SPICE Workshop on Chiral Phonons, July 29th - 31st 2025
Angela Hight Walker
For comparison of data from laboratories around the world, we must have consensus on terminology and measurands relevant to chiral phonon measurements. Even with linear polarization, errors or artifacts can be seen in published Raman data. Using universally available test materials, methodologies1 have been developed to decrease such issues and enable reproducible results. Underway now is such an effort for helicity resolved Raman measurements, including the impact of temperature and wavelength. While raw differences in Raman intensities have been used2 to characterize phonon chirality, we know that intensities are fraught with issues. Also, often spectrometers do not have the spectral resolution needed to differentiate Raman shifts between “handed” measurements. Our unique magneto-Raman spectroscopic capabilities include diffraction-limited, spatially-resolved Raman measurements while simultaneously varying the polarization (both incident and scattered), temperature (1.6 K to 400 K), laser wavelength (tunability from visible to near-infrared), and magnetic field (up to 9 T). Additionally, coupling to a triple grating spectrometer provides access to both low-frequency (down to 6 cm-1, or 0.75 meV) phonon and magnon modes and to the spectral resolution necessary for chiral measurements. By using broadband quarterwave plate (QWP), halfwave plate (HWP), and two polarizers we can reliably obtain out- or in-phase circularly polarized Raman measurements allowing us to get all four polarization combinations: RL, RL, RR and LL, where R and L refers to the handedness right of left for the incident or Raman scatter, respectively. Measuring spectra in these four configurations are essential to accurately extract the circular intensity difference (CID), which captures not only intensity variations but also spectral shifts, enabling us to distinguish the effects of optical selection rules from those arising due to chirality. Such measurement infrastructure provides confidence to measure unique phonon behavior such as chirality and phonon Zeeman splitting3.
1 arXiv:2505.16063
2 https://doi.org/10.1021/acsnano.5c00381
3 PRB 111, 104419 (2025)