Second-harmonic spectroscopy of Si nanocrystals embedded in silica

Extending previous second-harmonic generation (SHG) single-wavelength studies [1] of Si nanocrystals (NCs) embedded in fused silica, we implement spectroscopic cross-polarized two-beam SHG (XP2-SHG) by employing an optical parametric amplifier as a source of widely tunable (1.6 Eph 2.4 eV) femtosecond pulses. Subsequent isolation of the SH signal produced by the NCs yields the oscillator strength and phase of their nonlinear response. We obtain a broad resonance between bulk Si E1 and E2 critical points. Possible explanations for the spectroscopic result are discussed