Second harmonic generation from the surface of centrosymmetric particles in bulk solution

Second harmonic generation (SHG) is reported for the first time from the surfaces of centrosymmetric particles in bulk isotropic solution. Although SHG is generally described as electric dipole forbidden in centrosymmetric systems, we show that this requires the system to be centrosymmetric on length scales much less than the coherence length of the process. This condition is not satisfied for micron-size particles, and accordingly we have observed a strong SH signal from various particles of this length scale. This promising discovery provides a powerful spectroscopic method for the investigation of physical and chemical processes on the surfaces of microscopic centrosymmetric particles.