 These laser beams again overlap at the interface being studied, but can cover a wider range of resonances at once than a spectrometer operating in scanning mode and hence spectra can be acquired much faster, allowing the ability to perform time-resolved measurements with interfacial sensitivity.For a given nonlinear optical process, the polarization formula_1 which generates the output is given by

where formula_3 is the formula_4th order nonlinear susceptibility, for formula_5.It is worth noting that all the even order susceptibilities become zero in centrosymmetric media.