Raman scattering spectroscopy is a crucial technique for characterizing materials, providing insights into lattice structures and electron-photon/electron-phonon interactions. The process involves the excitation of ground-state electrons to intermediate energy levels by photons, followed by coupling to phonons and the emission of scattered photons with altered energies. Quantum interference among different Raman scattering pathways can lead to complex scattering effects. While quantum interference has been observed in a few 2D materials like electrostatically doped graphene and few-layer MoTe2, its role in chiral Raman scattering remains largely unexplored. This study focuses on rhenium dichalcogenides (ReX2, X=S or Se), layered transition metal dichalcogenides with triclinic symmetry. The distorted crystal structure of ReX2, arising from the displacement of Re atoms from ideal metal sites, results in anisotropic in-plane properties, including anisotropic carrier mobility, photoluminescence, and Raman scattering. Previous work showed chiral Raman scattering in thicker ReS2 flakes, attributed to anisotropic optical effects. However, the underlying fundamental interactions beyond optical effects remained unclear. This research aims to investigate the role of quantum interference in chiral Raman scattering in monolayer ReS2 and ReSe2, clarifying the interplay between photons, electrons, and phonons during this process.

The authors cite several key works on Raman scattering, including its applications in characterizing materials and probing electron-phonon interactions. They also review previous studies on quantum interference effects in Raman scattering in materials like graphene and MoTe2. The literature review highlights the anisotropic properties of ReS2 and ReSe2 and previous observations of chiral Raman scattering in ReS2, setting the stage for their investigation into the role of quantum interference in this phenomenon.

The experimental setup involved mechanically exfoliating single-layer ReS2 and ReSe2 onto fused silica substrates. Optical and atomic force microscopy (AFM) were used to confirm the monolayer thickness and quality. Chiral Raman scattering measurements were performed using a JY Horiba HR800 spectrometer with 1.96 eV and 2.33 eV excitation energies. A quarter-wave plate (QWP) controlled the circular polarization (RCP and LCP) of the excitation light. The Raman scattering intensities were measured as a function of the QWP rotation angle. Annular dark-field (ADF) scanning transmission electron microscopy (STEM) was employed to characterize the atomic structure and determine the orientation of the ReS2 and ReSe2 flakes. Density functional theory (DFT) calculations were performed to determine the eigenvectors of the Raman-active vibrational modes. First-order Raman scattering intensity was calculated using third-order perturbation theory, considering various interference patterns: no interference, intra-k interference, and full interference between all possible quantum pathways. The electronic band structure and phonon dispersion were calculated using Quantum ESPRESSO, and the electron-phonon coupling was calculated using the EPW package.

The experimental results revealed significant circular intensity differences (CID) in monolayer ReS2 and ReSe2, dependent on the Raman mode and excitation photon energy. The CID values were obtained for different Raman modes (I-VI) at 1.96 eV and 2.33 eV excitation energies. The study also revealed that the chiral Raman response was dependent on the orientation of the ReS2 and ReSe2 flakes, with enantiomers exhibiting opposite chiral Raman responses. The DFT calculations confirmed that the amplitudes of induced electric dipoles were identical at each k-point for both RCP and LCP excitations, but a phase difference was observed, leading to different interference patterns and distinct Raman scattering intensities. The calculated chiral Raman spectra using the full interference model closely matched the experimental results. This observation implies that the chiral Raman response arises from quantum interference between first-order Raman processes occurring at different k-points in the Brillouin zone. The study extended its findings to ReSe2, showing similar chiral Raman scattering behavior, validating the generality of the quantum interference effect.

The findings of this study demonstrate that quantum interference is a crucial mechanism responsible for the observed chiral Raman response in monolayer triclinic ReX2 (X=S, Se). The agreement between experimental and calculated results supports this conclusion. The absence of significant circular dichroism further strengthens the argument that the chirality originates from the quantum interference effect. The study reveals that even though the optical absorption intensities are the same for both RCP and LCP across the Brillouin zone, the k-resolved phases of the Raman tensors differ, causing the variation in Raman intensities due to quantum interference. The observation of this effect in two different triclinic materials suggests that quantum interference may be a general phenomenon in inelastic optical scattering. The pronounced chiral response observed under specific excitation energies emphasizes the importance of the interplay between excitation energy and quantum interference pathways.

This research demonstrates that quantum interference in Raman scattering leads to a strong chiral response in 2D enantiomers of single-layer triclinic ReX2. The observed helicity-dependent Raman scattering efficiencies are strongly correlated with the Raman mode and excitation photon energies, and are well-explained by quantum interference effects arising from phase differences between different k-points in the Brillouin zone. The study suggests that this quantum interference effect may be a general phenomenon in inelastic optical scattering, applicable to other materials as well, providing insights into manipulating light scattering properties.

While the study provides strong evidence for the role of quantum interference, the calculations are based on specific theoretical models, and approximations were made in the theoretical calculations. The experimental setup was restricted to specific excitation energies and Raman modes. Further investigations with a wider range of excitation energies and a more comprehensive theoretical model could strengthen the findings.