Abstract
We describe a high precision interferometer system to measure the pressure dependence of the refractive index and its dispersion in the diamond anvil cell (DAC). The reflective Fabry–Perot fringe patterns created by both a white light and a monochromatic beam are recorded to determine both the sample thickness and its index at the laser wavelength and to characterize the dispersion in the visible range. Advances in sample preparation, optical setup, and data analysis enable us to achieve 10^{-4} random uncertainty, demonstrated with an air sample, a factor of five improvement over the best previous DAC measurement. New data on {text{H}}_{2}text{O} liquid water and ice VI up to 2.21 GPa at room temperature illustrate how higher precision measurements of the index and its optical dispersion open up new opportunities to reveal subtle changes in the electronic structure of water at high pressure.
Highlights
We describe a high precision interferometer system to measure the pressure dependence of the refractive index and its dispersion in the diamond anvil cell (DAC)
In electro-magnetic theory, the complex refractive index can be defined as the square root of the complex relative dielectric constant[1,2]
= 2ts i n2s where ns and ts are the refractive index and thickness of the sample, nair is the refractive index of air (1.000278 at the laser wavelength L of 532 nm33), θi and θs are the incident and refractive angles of the beam which are related by Snell’s law, i is the wavelength of the incident beam, and A is the geometric calibration constant ( A = r/ tan θi ) which is related to the working distance of the objective lens and allows us to use the radius of interference pattern (r) instead of its incident angle[11]
Summary
We describe a high precision interferometer system to measure the pressure dependence of the refractive index and its dispersion in the diamond anvil cell (DAC). Refraction is one of the most common optical phenomena, indicating the bending of non-perpendicular incident waves at the interface between two different media It reflects the dynamic polarization of atoms and/or molecules under electromagnetic radiation inducing oscillations of electron clouds and/or rotations of polar m olecules[1,2]. According to Eq (1c), the refractive index of a sample in the DAC can be measured by analyzing the fringe patterns which are obtained from different sample thicknesses[9] or incident angles[22] at a known wavelength. This contrast loss can be utilized in the refractive index matching technique[12,21], but this simple method provides at most two refractive index values in the typical DAC
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