Optical constants as well as the gap, represent the signing of a material. They must be specific for this material. However, optical constants depend of the size of the crystallites which form the sample. The same constatation can be noted for the optical gap of a semiconductor material. Why they are called optical constants even if we notice the variation of their values with the size of crystallites ?
Dear Dr. Zoubir,
Optical constants and band gap depend on crystallite size when size becomes in nano range. However, for crystallite size in micro range and above, size effects were not observed.They were considered to be constants. Since nano size effect on the optical properties has been observed recently and hence by convention of earlier days we still call them optical constant even though they depend on size of crystallites due to quantum confinement effect.
The so called material constants (or properties) for given material composition, temperature, and pressure are constants for bulk crystals, i.e. 3D material. The electronic properties are obtained by solving the Schroedinger's equation; for a 3D material, the dimensions are infinite and there are no surfaces, and the electron wave-functions are Bloch functions. For 2D and 1D materials, the solutions of Schroedinger's equation are different; i.e. the band structure of 2D silicon will be different from that of 3D silicon or from that of 1D silicon, because of carrier confinement in one or two directions, respectively. There can be no exact (size) definitions of a 2D semiconductor or of a 1D semiconductor; hence the band structure of these materials may be size dependent, particularly, in the transition range from 3D to 2D and from 2D to 1D.
Regarding the word "constants": The refractive index n(E) and the extinction coefficient k(E) are often called "optical constants". Since they depend on the photon energy E, one should refrain from calling them "constants", to avoid any confusion. So, yes, in this sense, a misnomer.
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