The reflective properties of randomly rough surfaces at large incidence angles have been reported due to their potential applications in some of the radiative heat transfer research areas. The main purpose of this work is to investigate the formation mechanism of the specular reflection peak of rough surfaces at large incidence angles. The bidirectional reflectance distribution function (BRDF) of rough aluminum surfaces with different roughnesses at different incident angles is measured by a three-axis automated scatterometer. This study used a validated and accurate computational model, the rigorous coupled-wave analysis (RCWA) method, to compare and analyze the measurement BRDF results. It is found that the RCWA results show the same trend of specular peak as the measurement. This paper mainly focuses on the relative roughness at the range of 0.16<σ/λ<5.35. As the relative roughness decreases, the specular peak enhancement dramatically increases and the scattering region significantly reduces, especially under large incidence angles. The RCWA and the Rayleigh criterion results have been compared, showing that the relative error of the total integrated scatter increases as the roughness of the surface increases at large incidence angles. In addition, the zero-order diffractive power calculated by RCWA and the reflectance calculated by Fresnel equations are compared. The comparison shows that the relative error declines sharply when the incident angle is large and the roughness is small.

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In this work, the state-of-the-art infrared variable angle spectroscopic ellipsometry (IR-VASE) and first-principles molecular dynamics (FPMD) method were combined to obtain the infrared dielectric functions of MgO crystal in the spectral range 300-1000 cm(-1) and for temperatures up to 1950 K. The IR-VASE can measure the infrared dielectric functions of MgO crystal at temperatures ranging from 300 to 573 K and reproduce previous infrared-reflectivity experiments. As temperature increases, it demonstrates that the amplitude of dominant absorption peak centered around 400 cm(-1) reduces, the width broadens, and the position shifts to longer wavelength. Besides ellipsometry study, the FPMD method was implemented, seeking to theoretically predict the infrared spectra of MgO crystal at elevated temperatures. Comparing with experimental measurements, the FPMD method can reproduce the essential feature of ellipsometry and previous infrared-reflectivity experiments even at elevated temperatures, though with some deviations in predicting the exact position and amplitude of dominant absorption peak. On the other hand, the FPMD method can predict the temperature effect on the infrared dielectric functions of MgO crystal, e.g., redshift and broadened absorption peak with increasing temperature.