Akiyoshi Mitsuishi

Transmission Filters in the Far-Infrared Region

Transmission filters in the far-infrared region from 25 to 300 μ which cut off short-wave radiation are obtained from polyethylene sheet containing powders of reststrahlen crystals. The cut-off wavelength of these filters can be shifted with choice of restrahlen crystals. By using finer powder size and/or thinner polyethylene sheet, the steepness of the transmittance curve can be increased. These filters are very economical and convenient to use in far-infrared spectrometers.

Analysis of CaOSiO2 and CaOSiO2CaF2 glasses by Raman spectroscopy

Abstract Raman spectra of CaOSiO2 and CaOSiO2CaF2 glasses were measured to analyze three states of oxygen, i.e. bridging, non-bridging and free oxygen, in a similar way to the previous work on PbOSiO2 glasses. The stretching vibrational modes of the SiO bond in CaOSiO2 and CaOSiO2CaF2 glasses corresponded to the bands at 880, 920, 975 and 1050 cm−1. It is suggested from the comparison of the Raman spectra of the glasses with the Raman and infrared absorption spectra of crystalline silicates that these bands would arise from the SiO4 tetrahedron with four, three, two and one non-bridging oxygens, respectively. The fractions of bridging, non-bridging and free oxygens were calculated from the intensities of the four Raman bands and the composition of the glasses. They were in agreement with those obtained from the thermodynamical model for the CaOSiO2 glasses. When the content of CaF2 was smaller than 15–20 mol.% and the CaO SiO 2 ratio was smaller than unity, CaF2 contributed to the breakage of some SiO bonds.

Infrared Lattice Reflection Spectra of II-VI Compounds

The reflection spectra of cubic ZnS, ZnSe, ZnTe and CdTe, hexagonal CdS, CdSe and ZnS were measured in the spectral region from 20 µm up to about 100 µm. The Reststrahlen bands thus obtained were analysed using Drude dispersion relation and the following values of transverse optical mode frequency (νt) at the zone center were determined: cubic ZnS, 282 cm-1; ZnSe, 207 cm-1; ZnTe, 177 cm-1; hexagonal CdS, 234 cm-1 and 241 cm-1 for the vibrations parallel and perpendicular to the c axis respectively; hexagonal CdSe, 167 cm-1 for unpolarized radiation; hexagonal ZnS, 274 cm-1 for the vibrations of both polarizations. The longitudinal optical mode frequency at the zone center, the Szigeti effective charge and the other dispersion parameters were also obtained. νt was also obtained from the transmission spectrum of an evaporated thin film and was found to be in good agreement with the value obtained from the reflection analysis.

Raman spectra of amorphous SiC

Abstract It has been found that the condition of the sample preparation exerts large influence on the structural disorder of the films. For the sample prepared by sputtering method, the dependence of CC band on substrate temperature T s and partial pressure of SiH 4 during the sputtering has been measured.

Characterization of the free‐carrier concentrations in doped β‐SiC crystals by Raman scattering

LO phonon‐overdamped plasmon coupled modes in n‐type epitaxial films of β‐SiC have been measured in the carrier concentration range from 6.9×1016 to 2×1018 cm−3. The carrier concentrations and damping constants are determined by line‐shape fitting of the coupled modes and compared with the values derived from Hall measurements. The concentrations obtained from the two methods agree fairly well. The Faust–Henry coefficient determined from the fitting is 0.35. The line‐shape analysis of the coupled mode has shown that the dominant scattering mechanisms in β‐SiC are deformation‐potential and electro‐optic mechanisms.

Metal Mesh Filters in the Far Infrared Region

The transmittance and reflectivity of metal meshes made of brass wire were measured in the far infrared region. The reflectivity of these meshes is very low in the wavelength region up to the wavelength of about 1.3 times the mesh constant, but then increases steeply so that the mesh becomes a nearly perfect reflector beyond the wavelength of about twice the mesh constant. This reflecting property is excellent for a reflection filter, compared with the usual far infrared filters.

Far Infrared Spectrograph for Use from the Prism Spectral Region to about 1-mm Wavelength

A far infrared spectrograph is described. The mounting is of the Czerny-Turner type. The radiation sources are a heated platinum strip used in the short-wave region and a high-pressure mercury lamp used in the long-wave region. Golay cells with diamond or quartz windows are used as detectors. Holders for filters to eliminate short-wave radiation and for reflection or transmission samples are designed for convenient use. Special chambers provide optical paths for long gas cells and provide holders for small solid samples. All parts of the optical system are in a vacuum tank and are remote-controlled. Spectra are automatically recorded on a strip-chart recorder. Spectra in the region from 18 microns to about 1 mm, obtained with six echelette gratings, are shown.Reproducibility of wavelength settings is one part in 5000. Resolution of better than 0.5 cm−1 throughout the entire spectral region has been achieved.

Indices of refraction of ZnS, ZnSe, ZnTe, CdS, and CdTe in the far infrared

Abstract The indices of refraction of pure ZnS, ZnSe and ZnTe at 2, 80, and 300°K have been measured in the spectral region from 10 to 100 cm -1 . The indices of refraction can be described by the simple dispersion formula for an undamped harmonic oscillator. The static dielectric constants of these materials are obtained from the best fit parameters using a simple dispersion equation. The average indices of refraction of pure CdS and CdTe at 5°K in the spectral region from 10 to 100 cm -1 have also been measured.

Reflection Measurements on Reststrahlen Crystals in the Far-Infrared Region

The reflectivities of various reststrahlen crystals were accurately measured or remeasured in the far-infrared region. The crystals measured were NaF, CaF2, BaF2, NaCl, KCl, KBr, KI, CsBr, CsI, TlCl, TlBr, KRS-6, and KRS-5. A side band was newly observed for each of BaF2, Kl, CsBr, and CsI. Reflectivities of NaCl, KCl, and KBr were also measured at temperatures of 100° and 200°K. As the temperature decreased, the peak reflectivity increased with an overall shift of the band to the shorter-wave side.

Polarizer for the Far-Infrared Region

Pile-of-plates polarizers made of thin sheets of polyethylene were constructed for use in a far-infrared spectrograph. The degree of polarization with 15 sheets is more than 97% except in the spectral region in which constructive interference of light reflected from both surfaces of a sheet increases the transmission of s component and decreases the polarization. The transmission is more than 75% of the incident plane polarized light. Limitations due to interference were decreased by combining polyethylene sheets of two thicknesses. The reflectivities of a filter grating, dispersion gratings, and reststrahlen crystals were measured for s and p plane polarized radiation.