A nonlinear medium is a material in which the response to an external influence, such as light or sound, is not directly proportional to the intensity of that influence. This means that when energy is applied, the medium can behave in complex ways, leading to phenomena like frequency mixing or wave distortion. Nonlinear media are essential in various applications, including telecommunications and laser technology. In optics, nonlinear media can generate new wavelengths of light through processes like second-harmonic generation. This occurs when photons interact with the medium in such a way that they combine to form new photons with different energies. Understanding nonlinear media is crucial for advancing technologies in fields such as fiber optics and quantum computing.