Van der Waals Material Photonic Devices—Optical Vortex Generator

Van der Waals materials have become important in almost all fields of research, including electronics, photonics, and materials engineering. To date, numerous favorable characteristics of van der Waals materials have been reported. For instance, graphene has exceptionally high thermal and electrical conductivities, while transition metal dichalcogenides (TMDs) have a direct energy bandgap in their monolayer form, enabling the use of 2D materials as efficient light-emitting materials. Several 2D material-based quantum light sources have been reported so far, possessing promising features for various quantum applications.
In the early stages of 2D material-based devices, particularly for photonic applications, 2D materials were integrated with devices made of conventional bulk materials such as SiN and SiO2. Recently, researchers have begun to develop optical devices using exclusively 2D materials. In this talk, an overview of recent developments in the use of stand-alone van der Waals materials for optical components will be covered. The large birefringence in van der Waals materials is particularly interesting, and recent work on optical vortex generation will be introduced. We propose and demonstrate a fabrication-free optical vortex generator using van der Waals materials, leveraging their giant birefringence to induce spin-orbit coupling and generate an optical vortex beam. We focus on hexagonal boron nitride (hBN) and molybdenum disulfide (MoS₂) for their high birefringence—hBN being transparent from visible to infrared with birefringence indices of n_o ∼ 2.15 and n_e ∼ 1.86, and MoS₂ showing high optical anisotropy with n_o ∼ 4.7 and n_e ∼ 2.7. The large birefringence facilitates efficient spin-orbit conversion, allowing for shorter propagation lengths to achieve vortex generation. We experimentally demonstrate the optical vortex beam with an 8-µm-thick hBN slab, verifying the topological charge through interferometry and simulations.