Interaction between a Light Beam and a Gunn Oscillator near the Fundamental Edge of GaAs

Abstract

We have examined how Gunn domains could modulate the transmission of infrared light. All the phenomena related to the free‐carrier absorption cannot give an appreciable effect due to the relatively low carrier density in the devices (10¹⁴–10¹⁵ cm⁻³). Modulation by the linear electro‐optic effect is difficult to achieve experimentally. On the other hand, the high value of the electric field inside moving domains produces a shift of the fundamental edge towards longer wavelengths (Franz‐Keldysh effect). We have performed experiments using a GaAs laser as a light source, and a marked modulation was observed. The maximum value that we have observed is 66% at 9050 Å. The electric field deduced from the Franz theory is of the correct order‐of‐magnitude when compared to the fields inside the domain. Another effect of the illumination is to produce extra carriers in the material. Thus, each time a dipole crosses the perturbed region, the current shows a deformation. If the laser is cooled down, the domains can disappear and this provides a means of changing the frequency of the current oscillations. A frequency ratio of 6 has been obtained. The deformation shows a relatively long decay time typically of the order of 100 nsec. We discuss the marked similarity which exists between the transient changes in conductivity occurring in this experiment and those present when a sample containing a high field domain is subjected to an additional voltage pulse. We have found that the same trap level is involved in both experiments.