Second harmonic generation (SHG) is a second order nonlinear optical effect where two photons of frequency ω are converted to one photon of frequency 2ω. SHG is allowed only in media without inversion symmetry.

SHG is a sensitive method used to characterize molecules at surfaces or interfaces because inversion symmetry is broken at the interface.

SHG measurements provide information about: surface coverage, molecular orientation, adsorbtion-desorbtion processes, and reactions at interfaces.