Maki Tachikawa

Undamped undulation superposed on the passive Q-switching pulse of a CO2 laser

An undamped undulation superposed on the pulse tail of the passive Q-switching is observed using HCOOH gas as a saturable absorber. The pulse shapes with the undulation are nicely reproduced through the rate-equation analysis in which the laser gain medium is described as a three-level system. Good agreements between the observation and the calculation are also obtained in the dependence of the period and the width of passive Q-switching pulse on laser parameters. The mechanism of the undulation is interpreted as the relaxation oscillation attributed to the relaxation from the lower laser level. The collisional rate constant of HCOOH molecule is also obtained.

Comprehensive interpretation of passive Q switching and optical bistability in a CO 2 laser with an intracavity saturable absorber

A CO2 laser with a saturable absorber shows passive Q switching or optical bistability, depending on the lasing conditions. These characteristics are theoretically analyzed on a comprehensive basis. The conditions under which passive Q switching or optical bistability occurs are investigated on phase diagrams by applying the linear stability analysis to the rate equations that describe the laser system. Exclusive occurrence resulting from the phase-diagram analysis is in good agreement with the present experimental results. The passive-Q-switching pulse with an undamped undulation is nicely reproduced through numerical integration of the rate equations.

Laser instability and chaotic pulsation in a CO 2 laser with intracavity saturable absorber

A CO2 laser with a gaseous saturable absorber shows a variety of periodic self-pulsation [passive Q switching (PQS)], depending on the lasing conditions and the characteristics of the absorbing molecules. We present a novel rate-equation model that comprehensively describes the transient pulse structures of PQS in the CO2 laser system. The numerical calculation based on the present model predicts that a chaotic PQS pulsation also is realized in a limited parameter region.

Collision Cooling of Ions Stored in Quadrupole Radio-Frequency Trap

We present a novel model for collisional energy transfer between ions and neutral molecules in the quadrupole radio-frequency trap. Calculated results quantitatively reproduce the dependence of the ion temperature Ti on the buffer gas pressure Pn, observed on Mg+ ions and several collision partners (H2, He, and CH4). It is deduced from the present model that Ti is proportional to Pn-1/3 in the low-pressure range below 10-6 Torr, in good agreement with the experimental results.

Raman spectroscopy of optically levitated supercooled water droplet

By use of an optical trap, we can levitate micrometer-sized drops of purified water and cool them below the melting point free from contact freezing. Raman spectra of the OH stretching band were obtained from those supercooled water droplets at temperatures down to -35 °C. According to the two-state model, an enthalpy change due to hydrogen-bond breaking is derived from temperature dependence of the spectral profile. The isobaric heat capacity calculated from the enthalpy data shows a sharp increase as the temperature is lowered below -20 °C in good agreement with conventional thermodynamic measurements.

Sub-Doppler Spectroscopy of Cs Atoms Optically Pumped in a Thin Cell

Optical pumping in a thin Cs vapor cell (end-wall gap 0.5–10 mm) transfers only atoms with small velocity components to a certain energy state. The non-Maxwellian velocity distribution is detected as a hyperfine-resolved spectrum by a double resonance technique using diode lasers. The observed spectral profiles are quantitatively reproduced by the rate-equation analysis in which the velocity-dependent surface relaxation process is taken into account.

Laser trapping of ice crystals

Toward application to crystal physics, we demonstrate optical trapping of ice crystals as well as supercooled water droplets using counterpropagating laser beams. Confinement of an ice crystal is evidenced by the angular distribution of laser light deflected from the crystal faces. The average trap time, limited by air currents, is 5s for ice crystals and much longer for water drops.

Dynamics of Trapped Ions in the Presence of Laser Cooling and Radio-Frequency Heating

We establish a theoretical model which quantitatively describes the dynamics of laser cooled ions confined in a radio-frequency (rf) trap. In our model the laser cooling efficiency and the rate of collisional radio-frequency (rf) heating are explicitly formulated. Considering the balance between these two effects, the ion temperature is determined at each trap parameter. This model precisely reproduces the laser-induced-fluorescence (LIF) line profile observed for 24 Mg+ ions.

Instability in a CO 2 sequence-band laser with a saturable absorber and vibration-to-vibration energy transfer processes

Several types of instability are observed in the passive Q-switching (PQS) pulsation of a CO2 sequence-band laser with a saturable absorber. The observed pulse shapes are different from those of a regular-band CO2 laser in detailed structure. We propose a novel model for the PQS instability of a CO2 sequence-band laser by considering two vibration-to-vibration energy-transfer processes. We find that the sequence-band PQS pulse width critically depends on the rate of vibrational relaxation in the ν3 mode. The vibration-to-vibration relaxation rate is estimated from the analysis of the pulse shapes.

Higher order interactions in molecular collisions

The characteristics of three types of collisional interactions in molecular collisions (dipole–dipole, dipole–quadrupole, and dipole–induced dipole) have been studied by a novel laser spectroscopic method. The rotational relaxation rate constants of CH3CN against various kinds of collision partners are measured. It has been found that the relaxation rate constant critically depends on the energy level structure of collision partners for the long range interaction while it is less sensitive to the level structure for the short range interactions.