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

Abstract

Atomically thin transition metal dichalcogenides (TMDCs) present a promising platform for numerous photonic applications due to excitonic spectral features, possibility to tune their constants by external gating, doping, or light, and mechanical stability. Utilization of such materials for sensing or optical modulation purposes would require a clever optical design, as by itself the 2D materials can offer only a small optical phase delay – consequence of the atomic thickness. To address this issue, we combine films of 2D semiconductors which exhibit excitonic lines with the Fabry-Perot resonators of the standard commercial SiO2/Si substrate, in order to realize topological phase singularities in reflection. Around these singularities, reflection spectra demonstrate rapid phase changes while the structure behaves as a perfect absorber. Furthermore, we demonstrate that such topological phase singularities are ubiquitous for the entire class of atomically thin TMDCs and other high-refractive-index materials, making it a powerful tool for phase engineering in flat optics. As a practical demonstration, we employ PdSe2 topological phase singularities for a refractive index sensor and demonstrate its superior phase sensitivity compared to typical surface plasmon resonance sensors.

Optical Anisotropy and Excitons in MoS2 Interfaces for Sensitive Surface Plasmon Resonance Biosensors

“…Our result complements the previous works reporting orders of magnitude of increase in the phase sensitivity upon deposition of ultra-thin layers of materials [ 46 , 47 ]. Importantly, the singularity in the sensitivity is present in cases of both isotropic and anisotropic dielectric tensors for the MoS 2 layer, which we attribute to the topological protection of zero-reflection points, demonstrated recently [ 48 ]. However, there are a few obstacles that limit the applicability and maximum sensitivity of phase-interrogation of SPR biosensors.…”

Extreme Optical Anisotropy in the Type-II Dirac Semimetal NiTe2 for Applications to Nanophotonics

“…These results provide the groundwork for an efficient harnessing of topological semimetals in nanophotonic applications. Of particular interest for future studies will be NiTe 2 -based refractive index sensors, perfect absorbers for bio/chemical sensing, photodetection, solar energy harvesting, and even photothermal membrane distillation …”

“…In recent years, transition-metal dichalcogenides (TMDs) TMX 2 (TM = Mo, W, Pd, Pt; X = S, Se, Te) have stimulated considerable research efforts in view of their peculiar electronic and photonic properties. 1 Bulk TMDs are van der Waals crystals (i.e., stacks of atomically thin layers) and, in their twodimensional (2D) version, they have been harnessed for a variety of photonic effects and applications, such as efficient optical sensors, high refractive index, 2 nonreciprocal photonic devices, 3 modulation of second-harmonic generation, 4 and THz photodetectors. 5 Owing to the underlying van der Waals structural properties, bulk TMDs may exhibit a strong optical anisotropy, 6 possibly with hyperbolic character (i.e., oppositely signed in-plane and out-of-plane components).…”

Anomalous optical response of graphene on hexagonal boron nitride substrates

“…To enhance the sensitivity of our spectroscopic imaging technique, we assembled a specific configuration of layers giving a rise to a larger difference in optical responses from the substrate with and without graphene layer, and thus, to a higher sensitivity of ellipsometric parameters to graphene optical constants. This is achieved in the vicinity of topological phase singularities, which arise owing to intersection of graphene optical constant's dispersion with the substrate zero-reflection surface 32 . Here, we dry-transferred another graphene/hBN heterostructure on top of a thick 200 nm Au film to ensure an appropriate formation of a cavity, shown in the schematics and an optical image in Figure 3 ((a) and (b)), for realization of topological phase singularity in the vicinity of ellipsometer's best sensitivity (around ~500 nm).…”