Measurement of the nonlinear optical phase response of liquefying gallium

Summary form only given. Recently we showed that the optical reflectivity of an interface formed by the elemental metal gallium and silica can become highly nonlinear when held at a temperature just below the bulk melting point of the metal. We have shown that such an interface can exhibit as much as 40% reversible change in reflectivity when excited with just a few mW of optical power. The phenomenon is extremely broadband (extending from at least 450 nn to 1700 nm), and is relatively fast (<1 ns rise time and <1 /spl mu/s recovery time). We believe that the effect is due to an optically-induced, surface-assisted phase transition of the confined /spl alpha/-gallium surface to some as yet unidentified metastable phase of a more metallic nature. We have already used the effect for a number of applications including the construction of broadband optical switches, and passive Q-switching of both erbium and ytterbium fiber lasers. To date our nonlinear reflectivity experiments and applications have only been sensitive to the magnitude of the induced reflectivity change, and no direct measurements have been made of any associated nonlinear optical phase change. We present what we believe to be the first such measurement. The data is important as it provides further more detailed information on the microscopic processes underlying the phenomena, and allows accurate assessment of the performance of the technology in optical phase based sensitive devices and applications.