Shaping terahertz waves using anisotropic shear modes in a van der Waals mineral

TL;DR

This study demonstrates how anisotropic shear modes in a van der Waals mineral can be used to manipulate terahertz waves, revealing potential for vdW materials in advanced photonic applications.

Contribution

The paper provides experimental and theoretical insights into shear phonon modes in clinochlore, showing their role in reshaping terahertz wave polarization and enabling new control mechanisms.

Findings

Observation of a strong anisotropic resonance at 1.13 THz in clinochlore

Polarization state of THz waves is reshaped by shear phonon anisotropy

Theoretical analysis of phononic symmetries and polarization eigenstates

Abstract

Naturally occurring van der Waals (vdW) materials are currently attracting significant interest due to their potential as low-cost sources of two-dimensional materials. Valuable information on vdW materials' interlayer interactions is present in their low-frequency rigid-layer phonon spectra, which are experimentally accessible by terahertz spectroscopy techniques. In this work, we have used polarization-sensitive terahertz time-domain spectroscopy to investigate a bulk sample of the naturally abundant, large bandgap vdW mineral clinochlore. We observed a strong and sharp anisotropic resonance in the complex refractive index spectrum near 1.13 THz, consistent with our density functional theory predictions for shear modes. Polarimetry analysis revealed that the shear phonon anisotropy reshapes the polarization state of transmitted THz waves, inducing Faraday rotation and ellipticity. Furthermore, we used Jones formalism to discuss clinochlore phononic symmetries and describe our observations in terms of its eigenstates of polarization. These results highlight the potential of exploring vdW minerals as central building blocks for vdW heterostructures with compelling technological applications.