Achieving larger electromagnetic enhancement using a nanogap between neighboring metallic nanostructures has been long pursued for boosting light–matter interactions. However, the quantitative probing of this enhancement is hindered by the lack of a reliable experimental method for measuring the local fields within a subnanometer gap. Here, we use layered MoS2 as a two-dimensional atomic crystal probe in nanoparticle-on-mirror nanoantennas to measure the plasmonic enhancement in the gap by quantitative surface-enhanced Raman scattering. Our designs ensure that the probe filled in the gap has a well-defined lattice orientation and thickness, enabling independent extraction of the anisotropic field enhancements. We find that the field enhancement can be safely described by pure classical electromagnetic theory when the gap distance is no <1.24 nm. For a 0.62 nm gap, the probable emergence of quantum mechanical effects renders an average electric field enhancement of 114-fold, 38.4% lower than classical predictions.

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS 2 ) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS 2 relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

to the photon-assisted oxygen desorption from the nanostructure surface, resulting in a decrease of hole concentration. [2] Also, in the case of n-type InAs nanowires, the hot carrier trapping process at the surface gives the reduction of photoexcited charge carriers. [3] Therefore, additional electronic states are always required to compensate photoexcited charge carriers for accessing NPC situation. Nanostructured materials with plenteous surface sites can potentially generate a high density of localized energy states to trap photoexcited charge carriers, which are promising for constructing optoelectronic devices with high-speed frequency response and low power consumption. [3,4] 2D materials have proved to be one of the most promising materials of which the focus has expanded beyond graphene to other layered van der Waals materials with a variety of distinct properties. [5][6][7][8][9][10] A burgeoning research direction goes toward the newly emerged metal phosphorus trichalcogenides (MPTs). [11][12][13][14] MPTs have many fascinating properties as featured in photoelectronic and electronic fields, such as a wide range of indirect bandgap from 1.3 to 3.5 eV, [11] and high carrier mobility. [15] In addition, the introduction of magnetic metal atoms can bring new physical phases, such as antiferromagnetic or ferromagnetic order. [16,17] With these advantages, some works have demon strated that MPTs obtain a remarkable photo response under ultraviolet (UV) illumination 2D materials have aroused tremendous attention in the last decade, and magnetic materials of the 2D scale are rising to prominence in recent years, which may revolutionize current optical and electronic applications. Photoconductivity is a well-known optical and electrical property, which should be positive as the conductivity of material increases typically upon the absorption of electromagnetic radiation. Here, a controllable switch from negative to positive photoconductivity effect of FePS 3 nanosheets is enabled by the modulation of the excitation wavelengths. These effects originate from an ultrafast process of hot carrier trapping, as visualized and investigated by the transient absorption microscopy at a quantitative level. This hot carrier trapping process is about 1.25 ps resulting in a lower diffusion constant state (≈3.5 cm 2 s −1 ), which provides deep insight into the intrinsic properties of 2D FePS 3 for further study. The results demonstrate experimentally the possibility to tune the electronic properties of 2D magnetic materials by an optical way, which not only induces the change in the magnitude but also in the sign, leading to more understandings and possibilities in 2D optoelectronic devices with many unexpected and multifunctional properties.

Circularly polarized light (CPL) is essential for optoelectronic and chiro-spintronic applications. Hybrid perovskites, as star optoelectronic materials, have demonstrated CPL activity, which is, however, mostly limited to chiral perovskites. Here, we develop a simple, general, and efficient strategy to stimulate CPL activity in achiral perovskites, which possess rich species, efficient luminescence, and tunable bandgaps. With the formation of van der Waals heterojunctions between chiral and achiral perovskites, a nonequilibrium spin population and thus CPL activity are realized in achiral perovskites by receiving spinpolarized electrons from chiral perovskites. The polarization degree of room-temperature CPL in achiral perovskites is at least one order of magnitude higher than in chiral ones. The CPL polarization degree and emission wavelengths of achiral perovskites can be flexibly designed by tuning chemical compositions, operating temperature, or excitation wavelengths. We anticipate that unlimited types of achiral perovskites can be endowed with CPL activity, benefiting their applications in integrated CPL sources and detectors.

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