The factorized plasmon-phonon polariton description of the infrared dielectric function is generalized to include an additional factor to account for the effects of interband electronic transitions. This new formalism is superior to the usual Drude–Lorentz summation of independent oscillators, especially in materials with large transverse-longitudinal optical phonon splittings, multiple infrared-active phonon modes, or high concentrations of free carriers, if a broadband description of the dielectric function from the far-infrared to the vacuum-ultraviolet spectral region is desired. After a careful comparison of both approaches, the factorized description is applied to the dielectric function of undoped and doped semiconductors (GaAs, GaSb, and InAs) and metal oxides from 0.03 to 9.0 eV. Specifically, the authors find that both descriptions of the far-infrared dielectric function yield the same carrier density and mobility, at least for a single species of carriers. To achieve valid results for moderately high doping concentrations, measurements to lower energies would be helpful.

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