanning the EUV-soft X-ray spectral region using a single grating with an aberration-free spatial profile. Such capability allows coherent diffractive imaging applications to be conducted with highly monochromatic light in a broad spectral range and extended to the soft X-ray region with minimal photon loss, thus facilitating state-of-the-art imaging of intricate nano- and bio-systems, with a significantly enhanced spatiotemporal resolution, down to the nanometer-femtosecond level.

High Performance Ytterbium Regenerative Amplifier Based on Yb:CALYO with High Energy 100 fs Pulses

2023-06-26 · 1 citations

The Yb-based chirped pulse amplification laser systems have been established as a viable technology for generating femtosecond ultrashort pulses of high multi-mJ energy at multi-kHz repletion rates for numerous applications. The laser transitions between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${}^{2}\mathrm{F}_{5/2}-{}^{2}\mathrm{F}_{7/2}$</tex> manifold energy levels determine the unique properties of the Yb-doped crystalline materials of relatively broad, overlapping absorption and luminescence bands that can be pumped by widespread InGaAs laser diodes. However, one of the remaining challenges in the development of femtosecond Yb-based lasers is achieving high pulse energy of multi mJ-levels and high average power, combined with short pulse durations of sub-100 fs. Although there are Yb-crystals with broad bandwidth of emission such as Yb:KGW, disordered Yb:CaGdAlO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> (Yb:CALGO), and Yb:CaF<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, obtaining high energy amplified pulses of more than 1 mJ with pulse duration down to 100 fs is still a technological quest. While Yb:CALGO is relatively well explored for generation of shorter femtosecond pulses, the state-of-the-art systems have either low energy of sub-60 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{J}$</tex> and short pulse duration of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 220$</tex> fs, or pulse energy at the mJ-level but with longer pulses, with most systems being limited to dual-crystal amplifying architectures [1]. On the other hand, Yb:CaYAlO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> (Yb:CALYO) is not well investigated, and the most advanced amplifiers developed to date have sub-100 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{J}$</tex> energies, at high repetition rates, with long pulse durations of 190 - 215 fs [2].

Multipass Spectral Broadening of Spatially Chirped Pulses

2023-06-26

External pulse compression using a multipass gas cell (MPC) is an extremely powerful technique for shortening the duration of sub-ps laser pulses [1]. Despite its tremendous success and versatility, the MPC method faces a challenge of handling high energy due to an unfavourable geometrical scaling requiring a rapid increase of the distance separating concave mirrors, and the corresponding size of the vacuum chamber. In this contribution, we propose and experimentally demonstrate the feasibility of reducing the MPC length relative to the target pulse energy by arbitrarily increasing the beam spot sizes on the mirrors, which is achieved through adding a spatial chirp on the collimated output beam from a grating-pair pulse compressor in a fs Yb CPA system.

Coherent EUV - Soft X-ray Light with Continuous Red and Blue Wavelength Tunability

2023-01-01

We demonstrate bright, narrow-bandwidth, well-spaced (5 eV), high-order harmonics, continuously tunable towards higher and lower EUV – X-ray photon energies (&gt;120 eV), ideal for resonant, ultrafast, coherent diffractive imaging at the absorption edges of ferromagnetic materials.