Two-dimensional (2D) crystals have renewed opportunities in design and assembly of arti cial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high delity. Recent availability of uniform, waferscale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm 2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers.
A rapid surge in global energy consumption has led to a greater demand for renewable energy to overcome energy resource limitations and environmental problems. Recently, a number of van der Waals materials have been highlighted as efficient absorbers for very thin and highly efficient photovoltaic (PV) devices. Despite the predicted potential, achieving power conversion efficiencies (PCEs) above 5% in PV devices based on van der Waals materials has been challenging. Here, we demonstrate a vertical WSe 2 PV device with a high PCE of 5.44% under one-sun AM1.5G illumination. We reveal the multifunctional nature of a tungsten oxide layer, which promotes a stronger internal electric field by overcoming limitations imposed by the Fermi-level pinning at WSe 2 interfaces and acts as an electron-selective contact in combination with monolayer graphene. Together with the developed bottom contact scheme, this simple yet effective contact engineering method improves the PCE by more than five times.
Light-matter coupling in excitonic materials has been the subject of intense investigation due to emergence of new excitonic materials. Two-dimensional layered hybrid organic/inorganic perovskites (2D HOIPs) support strongly bound excitons at room-temperatures with some of the highest oscillator strengths and electric loss tangents among the known excitonic materials. Here, we report strong light-matter coupling in Ruddlesden-Popper phase 2D-HOIPs crystals without the necessity of an external cavity. We report concurrent occurrence of multiple-orders of hybrid lightmatter states via both reflectance and luminescence spectroscopy in thick (> 100 nm) crystals and near-unity absorption in thin (< 20 nm) crystals. We observe resonances with quality factors > 250 in hybridized exciton-polaritons and identify a linear correlation between exciton-polariton mode splitting and extinction coefficient of the various 2D-HOIPs. Our work opens the door to studying polariton dynamics in selfhybridized and open cavity systems with broad applications in optoelectronics and photochemistry.
Metasurfaces enable flat optical elements by leveraging optical resonances in metallic or dielectric nanoparticles to obtain accurate control over the amplitude and phase of the scattered light. While highly efficient, these resonances are static and difficult to tune actively. Exciton resonances in atomically thin 2D semiconductors provide a novel and uniquely strong resonant light–matter interaction, which presents a new opportunity for optical metasurfaces. Their resonant properties are intrinsic to the band structure of the material, do not rely on nanoscale patterns, and are highly tunable using external stimuli. In this tutorial, we present the role that exciton resonances can play for atomically thin optics. We describe the essentials of metasurface physics and provide background on exciton physics and a comprehensive overview of excitonic materials. Excitons demonstrate to provide new degrees of freedom and enhanced light–matter interactions in hybrid metasurfaces through coupling with metallic and dielectric metasurfaces. Using the high sensitivity of excitons to the medium's electron density, the first demonstrations of electrically tunable nanophotonic devices and atomically thin optical elements are also discussed. The future of excitons in metasurfaces looks promising, while the main challenge lies in large-area growth and precise integration of high-quality materials.
Metamaterials and metasurfaces operating in the visible and near‐infrared (NIR) offer a promising route towards next‐generation photodetectors and devices for solar energy harvesting. While numerous metamaterials and metasurfaces using metals and semiconductors have been demonstrated, semimetals‐based metasurfaces in the vis‐NIR range are notably missing. This work experimentally demonstrates a broadband metasurface superabsorber based on large area, semimetallic, van der Waals platinum diselenide (PtSe2) thin films in agreement with electromagnetic simulations. The results show that PtSe2 is an ultrathin and scalable semimetal that concurrently possesses high index and high extinction across the vis‐NIR range. Consequently, the thin‐film PtSe2 on a reflector separated by a dielectric spacer can absorb >85% for the unpatterned case and ≈97% for the optimized 2D metasurface in the 400–900 nm range making it one of the strongest and thinnest broadband perfect absorbers to date. The results present a scalable approach to photodetection and solar energy harvesting, demonstrating the practical utility of high index, high extinction semimetals for nanoscale optics.
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