Second-harmonic generation (SHG) is a nonlinear optical process that occurs when two photons with the same frequency combine within a nonlinear material, resulting in the generation of a new photon with twice the frequency (and therefore half the wavelength) of the original photons. This phenomenon is a specific case of second-order nonlinear optical effects.
Key points about second-harmonic generation include:
Nonlinear optical process: SHG is a nonlinear optical effect, meaning that the response of the material is not directly proportional to the intensity of the incident light. It becomes significant when the intensity of the light is relatively high.
Frequency doubling: In SHG, the original photons have a certain frequency (ω), and the generated photon has a frequency of 2ω. This corresponds to the doubling of the frequency and halving of the wavelength.
Conservation of energy: While the frequency doubles, the energy of the second-harmonic photon remains the same as that of the combined input photons. The process conserves energy.
Applications: Second-harmonic generation has various applications in scientific research and technology. It is commonly used in nonlinear optics, microscopy (e.g., in second-harmonic imaging), and studies of material properties.
Crystal symmetry: The efficiency of second-harmonic generation is influenced by the crystal symmetry of the material. Non-centrosymmetric crystals are particularly suitable for SHG because they lack inversion symmetry.
Medical imaging: In biological and medical applications, second-harmonic generation microscopy is used for imaging structures with intrinsic nonlinear optical properties, such as collagen fibers in tissues.
Understanding and controlling second-harmonic generation can provide valuable insights into the properties of materials and structures. Researchers and engineers use SHG in various fields, ranging from fundamental studies of light-matter interactions to practical applications in imaging and technology.