Coherent backscattering of optical second-harmonic generation with long-range surface plasmons.

We report the observation of coherent backscattering of optical second-harmonic generation that is due to excitation of the long-range surface plasmon mode at a silver-quartz interface. A peak-to-background ratio of greater than 2 is seen, which is possible for a coherent scattering process in nonlinear optics. The backscattering pattern has an angular half-width of 15 mrad in the incident plane but 90 mrad in the transverse plane. This effect is consistent with simple wave-vector considerations. Backscattering is observed only over a limited range of silver film thicknesses that correspond to a maximum propagation length for the surface mode.