Spectral reflectance of GaAs from infrared (IR) to ultra-violet (UV) bands is calculated from first principles. We first calculate the spectral dielectric function which is determined by the response of GaAs to external electromagnetic field. Two mechanisms exist for different wavelengths, namely, phonon absorption in the far-IR region and the electronic absorption in the near-IR to UV region. With plane-wave pseudopotential method, we determined the dielectric function of GaAs with the the initial structure as the only input. For the far-IR region, phonon calculations are carried out. By analyzing the phonon modes, low-frequency dielectric constant is calculated. For the near-IR to UV band, the electronic band structure of GaAs is calculated, and the imaginary part of the dielectric function is determined from the band structure using Fermi’s Golden rule. The real part of spectral dielectric function is then derived from Kramer-Kronig transformation. The reflectance is then calculated using Maxwell’s equations.

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