Fabrication and Characterization of WS2 Based Photonic Structures

Transition metal dichalcogenides (TMDs) are gaining momentum as powerful and versatile materials thanks to the outstanding physical properties that arise when they are reduced to monolayers. TMDs are also interesting for applications in photonics because of the high refractive index through the visible and near-infrared spectrum. However, the extremely complex fabrication process due to the difficult production of large area, homogeneous and high quality samples hinders the development of TMD-based photonic structures. Moreover, the complex chemistry makes high-resolution etching of the TMD film extremely difficult. In this work we characterized the fundamental optical properties of a thick, bulk-like film of tungsten sulphide (WS2). We deposited a tungsten oxide film with atomic layer deposition (ALD) and then annealed it in presence of H2S. ALD allows the conformal growth of large area, uniform and high quality films. Spectroscopic ellipsometry measurements of the optical constants show that the refractive index is higher than 3 on the entire visible and near-infrared spectrum, even where the extinction coefficient is negligible. Motivated by this remarkable result, we fabricated and characterized a photonic structure by etching the oxide before the conversion, overcoming the fabrication limits posed by chemistry. Therefore, we demonstrated that a thick WS2 layer can be exploited to modulate the optical properties of photonic structures. The combination of high refractive index with low extinction coefficient over a large portion of the electromagnetic spectrum validates the importance of TMDs and endorses their application to photonic devices.