Nissim N, Eliezer S, Bakshi L, Perelmutter L, Pasternak M P
Applied Physics Department, Soreq NRC, Yavne 81800, Israel.
Rev Sci Instrum. 2011 Mar;82(3):033905. doi: 10.1063/1.3560901.
High pressure ellipsometry (HPE) method was developed for determining the index of refraction of opaque materials in a diamond anvil cell (DAC). A main difficulty in DAC-based HPE, namely, the pressure-induced birefringence developed in the diamond, was overcome enabling the extraction of the ellipsometric parameters of the sample. The method used was based on the fact that an unpolarized light is unaffected by a retarding optical element and thus reduces the number of unknown parameters in the problem. Because of technical difficulties in using unpolarized light, a linear combination of orthogonal polarizations was applied. In the experimental procedure, multiangle measurements of the ellipsometric parameter ψ are collected at each pressure and the data is fitted together with a measurement of the near normal reflectivity, in order to extract the complex index of refraction. As a test case, this procedure was used to measure the high pressure index of refraction of iron up to 30 GPa for light with wavelengths of 532 and 633 nm. From the index of refraction as a function of pressure the diamond-iron interface emissivity for different pressures was derived and from which the phase transition α → ε could be identified and characterized. The emissivity increases with pressure both at the α (0-9 GPa) and the ε phase (21-30 GPa) however decreases at the mixed α - ε (9-21 GPa) range. From the imaginary part of the index of refraction the pressure dependence of the energy skin depth of iron was extracted. It was found that the energy skin depth increases by an order of magnitude at 30 GPa relative to ambient conditions.
高压椭圆偏振测量法（HPE）是为在金刚石对顶砧（DAC）中测定不透明材料的折射率而开发的。基于DAC的HPE的一个主要困难，即金刚石中产生的压力诱导双折射，得以克服，从而能够提取样品的椭圆偏振参数。所采用的方法基于这样一个事实：非偏振光不受延迟光学元件的影响，从而减少了问题中未知参数的数量。由于使用非偏振光存在技术困难，因此采用了正交偏振的线性组合。在实验过程中，在每个压力下收集椭圆偏振参数ψ的多角度测量值，并将数据与近正反射率的测量值一起进行拟合，以提取复折射率。作为一个测试案例，该程序用于测量波长为532和633 nm的光在高达30 GPa压力下铁的高压折射率。根据折射率随压力的变化关系，推导出不同压力下金刚石-铁界面的发射率，并据此识别和表征α→ε的相变。在α相（0-9 GPa）和ε相（21-30 GPa），发射率均随压力增加，但在α-ε混合相（9-21 GPa）范围内降低。从折射率的虚部提取了铁的能量趋肤深度与压力的关系。结果发现，相对于环境条件，在30 GPa时能量趋肤深度增加了一个数量级。
