Emergent all-vdW plate-type beam splitters

Over the past decades, the discovery of two-dimensional (2D) crystals has significantly reshaped approaches to material design. Stacking these atomically thin layers into artificial van der Waals (vdW) heterostructures has enabled exceptional control over electronic, photonic and even topological properties. Although many of these developments are commonly linked to unique phenomena found in 2D crystals, researchers from the Institute of Physics at Yerevan State University and the Institute for Functional Intelligent Materials at the National University of Singapore have shown that their bulk counterparts: vdW crystals, can also provide valuable opportunities when arranged into artificial heterostructures. In particular, such heterostructures hold strong potential for optical design. The combination of contrasting refractive indices, low optical losses, and atomically smooth interfaces in bulk vdW crystals makes them promising building blocks for compact optical components. One example, demonstrated by the group of researchers, is the ultrathin planar beam splitters, where functionality is achieved by alternating optically thick high- and low-index layers with layer thickness acting as the key parameter for tuning device performance.

For this purpose, the researchers selected a pair of vdW crystals (Fig. 1(a) and 1(b)): GaSe_0.8Te_0.2 and hBN. As reported in their recent study published in Scientific Reports, the former is a ternary compound derived from the group-III vdW monochalcogenide semiconductors of GaSe and GaTe. It provides simultaneous access to a high refractive index in the red-light wavelength region (and beyond) while still maintaining relatively low optical losses below its absorption edge. Their findings position vdW GaSe_0.8Te_0.2 as a promising layered semiconductor for a use as the high-index component in the architecture of the proposed compact plate-type beam splitters. In contrast, the latter vdW hBN was chosen to serve as a low-index dielectric counterpart due to its broadband optical transparency and compatibility for the layered assembly, making it a natural option for many multilayer vdW heterostructures.

Next, the authors employed a generalized 4×4 transfer-matrix method to calculate the beam-splitting characteristics of vdW GaSe_0.8Te_0.2/hBN few multilayer heterostructures for commercially relevant splitting ratios taking into account the out-of-plane anisotropy of the individual layers and oblique angles of incidence (see Fig. 1 (c–f)). Their simulations for sub-micron-thick stacks demonstrate controllable reflectance-to-transmittance beam-splitting channels, which arise from the optical dispersion of the constituent layers as well as multiple interference effects occurring within the heterostructure. Importantly, the qualitative characterization of the dielectric response of the individual layers establishes a basis for predictive multilayer modelling, since even minor variations in optical dispersion can significantly influence phase accumulation and the resulting band-splitting behavior. As a proof-of-principle demonstration, the researchers further fabricated and studied a simplified four-layer GaSe_0.8Te_0.2/hBN stack using a deterministic dry-transfer technique depositing it onto a fused silica substrate. Their findings confirm the predicted spectral characteristics and show that variations in layer thickness shift the operational near-infrared (NIR) spectral band rather than eliminating it.

Provided by Yerevan State University