second-harmonic generation

Second-harmonic generation is a process where certain materials convert incoming light at one frequency into light at twice that frequency. When a laser shines on such a material, it produces a new beam with a wavelength that is half—i.e., twice as energetic—compared to the original. This phenomenon is used in laser technology to generate new colors of light, such as converting infrared to visible green. It relies on the material's nonlinear properties, meaning its response to light changes with the intensity, allowing the material to effectively "combine" two photons into one with higher energy.