Far infrared reflection spectroscopy of solids

Abstract

The optical properties of a selection of crystalline solids have been studied in the far infrared by dispersive Fourier transform spectroscopy. This technique allows both the amplitude and phase to be determined without any approximation. The measurements were performed on a modified Michelson interferometer, in which one of the mirrors is replaced with the sample to obtain the dispersive interferogram. The instrument was improved by replacing the stepper motor drive with a hydraulic piston and a high precision linear bearing. Sampling control was obtained by monitoring the cosine fringes from a secondary laser beam passed down the optic axis. The far infrared amplitude and phase reflection spectra of eight zincblende structure binary semiconductors and two alkaline earth flourides have been measured, in most cases for the first time. Then, by using the equations of Maxwell and Fresnel, the refractive index, absorption index, and the real and imaginary parts of the dielectric response were obtained. From these functions the frequency dependence of the anharmonic self-energy function of the transverse optic mode at the centre of the Brillouin zone was calculated. Features observed in the self-energy functions of the zincblende crystals are attributed to phonon combination bands due to the decay of the lattice resonant mode. These are assigned by critical point analysis with the aid of lattice dynamical models to provide an experimental test of the phonon frequencies. Finally, the amplitude and phase reflectivity of a GaAs-AlGaAs multiple quantum well structure and a GaAs-AlAs super lattice are presented for the first time. In the case of the multiple quantum well structure the results are shown to be in good agreement with calculations based on macroscopic dielectric theory.