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Quantification of Chromatic Aberration In the Laser- Heated Diamond Anvil Cell Emily England, Wes Clary, Daniel Reaman, Wendy Panero School of Earth Sciences, The Ohio State University Introduction Introduction • Earth materials subjected to the high Earth materials subjected to the high pressure and temperature conditions of pressure and temperature conditions of planetary interiors display unique changes planetary interiors display unique changes in crystal structure, melting temperature, in crystal structure, melting temperature, and transport properties. and transport properties. • The laser-heated diamond anvil cell The laser-heated diamond anvil cell (LHDAC) is a tool to make (LHDAC) is a tool to make in situ in situ measurements of material properties, measurements of material properties, specifically at pressures up to 1 Mbar (100 specifically at pressures up to 1 Mbar (100 GPa) and temperatures from 1500 to 7000 K. GPa) and temperatures from 1500 to 7000 K. Temperatures above 1500 K are measured by Temperatures above 1500 K are measured by the spectroradiometry of the thermal the spectroradiometry of the thermal emission that passes through the diamond emission that passes through the diamond anvil and is collected on a intensity- anvil and is collected on a intensity- calibrated spectrometer and coupled-charged calibrated spectrometer and coupled-charged device (CCD). device (CCD). • The accuracy of the measured temperatures The accuracy of the measured temperatures is not always reliable, due to optical is not always reliable, due to optical dispersion of the diamond and lenses in the dispersion of the diamond and lenses in the system. The size of the laser-heated spot system. The size of the laser-heated spot on the sample is only about 20 micrometers. on the sample is only about 20 micrometers. This small hot spot leads to large spatial This small hot spot leads to large spatial variations in intensity as a function of variations in intensity as a function of wavelength. This variation requires wavelength. This variation requires focusing and magnification of light through focusing and magnification of light through the optical system with minimal chromatic the optical system with minimal chromatic aberrations. aberrations. X-ray diffraction (pressure, structure, density) The Laser Heated Diamond Anvil Cell The Laser Heated Diamond Anvil Cell (LHDAC) (LHDAC) 2 cm Pressure = Force/Area Spectroradiometry (temperature) Heating laser 0.1 mm X-ray Methods Methods • To quantify the chromatic aberrations, we will use To quantify the chromatic aberrations, we will use a high birefringence material to test each a high birefringence material to test each component (lenses, mirrors, beam splitters, etc.) component (lenses, mirrors, beam splitters, etc.) of the laser system. of the laser system. • The material we have chosen to measure the The material we have chosen to measure the system’s chromatic aberration a thin section of system’s chromatic aberration a thin section of olivine. This will provide a “color standard” to olivine. This will provide a “color standard” to test how well the system focuses each wavelength of test how well the system focuses each wavelength of visible light (400-900 nm). visible light (400-900 nm). • Specifically, light will pass through the olivine Specifically, light will pass through the olivine between crossed-polarizers then through the between crossed-polarizers then through the diamond, then the rest of the system. diamond, then the rest of the system. Path of Light Through System Path of Light Through System Walter and Koga, 2004 Olivine as a Image and Spectrum Test Olivine as a Image and Spectrum Test • The origin of chromatic aberration is the The origin of chromatic aberration is the wavelength-dependant index of refraction, or wavelength-dependant index of refraction, or dispersion, of refracting optical components, dispersion, of refracting optical components, especially the diamond. especially the diamond. • Specifically, Specifically, Birefringence Birefringence is the effect that is the effect that the refractive index which a light beam the refractive index which a light beam experiences in a non-isotropic medium depends experiences in a non-isotropic medium depends on the polarization direction. on the polarization direction. Birefringence colors in Birefringence colors in thin materials thin materials Laser System Laser System Olivine Images Olivine Images This is a image of the This is a image of the olivine as focused on olivine as focused on the spectrometer. the spectrometer. Image and spectrum of a 1 Image and spectrum of a 11 mm slit through the slit through the middle of the above image. The spectrometer middle of the above image. The spectrometer grating is centered at 600nm. This disperses grating is centered at 600nm. This disperses the light with red falling to the right and blue the light with red falling to the right and blue falling to the left.. falling to the left.. Olivine + Diamond Olivine + Diamond Images Images This is an image of This is an image of the olivine as the olivine as viewed through the viewed through the diamond and focused diamond and focused on the spectrometer. on the spectrometer. Image and spectrum after spectrometer Image and spectrum after spectrometer mirror was set to 600nm. Notice that mirror was set to 600nm. Notice that wavelengths are not focused as well. wavelengths are not focused as well. Conclusions Conclusions • The refraction of the diamond (dispersion = 0.044) causes The refraction of the diamond (dispersion = 0.044) causes loss of both image and spectral resolution at the imaging loss of both image and spectral resolution at the imaging spectrometer. spectrometer. • Further research is needed to quantify the magnitude of the Further research is needed to quantify the magnitude of the problem and its effect on the measurement of temperatures and problem and its effect on the measurement of temperatures and temperature measurements in the laser-heated diamond anvil temperature measurements in the laser-heated diamond anvil cell. cell. • Likely solutions include duplication of the measurement to Likely solutions include duplication of the measurement to use images of the total intensity paired with spectra of the use images of the total intensity paired with spectra of the central portion of the hotspot (See Clary et al) central portion of the hotspot (See Clary et al) References References • Benedetti, L. R., N. Guignot, and D. L. Farber, Achieving Benedetti, L. R., N. Guignot, and D. L. Farber, Achieving accuracy in spectroradiometric measurements of temperature accuracy in spectroradiometric measurements of temperature in the laser-heated diamond anvil cell: Diamond is an in the laser-heated diamond anvil cell: Diamond is an optical component. optical component. Journal of Applied Physics, 101 Journal of Applied Physics, 101 , 013109, 2007. , 013109, 2007. • Walter, M. J. and K. T. Koga, The effects of chromatic Walter, M. J. and K. T. Koga, The effects of chromatic dispersion on temperature measurement in the laser-heated dispersion on temperature measurement in the laser-heated diamond anvil cell, diamond anvil cell, Phys. Earth. Planet Int., 143-144 Phys. Earth. Planet Int., 143-144 , 541-558, 2004. , 541-558, 2004. 2.5 mm Diamond and Dispersion Diamond and Dispersion The index of refraction of diamond is higher for blue light than red, causing blue light to be refracted at a greater angle as it travels through the diamond. Diamond is a high- dispersion material and in the LHDAC, the thermal emission must refract through the diamond anvil in order to reach the spectrometer. Benedetti et al., 2007 Dispersion of Light Through System Dispersion of Light Through System Walter and Koga, 2004 The effect of dispersion is to focus blue more tightly than red. Therefore, an image that appears in focus in one color of light will be out of focus in another color. This leads to a loss of both spectral and image resolution. 2.5 mm •The birefringence color seen depends on the thickness of the material (horizontal lines) and the birefringence (diagonal lines) •Olivine has a birefringence of about 0.03-0.05, labeled forsterite (Mg 2 SiO 4 ) and fayalite (Fe 2 SiO 4 ) at right. The observed color is a function of both composition and orientation of the grain with respect to the polarizer. •Our specific olivine sample has an unusual color variation with position, varying from blue to red with a sharp boundary between, providing a spatially dependent color signal. The light passes from the source (“object plane”) through an objective and focusing lens. The light is then focused on the image plane of the imaging spectrometer where a thin slit of light (11 mm) is dispersed with a 600 groove per inch spectrometer in a 0.125 m spectrometer. Olivine Thin Section Olivine Thin Section Olivine in plane light Olivine in plane light Olivine in crossed polarized Olivine in crossed polarized light. light. The image and spectral test was The image and spectral test was performed along the black line. performed along the black line. Image of olivine through TV camera Image of olivine through TV camera position wavelength blue light red light wavelength positi on blue light red light 2.5 mm
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