Observations of transient nonlinear optical effects on reflection from opaque media and novel polarisation phenomena in crystals
The degenerate cubic optical nonlinearity of an indium-glass interface in the vicinity of the melting point has been measured for the first time at 1260 nm using a spectroscopic technique based on the pump probe specular inverse Faraday effect. A considerable enhancement of the nonlinearity was observed approaching the melting point from below with a magnitude of 6.3 x 10−7 e.s.u. (8.8 x 10−15 m2Volt−2) The nonlinear response was faster than 90 fs. Away from the melting the nonlinearity is attributed to the spin flipping mechanism. Near the melting point it is associated with weakly bound highly unharmonic electrons. An optical nonlinearity of a gallium dielectric interface has been used to achieve a broadband light by light control at milliwatt power levels. The broadband optical nonlinearity of an α-gallium-dielectric interface is shown to enhance near the bulk melting point (30°). The magnitude of the nonlinearity is ~1 e.s.u. (1.4x10−8 m2Volt−2). The nonlinear response was found to have various components ranging from at least in the micro to nanosecond range. Up to a 30% modulation of a probe beam in reflected light intensity has been observed. In a fiberised architecture the nonlinearity is exploited to make an optical switch at 1.5 µm. Cross wavelength switching is also presented at 1.3 µm and 1.5 µm where the modulation in reflected light exceeded 40% with dynamics of ~140ns. Based on the same nonlinearity a new passive self-starting Q-switching ring laser is presented. The pulse duration is ~1.5 µs with peak powers exceeding >100mW. The nonlinearity is attributed to the metallisation and recrystallisation velocities of gallium. The nonreciprocity of natural polarisation has been observed in the optically active crystal Bi12SiO20 at 632.8, 514.5, 501.7, 496.5, 476.5, and 457.9 nm. A nonreciprocal component of the polarisation rotation of radians was detected on the background of the polarisation rotation due to conventional optical activity. The nonreciprocal rotation has been attributed to the presence of the symmetric part of the nonlocality tensor Re[γxyz + γxyz] ~ 5 x 10−12 cm at 457.9 nm. The giant birefringence (0.03 < dn < 0.3) due to the quadratic term in the constitutive equation for ZnSe along the  direction in the excitonic region in reflection was measured for the first time. Using this effect a reflective configuration technique for studying crystallographic domains in cubic crystals has been developed.