Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

Abstract

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

In‐Plane and Out‐of‐Plane Optical Properties of Monolayer, Few‐Layer, and Thin‐Film MoS2 from 190 to 1700 nm and Their Application in Photonic Device Design

“…A recent pre‐print also indicates the presence of anisotropy in the optical properties of exfoliated MoS 2 . [ 35 ] Our newly reported optical property data should be particularly useful in designing novel MoS 2 ‐based photonic devices such as UV photodetectors [ 36 ] with monolayer and few‐layer MoS 2 and photovoltaics [ 37 ] with thin‐film MoS 2 .…”

Spectroscopic Ellipsometry Investigation of Au‐Assisted Exfoliated Large‐Area Single‐Crystalline Monolayer MoS2

“…We should note that the single-crystalline MoS 2 film is anticipated to exhibit strong optical anisotropy in the in-plane and out-of-plane directions, which has been reported very recently in exfoliated MoS 2 thin films. [29] However, for our ultrathin monolayer MoS 2 film with a thickness of only 0.65 nm, common spectroscopic techniques such as reflectance, [7] transmission, [30] and ellipsometry [31] are known to have low sensitivity to the out-of-plane dielectric component and the ellipsometric response is predominately determined by the in-plane component. In addition, the large in-plane refractive index is also an important reason for the insensitivity of ellipsometry to the out-of-plane component as the probed electric field of the refracted light mainly lies in plane in the ellipsometry configuration.…”

“…In addition, the large in-plane refractive index is also an important reason for the insensitivity of ellipsometry to the out-of-plane component as the probed electric field of the refracted light mainly lies in plane in the ellipsometry configuration. [29] Therefore, the extracted complex dielectric functions are considered to be pseudodielectric functions representing the in-plane component. [31] Nevertheless, we should note that the out-of-plane dielectric component of layerd materials such as MoS 2 can be extracted by thicker samples with substrates that can provide large interference enhancement such as the SiO 2 /Si substrate.…”

Topological phase singularities in atomically thin high-refractive-index materials

“…40 This crystal configuration naturally has high geometrical and, therefore, high optical anisotropy (see Supplementary Note 3). 41,42 To investigate anisotropic optical response, we measured Mueller matrices (Methods), which nonzero off-diagonal elements relate to sample anisotropy (see Supplementary Note 3). Interestingly, Mueller matrix' elements vary from point to point, as seen from Figure 3i-j.…”