Second-harmonic amplitude and phase spectroscopy by use of broad-bandwidth femtosecond pulses

Abstract

We present an in-depth experimental study of frequency-domain (FD) methods for measuring second-harmonic (SH) amplitude and phase spectra of surfaces by use of a 60-nm bandwidth femtosecond source and spectral dispersion of generated SH light. We directly compare FD with conventional scanning approaches, in which a narrowband laser is tuned over resonant features, by applying them to common Si1-xGex,Si1-x-yGexCy, and Si(001)–SiO2–Cr metal–oxide–semiconductor (MOS) samples. FD methods yield chirp-independentχ(2) amplitude spectra in good agreement with more time-consuming conventionally measured spectra. FD interferometric SH (FDISH) phase spectroscopy avoids the need for an interferometer scan at each frequency and yields detailed, reproducible phase spectra of the MOS capacitor. To validate the measured phase spectra, we reproduce their bias-dependent features in detail with a model of a resonant electric-field-induced SH polarization superposed coherently upon a field-independent background.