Wolf Luis Mochán Backal

New light from silicon nano-interfaces

Prepared by Mike Downer

Silicon nanocrystals embedded in glass behave in a most un-silicon-like manner:  they glow brightly, a feature that is giving birth to silicon light-emitting devices, including lasers.   Unusual bonding structures at the nanocrystal-glass interfaces are believed responsible, but effective experimental probes of these nano-interfaces have eluded the surface scientist.  In this paper, Figliozzi et al. report a simple re-configuration of optical second-harmonic generation -- a widely used spectroscopic probe of planar interfaces - that enables it to probe such sharply curved nano-interfaces.  Two beams of red laser pulses intersect inside a composite of spherical silicon nanocrystals in glass, and generate blue second-harmonic light along their bisector.  In contrast to the standard single-beam configuration, which generates barely perceptible second-harmonic radiation from the nano-composite, the new two-beam configuration generates a robust second-harmonic beam that is sensitive to chemical modifications of the nanocrystal-glass interfaces.  It behaves in accordance with a phenomenological model that is applicable to any composite of small spherical particles.  Results so far were obtained with incident pulses of a single wavelength, but the authors plan to use tunable pulses to generate a spectrum of the silicon nano-interfaces to elucidate their poorly understood electronic structure.