Xavier Gerbaux

Translational Frequencies of Molecules in Naphthalene, Durene, and Anthracene Single Crystals at Low Temperatures

Among the three translational frequencies of these monoclinic molecular crystals, two are expected when the electric field of the radiation is parallel to the monoclinic b axis, and one when the field is perpendicular to b. The infrared absorption spectra show that the last band is obtained in each case for an electric field parallel to the easy cleavage plane, at 66 cm−1 for naphthalene, 65 cm−1 for durene, 61 cm−1 for anthracene. This polarization is explained for naphthalene in terms of the tensor of polarizabilities of the molecule, and of its orientation in the crystal. For E parallel to the monoclinic axis the two expected absorption bands are found at 37 and 68 cm−1 for durene, 101 and 120 cm−1 for anthracene, and around 96 cm−1 for naphthalene where they have not been separated. All these bands are shifted towards higher frequencies when the crystal is cooled down to liquid‐nitrogen temperature.

Sur le comportement différent des cristaux et des verres dans l’absorption de l’infrarouge lointain (40–1500μ) à la température de l’hélium liquide

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, Al2O3, SiO2, MgO, and LaCl3. 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.

Infrared and millimeter wave absorber structures for thermal detectors

Abstract In a classical (i.e. Type I) pyroelectric detector, a crystal plate is coated with two metal electrodes. In order to achieve an absorbing power as close as possible to 100%, different structures have been used in the past: (i) a metal-black coating on the front electrode and (ii) a very thin front electrode having a square resistance matching the impedance of vacuum, with the back electrode making a quarter wavelength structure. However, the quarter wavelength structure becomes inefficient when the absorption length becomes smaller than the plate thickness. The simpler solution is to use a transparent antireflective layer, so that the whole radiant energy would enter the pyroelectric plate and be absorbed. This can be a perfect solution when the double path through the layer matches the phase shift on reflection at the pyroelectric material, and a nearly perfect one for a broad band centered at that wavelength. Transformation of radiant power into heat occurs directly within the pyroelectric. It is shown that a number of semiconductors can be used to make such antireflective layers, and practical solutions are feasible for many pyroelectrics. The advantages of this simple solution over λ/4 structures stuck to the pyroelectric plate, as proposed by Parsons et al., are a negligible added heat capacity as the antireflective coating can generally have enough electrical conductivity to be used as an electrode, and a negligible lateral heat conductivity. When such a solution is not feasible (i.e. reflectivity cannot be cancelled completely), we can still use a very thin metal electrode. It is shown that the electrical conductivity may be 2 orders of magnitude smaller than for the bulk. This leads to small indexes in the IR (e.g. n3≅k3≅ 6), very convenient to give the structure a null reflectivity, when the granular metal layer is covered by a suitable, experimentally available coating (index of refraction n2 = √2n3. Additionally, the determination of the wavelengths and values of the absorption maxima give useful information on the complex refractive index of the coated crystal at wavelengths where reflectivity and transmission measurements are often inaccurate or impossible (T0).

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.

Electrical properties and infrared spectra of LiNH4SO4 between 4 and 300 K

A new phase transition at 258K is suggested by a weak anomaly of the dielectric constant without modification of the infrared spectra for ν > 10 cm-1. It should be a polar-polar phase transition like in the 285 K well-known transition.On the opposite, another phase transition is found from modification in the infrared spectrum between 4 and 80 K, without significant change of the static dielectric constant.

Drastic modifications of electrical properties of ferroelectric crystal plates with thickness, the case of triglycine sulphate

The electrical properties of triglycine sulphate (TGS) single crystal plates are completely modified when the thickness is decreased down to 1 μm. The dielectric constant at the Curie temperature T0 is reduced by two orders of magnitude, coercive field is increased in the same ratio, and hysteresis loops are observed at several degrees above T0. Phenomenologically these observations can be explained in the frame of Devonshires theory by assuming an internal electric bias Eb ≃ 1 MV/m for the thinnest plates.To explain such a high bias field we have been led to a tentative model with a surface layer where spontaneous polarization is smoothly decreasing from the bulk value PSB to a finite one PSB at the crystal surface. It is in this layer that a high polarizing field is calculated at room temperature. Its values may be slightly reduced when free charges of opposite sign are assumed to be injected by each electrode (Schottky barriers). These free charges will partially stay when temperature is raised above ...

Reversible domain switching in ferroelectric triglycine sulphate (TGS) by Laser

It is known1 that laser pulses can lead to reversible domain switching in ferroelectric TGS. A tentative explanation in terms of a depolarizing electric field issued from bound charges due to thermal shock was proposed.2Experiments are now given showing that a slow quasistatic illumination leads to the same reversible domain switching. In this case the electric field inside the crystal is easier to calculate. In a rough approximation assuming known values of pyroelectric coefficient, thermal conductivity and dielectric constant, it is shown that close to the surface the electric field is parallel to the spontaneous polarization. Inside the crystal it is opposed to it with values of the same order of magnitude as the coercive field at room temperature measured by classical methods. It is thus demonstrated that such a field is really responsible for the domain switching.The pyroelectric probe technique is used to inspect the surface opposite to a fixed illuminating spot and has shown the end of a domain whi...

Residual losses of superconducting thin films of YBa2Cu3O7−δ in the far infrared and microwaves Applications

Abstract 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−1 very far infrared (VFIR) range. A previous phenomenological two-fluid model has been refined and may explain not only all the VFIR data, but also microwave measurements. Quasi-particles transform into Cooper pairs according to a Θ1.5 temperature law. The scattering frequency is decreasing rapidly when the temperature Θ is reduced by a few K under the critical temperature Θc, then more slowly with a value around vc = 5 cm−1 of Θ= 18 K. The model is used to predict the performances of a Perot-Fabry interferometer made by two MgO plates coated with a thin film of YBaCuO. A rough experiment confirms the model with a peak transmission T M ≈ 45% and a finesse F ≈ 30 at v = 29 cm−1.

Applications of the pyroelectric probe technique to the study of domain wall motion in ferroelectric NaNO2 and TGS

Abstract 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.”

Narrow band metallic grid filters for the very far infrared

Abstract Transmission measurements with a Michelson Interferometer have shown that by suitably choosing grid separation, a narrow band filter for the far infrared can be made with a two non-parallel grids arrangement showing band-width one-half of that so far reported.