Quadratic power dependence of the magneto-optic coupling coefficient in the magnetostatic wave–optical interaction (abstract)

Abstract

The transverse magnetostatic wave (MSW)–optical interaction was first observed at relatively low microwave power levels.1 Recent investigations show that a change in the demagnetizing field due to an increase in the dynamic magnetization causes a shift in the microwave passband2 and consequently a shift in the MSW–optical interaction spectrum.3 We now focus our efforts on determining the effects of high microwave power on the magneto-optic coupling coefficient (κ). A microstrip T transducer excited forward volume MSWs in a bismuth–lutetium–iron–garnet film. An optical beam is edge coupled into the film and the transverse MSW–optical interaction is observed at high microwave power levels. Using the coupled mode equations, we show that the amplitude of the diffracted beam is a product of κ and the phase mismatch between the optical guided modes and the MSWs. Taylor series expansions for κ and the MSW wave number (β) are used to obtain an analytic expression for the MSW–optical interaction. The dependence on β causes a shift in the interaction passband with increasing microwave power. After correcting for this nonlinear shift, the residual dependence of the output optical intensity on input microwave power is attributed to a quadratic power dependence in κ. The coefficient of the quadratic term was observed to be ∼1% of the coefficient of the linear term and its effect at high power is comparable to that of the nonlinear frequency shift.