Validation of Helicity-Resolved Raman Spectroscopy Using Chiral α-Quartz Phonons

For comparison of data from laboratories around the world, we must garner consensus on terminology and measurands relevant to chiral phonon measurements[1], especially for low dimensional or 2D materials.  Even with linear polarization, errors or artifacts can be seen in published Raman data.  Using universally available test materials, methodologies [2] and instrumental parameters have been developed to decrease such issues and enable reproducible results.   Underway now is such an effort for helicity-resolved Raman spectra, including the impact of temperature and excitation wavelength.    Our unique magneto-Raman measurement capabilities afford 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 confidently obtain in- or out-phase circularly polarized Raman measurements allowing us to probe all four combinations: RL,RL, RR and LL, where R and L refers to the handiness right of left respectively for the incident or Raman scatter. These four data sets are collected to define a circular intensity difference (CID) which goes beyond intensity to observe shifts and permits differentiating the impact of selection rules verses chirality on the test material of α-Quartz as a function of temperature and wavelength of excitation. Such measurement infrastructure provides confidence to measure unique phonon behavior such as chirality Zeeman splitting [3].

[1] Nature Phys. 2025, 21 1532

[2]arxiv.org/abs/2505.16063

[3] PRB 2025 111, 104419