Volume 61, number 5 OPTICS COMMUNICATIONS 1 March 1987 THEORY OF OPTICAL BISTABILITY IN METAL MIRRORED FABRY-PEROT CAVITIES CONTAINING THERMO-OPTIC MATERIALS D.C. HUTCHINGS, A.D. LLOYD, I. JANOSSY 1 and B.S. WHERRETT Department of Physics, Heriot-Watt University, Edinburgh, Scotland, UK Received 23 October 1986 We present and analyse a new method for achieving all-optical bistability, in metal mirrored Fabry-Perot cavities containing thermo-optic material. It is shown how "butterfly" bistability is achieved in metal/metal mirrored cavities. Optimisation for low switching powers is considered for a dielectric/metal mirrored cavity. The use of nematie liquid crystals with such a system makes submilliwatt switching powers easily achievable. 1. Introduction In a recent paper [ 1 ] we discussed the optimisa- tion of nonlinear narrow-pass band interference fil- ters for low power optical bistability. One of the conclusions of this work was that the source of heat- ing of the central spacer of any nonlinear Fabry-Perot does not need to be absorption in the spacer material itself. It is necessary only that the absorbing region experiences the optical feedback of the cavity. It is perhaps intuitively obvious, and we have confirmed by numerical calculations for nonlinear multilayer dielectric filters, that the optimum position for the absorber is at the back reflective surface of the cav- ity. This gives a direct-contact thermal source whilst avoiding the loss of finesse that occurs for high absorption internal to the cavity. Very simple nonlinear cavities can therefore be constructed with thermo-optic materials as the spacers, in good thermal contact with thin metallic mirrors. We analyse here the optical responses of such cavities and of alternative structures with dielectric reflectors at the front face of the spacer and metallic back-face reflectors, fig. 1. In the former case we obtain and explain a novel "butterfly"-bistability. Experimental observations of the butterfly are reported in the accompanying power Present address: Hungarian Academy of Science, Central Research Institute for Physics, H-1525 Budapest, Hungary. 0 030-401/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division ) THERMO-OPTIC SPACER MATERIAL C=251~m) \ INCIDENT .~.~. ~ RADIATION THINMETAL J FILMSC=lOnm)" DIELECTRIC REFLECTIVE STACK Fig. 1. Schematicof the nonlinearFabry-perot schemesanalysed in this paper. [ 2 ], for simple, liquid spacer-materials between metal mirrors. For dielectric/metal cavities we show that lower power bistability than observed in dielectric- /dielectric systems is to be expected for optimised cases. Ref. [2] reports on submilliwatt bistability (of the conventional anti-clockwise hysteresis-loop vari- ety) in such a dielectric material cavity, containing a nematic liquid crystal. The analyses given here apply equally to semiconductor or other solid spacers and to liquid media. 345