The Principle, Application and Imaging of SHG

The second harmonic microscope (SHIM) is based on a nonlinear optical effect called second harmonic generation (SHG). SHIM has been established as a viable microscope imaging contrast mechanism for visualizing the structure and function of cells and tissues. The second harmonic microscope obtains the contrast from the change of the second harmonic coefficient of the incident light of the sample, while the traditional optical microscope obtains the contrast by detecting the change of the optical density, optical path or refractive index of the sample. SHG requires intense laser light to pass through materials with non-centrosymmetric molecular structures. The second harmonic light emitted by the SHG material is exactly half the wavelength (frequency doubled) of the light entering the material. Although two-photon excited fluorescence (TPEF) is also a two-photon process, TPEF loses some energy during the relaxation of the excited state, and there is no energy loss. Generally, inorganic crystals are used to generate SHG light, such as lithium niobate (LiNbO \(_3\) ), potassium titanyl phosphate (KTP \(=\) KTiOPO \(_4\) ) and lithium triborate (LBO \(=\) LiB \(_3\) O \(_5\) ). Although SHG requires a certain material to have a specific molecular orientation in order to double the frequency of incident light.