Pierre Strimer

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.

Low temperature far infrared spectra of SiO2 polymorphs

Abstract Far-infrared transmission measurements of SiO 2 polymorphs (α-quartz, fused quartz, α-tridymite and α-cristobalite) have been extended into longer wavelengths (6–1000 μ) and to lower temperatures (the boiling points of liquid nitrogen, hydrogen and helium). The α-cristobalite sample was converted from fused quartz and the spectra taken before and after conversion. Differences were observed between the character of the low frequency resonance bands and that of the medium and high frequency bands. A possible explanation is put forward.

Far Infrared Reflection Spectra of AgCl, AgBr, and AgI 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.

Optical properties of lithium thioindate

The optical properties of LiInS2 suggested it as a promising material for generation of coherent radiation in the mid-IR region. Before investigating such capabilities its optical and mechanical properties have to be characterized precisely, and especially their evolution with temperature. Sufficiently large and suitably oriented crystals of good optical quality were studied. We first deduced the transparency range of these samples, as well as the frequencies of the optical phonons. We observed a phase- matched second-harmonic generation, using a nanosecond-OPO in the range 2.4 - 2.6 microns as the pump source and estimated a first value of the type-II nonlinearity deff(XY) equals 7.4 pm/V. The thermal expansion, thermo-optic, piezoelectric and electro-optic coefficients were determined along the three principal directions of polarization from -20 degrees Celsius up to + 120 degrees Celsius by means of original interferometric methods. A so-called Fabry-Perot Thermal Scanning (FPTS) interferometric method has been developed to measure accurately the electro-optic coefficients. For LiInS2 the values of ri3 were found to be of the same order of magnitude as its piezoelectric coefficients, but around one order of magnitude smaller than the electro-optic coefficients of the well known KTiOPO4.

Phase-modulated temperature scanning interferometry for measurements of electrooptic coefficients: application to KTiOPO/sub 4/

This work describes an efficient and simple method to determine the effective electrooptic coefficients of crystals, along with the continuous recording of their temperature dependence. A sample with plane and parallel optical end faces acts as a Fabry-Pe/spl acute/rot resonator, which is submitted to a linear ramp of temperature. The thermooptic effect generates a shift of interference fringes that are phase-modulated by applying an appropriate electric field. Residual absorption of the material is considered to achieve the best accuracy. The case of KTiOPO/sub 4/ is given as an example to illustrate the method; first results on the thermal behavior of its effective coefficients r/sub i3/ are presented.

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.

Simultaneous determination of thermo- and electro-optic coefficients by Fabry-Perot thermal scanning interferometry

A proper implementation of electro-optic materials in laser systems requires an accurate knowledge of their electro-optic coefficients, along with their temperature dependence which could be of importance at high power level. A new technique has been developed for this purpose, which takes advantage of the thermodynamic equivalence of two intensive parameters, namely the temperature and applied electric field. A suitably oriented parallelepipedic shaped sample is exposed to a laser beam and acts as a Fabry-Perot interferometer which is submitted to a linear ramp of temperature. The interference pattern is observed by reflection and the shift of interference fringes generated by the thermo-optic effect is detected through amplitude modulation of the light beam and recorded as a function of temperature. We then switch from amplitude- to phase- modulation by applying a suitable electric field to the crystal: the signal features now the derivative of the thickness fringes generated by the electro-optic effect. The thermo- and electro-optic coefficients are obtained from the fringe shift recorded respectively through amplitude- and phase- modulated operating modes. The study of both KTiOPO4 and LiInS2 single crystals is given as an example to illustrate the so-called Fabry-Perot Thermal Scanning Interferometric (FPTSI) method.

Far-infrared electronic transitions in solids

Two years ago at the Third Quantum Electronic Conference we showed that at low temperatures there were a large number of infrared transparent crystalline matrices. These solids can be doped with other ions, and we have recently found several far-infrared electronic transitions in both pure and doped materials.

Interferometric measurements of thermo-optic coefficients of ZnS, CaF2, and Ge in the infrared

The thermo-optic coefficient (delta) n divided by (delta) T of CaF2, ZnS and Ge single crystals have been measured in the infrared from 20 degrees C to 100 degrees C. The laser interferometric method employed allows a determination of (delta) n divided by (delta) T with an accuracy close to 10-6K-1 in the case of nonabsorbing materials. For Ge the uncertainty is increased by a factor of 3 and is mainly due to its increasing absorption coefficient with temperature. The behavior of ZnS was examined at 1.06 micrometers and 10.6 micrometers laser radiations; CaF2 and Ge were investigated respectively at 1.06 micrometers and 10.6 micrometers.

Accurate measurement of weak absorption in pyroelectric optical materials

The pyroelectric effect exhibited by two nonlinear optical materials is used to measure the residual absorption in their transparency window. The polarization behavior of potassium titanyl phosphate KTiOPO4 and thallium arsenic selenide Tl3AlSe3 are first described to determine the value of their pyroelectric coefficient. The pyrospectroscopic technique is then applied for absorption coefficient measurements at discrete laser wavelengths with a relative uncertainty estimated around 10 percent.