To explore resonance phenomena in the nonlinear region, we show by experimental measurements and theoretical analyses that resonance happens in modulation instability (MI) from non-instantaneous nonlinearities in photorefractive crystals. With a periodic modulation in the external bias voltage, corresponding to a modulation in the nonlinear strength, an enhancement in the visibility of MI at resonant frequency is reported through spontaneous optical pattern formations. By modeling a nonlinear driving force to the system, theoretical curves obtained from a nonlinear non-instantaneous Schrodinger equation give good agreement to experimental data. As MI is a universal signature of symmetry-breaking phenomena, our observation on the resonance in MI provides a control on chaotic, solitary, and turbulence waves.
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