Experimental Study On Optical Constants Of Optical Window Materials At Low Temperature

As the wide application of infrared technics on the areas of space exploration, cryogenic infrared telescope and so on, the infrared devices have higher possibilities to be used in cryogenic surroundings. Optical window is one of the significant element of infrared system. The optical and radiative properties of the optical window at low temperature are required to insure the optical window working well. However, there are few researches on this field. Optical constants are the most fundamental radiative properties of optical materials. In this paper, optical constants of several optical window materials are obtained by spectroscopic ellipsometry method and double optical pathlength transmission method(DOPTM) when at low temperature, then an analysis about the effect of temperature on optical constants is given.In this work, the optical constants of two window materials including Mg F2 and Ca F2 are measured by ellipsometry within the temperature range of-60~20℃ in 14~20μm and 16.5~20μm band, respectively.The results show that the refraction index of the two materials decreases and the absorption index increases with increasing wavelength, while the absorption index of Mg F2 has a rapider rise than that of Ca F2.As the temperature reduces, the refraction index of Mg F2 shows a slightly reduce while the absorption index increase. The refraction index of Ca F2 shows a similar trend with Mg F2, while the absorption index decreases as the temperature reduces.Considering the limitation of ellipsometry method on transparent band of materials, the double optical pathlength transmission method(DOPTM) which can obtain reliable optical constants at this range of wavelength is used in measurement and data processing. In this paper, the optical constants of Ca F2, Ba F2 and Zn S in 2~9 μm, 2~13 μm and 2~14 μm band are obtained within the temperature range of-50~20℃ by DOPTM. The results show that the refraction index of Ca F2 and Ba F2 decreases slowly when wavelength increases, and the absorption index of Ca F2 and Ba F2 increases from nearly 5×10-7 to 5×10-4 and 1×10-8 to 8×10-4 with increasing wavelength, respectively. The absorption index of Ca F2 and Ba F2 changes slightly from 2~7 μm and 2~9 μm while that increases rapidly in the remaining band. The refraction index of Ca F2 and Ba F2 has little changes when temperature reduces. The absorption index of Ca F2 and Ba F2 both change slightly with decreasing temperature. The refraction index of Zn S decreases from 2.26 to 2.1 with the wavelength from 2 to 14 μm, and almost unchanges when temperature reduces. The absorption index of Zn S decreases in 2~9 μm and increases in 9~14 μm band. It is noticed that the absorption index decreases significantly when temperature reduces in 8~10 μm band.