Optical Modulation by Acoustoelectric Domains inp-GbSb

Abstract

Propagating domains of acoustic flux are spontaneously generated and strongly amplified by pulses of high voltage in $p$-type GaSb. These domains were previously detected by electrical probe measurements of the associated high resistance and high field in the domains. We have found that such domains can also strongly modulate the transmission of infrared light. The passage of a domain past a focused light beam produces a transient change in transmission. This signal contains considerable information about the properties of the domains. However, our main interest in this paper is the nature and mechanism of the modulation. The spectral dependence of the interaction was studied in the purest available as-grown $p$-GaSb and in Li-diffused material. The primary effect is a strong decrease in transmission which is restricted to wavelengths near the intrinsic absorption edge. The modulation corresponds to a transient shift and/or broadening of the edge towards longer wavelength. This effect is best observed in Li-diffused GaSb. In as-grown material, the tail of the absorption edge is broadened by contributions from impurity transitions. There the modulation is related not only to the shift of the edge, but also to domain-induced increase in electron occupation of deep acceptor impurity levels. Greater heating effects in as-grown material make it much less useful for studying domain-induced modulation effects. Correlations were made between the optical modulation and the local electric field strength in the domain, using both electrical and optical probe measurements. Uniform applied fields, equivalent to those produced locally in the domain, were insufficient to produce optical modulation. It was concluded that the modulation is caused by the intense acoustic flux and not by the electric field established in the domain. Several mechanisms of acoustic modulation are suggested by analogy with known thermal and static strain effects.