Keigo Nagasaka

Output performance of a liquid‐N2‐cooled, para‐H2 Raman laser

Stimulated rotational Raman scattering (SRRS) of a transversely excited atmosphere‐CO2 laser in para‐H2 was studied with a liquid‐N2‐cooled multiple‐pass cell. Line‐tunable output from 13.7 to 17.2 μm was obtained by changing CO2 pump lines. Threshold pump energies required for SRRS were measured to be 0.9 and 1.5 J for the 9P(20) and 10P(20) CO2 lines, respectively. Above the threshold, the Stokes output energies for the 10‐μm band pumping increased in proportion to the increase of the pump energies, while the increase of the Stokes output for the 9‐μm band pumping was limited. Para‐H2 pressure dependence of the Stokes output energy showed that the Stokes gain became constant for pressures above 250 Torr at 100 K.

Complete removal of paint from metal surface by ablation with a TEA CO2 laser

For high-speed metal surface cleaning, we applied TEA CO2 laser pulses to ablate painted materials on metal surfaces and examined the efficiency of removal under different surface and irradiation conditions. Surfaces treated with a line-focused laser beam were analyzed with an energy dispersive X-ray analyzer and inspected with optical microscopic observation. Although paints were selectively ablated from the metal surface, the cleaning efficiency was found to depend on surface conditions of substrates. An application of a small amount of dimethyl formamide was effective for completely removing of resin without scorching the surface.

Spin Polarization Dependent Far Infrared Absorption in Ga1-xMnxAs

Infrared and far infrared transmission spectra have been measured for the ferromagnetic Ga1-xMnxAs (x=0.034 and x=0.050) between 10 and 12000 cm-1. A broad peak is observed around 1600 cm-1, and this energy is close to the deep acceptor level of Mn impurity in GaAs. A finite featureless absorption, spreading between this peak and the band gap, due to inter valence band transitions is also observed. At the same time, a non-zero absorption is observed in the lowest frequency studied, which points to the absorption by mobile holes related to the metallic DC conductivity. The frequency dependence in this range, which differs markedly from the Drude model, demonstrates a strong influence of electrostatic and magnetic disorder on the optical conductivity. In particular, the existence of a strong coupling between the Mn spins and the holes explains the considerable increase of the far infrared absorption below the ferromagnetic ordering temperature.

Output performance of a liquid-N/sub 2/-cooled, para-H/sub 2/ Raman laser

Stimulated rotational Raman scattering (SRRS) of a transversely excited atmosphere‐CO2 laser in para‐H2 was studied with a liquid‐N2‐cooled multiple‐pass cell. Line‐tunable output from 13.7 to 17.2 μm was obtained by changing CO2 pump lines. Threshold pump energies required for SRRS were measured to be 0.9 and 1.5 J for the 9P(20) and 10P(20) CO2 lines, respectively. Above the threshold, the Stokes output energies for the 10‐μm band pumping increased in proportion to the increase of the pump energies, while the increase of the Stokes output for the 9‐μm band pumping was limited. Para‐H2 pressure dependence of the Stokes output energy showed that the Stokes gain became constant for pressures above 250 Torr at 100 K.

A twin optically-pumped far-infrared CH3OH laser for plasma diagnostics

A twin optically-pumped far-infrared CH3OH laser has been constructed for use in plasma diagnostics. The antisymmetric doublet due to the Raman-type resonant two-photon transition is reproducibly observed at 118.8 μm. With the 118.8-μm line, it is obtained from the frequency separation of the anti-symmetric doublet that CH3OH absorption line center is 16±1 MHz higher than the pump 9.7-μm P(36) CO2 laser line center. It is shown that the Raman-type resonant two-photon transition is useful in order to get several-MHz phase modulation for the far-infrared laser interferometer. Some preliminary performances of this twin laser for the modulated interferometer are described.

Optical Properties of Zinc Telluride in the Infrared

ZnTe crystals were grown in a high pressure Tammann Furnace and doped with various kinds of impurities up to a carrier concentration of 8×10 18 cm -3 . The transmission and reflection spectra of undoped ZnTe crystals were measured in the region 100∼500 cm -1 . From the reflection spectrum the frequency values of TO (=179 cm -1 ) and LO (=206 cm -1 ) at the zone center and the effective charge of 0.62 e were determined. Absorption maxima in the transmission spectra could be assigned with the assumption that TO=175 cm -1 , LO=157 cm -1 , TA=73 cm -1 , and LA=126 cm -1 at the zone boundaries. The reliability of these values was checked by comparison with the data of other II-VI compounds and by the Brout sum rule. The reflection from heavily doped ZnTe crystals was interpretated in terms of the interaction of plasmons with optical phonons. The effective mass of the heavy holes was determined from this analysis to be (1.5±0.1) m 0 .

Optical Measurements and Band Calculations of FeSi.

Reflectivity measurements of high quality FeSi single crystal have been performed at 6 and 300 K over a very wide spectral region from 0.05 to 30 eV. The optical conductivity is obtained by a Kramers-Kronig transformation, and it is discussed in connection with the recent band calculation in which the relativistic spin-orbit effects are taken into account and with other measurements done on FeSi before. The result of direct photoconductivity measurement in the spectral region from 200 to 800 cm -1 is also presented.

Absorption and oscillation characteristics of a pulsed Cr/sup 4+/:YAG laser investigated by a double-pulse pumping technique

Because of the low Cr/sup 4+/ concentration in Cr/sup 4+/: YAG crystals, the output energy of the Cr/sup 4+/:YAG laser is limited by the ground-state depletion of Cr/sup 4+/ when pumped by a Q-switched Nd:YAG laser. We propose and demonstrate a double-pulse pumping technique to improve the output power and energy, by which a Cr/sup 4+/ ion that returns to the ground state during lasing is repumped by the second pump pulse and can immediately contribute to lasing. The output power and energy were increased by factors of 5 and 4, respectively, from those in conventional single-pulse pumping. These enhancement factors were limited by the relaxation time from the higher excited state following excited-state absorption to the upper laser level. In addition to the fast relaxation reported so far, we found that there is a slow relaxation channel with a time constant of approximately 100 ns, which was also explored by using the double-pulse pumping technique.

Superconductivity in BEDT-TTF Based Organic Metals: Role of Uniaxial Pressure and Inverse Isotope Effect

Abstract The superconducting properties characteristic to the two most extensively studied BEDT-TTF based organic metals are compared. A whole family of β-(BEDT-TTF)2X salts, including its mixed-anion crystals, with T c varying from 8 K to below 1 K enables us to study various factors governing T c in a systematic way. A new member, κ-(BEDT-TTF)2Cu(NCS)2, with the highest Tc among organic superconductors, obviously constitutes another case. One of the important feature common to both β-(BEDT-TTF)2X and κ-(BEDT-TTF)2Cu(NCS)2 is seen in the effect of hydrostatic pressure on Tc. In order to clarify its origin, the role of uniaxial pressure is discussed with some recent experimental results. Finally, an estimation of the isotope effect on the superconductivity in TTF-analog based organic metals is made, which leads us to an inverse isotope effect.

Polarized reflectance spectra of organic metals α- and κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTFd8)2Cu(NCS)2

Polarized reflectance spectra of two different phases (α- and κ-) of (BEDT-TTF)2Cu(NCS)2 as well as κ-(BEDT-TTFd8)2Cu(NCS)2 were measured over the spectral range from 800 cm−1 to 9000 cm−1. A clear reflectance drop with the plasma-edge like dispersion was observed for polarization parallel to any directions in the conducting b-c plane in both κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTFd8)2Cu(NCS)2. Small in-plane anisotropy reflects the characteristics stacking configuration of dimerized BEDT-TTF molecules in the crystal structure similar to κ-(BEDT-TTF)2I3, rather than the highly two-dimensional electronic structure within the conducting plane. In contrast, optical properties of α-(BEDT-TTF)2Cu(NCS)2, having a herring-bone type stacking of donors, was found to show higher reflectivity when the polarization was perpendicular to the stacking direction, similar to the case of α-(BEDT-TTF)2I3.