Boosting Second-Harmonic Generation in Monolayer Rhenium Disulfide by Reversible Laser Patterning

Active modification and control of transition metal dichalcogenides (TMDs) properties are highly desirable for next-generation optoelectronic applications. In particular, controlling one of the most important characteristics of TMDs─their crystal structure and symmetry─may open means for manipulating their optical nonlinearities and electrical transport properties. Here, we show that a monolayer ReS2, which does not have a broken inversion symmetry due to its stable 1T′-distorted phase and correspondingly shows only weak second-harmonic generation (SHG), can produce a significantly enhanced (∼2 orders of magnitude) SHG upon reversible laser patterning. This enhancement can be explained by the laser-induced transition from centrosymmetric 1T′ to noncentrosymmetric 2H-phase. This hypothesis is confirmed by polarization-resolved SHG measurements, which reveal a gradual change from the 2-fold to 6-fold symmetry profiles upon laser patterning. Additionally, we found that laser patterning of the bilayer ReS2 samples, contrary to the monolayers, leads to a substantially reduced SHG signal. This result corroborates the 1T′-to-2H laser-induced phase transition. Finally, we show that the laser-induced patterning is reversible by heat. These results open a possibility to actively and reversibly control the crystal structure of mono- and few-layer ReS2 and thus its optical and electronic properties

Optical Constants of Several Multilayer Transition Metal Dichalcogenides Measured by Spectroscopic Ellipsometry in the 300-1700 nm Range: High Index, Anisotropy, and Hyperbolicity

Transition metal dichalcogenides (TMDs) attract significant attention due to their remarkable optical and excitonic properties. It was understood already in the 1960s and recently rediscovered that many TMDs possess a high refractive index and optical anisotropy, which make them attractive for nanophotonic applications. However, accurate analysis and predictions of nanooptical phenomena require knowledge of dielectric constants along both in- and out-of-plane directions and over a broad spectral range, information that is often inaccessible or incomplete. Here, we present an experimental study of optical constants from several exfoliated TMD multilayers obtained using spectroscopic ellipsometry in the broad range of 300-1700 nm. The specific materials studied include semiconducting WS2, WSe2, MoS2, MoSe2, and MoTe2, as well as in-plane anisotropic ReS2 and WTe2 and metallic TaS2, TaSe2, and NbSe2. The extracted parameters demonstrate a high index (n up to ∼4.84 for MoTe2), significant anisotropy (n∥ - n⊥ ≈ 1.54 for MoTe2), and low absorption in the near-infrared region. Moreover, metallic TMDs show potential for combined plasmonic-dielectric behavior and hyperbolicity, as their plasma frequency occurs at around ∼1000-1300 nm depending on the material. The knowledge of optical constants of these materials opens new experimental and computational possibilities for further development of all-TMD nanophotonics

Enhancing Vibrational Light-Matter Coupling Strength beyond the Molecular Concentration Limit Using Plasmonic Arrays

Vibrational strong coupling is emerging as a promising tool to modify molecular properties by making use of hybrid light-matter states known as polaritons. Fabry-Perot cavities filled with organic molecules are typically used, and the molecular concentration limits the maximum reachable coupling strength. Developing methods to increase the coupling strength beyond the molecular concentration limit are highly desirable. In this Letter, we investigate the effect of adding a gold nanorod array into a cavity containing pure organic molecules using FT-IR microscopy and numerical modeling. Incorporation of the plasmonic nanorod array that acts as artificial molecules leads to an order of magnitude increase in the total coupling strength for the cavity with matching resonant frequency filled with organic molecules. Additionally, we observe a significant narrowing of the plasmon line width inside the cavity. We anticipate that these results will be a step forward in exploring vibropolaritonic chemistry and may be used in plasmon based biosensors

Transition metal dichalcogenide metamaterials with atomic precision

The ability to extract materials just a few atoms thick has led to discovery of graphene, monolayer transition metal dichalcogenides (TMDs), and other important two-dimensional materials. The next step in promoting understanding and utility of the flatland physics beyond the state-of-the-art is to study one-dimensional edges of such two-dimensional materials as well as to control the edge-plane ratio. Edges typically exhibit properties that are unique and distinctly different from those of planes and bulk. Thus, controlling them allows to design principally new materials with synthetic edge-plane-bulk characteristics, that is, TMD metamaterials. However, the enabling technology to study such metamaterials experimentally in a precise and systematic way has not yet been developed. Here we report a facile and controllable anisotropic wet etching method that allows scalable fabrication of TMD metamaterials with atomic precision. We show that TMDs can be etched along certain crystallographic axes, such that the obtained edges are atomically sharp and exclusively zigzag-terminated. This results in hexagonal nanostructures of predefined order and complexity, including few nanometer thin nanoribbons and nanojunctions. The method thus enables future studies of a broad range of TMD metamaterials with tailored functionality through atomically precise control of the structure

Suppression of photo-oxidation of organic chromophores by strong coupling to plasmonic nanoantennas

Intermixed light-matter quasiparticles - polaritons - possess unique optical properties owned to their compositional nature. These intriguing hybrid states have been extensively studied over the past decades in a wide range of realizations aiming at both basic science and emerging applications. However, recently it has been demonstrated that not only optical, but also material-related properties, such as chemical reactivity and charge transport, may be significantly altered in the strong coupling regime of light-matter interactions. Here, we show that a nanoscale system, comprised of a plasmonic nanoprism strongly coupled to excitons in J-aggregated form of organic chromophores, experiences modified excited state dynamics and therefore modified photo-chemical reactivity. Our experimental results reveal that photobleaching, one of the most fundamental photochemical reactions, can be effectively controlled and suppressed by the degree of plasmon-exciton coupling and detuning. In particular, we observe a 100-fold stabilization of organic dyes for the red-detuned nanoparticles. Our findings contribute to understanding of photochemical properties in the strong coupling regime and may find important implications for the performance and improved stability of optical devices incorporating organic dyes.Comment: 5 figures; includes Supplementary Material

Transition metal dichalcogenide metamaterials with atomic precision

The ability to extract materials just a few atoms thick has led to the discoveries of graphene, monolayer transition metal dichalcogenides (TMDs), and other important two-dimensional materials. The next step in promoting the understanding and utility of flatland physics is to study the one-dimensional edges of these two-dimensional materials as well as to control the edge-plane ratio. Edges typically exhibit properties that are unique and distinctly different from those of planes and bulk. Thus, controlling the edges would allow the design of materials with combined edge-plane-bulk characteristics and tailored properties, that is, TMD metamaterials. However, the enabling technology to explore such metamaterials with high precision has not yet been developed. Here we report a facile and controllable anisotropic wet etching method that allows scalable fabrication of TMD metamaterials with atomic precision. We show that TMDs can be etched along certain crystallographic axes, such that the obtained edges are nearly atomically sharp and exclusively zigzag-terminated. This results in hexagonal nanostructures of predefined order and complexity, including few-nanometer-thin nanoribbons and nanojunctions. Thus, this method enables future studies of a broad range of TMD metamaterials through atomically precise control of the structure

Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators

Monolayer transition metal dichalcogenides (TMDCs) have recently been proposed as a unique excitonic platform for advanced optical and electronic functionalities. However, in spite of intense research efforts, it has been largely overlooked that, in addition to displaying rich exciton physics, TMDCs also possess a very high refractive index. This opens a possibility to utilize these materials for constructing resonant nanoantennas based on subwavelength geometrical modes. Here we show that nanodisks fabricated from exfoliated multilayer WS$_2$ support distinct Mie resonances and so-called anapole states that can be easily tuned in wavelength over the visible and near-infrared spectral range by varying the nanodisks' size and aspect ratio. We argue that the TMDC material anisotropy and the presence of excitons substantially enrich nanophotonics by complementing traditional approaches based on plasmonics and well-known high-index materials such as silicon. As a proof-of-concept, we demonstrate a novel regime of light-matter interaction, anapole-exciton polaritons, which we realize within a single WS$_2$ nanodisk. Our results thus suggest that nanopatterned TMDCs are promising materials for high-index nanophotonics applications with enriched functionalities and superior prospects

Optical material anisotropy in high-index transition metal dichalcogenide Mie nanoresonators

Resonant optical antennas provide unprecedented opportunities to control light on length scales far below the diffraction limit. Recent studies have demonstrated that nanostructures made of multilayer transition metal dichalcogenides (TMDCs) can exhibit well-defined and intense Mie resonances in the visible to the near-infrared spectral range. These resonances are realizable because the TMDC materials exhibit very high in-plane refractive indices, in fact higher than what is found in typical high-index dielectric materials like Si orGaAs. However, their out-of-plane refractive indices are comparatively low. Here we experimentally and theoretically investigate how this unusually large material anisotropy influences the optical response of individual TMDC nanoresonators made of MoS2. We find that anisotropy strongly affects the far-field optical response of the resonators, as well as complex interference effects, such as anapole and resonant Kerker conditions. Moreover, we show that it is possible to utilize the material anisotropy to probe the vectorial nature of the nanoresonator internal near fields. Specifically, we show that Raman spectra originating from individual MoS2 nanoresonators exhibit mode-specific anisotropic enhancement factors that vary with the nanoresonator size and correlate with specific modes supported at resonance. Our study indicates that exploring material anisotropy in novel high-index dielectrics may lead to new nanophotonic effects and applications. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen