Second Harmonic Generation (SHG) is a subcategory of nonlinear optics in physics. Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization P responds nonlinearly to the electric field E of the light. ... A black hole concept drawing by NASA. Physics (from the Greek, φυσικός (physikos), natural, and φύσις (physis), nature) is the science of the natural world dealing with the fundamental constituents of the universe, the forces they exert on one another, and the results produced by these forces. ...

When light of well defined wavelength hits a surface, a small percentage of photons might "clash" together when returning from nonlinearities of the surface material, forming a wavepacket of double the energy (half the wavelength) of the incoming photons. This is the SHG signal that corresponds to greater resolution than the incoming photons. In physics, the photon (from Greek φως phos, meaning light) is the quantum of the electromagnetic field, for instance light. ... In quantum mechanics, for particles in a region of space where no forces act, the Schrödinger equation admits solutions known as wave packets. ...

SHG is a rare phenomenon, compared to the quantity of incoming photons, hence the returned SHG intensity is low. A photo-multiplicator is often used to multiply the expected reflected SHG wave spectre so the signal exceeds the threshold of the sensor(s). In physics, a photo multiplicator is a device to multiply the quantity of incoming photons using the photoelectric effect. ... In most modern usages of the word spectrum, there is a unifying theme of a variety of possible cases between extremes at either end. ...

Because SHG is sensitive to nonlinearities in the surface it allows for great usability at well defined crystal surfaces as that of Silicon. General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance dark gray, bluish tinge Atomic mass 28. ...