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Optics Letters

Moving grating and dc external field in photorefractive GaP at 633 nm

Jian Ma, Ken-ichi Chino, Yoshinao Taketomi, Yeshaiahu Fainman, Joseph E. Ford, Sing H. Lee

Published Jul 15, 1991• 8 citations
Publisher

Abstract

We demonstrate that the photorefractive effect in GaP crystal at 633 nm can be enhanced using an externally applied dc field and a moving grating. A two-beam coupling gain coefficient of 2.5 cm(-1) and a steady-state phase-conjugate reflectivity of 1.9% were obtained.
DOI
10.1364/ol.16.001080

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We have observed two-beam coupling gain (0.273 cm−1) that exceeds absorption (0.145 cm−1) in InP:Fe without applying an external field at λ=1.32 μm. Measurements of the gain coefficient as a function of grating spacing fit the photorefractive theory for simultaneous electron-hole transport, with the effective density of traps NE = (3.86 ± 1.1)NE=(3.86±1.1)× 1015 cm−3 and ξ, the factor that accounts for competition between electrons and holes, equaling −0.729±0.026. Absorption measurements, combined with the photoionization cross sections of electrons and holes, confirm these results. The agreement between theory and experiment suggests how to choose impurity concentrations that optimize net gain.
Nonreciprocal energy transfer between two coherent laser beams during two-wave mixing requires a photorefractive phase shift between the interference pattern and the resulting space charge. The symmetry of the Stark geometry photorefractive quantum wells forbids a phase shift using either dc or ac fields. We present the first experimental demonstration of nonreciprocal energy transfer during two-wave mixing using moving gratings. Optical energy exchange is observed in photorefractive p-i-n quantum well diodes with photorefractive gains approaching 1000 cm−1 and exhibiting complex spatio-temporal dynamics.
Using numerical simulations of the fully nonlinear equations that describe the steady-state optical fields that develop within photorefractive phase-conjugate mirrors, we have shown that the phase-conjugate reflectivity can be enhanced in the presence of linear absorption. Our investigations focused on the development of the optical fields that arise when the reflectivity is nonmonotonically dependent on the linear absorption both near and away from self-oscillation. We have identified several significant features associated with the enhancement. Near self-oscillation and with strong pumps, an increase in the loss can lead to a more efficient energy transfer to the probe by altering the phase of the coupling, resulting in an increase in the index grating amplitude. Away from self-oscillation and with strong pumps, the loss alters the distribution of the grating within the crystal, and maximum reflectivity is associated with an optimum distribution. Reflectivity enhancement that is due to linear absorption was shown to occur even in the weak-pump case, thus providing unequivocal evidence of the veracity of this phenomenon.