5:15 PM - 7:15 PM
[SMP29-P07] Chiral phonon study of minerals
Keywords:chiral phonon, Raman spectroscopy, circularly polarized light, quartz
Oishi et al. (2024) discovered that when Raman measurements are taken by incident a circularly polarized laser beam on a quartz crystal from the c-axis direction, the Raman peak position shifts for right- and left-handed circular polarization, and that this is due to chiral phonons. The shift occurs only in the E mode, and the shift direction is opposite for right- and left-handed crystals. The shift of the largest mode, 695 cm-1, reaches 1.5 cm-1 and can be measured by a Raman spectrometer with moderate resolution (although circular polarization is required). Chiral phonons have also been found in cinnabar and are explained by the interaction between the helical structure and the circularly polarized phonons (Sato et al., 2024).
From a mineralogical point of view, this means that this can be used as a new chirality (right-left) determination method, although it is limited to structures with helical structures. Using micro-Raman spectroscopy, it would be possible to quickly and nondestructively determine chirality for even the smallest crystals. In this presentation, we describe how we modified an existing Raman microspectrometer to be able to measure chiral phonons and the results of measurements using this device.
In our case, a quarter-wave plate is inserted into the optical path of a 488 nm laser pumped micro-Raman spectrometer with a rotating mount just before the objective lens, and circular polarization is generated by rotating the quarter-wave plate. When a linearly polarized laser enters a quarter-wave plate rotated 45 degrees to the right from the optical axis, right circular polarization is generated, and when rotated 45 degrees to the left, left circular polarization is generated. We use a piezo-driven rotating mount (ELL14) from Thorlab, and the rotation angle can be freely controlled from a PC using python code. In this arrangement, Raman scattering also passes through the quarter-wave plate. In addition, a polarizer (analyzer) is placed on the rotating mount in the collimated optical path in front of the spectrometer. As a side note, by replacing the quarter-wave plate with a half-wave plate, the spectrometer can be used as an angle-resolved polarized Raman microscope.
To check the performance of the modified device, we first performed a reproduction of Oishi et al. The samples used were right- and left-quartz 0001-plane single-crystal sheets purchased from Crystal Base. These samples were converted to circularly polarized light by an 80 mW linearly polarized laser, focused on the sample with a 20x objective lens, and the Raman scattering generated from the laser was observed. Measurements were made with right or left circularly polarized light and 180 second exposures at an analyzer orientation where almost only the E-mode was observed. The spectrometer used had a focal length of 500 mm and a diffraction grating of 1200 g/mm. A liquid nitrogen cooled CCD detector was used.
The results for the 695 cm-1 mode are shown in Fig. 1. The upper and lower spectra are for the left and right crystals, respectively, irradiated with right circularly polarized light (red) and left circularly polarized light (blue). There is a clear shift between the two crystals, with the left and right crystals in opposite directions. We were able to quantitatively reproduce the results of Oishi et al. (2024) for the other E-modes. This can be used to determine the chirality of quartz crystals. The same measurements were made on quartz spheres, and they corresponded to the left and right sides determined by the Airy spiral. Although the chirality of quartz can be determined by other methods, this method is a nondestructive measurement and can be applied regardless of the shape. However, it is necessary to orient the crystal in the c-axis direction.
Currently, we are trying to see if chiral phonons can be observed in cristobalite and keatite, which have similar spiral structures, and these will be presented on the meeting.
References
Oishi et al. (2024) DOI: 10.1103/PhysRevB.109.104306
Sato et al. (2024) JPSJ, vol. 79, 123.
From a mineralogical point of view, this means that this can be used as a new chirality (right-left) determination method, although it is limited to structures with helical structures. Using micro-Raman spectroscopy, it would be possible to quickly and nondestructively determine chirality for even the smallest crystals. In this presentation, we describe how we modified an existing Raman microspectrometer to be able to measure chiral phonons and the results of measurements using this device.
In our case, a quarter-wave plate is inserted into the optical path of a 488 nm laser pumped micro-Raman spectrometer with a rotating mount just before the objective lens, and circular polarization is generated by rotating the quarter-wave plate. When a linearly polarized laser enters a quarter-wave plate rotated 45 degrees to the right from the optical axis, right circular polarization is generated, and when rotated 45 degrees to the left, left circular polarization is generated. We use a piezo-driven rotating mount (ELL14) from Thorlab, and the rotation angle can be freely controlled from a PC using python code. In this arrangement, Raman scattering also passes through the quarter-wave plate. In addition, a polarizer (analyzer) is placed on the rotating mount in the collimated optical path in front of the spectrometer. As a side note, by replacing the quarter-wave plate with a half-wave plate, the spectrometer can be used as an angle-resolved polarized Raman microscope.
To check the performance of the modified device, we first performed a reproduction of Oishi et al. The samples used were right- and left-quartz 0001-plane single-crystal sheets purchased from Crystal Base. These samples were converted to circularly polarized light by an 80 mW linearly polarized laser, focused on the sample with a 20x objective lens, and the Raman scattering generated from the laser was observed. Measurements were made with right or left circularly polarized light and 180 second exposures at an analyzer orientation where almost only the E-mode was observed. The spectrometer used had a focal length of 500 mm and a diffraction grating of 1200 g/mm. A liquid nitrogen cooled CCD detector was used.
The results for the 695 cm-1 mode are shown in Fig. 1. The upper and lower spectra are for the left and right crystals, respectively, irradiated with right circularly polarized light (red) and left circularly polarized light (blue). There is a clear shift between the two crystals, with the left and right crystals in opposite directions. We were able to quantitatively reproduce the results of Oishi et al. (2024) for the other E-modes. This can be used to determine the chirality of quartz crystals. The same measurements were made on quartz spheres, and they corresponded to the left and right sides determined by the Airy spiral. Although the chirality of quartz can be determined by other methods, this method is a nondestructive measurement and can be applied regardless of the shape. However, it is necessary to orient the crystal in the c-axis direction.
Currently, we are trying to see if chiral phonons can be observed in cristobalite and keatite, which have similar spiral structures, and these will be presented on the meeting.
References
Oishi et al. (2024) DOI: 10.1103/PhysRevB.109.104306
Sato et al. (2024) JPSJ, vol. 79, 123.