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A new cryostat, a Fourier transform grid-spectrometer, and a new interferometric method have led to more accuracy for transmission spectra of YBaCuO films carefully deposited on MgO plates, than reported before in the 10–40 cm−1very far infrared (VFIR) range.

The Miller-Savage law is verified for low intensity electric fields and the power law for high fields, in the case of thick samples. For very thin crystal plates (e < 10 μm), the Miller-Savage law must be replaced by an exponential law. In all cases the different parameters can be kept constant in a small range of thickness by using an "effective electric field."

In a very rough experiment using two MgO plates, coated with a 200 Å thick YBaCuO film to increase their reflectivity, and facing each other their YBaCuO films to make a Perot-Fabry (PF) interferometer, we have observed a 43% maximum transmission and a finesse around 33 at v=29 cm−1, in good accordance with a new phenomenological model for YBaCuO. Much higher performances can be expected by optimizing the YBaCuO film thickness.

Far IR transmission spectra of very thin YBaCuO films in the 10–40 cm−1 range have now sufficient accuracy and reproducibility to show that the number of quasiparticles at temperature Θ<Θc, Θc being the transition temperature, do not follow a Θ4 law, but rather a Θ1.5 one.

Far infrared spectra of TGS from room temperature to liquid helium temperature in the spectral range extending from 3 to 400 cm-1, and description of a triglycine sulphate pyroelectric detector working at frequencies as high as 1 Megacycle per second.

Absorption spectra at liquid helium temperature in the far infrared are given for thick plates of CsI, CsBr, TlBr, TlCl, KI, NaCl, InSb, KBr, CuCl, Al_2O_3, SiO_2, MgO, and LaCl_3. For a thickness of about 3 mm all these crystals, completely opaque at room temperature, become quasitransparent at very low temperature. This supertransparency of crystals at low temperature gives several far infrared filters with remarkable cutoffs and a lot of available materials to the spectroscopist in a part of the spectrum where only quartz and polyethene were usable up to now. This is also evidence that the main part of the absorption at room temperature comes from the lattice phonons. It decreases with their number when the crystal is cooled (2 phonons difference process). The small absorption which remains in a few cases—KBr, —NaCl, KI—could arise from 2 phonons addition process nearly independent of temperature and much less efficient to give absorption. In the case of glasses the absorption coefficient is higher by 2 or 3 orders of magnitude and keeps constant when the glass is cooled. For quartz glass, the index of refraction also seemed invariant. An explanation is proposed in terms of one single phonon process, independent of temperature and allowed by the disorder characteristic of glassy states.

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 ν_1 corresponds to the TO transition at point Γ, the second one ν_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 Γ are, for AgCl: 105 cm^−1 (290°K), 114 cm^−1 (80°K), 119 cm^−1 (25°K); for AgBr: 79 cm^−1 (290°K), 84 cm^−1 (80°K), 85 cm^−1 (25°K), and for AgI: 103 cm^−1 (290°K), 106 cm^−1 (80°K), 106 cm^−1 (25°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.