Optical Bistability by Photothermal Displacement

Abstract

We have demonstrated that photothermal displacement is a new mechanism for optical bistability (OB) in integrated optics. The bistable elements investigated are prism couplers on planar waveguides. Input power P is the power of the laser beam incident on the prism coupler, output power P the reflected power (see Fig. 1). Bistability means that at constant input Sower P the output power PR can take on either of two different constant values. The system can be switched from one state to the other one by a momentary increase or decrease, respectively, of the input power P. The origin of the OB is the following: a part Pa =na P of the power coupled into the waveguide is absorbed in the coupling region,either in the waveguiding film F itself, if it is absorbing (as in the configuration shown in Fig. la), or in a metal film M (in the configurations shown in Figs. lb and c). This heating causes a local temperature increase and, consequently, a thermal expansion of the prism and the substrate S or the substrate SM. (Since the waveguiding film F and the metal film M are very thin, their thermal expansions have a negligible effect.) This expansion produces a photothermal displacement or ‘buckling’ of the surfaces of the waveguiding film F and of the prism or of the metal film M on the substrate SM respectively. This buckling leads to a local reduction in the width d of the air gap C in the coupling region. The incoupling efficiency na is a function of d.