Thermo- and electro-modulation spectroscopy of thin evaporated films

Abstract

Thin evaporated films have been used to study the effects on reflection and transmission of visible and near visible light through solids by modulating the temperature of the specimen and the electric field in which it is situated. The principal aims of the studies were;(a) to obtain reliable data for comparison with electronic band structure calculations(b) to investigate a possible device application. Thermally modulated reflectance measurements have been performed on thick absorbing films of gold and copper and on the heavy rare earth metals, gadolinium, terbium, dysprosium and erbium. The modulated reflectance results have been transformed to Deltae2, the change in the imaginary part of the dielectric constant, by the Kramers-Kronig method of analysis. The Ae2 spectra have been correlated with electronic band structure calculations. Shifts in the principal absorption edges of gold and copper with temperature (shifts of - 2.3 x 10 4 eV/k and - 1.2 x 10 4 eV/K respectively), not - previously observed in thermal modulation experiments although theoretically predicted by expanded lattice band structure calculations, have been detected, The technique has been found to be capable of locating weak structure in the Deltae2 dispersion curve. New thermal modulation experiments on the heavy rare earth metals have enabled changes in the DeltaRp/Rp spectra to be observed as the temperature of the sample is decreased. These changes occur at the magnetic ordering temperatures and have not previously been observed using conventional static reflectance. By using electro-modulation, an integrated thin film optical modulator has been produced. The device has been constructed from thermally evaporated thin films of aluminium, silicon monoxide and cadmium sulphide, the latter being the active medium. Modulation degrees of between 10 and 15% have been observed for very low modulating voltages ~ 15 volts. However the device is limited by the very large insertion loss at the centre of the cadmium sulphide absorption edge. Some studies have been made on the electro-modulation of metals using a "dry-package" configuration. No electro-modulation response of the metal was observed. This null result is in contradiction to electrolytic measurements. Computational analysis has been performed to demonstrate that the electroreflectance spectra observed using the electrolytic technique may not be an electro-modulation of the optical constants of the metal but an absorption induced by the modulating electric field in the Gouy region of the electrolyte.