Yasuyuki Hirakawa

Generation of high-order rotational lines in hydrogen by four-wave Raman mixing in the femtosecond regime

More than 40 rotational Raman lines are generated using an 800-fs Ti:sapphire laser. The spectral region extends from the near-infrared to the far-ultraviolet with a considerably flat intensity distribution. The effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, self-focusing, and harmonic generation are investigated. A white light continuum, generated by self-phase modulation, acts as a seed beam for the generation of high-order rotational lines through four-wave Raman mixing. Strong self-phase modulation, however, suppresses the generation of the Raman emission, due to a line broadening of the pump beam. Thus, optimization of experimental conditions is necessary for the efficient generation of high-order rotational lines.

Diffractive optical chemical sensor based on light absorption

A new type of chemical sensor based on light absorption is proposed. An array of zones alternatively containing the pH indicator thymolphthalein is formed in a gelatin film. By changing the sample solution from acidic to alkaline, a blue stripe appears in the gelatin film. This acts as a transmission grating and diffracts the introduced laser beam. Theory predicts that this method, which is based on light absorption/beam diffraction, is as sensitive as or more sensitive than fluorometry.

Molecular-optic modulator

An ultrafast light-intensity modulator, based on stimulated Raman scattering, is described. The intensity of a continuous wave laser is fully modulated at 17THz using hydrogen in a high-finesse cavity. The modulation frequency is determined by the molecular constant of the Raman medium, i.e., the Raman shift frequency. The modulation frequency can be changed in the tetrahertz range by replacing the Raman medium. Due to the accurate modulation frequency and the high beam coherence, this device is amenable to a variety of applications such as in basic science and also in advanced industrial technology.

Laser diagnostics of edge plasmas and laser diagnostics of plasmas for industrial applications (invited)

In plasma physics, laser diagnostics were first developed to probe the core region of magnetically confined high‐temperature plasmas, but the advent of various new lasers has turned out to be useful for edge plasma studies. The recent developments of various arc and glow discharges for industrial plasma applications have required measurement of the same quantities as in the edge plasma studies and expertise in the latter has been fully exploited for the former. The experience thus gained has then been used for the advantage in the studies of edge plasma behavior in high‐temperature plasmas. The interplay of these two fields of plasma studies, where laser diagnostics of plasmas (which we call ‘‘laser‐aided plasma diagnostics’’) is extensively used, is discussed.

Emission spectrum of hard x-rays, generated from iron, copper, and molybdenum targets by subpicosecond KrF laser pulses

K-shell emission spectra of iron, copper, and molybdenum in the range of 1.5–20 keV using a 500 fs KrF excimer laser have been observed and are reported herein.

Generation of tunable coherent XUV radiation in a hydrogen gas jet by two-photon resonant four-wave mixing

Abstract Tunable coherent XUV radiation down to 75 nm was generated by two-photon resonant four-wave mixing of a tunable ArF excimer laser with a dye laser in a molecular gas jet of hydrogen. The obtained XUV spectral profiles reflected the double resonance effect at Rydberg states in H 2 molecules. This scheme has a possibility to extend the shortest wavelength limit of the tunable radiation down to 66 nm by using a frequency-doubled dye laser.

Elemental analysis using hard x-ray emission from a laser-produced plasma, induced by a femtosecond laser pulse

A new approach for elemental analysis, which is based on the measurement of the hard x ray emitted from a laser-produced plasma is reported here. Since the K-shell emission is characteristic of the element in a sample, Cu, Zn, Fe, Co, and Ni were readily confirmed to be present in brass and kovar alloys by measuring the x-ray emission spectrum over an energy range from 2 to 30 keV.

Resonant Third-Harmonic Generation of ArF Laser in H2 and Kr Gas Jets at 64 nm

Two-photon resonant third-harmonic generation (THG) in H2 and Kr gas jets at 64 nm was investigated by using a tunable ArF excimer laser. It was found that the THG efficiency of H2 was more than one order of magnitude higher than that of Kr. This technique will be useful in extending the tunable XUV range down to 66 nm by four-wave mixing with a dye laser.

Laser Speckle Microscope for Instantaneous Determination of Condition of Single Living Cell

A laser speckle microscopy imaging system for observation of living cells was developed. This system consists of an optical microscope, a laser source, a single optical fiber, and a video camera. The system has an advantage in that cellular conditions could be instantaneously determined by observing speckle movements. In the experiments, the speckle fluctuations of living and fixed HeLa cells were compared with the change in the temperature of the culture medium around the cells, and their video images were quantitatively analyzed. By applying this system, it is possible to estimate not only cellular activity but also real-time movements of intercellular materials including the cytoplasm without requiring complicated labeling processes.

Effect of laser pulsewidth on the generation of multi-color laser emission by stimulated Raman scattering and four-wave Raman mixing in a KGd(WO4)2 crystal

Abstract Picosecond and femtosecond lasers having different pulsewidths were employed for the generation of multi-color laser emission via stimulated Raman scattering and four-wave Raman mixing using a KGd(WO 4 ) 2 crystal. Seven emission lines, consisting of high-order Stokes and anti-Stokes emissions, were observed at a laser pulsewidth of 20 ps and a pulse energy of 70 μJ.