Time-Domain Infrared-Visible-Visible Sum-Frequency Generation for Surface Vibrational Spectroscopy

We propose a new surface-sensitive, time-domain measurement of low-frequency vibrational coherence. A transverse optical phonon was coherently excited at 180 cm(-1) on a TiO(2) (110) wafer by femtosecond terahertz pulses of the far-infrared region. The probability of second harmonic generation was modulated by the coherent lattice vibrations and probed using visible light pulses. The observable depth was limited as small as 3 nm by tuning the wavelength of the second harmonic light to be resonant with the electronic absorption of TiO(2).