Visualizing Spatial Variations of Plasmon-Exciton Polaritons at the Nanoscale Using Electron Microscopy.

Polaritons are compositional light-matter quasiparticles that have enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in material science. Recently, plasmon-exciton polaritons (plexcitons) have been realized in hybrid material systems composed of transition metal dichalcogenide (TMDC) materials and metal nanoparticles, expanding polaritonic concepts to room temperature and nanoscale systems that also benefit from the exotic properties of TMDC materials. Despite the enormous progress in understanding TMDC-based plexcitons using optical-based methods, experimental evidence of plexcitons formation has remained indirect and mapping their nanometer-scale characteristics has remained an open challenge. Here, we demonstrate that plexcitons generated by a hybrid system composed of an individual silver nanoparticle and a few-layer WS2 flake can be spectroscopically mapped with nanometer spatial resolution using electron energy loss spectroscopy in a scanning transmission electron microscope. Experimental anticrossing measurements using the absorption-dominated extinction signal provide the ultimate evidence for plexciton hybridization in the strong coupling regime. Spatially resolved EELS maps reveal the existence of unexpected nanoscale variations in the deep-subwavelength nature of plexcitons generated by this system. These findings pioneer new possibilities for in-depth studies of the local atomic structure dependence of polariton-related phenomena in TMDC hybrid material systems with nanometer spatial resolution.

Transition metal dichalcogenide nanodisks as high-index dielectric Mie nanoresonators.

Monolayer transition metal dichalcogenides (TMDCs) have recently been proposed as an excitonic platform for advanced optical and electronic functionalities1-3. However, in spite of intense research efforts, it has not been widely appreciated that TMDCs also possess a high refractive index4,5. This characteristic opens up the possibility to utilize them to construct resonant nanoantennas based on subwavelength geometrical modes6,7. Here, we show that nanodisks, fabricated from exfoliated multilayer WS2, support distinct Mie resonances and anapole states8 that can be tuned in wavelength over the visible and near-infrared range by varying the nanodisk size and aspect ratio. As a proof of concept, we demonstrate a novel regime of light-matter interaction-anapole-exciton polaritons-which we realize within a single WS2 nanodisk. We argue that the TMDC material anisotropy and the presence of excitons enrich traditional nanophotonics approaches based on conventional high-index materials and/or plasmonics.

Collective Strong Light-Matter Coupling in Hierarchical Microcavity-Plasmon-Exciton Systems.

Polaritons are compositional light-matter quasiparticles that arise as a result of strong coupling between the vacuum field of a resonant optical cavity and electronic excitations in quantum emitters. Reaching such a regime is often hard, as it requires materials possessing high oscillator strengths to interact with the relevant optical mode. Two-dimensional transition metal dichalcogenides (TMDCs) have recently emerged as promising candidates for realization of strong coupling regime at room temperature. However, these materials typically provide coupling strengths in the range of 10-40 meV, which may be insufficient for reaching strong coupling with low quality factor resonators. Here, we demonstrate a universal scheme that allows a straightforward realization of strong coupling with 2D materials and beyond. By intermixing plasmonic excitations in nanoparticle arrays with excitons in a WS2 monolayer inside a resonant metallic microcavity, we fabricate a hierarchical system with the collective microcavity-plasmon-exciton Rabi splitting exceeding ∼500 meV at room temperature. Photoluminescence measurements of the coupled systems show dominant emission from the lower polariton branch, indicating the participation of excitons in the coupling process. Strong coupling has been recently suggested to affect numerous optical- and material-related properties including chemical reactivity, exciton transport, and optical nonlinearities. With the universal scheme presented here, strong coupling across a wide spectral range is within easy reach and therefore exploration of these exciting phenomena can be further pursued in a much broader class of materials.

Strong Light-Matter Coupling between Plasmons in Individual Gold Bi-pyramids and Excitons in Mono- and Multilayer WSe2.

Monolayer transition-metal dichalcogenides (TMDCs) have attracted a lot of research attention recently, motivated by their remarkable optical properties and potential for strong light-matter interactions. Realization of strong plasmon-exciton coupling is especially desirable in this context because it holds promise for the enabling of room-temperature quantum and nonlinear optical applications. These efforts naturally require investigations at a single-nanoantenna level, which, in turn, should possess a compact optical mode interacting with a small amount of excitonic material. However, standard plasmonic nanoantenna designs such as nanoparticle dimers or particle-on-film suffer from misalignment of the local electric field in the gap with the in-plane transition dipole moment of monolayer TMDCs. Here, we circumvent this problem by utilizing gold bi-pyramids (BPs) as very efficient plasmonic nanoantennas. We demonstrate strong coupling between individual BPs and tungsten diselenide (WSe2) monolayers at room temperature. We further study the coupling between multilayers of WSe2 and BPs to elucidate the effect of the number of layers on the coupling strength. Importantly, BPs adopt a reduced-symmetry configuration when deposited on WSe2, such that only one sharp antenna tip efficiently interacts with excitons. Despite the small interaction area, we manage to achieve strong coupling, with Rabi splitting exceeding ∼100 meV. Our results suggest a feasible way toward realizing plasmon-exciton polaritons involving nanoscopic areas of TMDCs, thus pointing toward quantum and nonlinear optics applications at ambient conditions.

Observation of Tunable Charged Exciton Polaritons in Hybrid Monolayer WS2-Plasmonic Nanoantenna System.

Formation of dressed light-matter states in optical structures, manifested as Rabi splitting of the eigen energies of a coupled system, is one of the key effects in quantum optics. In pursuing this regime with semiconductors, light is usually made to interact with excitons, electrically neutral quasiparticles of semiconductors; meanwhile interactions with charged three-particle states, trions, have received little attention. Here, we report on strong interaction between localized surface plasmons in silver nanoprisms and excitons and trions in monolayer tungsten disulfide (WS2). We show that the plasmon-exciton interactions in this system can be efficiently tuned by controlling the charged versus neutral exciton contribution to the coupling process. In particular, we show that a stable trion state emerges and couples efficiently to the plasmon resonance at low temperature by forming three bright intermixed plasmon-exciton-trion polariton states. Our findings open up a possibility to exploit electrically charged polaritons at the single nanoparticle level.

Zeeman splitting via spin-valley-layer coupling in bilayer MoTe2.

Atomically thin monolayer transition metal dichalcogenides possess coupling of spin and valley degrees of freedom. The chirality is locked to identical valleys as a consequence of spin-orbit coupling and inversion symmetry breaking, leading to a valley analog of the Zeeman effect in presence of an out-of-plane magnetic field. Owing to the inversion symmetry in bilayers, the photoluminescence helicity should no longer be locked to the valleys. Here we show that the Zeeman splitting, however, persists in 2H-MoTe2 bilayers, as a result of an additional degree of freedom, namely the layer pseudospin, and spin-valley-layer locking. Unlike monolayers, the Zeeman splitting in bilayers occurs without lifting valley degeneracy. The degree of circularly polarized photoluminescence is tuned with magnetic field from -37% to 37%. Our results demonstrate the control of degree of freedom in bilayer with magnetic field, which makes bilayer a promising platform for spin-valley quantum gates based on magnetoelectric effects.Monolayer transition metal dichalcogenides host a valley splitting in magnetic field analogous to the Zeeman effect. Here, the authors report that the Zeeman splitting still persists in bilayers of MoTe2 without lifting the valley degeneracy, due to spin-valley-layer coupling.

Observation of Mode Splitting in Photoluminescence of Individual Plasmonic Nanoparticles Strongly Coupled to Molecular Excitons.

Plasmon-exciton interactions are important for many prominent spectroscopic applications such as surface-enhanced Raman scattering, plasmon-mediated fluorescence, nanoscale lasing, and strong coupling. The case of strong coupling is analogous to quantum optical effects studied in solid state and atomic systems previously. In plasmonics, similar observations have been almost exclusively made in elastic scattering experiments; however, the interpretation of these experiments is often cumbersome. Here, we demonstrate mode splitting not only in scattering, but also in photoluminescence of individual hybrid nanosystems, which manifests a direct proof of strong coupling in plasmon-exciton nanoparticles. We achieved these results due to saturation of the mode volume with molecular J-aggregates, which resulted in splitting up to 400 meV, that is, ∼20% of the resonance energy. We analyzed the correlation between scattering and photoluminescence and found that splitting in photoluminescence is considerably less than that in scattering. Moreover, we found that splitting in both photoluminescence and scattering signals increased upon cooling to cryogenic temperatures. These findings improve our understanding of strong coupling phenomena in plasmonics.

Correlación adenosindesaminasa/linfocitos en el líquido pleural de origen tuberculoso / Adenosindeaminase/lymphocyte correlation in pleural liquid of tuberculosis

El presente estudio fue llevado a cabo para evaluar prospectivamente la correlación entre la actividad de la adenosindesaminasa, (ADA) y la proporción de linfocitos en líquido pleural en una región de alta prevalancia de tuberculosis. Durante el periodo de 1991 a 1996 se estudiaron 222 pacientes que ingresaron al Instituto Nacional de Enfermedades Respiratorias con diagnóstico de derrame pleural tuberculoso (TB) y secundario a cáncer (Ca). Hubo 133 TB y 89 Ca, (146 hombres y 76 mujeres). Todos los pacientes fueron diagnosticados con los métodos convencionales. La edad promedio de los pacientes con TB fue 42 ñ 17 y los Ca 61 ñ 13, (-x ñ D.E.). El nivel medio de la actividad de la ADA en el grupo TB (101.6 ñ 41.3 U/L) fue significativamente más alto (p< 0.0001) que en Ca (24.3 ñ 19.1). El porcentaje de linfocitos en el líquido pleural de los pacientes con TB fue 75.5 ñ 16.6 contra 66.8 ñ 17.1 de los Ca (p < 0.0001). La relación entre los niveles de ADA y el porcentaje de linfocitos en líquido pleural de los dos grupos estudiados produjo una significativa curva de regresión (r = 0.750, p< 0.0001), la cual mostró una correlación positiva entre estos dos parámetros. Estos resultados nos sugieren: 1) que la ADA puede ser un buen marcador de inmunidad mediada por células y 2) que existe una buena correlación entre los niveles de ADA y la proporción de linfocitos en líquido pleural tuberculoso

Tratamiento de la Alopecia Areata con dinistroclorobenceno (DNCB) en el Hospital Dermatológico Nacional en el período septiembre 1991-abril de 1992

La investigación es de tipo prospectivo y transversal, con un universo de estudio de 35 pacientes diagnósticados de Alopecia Areata que asistieron a consulta al Hospital Dermatológico Nacional, contando para ello con su consentimiento. La nuestra fue de 25 pacientes y se elaboró una hoja de flujo para la recolección de la información, que fue llevada desde inicios del tratamiento anotando las diferentes actividades realizadas. A todos los pacientes se les tomó control iconográfico antes y despues del estudio, todos fueron sensibilizados con DNCB al 2 por ciento en acetona en un área de 3 X 5 CM, directamente en la lesión o en la espalda, posteriormente fueron tratados con DNCB al 1 por ciento en acetona, con una frecuencia semanal hasta su repoblación