Kazumichi Yoshii

Measurement of molecular rotational temperature in a supersonic gas jet with high-order harmonic generation.

We apply high-order harmonic generation to sensitive measurements of the molecular rotational temperature in a thin supersonic gas beam. The method uses nonresonant pump and probe femtosecond laser pulses to generate harmonic radiation from coherently rotating molecules. The rotational temperature of molecules can be derived accurately with high spatial and temporal resolutions from the Fourier spectrum of time-dependent signals. The validity of this method was tested for an expanding flow of an N(2) beam with a rapid temperature decrease. The results show the versatile applicability of this method.

Probing molecular structure with alignment-dependent high-order harmonic generation

High-order harmonic generation (HHG) from nonadiabatically aligned molecules has been demonstrated to have new abilities to probe molecular dynamics and structures with femtosecond (fs) temporal resolutions [1–4]. In the recent study of HHG from coherently rotating molecules, the present authors have shown that HHG properties strongly depend on the degree of molecular alignment, and a low rotational temperature Trot and resulting high degree of alignment is strongly desired in the HHG experiment to retrieve the image of electronic distribution or orbital structure in a molecule [3].

Two-step frequency conversion for connecting distant quantum memories by transmission through an optical fiber

Long-distance quantum communication requires entanglement between distant quantum memories. For this purpose, photon transmission is necessary to connect the distant memories. Here, for the first time, we develop a two-step frequency conversion process (from a visible wavelength to a telecommunication wavelength and back) involving the use of independent two-frequency conversion media where the target quantum memories are nitrogen-vacancy centers in diamonds (with an emission/absorption wavelength of 637.2 nm), and experimentally characterize the performance of this process acting on light from an attenuated CW laser. A total conversion efficiency of approximately 7% is achieved. The noise generated in the frequency conversion processes is measured, and the signal-to-noise ratio is estimated for a single photon signal emitted by a nitrogen-vacancy (NV) center. The developed frequency conversion system has future applications via transmission through a long optical fiber channel at a telecommunication wavelength for a quantum repeater network.

High-precision spectroscopy using ultra-compact lasers

We demonstrate high-precision spectroscopy of molecular iodine using ultra-compact diode laser modules at 531, 561 and 594 nm. Entire hyperfine structures of iodine lines are obtained with a linewidth of a few MHz.

Dual-frequency injection-locked continuous-wave near-infrared laser

We report a dual-frequency injection-locked continuous-wave near-infrared laser. The entire system consists of a Ti:sapphire ring laser as a power oscillator, two independent diode lasers employed as seed lasers, and a master cavity providing a frequency reference. Stable dual-frequency injection-locked oscillation is achieved with a maximum output power of 2.8 W. We show its single longitudinal/transverse mode characteristics and practical power stability, as fundamental performance features of this laser system. We also demonstrate arbitrary selectivity of the two frequencies and flexible control of their relative powers by simply manipulating the seed lasers, as advanced features.

Attractive natures of a Raman frequency comb in the time and frequency domains

We show attractive features of a Raman comb that is produced by an adiabatic Raman process. One is the features in the time domain and the other in the frequency domain.

Attosecond temporal shape manipulation by arbitrarily designing spectral phases

We report a novel approach to generate an attosecond pulse train from a discrete spectrum in the UV-VIS-NIR range by positioning a set of thin dispersive materials on the optical path.

Angle-dependent high-order harmonic generation from a single N 2 and O 2 molecule

High-order harmonic generation (HHG) from nonadiabatically aligned molecules [1,2] provides an effective tool for studying structures and dynamics of gas phase molecules, where the angle-dependent harmonic yield around the molecular axis is the key to deduce the characteristic properties of a single molecule. The harmonic distribution observed in the HHG experiment, however, always includes some broadening of the molecular axis distribution that blurs the shape of a single molecule, due to the imperfect alignment of molecules.

Rotational Temperature Measurementsin a Molecular Beam with High-Order Harmonic Generation

We have developed a new method to measure molecular rotational temperature in a supersonic gas beam, using nonresonant pump and probe femtosecond laser pulses, where the pump forms a rotational wave packet of molecules, and the probe produces high-harmonic radiation from coherently rotating molecules. The rotational temperature can accurately be derived with high spatial and temporal resolutions from the Fourier spectrum of time-dependent harmonic signals. The validity of this method was demonstrated for an expanding supersonic flow of N2 beam with a rapid temperature decrease.

Ultrafast dynamics in strong-field interactions with molecules and solid surfaces: high-harmonic generation and nanostructuring

This paper describes two topics of our recent studies on ultrafast strong-field interactions with atoms, molecules and solid surfaces. One is concerned with the high-order harmonic generation (HHG) from molecules nonadiabatically aligned with intense femtosecond (fs) laser pulses in a pump and probe experiment. The HHG is very sensitive to the molecular orbital and its spatial orientation with respect to the laser polarization. Experimental and theoretical studies demonstrate the characteristic properties of HHG from coherently rotating molecules. The other topic is the periodic nanostructure formation observed in fs laser ablation of dielectric materials. The major interest is in the ultrafast interaction process of nanostructuring on solid surfaces, for the purposes of potential applications of fs lasers to nanoprocessing. The experimental results have shown that enhanced near-field initiates the ablation of surface area much smaller than the laser wavelength and the origin of nanoscale periodicity can be attributed to the excitation of surface plasmon polaritons in the surface layer.