Figure 6. Index of refraction versus wavelength for six materials. Theory (lines) may be compared to measurements (points). The theory was adjusted to calibrate the static electronic dielectric constant, ε∞.Results for the index of refraction as a function of wavelength are shown for six materials: germanium (Ge), silicon (Si), gallium arside (GaAs), magnesium oxide (MgO), lithium fluoride (LiF), and argon (Ar). The index, a unitless quantity denoted by n, is plotted as a function of wavelength, which is denoted by λ and is specified in µm. The wavelength range shown goes from 30 nm to 100 µm, and the range of values of index shown goes from 1 to 6. Experimental results are shown at discrete yet closely spaced values of wavelength, whereas theoretical results are shown as continuous curves. Experiment and theory agree reasonably well over most of the range shown. In Ge, Si and Ar, the index rises rapidly as one goes towards shorter wavelengths (because of effects of inter-band transitions), starting around 1 µm for Ge and Si, and around 0.1 µm for Ar, but the index approach a constant value around 4, 3.5, and 1.3, respectively, as one goes towards longer wavelengths. In GaAs, MgO and LiF, the index starts rising rapidly around 1 µm, 0.2 µm and 0.1 µm as one goes toward shorter wavelengths, respectively. However, the index also drops rapidly around 25 µm, 10 µm, and 8 µm, respectively, as one goes toward longer wavelengths (this is because of lattice vibrations in these three polar crystals). The index is nearly flat in-between for GaAs, MgO and LiF, where it is close to a value of 3.3, 1.8 and 1.4, respectively.
Online: April 2000