Quantification of Chromatic Aberration In the Laser-Heated Diamond Anvil Cell

Abstract

Earth materials subjected to the high pressure and temperature conditions of planetary interiors display unique changes in crystal structure, melting temperature, and transport properties. The laser-heated diamond anvil cell (LHDAC) is a tool to make in situ measurements of material properties, specifically at pressures up to 1 Mbar (100 GPa) and temperatures from 1500 to 7000 K. Temperatures above 1500 K are measured by the spectroradiometry of the thermal emission that passes through the diamond anvil and is collected on a intensity-calibrated spectrometer and coupled-charged device (CCD). The accuracy of the measured temperatures is not always reliable, due to optical dispersion of the diamond and lenses in the system. The size of the laser-heated spot on the sample is only about 20 micrometers. This small hot spot leads to large spatial variations in intensity as a function of wavelength.. This variation requires focusing and magnification of light through the optical system with minimal chromatic aberrations. State of the art LHDAC spectroradiometry systems underestimate the temperature gradients by as much as 100%. To quantify the chromatic aberrations, we will use a high birefringence material to test each component (lenses, mirrors, beam splitters, etc.) of the laser system. The material we have chosen to measure the system’s chromatic aberration is yttrium vanidate (YVO4), which has a birefringence of 0.205. The thickness of the material varies vertically, providing a spatially varying “color standard” to test how well the system focuses each wavelength of visible light (400-900 nm). Specifically, light will pass through YVO4 between crossed-polarizers to produce rainbows separated by 20 micrometers vertically. This rainbow then passes through a diamond and the rest of the optical system. Our test of the quality of the spectroradiometry system will improve temperature and temperature gradient measurements in the laser heated diamond anvil cell and provide an inter-lab cross-calibration standard.