Theoretical and Experimental Effects of Spatial Dispersion on the Optical Properties of Crystals

The classical dielectric theory of optical properties is a local theory, and results in a dielectric constant dependent only on frequency. This dielectric behavior can be written as a sum over resonances, each resonance occurring at a particular frequency. The spatial dispersion (i.e., nonlocal dielectric behavior) effect considered here is the effect of the wave-vector dependence of the resonant frequencies on optical properties. The additional boundary condition needed for the application of such a theory is discussed for the case in which the resonance is due to an exciton band and the wave-vector dependence to the finite exciton mass. Experimental data presented on the reflection peaks due to excitons in CdS and ZnTe exhibit gross departures from the reflectivities expected from classical theory. Particularly striking are sharp subsidiary reflectivity spikes. The departures from classical results are all well represented by calculations based on the theory of spatial resonance dispersion and a simple approximation to the drived boundary condition.