Avalanche Photodiodes for Optical Bistability

This has been one of the first topics considered by sub-group I “Silicon devices for optical logic” in ESPRIT WOIT. If the active region of an avalanche photodiode (APD) is a plane parallel silicon plate with good optical quality faces, a Fabry-Pérot resonator (FP) is realized by putting a reflection coating in place of the usual AR one on the input face giving the front mirror, the back mirror of the FP being the rear metallization of the silicon plate. Optical nonlinearities in silicon have two different origins — electronic and thermal — if silicon temperature is fixed: its refractive index decreases proportionally to the photo-carrier density. High optical excitation can lead to excess carrier density of the order of 1018 cm-3 and optical switching can be observed in pulsed regime where competition between electronic and thermal effects is present [1].Under cw illumination, optical nonlinearities in a silicon APD are of thermal origin and greatly enhanced by Joule effect due the large reverse applied voltage necessary to get photo-carrier multiplication in the preavalanche regime. In these conditions, an optothermal gain very much larger than in references [2,3] is expected giving much lower power thresholds for optical bistability. Such experiments have been carried out at IEF with a slightly modified RCA C30817 supplied by EG&G Canada. The experimental results have been analyzed and the potentiality of pixelated thin film APDs operating as optical bistable devices in 2D array has been evaluated.