The field of optics that deals with non – linear behavior of optical materials is known as non- linear optics.

The material for which the optical properties depends on the intensity of light and behave non – linearly is called non linear optical (NLO) Materials.

The physical properties of optical material depend on intensity of light.

Classification of nonlinear materials

The materials which are used to produce the nonlinear optical effects are classified into two categories, namely, passive and active.

Passive Materials

The materials which are simply used as catalysts without imposing their characteristic internal resonance frequencies onto the incident beam of light are known as passive materials and the effect is known as passive optical effect. The harmonic generations, frequency mixing, optical reflection, self focusing of light, etc. are some of the examples for passive optical effects.

Active Materials

The materials which impose their characteristic resonance frequencies onto an incident beam of light are known as active materials. The corresponding effect is known as active nonlinear effect. The following are some of the optical nonlinear effects-two-photon absorption, stimulated Raman, Rayleigh, Brillouin scattering, etc.

Examples of NLO Materials

1. Potassium dihydrogen phosphate (KDP)
2. Ammonium dihydrogen phosphate (ADP)
3. Barium titanate (BaTiO₃)
4. Lithium Iodate (LiIO₃)

Applications of NLO Materials:

1. Organic and inorganic NLO materials play an important role in device manufacture in electronic industry. Ex. Photonic devices, electro optic modulators optical switches.
2. NLO materials are used for light modulation, Q- switching, indicators and photo refractive materials.
3. NLO materials are widely used in lasers and electro-optics.
4. They are used in broad band telecommunications as optical switches and modulators.
5. They are used for higher harmonic generation or frequency shifting.