DeepLightSeekdefinition:
The factor by which the speed of light decreases in a medium.
The refractive index of transparent media is the factor by which the phase velocity vph decreases relative to the speed of light in vacuum:
It is assumed here that plane waves propagate linearly (with relatively low light intensity). The refractive index determines the refraction, reflection, and diffraction phenomena at the interface through phase velocity.
The wavelength of light in a medium is equal to one nth of the vacuum wavelength.
According to the relative dielectric constant ε and relative magnetic permeability μ of the material, its refractive index can be calculated:
It should be noted that here, ε and μ are values at the optical frequency, which differ greatly from their values at low frequencies. The magnetic permeability of ordinary optical materials is about 1.
The phenomenon in materials where the refractive index is related to the frequency or wavelength of light is called dispersion. The refractive index range of ordinary glass and crystals (such as laser crystals) in the visible light region is 1.4-2.8, and the shorter the wavelength, the higher the refractive index (normal dispersion). This is a result of the following phenomenon: in the visible light region (where the medium has a high transmittance), between two strong absorption regions: in the ultraviolet region, photon energy is greater than the bandgap, while vibration resonance occurs in the near-infrared or mid infrared region.
Figure 1: The refractive index (solid line) and group velocity (dashed line) of silica as a function of wavelength at temperatures of 0 ° C (blue), 100 ° C (black), and 200 ° C (red), respectively.
Semiconductors have a higher refractive index in transparent regions. For example, the refractive index of gallium arsenide is about 3.5 at 1 micron. This is because strong absorption occurs when the wavelength is smaller than the bandgap wavelength (about 870nm). The result of high refractive index is strong Fresnel reflection and a relatively large total reflection angle at the semiconductor air interface.
The phenomenon of refractive index wavelength dependence in transparent optical materials can be described by the Sellmeier equation, which includes some empirical parameters. The extended version of this equation can describe temperature characteristics, which is used in Figure 1.
The phase matching in nonlinear frequency conversion occurring in nonlinear crystal materials requires knowledge of the specific changes in refractive index with temperature and wavelength.
In non isotropic media, the refractive index is related to the polarization direction (see birefringence) and propagation direction (non isotropic). If the medium has an optical axis, the refractive index of light propagating on that axis is independent of the polarization direction.
The complex refractive index can not only quantitatively represent the phase change per unit length, but also represent the (imaginary part) optical gain or propagation loss (e.g. due to absorption).
There is also a refractive index called group refractive index, which can quantify the decrease in group velocity. In resonance, the refractive index will differ greatly from the group refractive index, which can be observed in some quantum optical experiments. When the group speed is very high or very low, (slow light) will be used.
Some photon metamaterials (usually including metal dielectric composite materials) can achieve negative refractive index, which was first achieved in the microwave region and is now also available in the field of optics. Negative refractive index can cause many unconventional phenomena. For example, the refracted beam at the interface between vacuum and the medium is located on the same side of the normal as the incident beam.
In a waveguide, each propagation mode corresponds to an effective refractive index, which is related to its phase velocity.
