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Enhancement of femtosecond surface nonlinear optical signals with spatiotemporal focusing.

Nonlinear optical reflection spectroscopies for surface studies often suffer from their relatively weak signals. To enhance the signal strength, femtosecond input pulses are generally preferred, but their intensity is usually limited by supercontinuum (SC) generation in a bulk medium. We show here that the problem can be largely resolved by means of spatiotemporal focusing (STF) of the input that effectively suppresses the SC, permits much higher input intensity on a medium, and greatly enhances the nonlinear output signal. We use second-harmonic generation (SHG) on oxide as examples and demonstrate that, with STF, the input intensity for the onset of the SC can be increased by ∼30 times in comparison with conventional focusing, and the reflected SHG can be increased by about three orders of magnitude.