Anisotropic Complex Refractive Indices of Atomically Thin Materials: Determination of the Optical Constants of Few-Layer Black Phosphorus

Lossy Mode Resonance Sensors Based on Anisotropic Few-Layer Black Phosphorus

Lossy mode resonance (LMR) sensors offer a promising avenue to surpass the constraints of conventional surface plasmon resonance (SPR) sensors by delivering enhanced label-free detection capabilities. A notable edge of LMR over SPR is its excitation potential by both transverse electric (TE) and transverse magnetic (TM) polarized light. Yet this merit remains underexplored due to challenges to achieving high sensing performance under both TM and TE polarization within a singular LMR model. This study introduces a theoretical model for an LMR prism refractive index sensor based on a MgF2-few layer black phosphorus-MgF2 configuration, which can achieve angular sensitivity nearing 90° refractive index unit-1 (RIU-1) for both polarizations. Leveraging the distinct anisotropic nature of black phosphorus, the figure of merit (FOM) values along its two principal crystal axes (zigzag and armchair) show great difference, achieving an impressive FOM of 1.178 × 106 RIU-1 along the zigzag direction under TE polarized light and 1.231 × 104 RIU-1 along the armchair direction under TM polarized light. We also provide an analysis of the electric field distribution for each configuration at its respective resonant conditions. The proposed structure paves the way for innovative applications of anisotropic-material-based LMR sensors in various applications.

Anisotropic Optical Response of WTe2 Single Crystals Studied by Ellipsometric Analysis

In this paper we report the crystal growth conditions and optical anisotropy properties of Tungsten ditelluride (WTe₂) single crystals. The chemical vapor transport (CVT) method was used for the synthesis of large WTe₂ crystals with high crystallinity and surface quality. These were structurally and morphologically characterized by means of X-ray diffraction, optical profilometry and Raman spectroscopy. Through spectroscopic ellipsometry analysis, based on the Tauc–Lorentz model, we identified a high refractive index value (~4) and distinct tri-axial anisotropic behavior of the optical constants, which opens prospects for surface plasmon activity, revealed by the dielectric function. The anisotropic physical nature of WTe₂ shows practical potential for low-loss light modulation at the 2D nanoscale level.