Probing optical anisotropy of nanometer-thin van der waals microcrystals by near-field imaging

Abstract

Most van der Waals crystals present highly anisotropic optical responses due to their strong in-plane covalent bonding and weak out-of-plane interactions. However, the determination of the polarization-dependent dielectric constants of van der Waals crystals remains a nontrivial task, since the size and dimension of the samples are often below or close to the diffraction limit of the probe light. In this work, we apply an optical nano-imaging technique to determine the anisotropic dielectric constants in representative van der Waals crystals. Through the study of both ordinary and extraordinary waveguide modes in real space, we are able to quantitatively determine the full dielectric tensors of nanometer-thin molybdenum disulfide and hexagonal boron nitride microcrystals, the most-promising van der Waals semiconductor and dielectric. Unlike traditional reflection-based methods, our measurements are reliable below the length scale of the free-space wavelength and reveal a universal route for characterizing low-dimensional crystals with high anisotropies.

Nanoscale Guiding of Infrared Light with Hyperbolic Volume and Surface Polaritons in van der Waals Material Ribbons

“…This hybridization occurs for decreasing momenta when the electromagnetic fields of the surface modes overlap and interact. Importantly, we see that for q → 0 the SM 0 ‐S mode approaches ω TO , indicating that this mode has no cutoff frequency (ω c ), similar to the behavior of the fundamental dielectric mode in a MoS 2 slab in a symmetric environment . In contrast, for the SM 0 ‐A mode we observe a kink in the dispersion curve at ω c ≈ 1445 cm −1 .…”

Recent Progress on Exciton Polaritons in Layered Transition‐Metal Dichalcogenides

“…Here ε ab and ε c are the ab ‐plane and c ‐axis dielectric constant of TMDs. According to previous literature, the real parts of and for various types of bulk TMDs are in the range of 4.5–5.5 and 2.5–3.5 respectively close to the A exciton energy (see Figure d), which define the up limits of ξ TE and ξ TM . Therefore, the ξ TE and ξ TM are expected to be close for different types of TMDs.…”

“…d) The s‐SNOM images with fringe profiles (left) and the corresponding Fourier transform profiles (right) of propagative waveguide modes in MoS 2 thin flakes with various thicknesses. Reproduced with permission . Copyright 2017, Springer Nature.…”

“…The TE modes far away from the exciton energy imaged in Ref. have a ξ TE close to 4.5 in a 198 nm thick MoS 2 flake, and it is expected to be higher close to the A exciton energy.…”

“…In a later report, near‐field imaging with s‐SNOM was performed to study the waveguide modes in TMDs away from the A exciton energy. Here the authors mapped the interference fringes formed in MoS 2 thin waveguide by using an s‐SNOM excited by a 1.55 µm lasers, corresponding to a photon energy of 0.8 eV (left panel of Figure d).…”