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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.

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 ≃ 1 μm) 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.

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.