Harmonic Generation is a nonlinear optical process where a light wave interacts with a material to produce new light waves at multiples of the original frequency. This occurs when intense light, such as that from a laser, passes through certain materials, causing the material to emit light at higher frequencies, known as harmonics. The most common type is second harmonic generation, where a photon from the original light is converted into two photons of double the frequency. This phenomenon is widely used in various applications, including medical imaging, spectroscopy, and the development of ultrafast lasers.