Optical second-harmonic generation (SHG) is used as a noninvasive probe of the interfaces of Si nanocrystals (NCs) embedded uniformly in an ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ matrix. Measurements of the generated SH mode verify that the second-harmonic polarization has a nonlocal dipole form proportional to $(\overrightarrow{E}·\nabla )\overrightarrow{E}$ that depends on inhomogeneities in the incident field $\overrightarrow{E}$, as proposed in recent models based on a locally noncentrosymmetric dipolar response averaged over the spherical NC interfaces. A two-beam SHG geometry is found to enhance this polarization greatly compared to single-beam SHG, yielding strong signals useful for scanning, spectroscopy, and real-time monitoring. This configuration provides a general strategy for enhancing the second-order nonlinear response of centrosymmetric samples, as demonstrated here for both Si nanocomposites and their glass substrates.

• Received 21 July 2004

DOI:https://doi.org/10.1103/PhysRevLett.94.047401