Higher-order combinational gratings play an important role in explaining so-called nonlinear cross talk between index gratings in photorefractive media. We present what are to our knowledge the first experimental results showing the existence of a new, strong combinational index grating in a three-wave mixing experiment. We investigate the kinetics of the first harmonic, the second harmonic, and the new combinational index gratings. As a novel result, we demonstrate that the rise time of the new grating is strongly dependent on the intensity ratio of the incident recording beams. For low values of the intensity ratio the rise time is more than twice the value of the ordinary second-harmonic grating. Furthermore, it is shown that the steady-state strength of the new grating can be more than five times greater than that of ordinary second-harmonic gratings and can lead to substantial nonlinear cross talk on the first- and second-harmonic gratings.
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