Abstract

Daum et al.¹ have demonstrated a strong two-photon-resonance band at 3.3 eV in the ${\chi }_{zzz}^{\left(2\right)}$ susceptibility component of the surface second-harmonic (SH) spectrum of Si(001)-2 × 1, due to direct electronic transitions in the strained near-surface region. By using 120-fs Ti:sapphire-laser pulses to maximize surface SH efficiency while minimizing surface heating, our experiments (1) measure the thermal red shift of this two-photon ${\chi }_{zzz}^{\left(2\right)}$ resonance and agree with the calculated thermal shrinkage rate of the bulk A₃-A₁ transition; (2) measure the temperature dependence of the weaker ${\chi }_{zxx}^{\left(2\right)}$ and ${\chi }_{xxz}^{\left(2\right)}$ components, which do not track the A₃-A₁ resonance and must therefore be attributed to other temperature-dependent surface mechanisms; (3) measure a blue shift of the ${\chi }_{zzz}^{\left(2\right)}$ resonance on hydrogen termination and explain it by a coupled anharmonic Si-Si/Si-H oscillator model.

© 1995 Optical Society of America