The linear and nonlinear optical properties of a material describe how the material interacts with light. Still, they differ in terms of their relationship to the intensity of the light and the underlying mechanisms:
- Linear optical properties describe the interaction of light with a material where the response (e.g., polarization, refractive index) is directly proportional to the incoming light's intensity (or electric field strength).
- Nonlinear optical properties arise when the material's response is not directly proportional to the intensity of the incoming light, usually observed at high light intensities. Here are some examples: Kerr effect, two-photon absorption, self-focusing, second harmonic generation etc.
In Linear optics, the study and manipulation of light in a linear and deterministic manner. The response of optical element is proportional to the amplitude of the incident light wave. Nonlinear optics is the study of the interaction of intense laser light with matter. The nonlinear susceptibility is a quantity that determines the nonlinear polarization of a material medium in terms of the strength of the applied electric field. There are many nonlinear optical processes like second harmonic generation, sum and difference frequency generation, optical parametric oscillations, Third harmonic generation, intensity dependent refractive index, saturable absorption, two-photon absorption. The electro optical effect is the phenomenon that the refractive index of a material is modified with an electric field. The linear electro-optic effect is the Pockels effect where the refractive index change is proportional to the electric field strength. Only non-centrosymmetric materials exhibit the linear electro-optic effect e.g. LiNbO3, KTP, BBO etc. On the other hand, all centrosymmetric media exhibit the quadratic electro-optic effect i.e. Kerr effect.
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