Four-wave mixing is a nonlinear optical process in which four electromagnetic waves interact within a medium to create new frequencies. This phenomenon occurs when three waves, typically a pump wave and two signal waves, interact with a medium to generate a fourth wave. Four-wave mixing can be used in various applications such as signal processing, wavelength conversion, and optical switching in telecommunications and quantum information processing. The process is highly dependent on the properties of the medium and the input wave frequencies, and researchers are continually exploring ways to optimize and control four-wave mixing for practical use in optical devices.