Nonlinear Optics
Introduction
Nonlinear optics is the branch of optics that studies the interaction of light with matter in which the response of the material system is nonlinearly dependent on the intensity of the incident light. This nonlinearity can give rise to a host of unusual and intriguing phenomena, such as second-harmonic generation, third-harmonic generation, optical parametric oscillation, and four-wave mixing, among others.
History
The field of nonlinear optics was essentially born with the invention of the laser in 1960. The high intensities achievable with lasers allowed for the observation of optical phenomena that were not detectable with traditional light sources. The first experimental observation of a nonlinear optical effect, second-harmonic generation, was reported by Peter Franken and his colleagues at the University of Michigan in 1961, using a ruby laser.
Nonlinear Optical Effects
Nonlinear optical effects can be classified into several categories, including second-order effects, third-order effects, and higher-order effects. These effects are typically observed only at very high light intensities, such as those provided by lasers.
Second-Order Effects
Second-order nonlinear optical effects occur in materials that lack inversion symmetry. The most well-known second-order effect is second-harmonic generation, in which two photons at the fundamental frequency are converted into a single photon at twice the frequency.
Other second-order effects include sum-frequency generation, difference-frequency generation, and optical parametric amplification. These effects are used in a variety of applications, from laser frequency conversion to optical signal processing.
Third-Order Effects
Third-order nonlinear optical effects occur in all materials, regardless of their symmetry properties. These effects include third-harmonic generation, four-wave mixing, and Kerr effect.
The Kerr effect, in particular, has found wide application in the field of optical telecommunications, where it is used for all-optical switching and signal regeneration.
Higher-Order Effects
Higher-order nonlinear optical effects are less commonly observed, due to the extremely high light intensities required. However, they can be important in certain specialized applications, such as high-intensity laser physics and X-ray optics.
Nonlinear Optical Materials
The properties of nonlinear optical materials are crucial for the observation and exploitation of nonlinear optical effects. These materials can be broadly classified into organic and inorganic materials.
Organic Materials
Organic materials, such as certain dyes and polymers, often exhibit large nonlinear optical coefficients, making them attractive for many applications. However, they also tend to have lower damage thresholds than inorganic materials, limiting their use in high-intensity applications.
Inorganic Materials
Inorganic materials, such as crystals and glasses, are widely used in nonlinear optics. These materials typically have lower nonlinear optical coefficients than organic materials, but they also have higher damage thresholds, making them suitable for high-intensity applications.
Applications
Nonlinear optics has a wide range of applications in various fields, including telecommunications, medicine, and scientific research.
In telecommunications, nonlinear optical effects are used for all-optical switching, wavelength conversion, and signal regeneration. In medicine, they are used for imaging and therapy, particularly in the field of ophthalmology. In scientific research, they are used for frequency conversion of lasers, generation of ultrafast pulses, and study of ultrafast phenomena.