Absorption saturation and photodarkening in semiconductor doped glasses

Abstract

Semiconductor doped glass [1] (SDG) is a promising composite material for third-order nonlinear integrated optics. It shows a good compromise between speed of response and magnitude of the nonlinearity. The glasses content sodium and are therefore good candidates for waveguide fabrication by ion-exchange [2]. Devices based on SDG waveguides may be useful in all optical switching and for optical logic. The nonlinearity stems from bandfilling [3] in small semiconductor crystallites. Since the response of an absorptive nonlinearity is limited by the recombination rate of the carriers, much interest has been focused on time resolved measurements [4,5]. Apparent contradictory results regarding the recombination time were explained by Roussignol et. al. [6]. They found that a photochemical process permanently changes the degenerate four wave mixing (DFWM) response of the material. A "fresh" unexposed glass shows a slow response - several nanoseconds, whereas after exposure of intense absorbing light, the response can be several orders of magnitude faster. After long exposures, a darkened area on the glass is also seen. Recent results by Mitsunaga et. al. [7] show recombination times as short as 7 ps for a darkened glass.