opticalconstant

Optical constants are parameters that characterize how a material interacts with electromagnetic radiation in the optical range. They describe, for a given wavelength or frequency, how light propagates through, is absorbed by, or is reflected from a material, and are essential for predicting optical performance in devices and coatings.

The two most common constants are the refractive index n(λ) and the extinction coefficient k(λ). They form the complex refractive index N(λ) = n(λ) + i k(λ). The extinction coefficient is related to absorption: larger k means stronger attenuation within the material. The square of N gives the complex dielectric function ε(ω) = ε1(ω) + i ε2(ω), since ε = (n + i k)^2. Note that some sources use N = n − i k depending on the chosen time dependence convention.

Measurement and modeling: Optical constants are obtained by spectroscopic ellipsometry, reflectance/transmittance measurements, and various retrieval methods. Kramers-Kronig relations provide ε from reflectance data under suitable conditions. Physical models such as Drude, Lorentz oscillators, or Sellmeier/Cauchy parameters are used to describe dispersion across spectral regions and to interpolate or extrapolate data. Experimental databases compile measured constants for many materials under standard conditions.

Applications: Knowledge of optical constants enables design of lenses, coatings, waveguides, photonic devices, and solar cells, as well as interpretation of optical spectra. They reveal intrinsic material properties such as band structure and free-carrier response and are used in simulations of optical stacks, plasmonics, and metamaterials.