complex device functions, it is desirable to develop polarization-sensitive 2D materials or hybrid structures which will boost the development of smart optoelectronic devices, e.g., polarized photodetectors [7,8] or light emitting diodes. [9] Group-VI transition metal dichalcogenide (TMDC) monolayers, with the form of MX 2 (M = Mo, W and X = S, Se, etc.), possess two energy-degenerate but inequivalent valleys (K and -K) in the six corners of the Brillouin zone, which respond differently to the left-and rightcircularly polarized light. [10] However, the valley polarization (defined as circular dichroism) is typically quite low, especially at room temperature, owing to the effects such as electron-hole exchange interaction and phonon-assisted intervalley scattering. [11,12] To gain an obvious valley polarization, resonant excitation condition and cryogenic temperature are required, which largely limits the potential for practical applications. Similar requests also apply to a decent valley coherence for TMDC materials, in which linearly polarized light emission can be obtained as a result of coherent superposition of circularly polarized photon emission from K and -K valleys. [13] Different from 2H phase Group-VI TMDC, there are also 2D materials with an anisotropic crystal structure which naturally leads to polarization-sensitive absorption or emission properties. [14,15] For example, orthorhombic black phosphorus (BP) shows higher absorption and photoluminescence (PL) along the armchair direction than along the zigzag direction. [16,17] Meanwhile, germanium selenide (GeSe) also has the orthorhombic crystal structure which enables anisotropic absorption with a maximum anisotropic absorption ratio of 3.02 at 808 nm. [18] Currently, the limitations of these anisotropic 2D semiconductors mainly lie in two points: 1) Some of the materials, e.g., BP, suffers from low chemical stability so that the preparation and fabrication procedures have to be conducted in an inert atmosphere; 2) The anisotropic ratio for both absorption and PL emission is usually low and needs further enhancement towards practical applications.2D materials are famous for their flexibility and convenience for integration with other materials or nanostructures. By coupling monolayer semiconductors to plasmonic or dielectric As contemporary star materials, 2D monolayer semiconductors have drawn huge research interests owing to their striking electrical and optical properties, rendering them ideal candidates as building blocks for novel optoelectronic devices. Towards light emitting devices with extended functions, it is necessary to manipulate the polarization of light emission from monolayer semiconductors. However, most of these monolayer semiconductors exhibit no or very limited polarization sensitivity inherited from their structural anisotropy, making it challenging to develop highly polarized light sources. Herein, by embedding monolayer tungsten diselenide (WSe 2 ) in a nanowireon-film nanocavity, highly polarized light emission is demonstrated...

Vibrational strong coupling (VSC), the strong coupling between optical resonances and the dipolar absorption of molecular vibrations at mid‐infrared frequencies, holds the great potential for the development of ultrasensitive infrared spectroscopy, the modification of chemical properties of molecules, and the control of chemical reactions. In the realm of ultracompact VSC, there is a need to scale down the size of mid‐infrared optical resonators and to elevate their optical field strength. Herein, by using single quartz micropillars as mid‐infrared optical resonators, the strong coupling is demonstrated between surface phonon polariton (SPhP) resonances and molecular vibrations from far‐field observation. The single quartz micropillars support sharp SPhP resonances with an ultrasmall mode volume, which strongly couples with the molecular vibrations of 4‐nitrobenzyl alcohol (C7H7NO3) molecules featuring pronounced mode splitting and anticrossing dispersion. The coupling strength depends on the molecular concentration and reaches the strong coupling regime with only 7300 molecules. The findings pave the way for promoting the VSC sensitivity, miniaturing the VSC devices, and will boost the development of ultracompact mid‐infrared spectroscopy and chemical reaction control devices.

Plasmonic nanocavities, with the ability to localize and concentrate light into nanometer-scale dimensions, have been widely used for ultrasensitive spectroscopy, biosensing, and photodetection. However, as the nanocavity gap approaches the subnanometer length scale, plasmonic enhancement, together with plasmonic enhanced optical processes, turns to quenching because of quantum mechanical effects. Here, instead of quenching, we show that quantum mechanical effects of plasmonic nanocavities can elevate surface-enhanced infrared absorption (SEIRA) of molecular moieties. The plasmonic nanocavities, nanojunctions of gold and cadmium oxide nanoparticles, support prominent mid-infrared plasmonic resonances and enable SEIRA of an alkanethiol monolayer (CH 3 (CH 2 ) n−1 SH, n = 3−16). With a subnanometer cavity gap (n < 6), plasmonic resonances turn to blue shift and the SEIRA signal starts a pronounced increase, benefiting from the quantum tunneling effect across the plasmonic nanocavities. Our findings demonstrate the new possibility of optimizing the field enhancement and SEIRA sensitivity of mid-infrared plasmonic nanocavities.

As the strong light–matter interaction between molecular vibrations and mid-infrared optical resonant modes, vibrational strong coupling (VSC) has the potential to modify the intrinsic chemistry of molecules, leading to the control of ground-state chemical reactions. Here, by using quartz as an epsilon-near-zero (ENZ) substrate, we have realized VSC between organic molecular vibrations and mid-infrared plasmons on metallic antennas. The ENZ substrate enables sharp mid-infrared plasmonic resonances (Q factor ∼50) which efficiently couple to the molecular vibrations of polymethyl methacrylate (PMMA) molecules with prominent mode splitting. The coupling strength is proportional to the square root of the thickness of the PMMA layer and reaches the VSC regime with a thickness of ∼300 nm. The coupling strength also depends on the polarization of the incident light, illustrating an additional way to control the molecule–plasmon coupling. Our findings provide a new, to the best of our knowledge, possibility to realize VSC with metallic antennas and pave the way to increase the sensitivity of molecular vibrational spectroscopy.

Two-dimensional semiconducting transition-metal dichalcogenides (TMDCs) have attracted extensive attention as building blocks of miniaturized electronic and optical devices. However, as the characteristics of TMDC devices are predominately determined by their device structures, the function of TMDC devices is fixed once fabricated, leaving the reconfigurable active device and circuit a challenge. Here, we have demonstrated the current rectification switching in TMDC vertical diodes using a liquid metal (EGaIn) top electrode with a reconfigurable contact area. The rectification switching is closely related to the ultrathin gallium oxide layer on the surface of EGaIn. Under the small contact, with the existence of gallium oxide, photocurrent dominates the electrical transport showing a negative rectification, while as the contact increases, the broken gallium oxide leads to rectification switching to the positive bias direction. Such rectification switching applies to thin TMDC flakes down to 3 nm, benefitting from the soft electrical contact between the TMDC and the EGaIn electrode. Our work shows the new possibility of actively reconfigurable TMDC vertical diodes enabled by the liquid metal electrode and will promote promising applications of flexible and tunable TMDC-based nanoelectronic devices.