Sum frequency generation spectroscopy

Sum frequency generation spectroscopy (SFG) is a technique used to analyze surfaces and interfaces. This nonlinear laser spectroscopy method was developed in 1987 and rapidly applied to deduce the composition, orientation distributions, and some structural information of molecules at gas–solid, gas–liquid and liquid–solid interfaces. In a typical SFG setup, two laser beams mix at a surface and generate an output beam with a frequency equal to the sum of the two input frequencies. SFG has advantages in its ability to be monolayer surface sensitive, ability to be performed in situ (for example aqueous surfaces and in gases), and not causing much damage to the sample surface. SFG is comparable to second harmonic generation (SFG is a more general form) and Infrared and Raman spectroscopy.

IR-visible sum frequency generation spectroscopy uses two laser beams that overlap at a surface of a material or the interface between two materials. An output beam is generated at a frequency of the sum of the two input beams. The two input beams have to be able to access the surface, and the output beam needs to be able to leave the surface to be picked up by a detector. One of the beams is a visible wavelength laser held at a constant frequency and the other is a tunable infrared laser. By tuning the IR laser, the system can scan over resonances and obtain the vibrational spectrum of the interfacial region.

For a given nonlinear optical process, the polarization ${\displaystyle {\overrightarrow {P}}}$ which generates the output is given by

where ${\displaystyle \chi ^{(i)}}$ is the ${\displaystyle i}$th order nonlinear suspectibility, for ${\displaystyle i\in [1,2,3,\dots ,n]}$.

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