Memory effects in a semi-insulating GaAs optical modulator

Abstract

A beam of light from a light emitting diode (LED) is passed through a semi-insulating GaAs sample consisting of electrodes on both surfaces. The LED spectrum is centered on the absorption edge of the GaAs sample. An electric field across the electrodes causes absorption of below band gap light due to the electroabsorption effect. When an optical pulse hits the sample in the presence of the electric field, after the pulse, the LED light is further absorbed. The enhanced absorption is maintained as long as the voltage is present giving rise to a memory effect. Experiments suggest that the effect might be caused by trapped carriers in the semi-insulating GaAs sample. The optical pulse generated carriers are maintained at high temperature by the electric field. The trapped carriers are released to the conduction band to reach thermal equilibrium with conducting high-temperature carriers. During the process some traps are emptied, and further absorption of below band gap LED light takes place.