Photon, an elementary particle that transmits electromagnetic interaction, is a kind of gauge boson. Photons are the carriers of electromagnetic radiation, and photons are considered as the medium of electromagnetic interaction in quantum field theory. Optical material refers to a general term for optical materials that make use of the principle of changes in optical properties (such as refractive index or induced polarization) under the action of external fields (electricity, light, magnetism, heat, sound, force.) According to the specific mechanism or application purpose, optical functional materials can be further divided into electro-optic materials, magneto-optic materials, elastic-optic materials, acousto-optic materials, thermo-optical materials, nonlinear optical materials and laser materials.

 

 

Electro-optical material: It usually refers to the material in which the refractive index changes birefringently under the action of an external electric field (DC or alternating field). One of its working principles is based on the linear electro-optic effect (Pockels effect), and the other is based on the secondary electro-optic effect (optical Kerr effect). The characteristic of the linear electro-optic effect is that the change of the induced refractive index is proportional to the power of the external electric field, so the material that produces the effect must be an anisotropic crystal with no symmetrical center. The characteristic of the Kerr effect is that the change of the induced refractive index is proportional to the quadratic power of the applied electric field, and the material producing the effect can be a crystal with arbitrary symmetry or anisotropic medium. The commonly used linear electro-optic effect materials are crystals without central symmetry, such as potassium dihydrogen phosphate (KDP), ammonium dihydrogen phosphate (ADP), lithium niobate (LiNbO) lithium iodate (LiIO) and so on. The commonly used materials of secondary electro-optic effect are some organic liquids with large Kerr constant, such as nitrobenzene. The above two kinds of electro-optic materials are usually used to make optical switch elements or optical modulation elements, which are controlled by external electric field.

 

Magneto-optical material: It refers to a kind of optical material in which the refractive index changes inductively under the action of an external magnetic field. Its principle is based on a variety of magneto-optical effects, such as Faraday magnetic rotation, magnetic dichroism and magneto-induced birefringence. Magneto-optical materials are divided into diamagnetic and paramagnetic materials. The commonly used diamagnetic materials are ultra-high lead glass, arsenic sulfide glass and so on. Paramagnetic materials are (TbO) glass containing terbium oxide and europium oxide (EuO), europium selenide (EuSe) crystals, as well as ferromagnetic crystals such as iron fluoride (FeF) yttrium iron garnet (YFeO) and so on. In laser technology, the Faraday rotator made of ultra-high lead glass materials can play the role of not only a fast optical switch, but also a reverse optical isolator, which has great application value.

 

Acousto-optical material: It refers to a kind of transparent optical medium whose reflectivity characteristics change inductively under the action of the sound field. Under the action of sound field, the density inside the material fluctuates periodically, which leads to the periodic fluctuation of refractive index. This makes the medium itself equivalent to a phase grating, thus diffracting the directional incident beam. The requirement for acousto-optic materials is to have a higher quality factor of acousto-optic interaction, as well as lower acousto-optic loss and light loss. The commonly used acousto-optic materials are fused quartz, high lead glass, lead molybdate (PbMoO) tellurium dioxide (TeO) and gallium phosphide (GaP) crystal. Acousto-optic materials are usually made into acousto-optic switches to modulate light, or acousto-optic deflectors to control the direction of light speed.

 

Elastooptic material: It usually refers to a kind of optical material whose refractive index changes induced birefringence under the action of external force field. Its action principle is based on elastic-optical effect, that is, under the action of external force field, the material itself produces elastic strain, which leads to the induced change of refractive index. The commonly used elastic-optical materials are some transparent optical media with large elastic-optical coefficient, such as glass, crystal, plastic and so on. They are often used in the study of photoelasticity.