Yasuo Nabekawa

Femtosecond laser pulses in a Kerr lens mode-locked thin-disk ring oscillator with an intra-cavity peak power beyond 100 MW

We report 860 W average power, ∼440 fs laser pulse generation inside a 15.2 MHz thin-disk ring oscillator under development for intra-cavity high-order harmonic generation (HHG). This has been achieved by placing a Kerr lens mode-locking setup in a low-pressure chamber. The intra-cavity pulse energy is ∼57 µJ with an estimated peak power of 130 MW, which is the highest, to the best of our knowledge, in ring type oscillators. By focusing these laser pulses, we can expect a peak intensity high enough for demonstration of intra-cavity HHG at the multi MHz repetition rate to facilitate experiments in photoelectron spectroscopy.

Settling time of a vibrational wavepacket in ionization

The vibrational wavepacket of a diatomic molecular ion at the time of ionization is usually considered to be generated on the basis of the Franck–Condon principle. According to this principle, the amplitude of each vibrational wavefunction in the wavepacket is given by the overlap integral between each vibrational wavefunction and the ground vibrational wavefunction in the neutral molecule, and hence, the amplitude should be a real number, or equivalently, a complex number the phase of which is equal to zero. Here we report the observation of a non-trivial phase modulation of the amplitudes of vibrational wavefunctions in a wavepacket generated in the ground electronic state of a molecular ion at the time of ionization. The phase modulation results in a group delay of the specific vibrational states of order 1 fs, which can be regarded as the settling time required to compose the initial vibrational wavepacket.

Nonlinear Fourier transformation spectroscopy of small molecules with intense attosecond pulse train

We have developed an attosecond nonlinear molecular spectroscopic method called nonlinear Fourier transformation spectroscopy (NFTS) that uses an intense attosecond pulse train (APT) to induce multiphoton ionization processes. In the NFTS method, in addition to characterization of the temporal profile of attosecond pulses, the nonlinear molecular responses are encoded in the interferometric autocorrelation traces depending on the molecular species, their fragment ions and their kinetic energy distributions. The principle and applicability of the NFTS method are described in this paper along with the numerical simulations. The method is applied to diatomic molecules (N2 , D2 and O2) and polyatomic molecules (CO2, CH4 and SF6). Our results highlight the fact that nonlinear spectroscopic information of molecules in the short wavelength region can be obtained through the irradiation of intense APT by taking advantage of the broad spectral bandwidth of attosecond pulses. The development of the nonlinear spectroscopic method in attoseconds is expected to pave the way to investigate the ultrafast intramolecular electron motion such as ultrafast charge migration and electron correlation.

Conical third-harmonic generation of optical vortex through ultrashort laser filamentation in air.

We experimentally generate third-harmonic (TH) vortex beams in air by the filamentation of femtosecond pulses produced in a lab-built Ti:sapphire chirped pulse amplifier. The generated TH beam profile is shown to evolve with increasing pump energy. At a sufficiently high pump energy, we observe a conical TH emission of the fundamental vortex and confirm that the conical radiation follows the conservation law for orbital angular momentum. We further investigate the far-field angularly resolved spectra of the TH wave to analyze the conical emission angle. We theoretically verify that the formation of the conical TH vortex results from the phase-matching between the fundamental and TH waves during the filamentation process.

Frequency-resolved optical gating technique for retrieving the amplitude of a vibrational wavepacket.

We propose a novel method to determine the complex amplitude of each eigenfunction composing a vibrational wavepacket of / molecular ions evolving with a ~10 fs time scale. We find that the two-dimensional spectrogram of the kinetic energy release (KER) of H+/D+ fragments plotted against the time delay of the probe pulse is equivalent to the spectrogram used in the frequency-resolved optical gating (FROG) technique to retrieve the complex amplitude of an ultrashort optical pulse. By adapting the FROG algorithm to the delay-KER spectrogram of the vibrational wavepacket, we have successfully reconstructed the complex amplitude. The deterioration in retrieval accuracy caused by the bandpass filter required to process actual experimental data is also discussed.

Attosecond Frequency Resolved Momentum Imaging of Two-Photon Dissociative Ionization Dynamics of Nitrogen Molecule

Two-photon dissociative ionization processes of nitrogen molecule are investigated with attosecond nonlinear Fourier transformation spectroscopy. Nonlinear absorption spectrum of nitrogen molecule are extracted by measuring interferometric autocorrelation of a-few-pulse attosecond pulse train using non-sequential two-photon processes of nitrogen molecule. The frequency resolved momentum images (FRMI) are reconstructed from the delay dependent momentum images of fragment ion \( {\text{N}}^{ + } \) and the resultant FRMIs show the attosecond nonlinear response of nitrogen molecule.

Development of an ultrafast thin-disk ring oscillator with an intra-cavity average power higher than 1 kW

We report intra-cavity average power upscaling of a Kerr lens mode-locked ring oscillator to 1060 W. Obtained pulse energy of 68 μJ is the highest inside a ring-type oscillator, to the best of our knowledge.

High-Order Harmonics Fourier Transform Spectroscopy of Two-Photon Dissociative Ionization of Hydrogen Molecules

High-order harmonic generation provides us a coherent femtosecond pulse in a vacuum ultraviolet wavelength region. We have investigated the two-photon dissociative ionization of deuterium molecules by Fourier transform spectroscopy with a high-order harmonic generation pulse. We measured the delay-dependent kinetic energy distribution of the deuteron fragments resulting from the dissociative ionization by scanning the delay between the two replicas of the high-order harmonic generation pulse. From the Fourier transform analysis, we successfully distinguished the harmonic order, which induced the photodissociation and provided the specific kinetic energy to the fragments via the repulsive electronic excited state.

Simultaneous Observation of Vibrational Wavepackets of Nitrogen Molecule in Neutral and Singly-Charged Manifolds

Vibrational wavepackets of nitrogen molecule were created by attosecond pulse train and their time-evolutions were also probed by attosecond pulse train. Vibrational wavepackets are launched both in neutral electronic states and singly-charged electronic states of nitrogen molecule. The created vibrational wavepackets were excited to the dissociative states and detected as a momentum image of fragment ion using a velocity map momentum imaging ion spectrometer. The fast vibrational wavepackets \( \left( {T_{vib} = 1 5-20\,{\text{fs}}} \right) \) were ascribed to the vibrational wavepackets generated in the singly-charged electronic states. The slow vibrational wavepackets \( T_{vib} = 50-80\,{\text{fs}} \) were ascribed to the vibrational wavepackets generated in the weakly-bounded neutral electronic states. The co-existence of several vibrational wavepackets indicates the possibility to launch the electron wavepacket between these electronic states.