Pure sinusoidal photo-modulation using an acousto-optic modulator.

We use an arbitrary waveform generator to generate a clean sinusoidal modulation from the otherwise nonlinear acousto-optic modulator (AOM). A closed loop optimization script is applied to reduce high order harmonic distortion to less than 0.05% in a high AOM diffraction efficiency regime. This low level of distortion allows us to measure the nonlinear response to photoexcitation of many materials. We demonstrate this technique in a pump-probe experiment to measure the Nonlinear Photo-Modulated Reflectivity (NPMR) of surfaces. NPMR served us as the basis for developing super-resolution microscopy for non-fluorescence samples (label-free) as well as a tool in studying the ultrafast nonlinear response of photo-excited plasmonic nano-structures. Our methodology could be applied to other imaging systems in which measuring nonlinearity is desirable, such as fluorescence and photoacoustic microscopy.