SFG is a nonlinear optical process in which two light waves at frequencies  and  mix in a medium to generate a wave at the sum frequency

        Symmetry argument rules that it is forbidden in a medium with an inversion center but allowed at a surface or interface where the inversion symmetry is necessarily broken. Therefore, SFG (or SHG) is highly surface-specific, and in recent years has been developed into a most powerful and versatile surface probe. If  or  is near a resonance, SFG is resonantly enhanced. Thus, tuning  or  can yield SFG spectrum characteristic of a surface or interface. Figure 1 describes the SFG process in the reflection geometry often used in experiments. Matching of the wave-vector components along the surface  determines the direction of the reflected sum-frequency output. The efficiency of surface SFG is very low even with the use of very strong pulsed lasers a photon counting system is often needed to detect the sum-frequency output.

        The SFG surface vibrational spectroscopy allows in-situ selective detection and studies of surface molecules and structure with excellent temporal, spatial and spectral resolution. It is applicable to all interfaces accessible by light. Because of its high surface specificity, it is particularly useful for studies of systems where surface and bulk spectra overlap. We have successfully found many unique applications to important problems in various diciplines that cannot be easily investigated by other spectroscopic techniques.

        Examples are

• Studies of pure liquid interfaces. (Fig. 2 gives the vibrational spectrum of a free surface of water)
• Studies of liquid crystal interfaces. (Relevant for liquid crystal displays)
• Studies of polymer interfaces. (Important for many aspects of modern technology)
• Studies of Buried Interfaces.
• Studies of adsorption kinetics on metals, semiconductors, and insulators.
• Studies of Surface Magnetization.
• Studies of surface modification by surfactants
• Studies of surface reactions under real atmosphere.
• Studies of monolayer phase transitions.
• Studies of surface melting of ice.
• Studies of surface-induced ferroelectric ice.
• Studies of ultrashort surface dynamics.
• Surface microscopy.

        We are currently still working on extension of the technique to cover a wider spectral region, both in the infrared and in the UV, and to move into femtosecond time resolution. The improvements will allow us to explore another wide range of interesting problems that have never been explored before. We are also experimenting other nonlinear spectroscopic techniques that could become powerful surface analytical tools.