SFG is a process in which two input beams at frequencies w1 and w2 mix in a medium and generate an output beam at the sum frequency w = w1 + w2. As a nonlinear optical process, it is only allowed under electric-dipole approximation in media without inversion symmetry. At surfaces or interfaces, the inversion symmetry of the bulk is broken and therefore SFG is allowed.
Traditional femtosecond SFG is based on the 35-femtosecond laser system, compared with the scanning picosecond SFG, which can obtain a spectrum within seconds.
Phase-sensitive SFG can directly probe real and imaginary parts of the second-order nonlinear susceptibility, and determine the absolute average orientation (up or down) of molecules at the surface or interface, which can be hardly obtained by the traditional SFG.
Pump-probe SFG is a powerful tool to investigate the ultrafast energy transfer process occurring at surfaces and interfaces on the femtosecond time scale.
