Abstract

Previous studies of second-order nonlinear phenomena at surfaces employed second harmonic generation (SHG) with acollinear excitation geometry. The surface specificity of SHG has been clearly demonstrated by many workers. Since the nonlinear generation occurs over distances corresponding to a few monolayers, phase matching is inherently satisfied. Sum frequency generation (SFG) should also be observable for both collinear and noncollinear excitation geometries, although the phase matching requirement will be more stringent for the latter case due to angular variations of the linear Fresnel factors. We have demonstrated that doubly resonant SFG is enhanced by more than 2 orders of magnitude relative to resonant SHG for submonolayer coverages of rhodamine 6G, in good agreement with theory. A pulsed dye laser probed the S₁←S₀ transition of rhodamine 6G (~525 nm) in combination with the Nd:YAG fundamental (1064 nm) such that the sum frequency output was also resonant with the S₁←S₀ transition. In addition, results for the noncollinear excitation of second harmonic and sum frequency generation from rhodamine 6G coated substrates demonstrate phase matching is easily met for crossing angles < 15°. The output beam coherence can be exploited to achieve spatial separation from the incident beams.

© 1986 Optical Society of America