We have discovered an unusual form of optical bistability in hot-pressed ceramic PMN (PbMg0.3Nb0.5Ti0.1O3) in which the thermal focussing of laser light at very low power densities (<1 kW/cm2) oscillates between two metastable states. At low laser powersPthe phenomenon is highly aperiodic but extremely reproducible; the oscillation events occur at times tNempirically given by the equationtN= A/(P-PN), where the threshold powerPNfor theN-th switching event is found to beNPo, withPo= 45 mW [i.e., 3.6 W/cm2]. At higher laser power the switching events become periodic, ca. 8 Hz, and resemble the phenomenon reported by Vasnetsov in thin films of thermotropic liquids. One of the two metastable spatial light patterns is doughnut-shaped; the other resembles a Bessel function, with large intensity in the center. Therefore by setting a Si-diode detector in the center of the pattern, we can monitor the oscillation. The oscillation event times (or frequency) are strongly dependent upon convection from the sides of the PMN thermal lens. Therefore the device acts as an optical (“photonic”) sensor that is extremely acute to pressure and flow rate of the surrounding atmosphere. We have measured pressure changes of 10−2Torr and flow rate changes of 0.1 cu. ft/hr. (0.3 m/s) in air.