Harmonic generation refers to a nonlinear optical process in which incoming photons interact with a material and produce new photons at integer multiples of the frequency of the incoming photons. These new photons have energies (and thus wavelengths) that are multiples of the original photons' energy.
Harmonic generation occurs when the intensity of the incoming light is sufficiently high to induce nonlinear optical effects in the material. The most common form of harmonic generation is second harmonic generation (SHG), where two photons of the same frequency combine to produce a single photon with twice the frequency (and hence, half the wavelength) of the original photons. This process is governed by the second-order nonlinear susceptibility of the material.
Higher-order harmonic generation (e.g., third harmonic generation, fourth harmonic generation, etc.) involves the production of photons at frequencies that are three, four, or more times the frequency of the original photons, respectively. These processes are less efficient than second harmonic generation and typically require even higher intensities of the incoming light.
Harmonic generation is utilized in various fields of science and technology, including:
Nonlinear optics: Harmonic generation is used to study the nonlinear optical properties of materials and to investigate processes such as multiphoton absorption and optical rectification.
Frequency conversion: Harmonic generation can be used to convert light from one wavelength to another. For example, second harmonic generation is used to generate coherent light at shorter wavelengths, which is valuable in applications such as microscopy, spectroscopy, and laser-based machining.
Frequency doubling: Second harmonic generation is employed in frequency doubling crystals to generate laser light at wavelengths not directly accessible from common laser sources. This is important in applications such as laser-based medical treatments and precision metrology.
Optical parametric amplification: Harmonic generation can be used as part of optical parametric amplifiers to amplify specific wavelengths of light. This technique is valuable in ultrafast laser systems for producing intense, ultrashort pulses of light for applications in laser surgery, material processing, and scientific research.
Overall, harmonic generation is a fundamental process in nonlinear optics, providing a powerful tool for manipulating the properties of light and enabling a wide range of applications in science, technology, and engineering.