First-principles Calculations And Experimental Analyses On Optical Properties Of Germanium Under Pressure

The photoelastic coefficient of germanium is 4~6 times larger than other semiconductors. With good thermo-optic and photoelastic sensitivity, germanium becomes a popular sensing element. In the absorption region of wavelength less than 1850 nm, neither experimental data nor theoretical models of the refractive index of germanium is available for reference, and the interaction between light and germanium is yet unknown to people.This paper establishes a 3D model of single crystal germanium using the first-principles calculations software, CASTEP. Energy band structure and optical parameters such as permittivity, absorption coefficient and refractive index are derived through geometric optimization. We detail the analysis of the relevant parameters and get the simulated photoelastic coefficient of germanium by varying the pressure settings in the software. We discuss the effects of K-points on the final accuracy of the calculation results. K-points are set to 11*11*11 finally.A hydraulic press specially designed for optical fiber probe is customized for demonstration of the photoelastic coefficient. The hydraulic press that uses hydraulic oil as the pressure medium has a loading and unloading pressure range of 0~25MPa. 240 nm thick germanium film is vapor deposited on the end of a piece of optical fiber as the sensing probe. The refractive index of the germanium film changes with the pressure and experience model can be reached by measurement of the reflectivity of the fiber probe. The experimental results show that the thickness of germanium effects the performance of the sensing probe.In this paper, we model and calculate the optical constants of germanium using the first principles calculations, then analyze quantitatively the relationship between optical constants and pressure. Comparing the theoretical calculation to the experimental results, we conclude that refractive index of germanium reduces linearly as the pressure increases, which provides for the first time the experimental reference data of semiconductor optical constants in absorption region as we known.