Giant spectral transformations in plasmon-enhanced difference-frequency generation with polychromatic light

We theoretically study the generation of mid-infrared light through difference-frequency excitation using a single dual-wavelength coherent femtosecond laser source. The differencefrequency wave is generated from the surface of a thin gold film in the Kretschmann coupling geometry due to the surface nonlinearity of the film. We show a clear enhancement of the difference-frequency wave around the surface plasmon polariton coupling angle of the incident fundamental wave. We also show an enormous shift and modification of the differencefrequency spectrum near surface plasmon resonance. We discuss the discovered spectral change dependence on the source pulse duration and incidence angle of the fundamental wave. Our findings have an enormous potential for use in difference-frequency surface sensing and spectroscopy applications.