What is the Second Harmonic Generation

SHG is a secondary nonlinear optical phenomenon. In SHG, the energy of 2 photons entering a specimen is combined, producing energy in the form of light. That is, the wavelength of light observed is half of the incident wavelength (the frequency is doubled). An SHG signal is not produced unless molecules in the material are noncentrosymmetric (i.e. a center of inversion symmetry is absent).The signal is linear, so a transmitted light detector is needed. In addition, SHG signal intensity is proportional to the size of the potential, so changes in membrane potential in the vicinity of lipid bilayers of cells with a regular molecular structure can also be analyzed.