High-harmonic generation is a nonlinear optical process in which intense laser light interacts with a gas or solid material, creating extremely high-frequency electromagnetic radiation. This process typically occurs when a strong laser field interacts with a target material, causing the atoms or molecules to ionize and recombine in a high-energy state. This results in the emission of harmonics of the original laser frequency, which can reach up to the extreme ultraviolet or soft X-ray range. High-harmonic generation has important applications in areas such as attosecond spectroscopy, ultrafast imaging, and laser technology. It allows for the generation of coherent light sources with very short pulse durations and high photon energy, which can be used for studying ultrafast processes in physics, chemistry, and material science. Additionally, high-harmonic generation can also be applied in laser machining, microscopy, and medical imaging.