Plasmon Tunneling Research Articles

Dynamically tunable perfect absorption based on quantum plasmonic metal-insulator-metal mirror

Achieving perfect absorption in nanostructures plays a crucial role in applications such as photovoltaics, photoluminescence, and sensing. In recent years, a large number of scholars have devoted themselves to the research of metamaterial absorbers from microwave to optical frequencies. However, almost these absorbers are based on the resonant absorption resulting from strong electromagnetic coupling. In this study, we propose a dynamically tunable perfect absorber based on quantum plasmonic tunneling effect with nonelectromagnetic coupling in a metal–insulator–metal mirror, which has never been demonstrated before as far as we know. With the manipulation of tunneling current and tunneling damping via altering the external bias on the tunneling electrodes, the absorptivity of proposed absorber can be modulated at the Er:YAG laser wavelength (2940nm) with the modulation deep near 92%. Moreover, both the absorptivity and full width at half maximum of this absorber have strong robustness to the incidence angle. This ultra-thin perfect absorber, characterized by its rapid response, exceptional integration capabilities, robust tolerance to the incidence angle, and superior controllability, holds tremendous promise for a broad range of applications within the realm of infrared imaging and treatment.

Tuning Overbias Plasmon Energy and Intensity in Molecular Plasmonic Tunneling Junctions by Atomic Polarizability.

Plasmon excitation in molecular tunnel junctions is interesting because the plasmonic properties of the device can be, in principle, controlled by varying the chemical structure of the molecules. The plasmon energy of the excited plasmons usually follows the quantum cutoff law, but frequently overbias plasmon energy has been observed, which can be explained by quantum shot noise, multielectron processes, or hot carrier models. So far, clear correlations between molecular structure and the plasmon energy have not been reported. Here, we introduce halogenated molecules (HS(CH2)12X, with X = H, F, Cl, Br, or I) with polarizable terminal atoms as the tunnel barriers and demonstrate molecular control over both the excited plasmon intensity and energy for a given applied voltage. As the polarizability of the terminal atom increases, the tunnel barrier height decreases, resulting in an increase in the tunneling current and the plasmon intensity without changing the tunneling barrier width. We also show that the plasmon energy is controlled by the electrostatic potential drop at the molecule-electrode interface, which depends on the polarizability of the terminal atom and the metal electrode material (Ag, Au, or Pt). Our results give new insights in the relation between molecular structure, electronic structure of the molecular junction, and the plasmonic properties which are important for the development of molecular scale plasmonic-electronic devices.

Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions.

Plasmonic tunnel junctions are a unique electroluminescent system in which light emission occurs via an interplay between tunnelling electrons and plasmonic fields instead of electron-hole recombination as in conventional light-emitting diodes. It was previously shown that placing luminescent molecules in the tunneling pathway of nanoscopic tunnel junctions results in peculiar upconversion electroluminescence where the energy of emitted photons exceeds that of excitation electrons. Here we report the observation of upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions comprising gold and few-layer graphene electrodes separated by a ~2-nm-thick hexagonal boron nitride tunnel barrier and a monolayer semiconductor. We find that the semiconductor ground exciton emission is triggered at excitation electron energies lower than the semiconductor optical gap. Interestingly, this upconversion is reached in devices operating at a low conductance (<10-6 S) and low power density regime (<102 W cm-2), defying explanation through existing proposed mechanisms. By examining the scaling relationship between plasmonic and excitonic emission intensities, we elucidate the role of inelastic electron tunnelling dipoles that induce optically forbidden transitions in the few-layer graphene electrode and ultrafast hot carrier transfer across the van der Waals interface.

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.

Plasmonic Tunnel Junctions (Invited)

Plasmonic Tunnel Junctions (Invited)

Engineering the Outcoupling Pathways in Plasmonic Tunnel Junctions via Photonic Mode Dispersion for Low-Loss Waveguiding.

Outcoupling of plasmonic modes excited by inelastic electron tunneling (IET) across plasmonic tunnel junctions (TJs) has attracted significant attention due to low operating voltages and fast excitation rates. Achieving selectivity among various outcoupling channels, however, remains a challenging task. Employing nanoscale antennas to enhance the local density of optical states (LDOS) associated with specific outcoupling channels partially addressed the problem, along with the integration of conducting 2D materials into TJs, improving the outcoupling to guided modes with particular momentum. The disadvantage of such methods is that they often involve complex fabrication steps and lack fine-tuning options. Here, we propose an alternative approach by modifying the dielectric medium surrounding TJs. By employing a simple multilayer substrate with a specific permittivity combination for the TJs under study, we show that it is possible to optimize mode selectivity in outcoupling to a plasmonic or a photonic-like mode characterized by distinct cutoff behaviors and propagation length. Theoretical and experimental results obtained with a SiO2-SiN-glass multilayer substrate demonstrate high relative coupling efficiencies of (62.77 ± 1.74)% and (29.07 ± 0.72)% for plasmonic and photonic-like modes, respectively. The figure-of-merit, which quantifies the tradeoff between mode outcoupling and propagation lengths (tens of μm) for both modes, can reach values as high as 180 and 140. The demonstrated approach allows LDOS engineering and customized TJ device performance, which are seamlessly integrated with standard thin film fabrication protocols. Our experimental device is well-suited for integration with silicon nitride photonics platforms.

Electroluminescence as a Probe of Strong Exciton-Plasmon Coupling in Few-Layer WSe2.

The manipulation of coupled quantum excitations is of fundamental importance in realizing novel photonic and optoelectronic devices. We use electroluminescence to probe plasmon-exciton coupling in hybrid structures consisting of a nanoscale plasmonic tunnel junction and few-layer two-dimensional transition-metal dichalcogenide transferred onto the junction. The resulting hybrid states act as a novel dielectric environment that affects the radiative recombination of hot carriers in the plasmonic nanostructure. We determine the plexcitonic spectrum from the electroluminescence and find Rabi splittings exceeding 50 meV in the strong coupling regime. Our experimental findings are supported by electromagnetic simulations that enable us to explore systematically and in detail the emergence of plexciton polaritons as well as the polarization characteristics of their far-field emission. Electroluminescence modulated by plexciton coupling provides potential applications for engineering compact photonic devices with tunable optical and electrical properties.

Engineering the directionality of hot carrier tunneling in plasmonic tunneling structures

Tunneling metal–insulator–metal (MIM) junctions can exhibit an open-circuit photovoltage (OCPV) response under illumination that may be useful for photodetection. One mechanism for photovoltage generation is hot carrier tunneling, in which photoexcited carriers generate a net photocurrent that must be balanced by a drift current in the open-circuit configuration. We present experiments in electromigrated planar MIM structures, designed with asymmetric plasmonic properties using Au and Pt electrodes. Decay of optically excited local plasmonic modes preferentially creates hot carriers on the Au side of the junction, leading to a clear preferred directionality of the hot electron photocurrent and hence a preferred polarity of the resulting OCPV. In contrast, in an ensemble of symmetric devices constructed from only Au, polarity of the OCPV has no preferred direction.

Optical properties of plasmonic tunneling junctions.

Over the last century, quantum theories have revolutionized our understanding of material properties. One of the most striking quantum phenomena occurring in heterogeneous media is the quantum tunneling effect, where carriers can tunnel through potential barriers even if the barrier height exceeds the carrier energy. Interestingly, the tunneling process can be accompanied by the absorption or emission of light. In most tunneling junctions made of noble metal electrodes, these optical phenomena are governed by plasmonic modes, i.e., light-driven collective oscillations of surface electrons. In the emission process, plasmon excitation via inelastic tunneling electrons can improve the efficiency of photon generation, resulting in bright nanoscale optical sources. On the other hand, the incident light can affect the tunneling behavior of plasmonic junctions as well, leading to phenomena such as optical rectification and induced photocurrent. Thus, plasmonic tunneling junctions provide a rich platform for investigating light-matter interactions, paving the way for various applications, including nanoscale light sources, sensors, and chemical reactors. In this paper, we will introduce recent research progress and promising applications based on plasmonic tunneling junctions.

Tuning Light Emission Crossovers in Atomic-Scale Aluminum Plasmonic Tunnel Junctions.

Atomic-sized plasmonic tunnel junctions are of fundamental interest, with great promise as the smallest on-chip light sources in various optoelectronic applications. Several mechanisms of light emission in electrically driven plasmonic tunnel junctions have been proposed, from single-electron or higher-order multielectron inelastic tunneling to recombination from a steady-state population of hot carriers. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through these possibilities for the first time, finding quantitative agreement with theory in each regime. Improved plasmonic resonances in the energy range of interest increase photon yields by 2 orders of magnitude. These results demonstrate that the dominant emission mechanism is set by a combination of tunneling rate, hot carrier relaxation time scales, and junction plasmonic properties.

Spatial Control over Stable Light‐Emission from AC‐Driven CMOS‐Compatible Quantum Mechanical Tunnel Junctions

AbstractThe potential application of quantum mechanical tunnel junctions as subdiffraction light or surface plasmon sources has been explored for decades, but it has been challenging to create devices with subwavelength spatial control over the light or plasmon excitation. This paper describes spatial control over the electrical excitation of surface‐plasmon polaritons (SPPs) and photons in large‐area junctions of the form of Al–AlOX–Cu complementary metal‐oxide‐semiconductor (CMOS)‐compatible tunnel junctions. Nanoscale spatial control (smallest feature sizes of 150 nm) is achieved by locally fine‐tuning the thickness of the AlOX tunneling barrier resulting in large local tunneling currents and associated SPP excitation rates. Mostly, plasmonic tunnel junctions are studied under DC operation with a relatively large bias (and associated currents) to observe light emission at optical frequencies. Large voltages risk device failure and reduce device lifetimes. Here it is demonstrated that the operational lifetime of AC‐driven plasmonic tunnel junctions is improved by a factor of three. Under DC conditions, slow processes that lead to device failure (e.g., undesirable electromigration leading to shorts) readily occur, thus limiting the device decay time to 9.2 h; but under AC operation, such processes are slow with respect to the voltage changes prolonging the decay time beyond 18.0 h.

Plasmonic Tunnel Diode and Photodetector based on Layer‐Stacked AuNP‐Nanomembrane/p‐Si Heterojunction

AbstractElectronic devices with rectifying effect are typical building blocks for integrated circuits and useful for photodetectors. Herein, a new type of diode based on plasmonic tunnel hetero‐nanostructure is reported. By constructing a planar plasmoelectric junction between p‐Si and Au pad electrode with a few‐layer‐stacked 2D plasmonic AuNP‐nanomembrane as a medium layer, rectifying behavior of the device is achieved, though the direct contact between bulk Au and semiconductor usually tends to form ohmic contact. The electrically driven plasmons enable the 2D AuNP‐nanomembrane to be electron‐rich, while the asymmetric band alignment which causes bidirectionally different barrier at the (buried) interface results in the unidirectional rectifying behavior of the constructed plasmonic diode (p‐diode). The planar junction made of shell‐isolated Au@SiO2 NP‐nanomembrane (instead of AuNP‐nanomembrane) shows still rectifying behaviors but better photoresponse, due to structurally well‐defined/ordered nanostructures and intensified plasmonic tunneling effect of the nanomembrane. The as‐constructed p‐diode‐based photodetector with Au@SiO2 NP‐nanomembrane shows impressive photoresponse with a high photoresponsivity value of 6.496 A W‐1 and a detectivity value of 1.9859 × 1011 Jones under the 268 µW cm‐2 650 nm laser irradiance, comparable to the reported values of similar devices prepared from other 2D materials.

Optical Anisotropy in van der Waals materials: Impact on Direct Excitation of Plasmons and Photons by Quantum Tunneling

Inelastic quantum mechanical tunneling of electrons across plasmonic tunnel junctions can lead to surface plasmon polariton (SPP) and photon emission. So far, the optical properties of such junctions have been controlled by changing the shape, or the type of the material, of the electrodes, primarily with the aim to improve SPP or photon emission efficiencies. Here we show that by tuning the tunneling barrier itself, the efficiency of the inelastic tunneling rates can be improved by a factor of 3. We exploit the anisotropic nature of hexagonal boron nitride (hBN) as the tunneling barrier material in Au//hBN//graphene tunnel junctions where the Au electrode also serves as a plasmonic strip waveguide. As this junction constitutes an optically transparent hBN–graphene heterostructure on a glass substrate, it forms an open plasmonic system where the SPPs are directly coupled to the dedicated strip waveguide and photons outcouple to the far field. We experimentally and analytically show that the photon emission rate per tunneling electron is significantly improved (~ ×3) in Au//hBN//graphene tunnel junction due to the enhancement in the local density of optical states (LDOS) arising from the hBN anisotropy. With the dedicated strip waveguide, SPP outcoupling efficiency is quantified and is found to be ∼ 80% stronger than the radiative outcoupling in Au//hBN//graphene due to the high LDOS of the SPP decay channel associated with the inelastic tunneling. The new insights elucidated here deepen our understanding of plasmonic tunnel junctions beyond the isotropic models with enhanced LDOS.

Anti-Stokes Light Scattering Mediated by Electron Transfer Across a Biased Plasmonic Nanojunction

Light scattering from plasmonic nanojunctions is routinely used to assess their optical properties. However, the microscopic mechanism remains imperfectly understood, and an accurate description requires the experiment in a well-defined environment with a highly precise control of the nanojunction. Here we report on inelastic light scattering (ILS) from plasmonic scanning tunneling microscope (STM) junctions under ultrahigh vacuum and cryogenic conditions. We particularly focus on anti-Stokes continuum generation in the ILS spectra with a narrowband continuous-wave laser excitation, which appears when an electrical bias is applied between the tip and the surface. This anti-Stokes continuum is commonly observed for various STM junctions at ∼10 K, corroborating that it is a universal phenomenon in electrically biased plasmonic nanojunctions. We propose that the microscopic mechanism underlying the anti-Stokes continuum generation is explained by ILS accompanied by electron transfer across the STM junction, whereby the excess energy is provided by the applied bias voltage. This process occurs through either photoluminescence (PL) or electronic Raman scattering (ERS). By recording the ILS spectra in parallel with STM-induced luminescence, we show that ERS becomes dominant when the excitation wavelength matches the plasmonic resonance of the STM junction, whereas PL mainly contributes to the off-resonance excitation. Our results provide an in-depth understanding of ILS by plasmonic nanojunctions and demonstrate that the anti-Stokes continuum can arise from a nonthermal mechanism.

Constant amplitude driving of a radiofrequency excited plasmonic tunnel junction.

Constant-amplitude bias modulation over a broad range of microwave frequencies is a prerequisite for application in high-resolution spectroscopic techniques in a tunneling junction as e.g. electron spin resonance spectroscopy or optically detected paramagnetic resonance. Here, we present an optical method for determining the frequency-dependent magnitude of the transfer function of a dedicated high-frequency line integrated with a scanning probe microscope. The method relies on determining the energy cutoff of the plasmonic electroluminescence spectrum, which is linked to the energies of the electrons inelastically tunneling across the junction. We develop an easy-to-implement procedure for effective compensation of an RF line and determination of the transfer function magnitude in the GHz range. We test our method with conventional electronic calibration and find a perfect agreement.

Klein scattering of spin-1 Dirac-Weyl wave and localized surface plasmon

Localized surface plasmon and Klein tunneling resonances are two phenomena that were previously thought to be unrelated, where the former plays an important role in subwavelength optics while the latter is fundamental to relativistic quantum mechanics and physics of Dirac materials. We develop a rigorous theory for spin-1 Dirac-Weyl particles, which establishes a striking analogy between the two phenomena and unveils a deep connection between the distinct physical contexts, paving the way for gate-controlled surface plasmon mimetic electronics as well as for realizing localized spoof plasmons in a scope wider than previously thought. A possible experimental scheme is articulated.

Thousand-fold Increase in Plasmonic Light Emission via Combined Electronic and Optical Excitations.

Surface plasmon enhanced processes and hot-carrier dynamics in plasmonic nanostructures are of great fundamental interest to reveal light-matter interactions at the nanoscale. Using plasmonic tunnel junctions as a platform supporting both electrically and optically excited localized surface plasmons, we report a much greater (over 1000× ) plasmonic light emission at upconverted photon energies under combined electro-optical excitation, compared with electrical or optical excitation separately. Two mechanisms compatible with the form of the observed spectra are interactions of plasmon-induced hot carriers and electronic anti-Stokes Raman scattering. Our measurement results are in excellent agreement with a theoretical model combining electro-optical generation of hot carriers through nonradiative plasmon excitation and hot-carrier relaxation. We also discuss the challenge of distinguishing relative contributions of hot carrier emission and the anti-Stokes electronic Raman process. This observed increase in above-threshold emission in plasmonic systems may open avenues in on-chip nanophotonic switching and hot-carrier photocatalysis.

Hot-carrier enhanced light emission: The origin of above-threshold photons from electrically driven plasmonic tunnel junctions

Understanding the origin of above-threshold photons emitted from electrically driven tunnel junctions (ℏω&amp;gt;eVb with Vb being the applied voltage bias) is of current interest in nano-optics and holds great promise to create novel on-chip optoelectronic and energy conversion technologies. Here, we report experimental observation and theoretical analysis of above-threshold light emission from electromigrated Au tunnel junctions. We compare our proposed hot-carrier enhanced light emission theory with existing models, including blackbody thermal radiation, multi-electron interactions, and an interpretation involving finite temperature effects. Our study highlights the key role of plasmon-induced hot carrier dynamics in emitting above-threshold photons and the need to further explore the underlying mechanisms and optimization of upconversion effects in plasmonically active nanostructures.

Programmable Organic-Free Negative Differential Resistance Memristor Based on Plasmonic Tunnel Junction.

A novel negative differential resistance (NDR) phenomenon is reported herein based on planar plasmonic tunnel junction, resulting from plasmon-assisted long-range electron tunneling (P-tunneling) and electronic caching effect of Au@SiO2 nanoparticles. The tunnel junction is made of shell-insulated Au@SiO2 nanoparticle nanomembrane, in which SiO2 shells act as a tunable tunneling barrier, while the Au core not only support the plasmonic effect to enable P-tunneling, but also act as electronic caches to render NDR responses. The NDR peak voltage and current can be programmably controlled by varying the thickness of SiO2 shell and the size of Au core to tune barrier level for electron transport. In addition, light induced plasmonic effect can be further managed to regulate the NDR behavior by fine-tuning P-tunneling. The phenomenon is exploited for robust use as memristors. The work provides a new mechanism for the generation of NDR effect and may open a way for the development of robust and new conceptual nanoelectronic devices.

Electrically Driven Hot-Carrier Generation and Above-Threshold Light Emission in Plasmonic Tunnel Junctions

Above-threshold light emission from plasmonic tunnel junctions, when emitted photons have energies significantly higher than the energy scale of incident electrons, has attracted much recent interest in nano-optics, while the underlying physics remains elusive. We examine above-threshold light emission in electromigrated tunnel junctions. Our measurements over a large ensemble of devices demonstrate a giant (∼104) material-dependent photon yield (emitted photons per incident electrons). This dramatic effect cannot be explained only by the radiative field enhancement due to localized plasmons in the tunneling gap. Emission is well described by a Boltzmann spectrum with an effective temperature exceeding 2000 K, coupled to a plasmon-modified photonic density of states. The effective temperature is approximately linear in the applied bias, consistent with a suggested theoretical model describing hot-carrier dynamics driven by nonradiative decay of electrically excited localized plasmons. Electrically generated hot carriers and nontraditional light emission could open avenues for active photochemistry, optoelectronics, and quantum optics.