# Linear and Nonlinear Photonics

**- Nonlinear Optics**

Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density *P* responds non-linearly to the electric field* E *of the light. The non-linearity is typically observed only at very high light intensities (values of atomic electric fields, typically 10^{8} V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.

**- Linear Optics**

Linear optics is a sub-field of optics, consisting of linear systems, and is the opposite of nonlinear optics. Linear optics includes most applications of lenses, mirrors, waveplates, diffraction gratings, and many other common optical components and systems.

If an optical system is linear, it has the following properties (among others):

- If monochromatic light enters an unchanging linear-optical system, the output will be at the same frequency. For example, if red light enters a lens, it will still be red when it exits the lens.
- The superposition principle is valid for linear-optical systems. For example, if a mirror transforms light input A into output B, and input C into output D, then an input consisting of A and C simultaneously give an output of B and D simultaneously.
- Relatedly, if the input light is made more intense, then the output light is made more intense but otherwise unchanged.

These properties are violated in nonlinear optics, which frequently involves high-power pulsed lasers. Also, many material interactions including absorption and fluorescence are not part of linear optics.

### - Nonlinear photonics

Nonlinear photonics is the technology which exploits the optical nonlienarities of materials to induce new phenomena and achieve new functions. This Research Topic aims at introducing scientists with an optics background to the concepts of nonlinear photonics, focusing on the hot topics that are driving.

**[More to come ...]**