Technique for characterization of laser-induced dielectric gratings: Application to non-photorefractive cubic KTaO3:Cu

Two constant overlapping monochromatic laser beams can interfere in any homogeneous medium to produce a spatially sinusoidal variation Δ ε ⇀ ⇀ Ae in the complex dielectric tensor (i.e. an "optical grating").1 In most experiments in insulators to date, the optical grating arose either from the photorefractive effect or from optically excited impurities such as Nd3+ in YAG. In general Δ ε ⇀ ⇀ may be written as a superposition of complex sinusoidal variations that are in and out of phase with the optical beam interference pattern. In this paper we describe a simple and efficient technique for measuring both the spatially in-phase and out-of-phase components of at least one complex tensor component of Δ ε ⇀ ⇀ . In those rare cases where desired information resides in the other tensor components, our method can be extended to measure them. We illustrate our technique by measuring an optical grating produced by 633 nm beams in a cubic KTaO3:Cu crystal which is not photorefractive. Nevertheless, the grating is comparable in magnitude to photorefractive gratings. Also the grating is quite visible to our technique, even though there is almost no steady-state energy exchange between the beams. Our method is an extension of one used by Zha, et al.2 to monitor photorefractive gratings.