A fully nonlinear frequency response of a moving grating in bismuth silicon oxide, including the effects of an applied electric field, is modeled by solution of the time-dependent Kukhtarev equations for photorefractive materials. The numerical results are used to define fully the nonlinear response function , where is the modulation index in the intensity pattern, to yield the unknown quantity over a broad range of detuning frequencies . For low , the response is superlinear with , and for relatively large it is sublinear with . In the midrange we predict, for the first time to our knowledge, a characteristic frequency at which and the response is linear, that is, , despite the presence of nonlinearly generated higher harmonics of the fundamental grating wave number. In view of this linear behavior, writing a hologram at the linear-response frequency might permit a more faithful reproduction of an object than that which is possible by writing at the frequency of maximum response at the resonance.
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