Light-Emitting Plasmonic Tunneling Junctions: Current Status and Perspectives.

Quantum tunneling, in which electrons can tunnel through a finite potential barrier while simultaneously interacting with other matter excitation, is one of the most fascinating phenomena without classical correspondence. In an extremely thin metallic nanogap, the deep-subwavelength-confined plasmon modes can be directly excited by the inelastically tunneling electrons driven by an externally applied voltage. Light emission via inelastic tunneling possesses a great potential application for next-generation light sources, with great superiority of ultracompact integration, large bandwidth, and ultrafast response. In this Perspective, we first briefly introduce the mechanism of plasmon generation in the inelastic electron tunneling process. Then the state of the art in plasmonic tunneling junctions will be reviewed, particularly emphasizing efficiency improvement, precise construction, active control, and electrically driven optical antenna integration. Ultimately, we forecast some promising and critical prospects that require further investigation.

Identification of twist-angle-dependent excitons in WS2/WSe2 heterobilayers.

Stacking atomically thin films enables artificial construction of van der Waals heterostructures with exotic functionalities such as superconductivity, the quantum Hall effect, and engineered light-matter interactions. In particular, heterobilayers composed of monolayer transition metal dichalcogenides have attracted significant interest due to their controllable interlayer coupling and trapped valley excitons in moiré superlattices. However, the identification of twist-angle-modulated optical transitions in heterobilayers is sometimes controversial since both momentum-direct (K-K) and -indirect excitons reside on the low energy side of the bright exciton in the monolayer constituents. Here, we attribute the optical transition at ∼1.35 eV in the WS2/WSe2 heterobilayer to an indirect Γ-K transition based on a systematic analysis and comparison of experimental photoluminescence spectra with theoretical calculations. The exciton wavefunction obtained by the state-of-the-art GW-Bethe-Salpeter equation approach indicates that both the electron and hole of the excitons are contributed by the WS2 layer. Polarization-resolved k-space imaging further confirms that the transition dipole moment of this optical transition is dominantly in-plane and is independent of the twist angle. The calculated absorption spectrum predicts that the so-called interlayer exciton peak coming from the K-K transition is located at 1.06 eV, but with a much weaker amplitude. Our work provides new insight into the steady-state and dynamic properties of twist-angle-dependent excitons in van der Waals heterostructures.

Plasmon-Directed On-Wire Growth of Branched Silver Nanowires with Chiroptic Activity.

Silver nanowires (Ag NWs) present prominent waveguiding properties of subwavelength light due to their nanoconfinement with propagating surface plasmons, which is of great importance for on-chip integration of nanophotonic devices and optical computation. Such propagating plasmons also exert plasmonic forces, which can be utilized to manipulate nanoparticles (NPs) beyond the diffraction limit. However, such controllability is spatially limited to the near fields, whereas a large portion of uncontrolled particles are randomly deposited on the chips, which could be detrimental to the integrated optical devices. Herein we shine continuous wave laser at one end of the Ag NW immersed in AgNO3 solution to launch the propagating surface plasmons. The laser irradiation also induces the photoreduction of Ag+ ions to locally generate tiny Ag NPs, which evolve into large Ag flake branches closer to the other end of the Ag NW. Such a peculiar growth is due to the synergistic effect of plasmonic forces and the thermophoretic/thermo-osmosis forces induced by temperature gradient. These branched Ag NWs with sharp angles are intrinsically chiral, which can be partially controlled by changing the irradiation location, forming plasmonic chiral enantiomers. The circular differential scattering (CDS) response of these branched Ag NWs can be as large as 40%, which can be used for chiral enantiomer sensing with spectral dissymmetric factor up to 4 nm induced by phenylalanine. This plasmon-directed on-wire growth not only offers a facile approach for generating plasmonic chiral nanostructures with remote controllability, but also provides significant insights on the synergistic effect of plasmonic forces and thermal-induced forces, which has great implications for self-assembly and integration of on-chip optics.

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals.

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 (WS2) 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 WS2 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.

Malus-metasurface-assisted polarization multiplexing.

Polarization optics plays a pivotal role in diffractive, refractive, and emerging flat optics, and has been widely employed in contemporary optical industries and daily life. Advanced polarization manipulation leads to robust control of the polarization direction of light. Nevertheless, polarization control has been studied largely independent of the phase or intensity of light. Here, we propose and experimentally validate a Malus-metasurface-assisted paradigm to enable simultaneous and independent control of the intensity and phase properties of light simply by polarization modulation. The orientation degeneracy of the classical Malus's law implies a new degree of freedom and enables us to establish a one-to-many mapping strategy for designing anisotropic plasmonic nanostructures to engineer the Pancharatnam-Berry phase profile, while keeping the continuous intensity modulation unchanged. The proposed Malus metadevice can thus generate a near-field greyscale pattern, and project an independent far-field holographic image using an ultrathin and single-sized metasurface. This concept opens up distinct dimensions for conventional polarization optics, which allows one to merge the functionality of phase manipulation into an amplitude-manipulation-assisted optical component to form a multifunctional nano-optical device without increasing the complexity of the nanostructures. It can empower advanced applications in information multiplexing and encryption, anti-counterfeiting, dual-channel display for virtual/augmented reality, and many other related fields.

Routing a Chiral Raman Signal Based on Spin-Orbit Interaction of Light.

Spontaneous Raman scattering is a second-order perturbation process with two photons linking the internal structures of the matter. The frequency-shifted Raman peaks are sharp and carry rich information about the internal structures. However, encoding and manipulating this information have been barely explored up to now. Here, we report the high-fidelity routing of a chiral Raman signal into propagating surface plasmon polaritons along a silver nanowire based on spin-orbit interaction of light. A directionality up to 91.5±0.5% is achieved and can be quantitatively controlled by tuning the polarization of the incident laser and the position of excitation. The deterministic routing of the Raman signal is sensitively dependent on the local spin density of the plasmon field and the polarization of the Raman modes. This study extends the spin-orbit interaction of light to the Raman scattering regime and proposes a new perspective for the remote readout of local optical chirality, helicity-related directional sorting, and quantum information processing.

Electrically Driven Optical Antennas Based on Template Dielectrophoretic Trapping.

An electrically driven optical antenna (EDOA) provides a nanoscale light-emitting scheme that is appealing for biosensors, plasmonic displays, and on-chip optoelectronic circuits. The EDOA (consisting of metal nanoparticles (NPs)) excited by inelastic tunneling electrons has attracted broad interest due to its terahertz modulation bandwidth and microelectronics-compatible dimensions. Currently, the efficient fabrication of EDOA is hampered by the ultrasmall size of NPs and the requirement of controllable preparation. Here, we overcome this limitation by accurately positioning thiol-covered gold NPs onto predesigned electrodes using dielectrophoresis trapping. The combination of a high-quality molecule tunnel barrier and the template trapping ensures that the EDOA can operate stably in ambient conditions. More importantly, the template trapping allows fabrication of EDOA with different numbers and arrangements of NPs by controlling the size and orientation of the template. This technology provides a way to fabricate controllable optoelectronic devices based on NPs and is promising for compact and smart photonic devices.

Light-controlled nanoswitches: from fabrication to photoelectric switching.

Plasmon-assisted optical growth significantly improves the controllability of polymerization on the nanoscale, which greatly facilitates the fabrication of nanodevices and their integration. Traditional approaches for nanodevices strongly rely on nanofabrication techniques, which involve complicated multi-steps and expensive instrumentation. Here, we build photoelectric nanoswitches by precisely implanting photoelectric active polymers, polypyrrole, into metallic bowtie nanogaps via charge transfer induced oxidative polymerization, which show conductivity up to 4 µS m-1 with an optical switching response time of ∼50 ms. We further demonstrate their photoelectric switchability with different laser powers, wavelengths and amounts of polypyrrole in the gaps. This plasmon-assisted optical growth strategy not only provides a facile and accurate means to control the polymerization process, but also greatly expedites the fabrication of nanodevices and their integration.