Harmonic generation is a phenomenon in physics where a nonlinear material will generate new frequencies of light when illuminated by a high-intensity light source. This process involves the interaction of multiple photons within the material, resulting in the production of harmonics that are integer multiples of the original frequency. Harmonic generation has applications in fields such as laser technology, spectroscopy, and imaging, and is used in techniques such as second-harmonic generation (SHG) and third-harmonic generation (THG) for various scientific and technological purposes.