Tsuneto Kanai

Quantum interference during high-order harmonic generation from aligned molecules.

High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions. In the three-step model of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.

Generation of 5.0 fs, 5.0 mJ pulses at 1 kHz using hollow-fiber pulse compression

We demonstrate methods to increase the energy incident on hollow fibers for spectral broadening by self-phase modulation. We used chirped pulses for spectral broadening, lowering the optical intensity to avoid ionization of the gaseous medium. We also used helium as a nonlinear medium and demonstrated the generation of 5.0fs, 5.0mJ pulses at a repetition rate of 1kHz using a pressure gradient hollow-fiber pulse compressor.

10mJ class femtosecond optical parametric amplifier for generating soft x-ray harmonics

Power scaling of an infrared (IR) parametric amplifier chain, pumped at 800nm by a terawatt Ti:sapphire laser system has been reported. A total output energy exceeding 10mJ with 40fs pulse duration has been achieved in the IR region, which is the highest energy and peak power ever reported for an ultrafast optical parametric amplifier scheme. By applying the developed IR pulses to high-order harmonic generation, we have observed a significant cutoff extension compared to the case of 800nm driving wavelength. This source suits as a driver laser for extending high-order harmonic photon energy into the kiloelectronvolts region.

Numerical simulations of molecular orientation using strong, nonresonant, two-color laser fields

By numerically solving the Schrodinger equation, we show that an asymmetric potential created by the superposition of strong, nonresonant, two-color (/spl omega/+2/spl omega/) laser fields can be used to orient polar molecules if they are rotationally cold.

Observing the attosecond dynamics of nuclear wavepackets in molecules by using high harmonic generation in mixed gases

We probe the attosecond dynamics of nuclear wavepackets in H2 and D2 molecules by measuring the relative phase of high harmonics generated in each molecule by using a novel method with a mixed gas of H2 and D2. We find that not only the single molecule responses but also the propagation effects of harmonics differ between the two isotopes and we conclude that in order to discuss the dynamics of molecules in the single molecule responses, the propagation effects need to be excluded from the raw harmonic signals. The measured relative phase as well as the intensity ratio are found to be monotonic functions of the harmonic order and are successfully reproduced by applying Feynmans path integral method fully to the dynamics of the nuclei and electrons in the molecules.

Hollow-core-waveguide compression of multi-millijoule CEP-stable 3.2 μm pulses

We present the extension of high-power pulse compression deeper into the challenging IR spectral range around 3.2 μm wavelength, where the effects of material absorption, dispersion, and free electron disturbance on nonlinear propagation become increasingly limiting parameters. 5 mJ, 80 fs pulses from a KTA parametric amplifier were spectrally broadened in a large-core hollow fiber with argon as the nonlinear medium. Subsequent compression through anomalous dispersion in CaF2 yielded 2.5 mJ close-to-transform-limited two-cycle pulses exhibiting a passively stabilized carrier envelope phase (CEP). Furthermore, we outline the feasibility of generating sub-two-cycle pulses with good spatial and temporal characteristics.

Parametric amplification of 100 fs mid-infrared pulses in ZnGeP 2 driven by a Ho:YAG chirped-pulse amplifier

We report on the parametric generation of 100 fs sub-6-cycle 40 μJ pulses with the center wavelength at 5.2 μm using a 1 ps 2.1 μm pump laser and a dispersion management scheme based on bulk material. Our optically synchronized amplifier chain consists of a Ho:YAG chirped-pulse amplifier and white-light-seeded optical parametric amplifiers providing simultaneous passive carrier-envelope phase locking of three ultrashort longwave pulses at the pump, signal, and idler wavelengths corresponding, respectively, to 2.1, 3.5, and 5.2 μm. We also demonstrate bandwidth enhancement and efficient control over nonlinear spectral phase in the regime of cascaded χ2 nonlinearity in ZnGeP2.

Observation of a shape resonance of the positronium negative ion

When an electron binds to its anti-matter counterpart, the positron, it forms the exotic atom positronium (Ps). Ps can further bind to another electron to form the positronium negative ion, Ps− (e−e+e−). Since its constituents are solely point-like particles with the same mass, this system provides an excellent testing ground for the three-body problem in quantum mechanics. While theoretical works on its energy level and dynamics have been performed extensively, experimental investigations of its characteristics have been hampered by the weak ion yield and short annihilation lifetime. Here we report on the laser spectroscopy study of Ps−, using a source of efficiently produced ions, generated from the bombardment of slow positrons onto a Na-coated W surface. A strong shape resonance of 1Po symmetry has been observed near the Ps (n=2) formation threshold. The resonance energy and width measured are in good agreement with the result of three-body calculations.

Observation of a resonance in the photodetachment of positronium negative ions

The positronium negative ion, e−e+e−, is one of the simplest three body bound-states. As its constituents have a unique mass ratio, the ion is an attractive candidate for the investigation of the genuine three-body problem. In the present work, laser spectroscopy of this ion has been demonstrated in the UV region. A shape resonance near the positronium (n=2) formation threshold was clearly observed and the resonance energy was estimated.

Multi-millijoule Few-Optical-Cycle Pulses in Mid-IR: Scaling Power, Energy and Wavelength

We discuss prospects of the generation of high power mid-IR pulses by analyzing soliton self-compression of multi-mJ 4-μm pulses and the development of few-optical-cycle 6-μm optical parametric amplifier. Scalability of pump lasers is addressed.