Special pleochroism in the far-infrared spectra of ferroelectric triglycine sulfate.

A special dichroism is observed in the far-infrared spectra of triglycine sulfate crystals. The absorption of a few peculiar lines is dependent on the orientation of the sample surface while, indeed, the infrared electric vector is kept in a constant direction. A significant broadening of some lines in a plate cut perpendicular to the axis is explained by a heterogeneous electric field in the surface layer. New lines appearing only in a plate cut parallel may be interpreted either by a loss of symmetry in the surface parallel to the binary C2 axis or by more classic arguments derived from the complexity introduced by a high absorption index in a very anisotropic crystal. This dichroism is observed in reflectivity and, when transmission is considered, for some lines the dichroism is limited to very thin samples in good accordance with a surface layer explanation, while for other lines it is still observed for thick samples.

Far Infrared Reflection Spectra of AgCI, AgBr, and Agl at Low Temperatures.

The reflection spectra of AgCl, AgBr, and AgI are measured at liquid hydrogen, liquid nitrogen, and room temperatures. For AgCl, they show distinctly two components. For AgBr and AgI, the structure, though complex, is not resolved. Nevertheless, in all cases, except for AgI, at low temperature, the Lorentz analysis needs two oscillators. The stronger one v(1) corresponds to the TO transition at point Gamma, the second one v(2), about an order of magnitude smaller, corresponds to a TA + TO addition band for the NaCl type structure, and to a TA + LA for the zinc blende or wurtzite structure, confirming preliminary measurements made by a completely different experimental approach (transmission of thin evaporated films instead of reflection on bulk crystals). They are, however, slightly distorted towards high frequencies. The proposed frequencies for the TO branch at point Gamma are, for AgCl: 105 cm(-1) (290 degrees K), 114 cm(-1), (80 degrees K), 119 cm(-1) (25 degrees K); for AgBr: 79 cm(-1) (290 degrees K), 84 cm(-1) (80 degrees K), 85 cm(-1) (25 degrees K), and for AgI: 103 cm(-1) (290 degrees K), 106 cm(-1) (80 degrees K), 106 cm(-1) (25 degrees K). The oscillator strengths and the damping of both oscillators are studied vs temperature. A large decrease of the damping is still observed between liquid nitrogen and liquid hydrogen temperatures. The refractive index and the absorption index are determined in the whole ir for the first time at liquid hydrogen, liquid nitrogen, and room temperatures, showing large variations, particularly a high transparency in the very far ir at liquid hydrogen temperature. The maximum values of n and k range from 2 to 10. The real and the imaginary parts of the dielectric constants are also computed for the whole ir range at room and lower temperatures down to liquid hydrogen temperature.

Spectres de transmission et de reflexion de I'iodure de potassium, pur et dopé, aux basses températures.

The far ir reflectivity of a pure single crystal of KI at liquid helium, liquid nitrogen, and room temperature has been measured from 10 micro to 700 micro. Kramers-Kronig analysis, and a two-oscillators Lorentz analysis lead to close results concerning the refractive index n, and the absorption index k in the whole ir spectrum, with maximum values of n and k ranging from 4 to 8. The strongest Lorentz oscillator has a frequency at 80 degrees K (nu(1))= 110 cm(-1))) close to the transverse optic frequency recently determined from neutron scattering at 95 degrees K (TO = 107 +/- 1.3 cm(-1)). The strength of the second Lorentz oscillator decreases rapidly when the temperature is lowered. Its frequency (nu(2) = 150 cm(-1) at 80 degrees K) is such that it can be considered as the TO + TA combination at point L of the first Brillouin zone, which is ir active, and expected at 152.7 +/- 3 cm(-1) from neutron scattering. Absorption bands,locaized in the gap determined by neutron scattering as extending from 104-143 micro (96-70 cm(-1)), are induced by sodium, cesium, and chlorine ions impurities. A peak of absorption in the acoustic phonons spectrum is induced by sodium ion impurities at 63 cm(-1)) close to a maximum in the frequency distribution function for the normal modes of vibration of KI computed recently by Cowley et al. Bromine ions, on the other hand, lead to a general background absorption.

Far-infrared spectra of magnesium oxide.

High-resolution spectra from 20 to 200 cm(-1) for temperatures from 10 to 300 K give interference fringes that cannot be fitted by previous analysis in terms of four Lorentz oscillators. Parameters in the theory are revised to give a better fit both to the far IR and to the reststrahlen spectra.

Laser scanning microscope for pyroelectric display in real time.

The pyroelectric probe technique is now capable of inspecting a 100 x 100-microm TGS area in a few seconds with a 2-microm limit of resolution. The method is described in detail, and a calculation of the pyroelectric signal is presented. The longitudinal growth of a cylindrical domain is mathematically simulated and also the variation of pyroelectric signal at domain boundary. These calculations are in good accordance with experiment. With a 200-kHz chopping frequency only a thin layer of the crystal surface is inspected (thermal diffusion length l = 0.64 microm), while the lateral limit of resolution is given by the laser spot radius (R(1) = 1 microm). It is possible to decrease this limit by using a smaller laser spot, but the chopping frequency ought to be increased.

Application de la méthode des GETS à la préparation de films minces monocristallins de TGS et TGSe deutériés pour spectroscopie infrarouge.

The GETS method (germination epitaxique dans des trous submicroscopiques) needs only a drop of TGS or TGSe solution in heavy water to give a single crystal film. The thickness e (e approximately 1 microm) is perfectly suited for mid-ir spectroscopy with polarized radiations. New lines are observed. In opposition to previous studies, there is no observation of lines disappearing at the Curie temperature, but several lines are continuously broadening when the temperature is increased from 80 K to 300 K.

Far infrared study of the copper halides at low temperatures.

The far infrared spectra of the copper halides CuCl, CuBr, and Cul have been measured in transmission and in reflection at various fixed temperatures from room temperature to liquid-helium temperature. Marked changes of such parameters as resonance frquency and damping as a function of temperature have been observed, also a line-splitting in the case of CuCl. In addition, a considerable increase in the asymmetry of the transmission curves with decreasing temperature has been observed.