Yasuo Nabekawa

We demonstrated a perturbed three-channel waveform synthesizer for efficient isolated attosecond pulse (IAP) generation and characterization at a low repetition rate of 10 Hz. Intense IAPs centered at photon energies of 60 eV (227 as duration) in argon and 107 eV (128 as duration) in neon were generated from an optimized three-channel waveform synthesizer. By introducing a perturbed pulse to the waveform synthesizer, the IAPs can be controlled and characterized rapidly in a few minutes, which is an all-optical method and enables fast feedback for the tunability of the IAP source. The peak power of the soft x-ray IAPs reached subgigawatts, which is higher than that reported in the literature. The work provides a method for efficient generation and characterization of IAPs, which is very helpful for applications in attosecond pump–probe experiments.

We introduce historical and current studies on nonlinear optics using extreme ultraviolet (XUV) high-order harmonic pulses to explore the characterization of attosecond pulses and ultrafast dynamics of atoms and molecules.

We derived analytical formulas describing the joint energy spectrum of photo-electron and the dissociating deuterium molecular ion emerging under the irradiation of the pair of XUV attosecond pulse trains and the sub-10 fs UV pulse.

In recently published papers [M. J. J. Vrakking, Phys. Rev. Lett. 126, 113203 (2021); J. Phys. B 55, 134001 (2022)], Vrakking proposed an inventive scheme to control the entanglement or coherence of the vibrational states in a hydrogen molecular ion and a continuum electron, both of which were generated via the photoionization of a hydrogen molecule irradiated by a coherent pair of extreme ultraviolet (XUV) attosecond pulses and a few-femtosecond ultraviolet (UV) pulse. He clarified, for the first time to our knowledge, how the coherence of the XUV attosecond pulse pair is transferred to the molecular ion system accompanying a detached continuum electron by numerically solving a time-dependent Schrödinger equation (TDSE) governing the evolution of the ion and the electron in a rigorous manner. Nevertheless, it was not straightforwardly resolved how and why the specific characteristics of the resultant joint energy spectrogram emerged and how the entanglement or coherence was altered with the irradiation of the UV pulse. In this paper, we present an analytical solution of the TDSE using time-dependent perturbation theory, and we utilize the resultant solution to explain what causes the particular features in the entanglement or coherence between the electron and the ion spectra.

Time domain Ramsey-type interferometry is useful for investigating spectroscopic information of quantum states in atoms and molecules. The energy range of the quantum states to be observed with this scheme has now reached more than 20 eV by resolving the interference fringes with a period of a few hundred attoseconds. This attosecond Ramsey-type interferometry requires the irradiation of a coherent pair of extreme ultraviolet (XUV) light pulses, while all the methods used to deliver the coherent XUV pulse pair until now have relied on the division of the source of an XUV pulse in two before the generation. In this paper, we report on a novel technique to perform attosecond Ramsey-type interferometry by splitting an XUV high-order harmonic (HH) pulse of a sub-20 fs laser pulse after its generation. By virtue of the postgeneration splitting of the HH pulse, we demonstrated that the optical interference emerging at the complete temporal overlap of the HH pulse pair seamlessly continued to the Ramsey-type electronic interference in a helium atom. This technique is applicable for studying the femtosecond dephasing dynamics of electronic wavepackets and exploring the ultrafast evolution of a cationic system entangled with an ionized electron with sub-20 fs resolution.

By performing delay scans of two replicas of a few-pulse attosecond pulse train (APT), we investigated two-photon ionization/dissociation processes of C 2 H 2 . The correlation time of the C + yield is determined to be 300 as, which is, to the best of our knowledge, the shortest correlation time ever obtained in the delay-scan measurement of two APTs. We also extracted the contribution of the resonant excitation from the ground state to a specific excited state of C 2 H 2 + leading to C–C bond breaking by performing Fourier analyses of the yield and angular distribution of the C H + fragment ions, which are also generated within 300 as.

High-order harmonic generation (HHG) is currently utilized for developing compact table-top radiation sources to provide highly coherent extreme ultraviolet (XUV) and soft X-ray pulses; however, the low repetition rate of fundamental lasers, which is typically in the multi-kHz range, restricts the area of application for such HHG-based radiation sources. Here, we demonstrate a novel method for realizing a MHz-repetition-rate coherent XUV light source by utilizing intracavity HHG in a mode-locked oscillator with an Yb:YAG thin disk laser medium and a 100-m-long ring cavity. We have successfully implemented HHG by introducing two different rare gases into two separate foci and picking up each HH beam. Owing to the two different HH beams generated from one cavity, this XUV light source will open a new route to performing a time-resolved measurement with an XUV-pump and XUV-probe scheme at a MHz-repetition rate with a femtosecond resolution.

Few–cycle short–wave infrared (SWIR) pulses are useful tools for research on strong–field physics and nonlinear optics. Here we demonstrate the amplification of sub–cycle pulses in the SWIR region by using a cascaded BBO–based optical parametric amplifier (OPA) chain. By virtue of the tailored wavelength of the pump pulse of 708 nm, we successfully obtained a gain bandwidth of more than one octave for a BBO crystal. The division and synthesis of the spectral components of the pulse in a Mach–Zehnder–type interferometer set in front of the final amplifier enabled us to control the dispersion of each spectral component using an acousto–optic programmable dispersive filter inserted in each arm of the interferometer. As a result, we successfully generated 0.73–optical–cycle pulses at 1.8 μm with a pulse energy of 32 μJ.

We observed the dynamical evolution of the electron–nuclear wavepacket composed of the B 2 Σ g − and 3 ² Π _u states of O 2 + with an evolution period of 1.3 fs. We created a dissociative wavepacket of O 2 + by photoionization of O ₂ with the irradiation of an intense attosecond pulse train (APT) in the extreme ultraviolet (XUV) wavelength region, and probed it with another XUV APT by scanning delay between the two APTs. We revealed the evolution of the electron–nuclear wavepacket by the analysis of the spectral phase in the Fourier transform of the recorded delay dependent momentum spectrum of O ⁺ . We also found the correlation between the photon energy and the released kinetic energy originating from the dissociation of Rydberg states of O ₂ followed by the autoionization of the fragment O atom in the Rydberg states.