  The low-frequency collective excitations, which often occur in the terahertz
or multi-terahertz spectral region, play an essential role in many novel
emergent phenomena. Despite numerous studies in the bulk, detection of such
excitations at interfaces remains challenging owing to the lack of feasible
experimental techniques. Here, we show that interfacial low-frequency modes can
be characterized using surface-specific nonlinear terahertz spectroscopy. This
technique uses intra-pulse difference frequency mixing (DFM) process that can
extend the second-order optical spectroscopy to the terahertz range. As a
demonstration, the surface phonon of SrTiO3(001) at 2.8 THz was successfully
measured. This surface polarization originates from the excess of oxygen
vacancies or charge transfer at the interface. We have also developed an
analytical procedure for remote measurement of the interfacial potential of
complex oxides in a practical environment. Our method offers new opportunities
for in situ studies of the low-frequency excitations at interfaces in broad
disciplines.
