Electrically Driven Unidirectional Optical Nanoantennas

Revealing the Properties of Electrically Driven Optical Antennas via Conductive Atomic Force Microscope

Light emission from ultracompact electrically driven optical antennas (EDOAs) has garnered significant attention due to its terahertz modulation bandwidth. Typically, the EDOAs are fixed and nonadjustable once fabricated, thus hindering the attempts to investigate the influence of structural geometry on light emission properties. Here, we propose and demonstrate that the EDOAs can be constructed by carefully manipulating the gold-coated tips of atomic force microscopy operated in conductive mode into contact with the optical antennas covered by insulating film, where the position of the tunnel junction on the antenna surface can be controlled with high accuracy and flexibility. Taking gold nanorod antennas covered by HfO2 film as an example, we found that the highest light generation efficiency is obtained when the tunnel junction is located at the shoulder edge of the nanorod antenna, where the bonding dipolar surface plasmon mode in the junction is spectrally and spatially coupled with the longitudinal radiation mode of the EDOAs. Besides, position variation of the tunnel junction on the nanorod surface also strongly influences the far-field radiation angular distribution and emission spectrum. Numerical simulations are in good agreement with the experimental results. Our findings offer fundamental insights into the influence of structural parameters on the light emission performance of EDOAs, thus leading to better design of EDOAs with high light generation efficiency and powerful functionality.

Electrically Driven Deterministic Plasmon Light Sources Based on Arrays of Molecular Tunnel Junctions

Surface plasmons excited via inelastic tunnelling have led to plasmon light sources with small footprints and ultrafast response speeds, which are favored by integrated optical circuits. Self-assembled monolayers of organic molecules function as highly tunable tunnel barriers with novel functions. However, limited by the low effective contact between the liquid metal electrode and the self-assembled monolayers, it is quite challenging to obtain molecular plasmon light sources with high density and uniform emission. Here, by combining lithographic patterning with a solvent treatment method, we have demonstrated electrically driven deterministic plasmon emission from arrays of molecular tunnel junctions. The solvent treatment could largely improve the effective contact from 9.6% to 48% and simultaneously allow the liquid metal to fill into lithographically patterned micropore structures toward deterministic plasmon emission with desired patterns. Our findings open up new possibilities for tunnel junction-based plasmon light sources, laying the foundation for electrically driven light-emitting metasurfaces.

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.

Light-Emitting Electrochemical Cells Based on Nanogap Electrodes

In a light-emitting electrochemical cell (LEC), electrochemical doping caused by mobile ions facilitates bipolar charge injection and recombination emissions for a high electroluminescence (EL) intensity at low driving voltages. We present the development of a nanogap LEC (i.e., nano-LEC) comprising a light-emitting polymer (F8BT) and an ionic liquid deposited on a gold nanogap electrode. The device demonstrated a high EL intensity at a wavelength of 540 nm corresponding to the emission peak of F8BT and a threshold voltage of ∼2 V at 300 K. Upon application of a constant voltage, the device demonstrated a gradual increase in current intensity followed by light emission. Notably, the delayed components of the current and EL were strongly suppressed at low temperatures (<285 K). The results clearly indicate that the device functions as an LEC and that the nano-LEC is a promising approach to realizing molecular-scale current-induced light sources.

Tunable directional emission from electrically driven nano-strip metal-insulator-metal tunnel junctions

Electrically driven nanoantennas for on-chip generation and manipulation of light have attracted significant attention in recent times. Metal-insulator-metal (MIM) tunnel junctions have been extensively used to electrically excite surface plasmons and photons via inelastic electron tunneling. However, the dynamic switching of light from MIM junctions into spatially separate channels has not been shown. Here, we numerically demonstrate switchable, highly directional light emission from electrically driven nano-strip Ag-SiO2-Ag tunnel junctions. The top electrode of our Ag-SiO2-Ag stack is divided into 16 nano-strips, with two of the tunnel junctions at the centre (S L and S R) acting as sources. Using full-wave electromagnetic simulations, we show that when S L is excited, the emission is highly directional with an angle of emission of -30° and an angular spread of ∼11°. When the excitation is switched to S R, the emission is redirected to an angle of 30° with an identical angular spread. A directivity of 29.4 is achieved in the forward direction, with a forward-to-backward ratio of 12. We also demonstrate wavelength-selective directional switching by changing the width, and thereby the resonance wavelength, of the sources. The emission can be tuned by varying the periodicity of the structure, paving the way for electrically driven, reconfigurable light sources.

DNA-Templated Ultracompact Optical Antennas for Unidirectional Single-Molecule Emission

Optical antennas are nanostructures designed to manipulate light-matter interactions by interfacing propagating light with localized optical fields. In recent years, numerous devices have been realized to efficiently tailor the absorption and/or emission rates of fluorophores. By contrast, modifying the spatial characteristics of their radiation fields remains challenging. Successful phased array nanoantenna designs have required the organization of several elements over a footprint comparable to the operating wavelength. Here, we report unidirectional emission of a single fluorophore using an ultracompact optical antenna. The design consists of two side-by-side gold nanorods self-assembled via DNA origami, which also controls the positioning of the single-fluorophore. Our results show that when a single fluorescent molecule is positioned at the tip of one nanorod and emits at a frequency capable of driving the antenna in the antiphase mode, unidirectional emission with a forward to backward ratio of up to 9.9 dB can be achieved.

Geometric control over surface plasmon polariton out-coupling pathways in metal-insulator-metal tunnel junctions

Metal-insulator-metal tunnel junctions (MIM-TJs) can electrically excite surface plasmon polaritons (SPPs) well below the diffraction limit. When inelastically tunneling electrons traverse the tunnel barrier under applied external voltage, a highly confined cavity mode (MIM-SPP) is excited, which further out-couples from the MIM-TJ to photons and single-interface SPPs via multiple pathways. In this work we control the out-coupling pathways of the MIM-SPP mode by engineering the geometry of the MIM-TJ. We fabricated MIM-TJs with tunneling directions oriented vertical or lateral with respect to the directly integrated plasmonic strip waveguides. With control over the tunneling direction, preferential out-coupling of the MIM-SPP mode to SPPs or photons is achieved. Based on the wavevector distribution of the single-interface SPPs or photons in the far-field emission intensity obtained from back focal plane (BFP) imaging, we estimate the out-coupling efficiency of the MIM-SPP mode to multiple out-coupling pathways. We show that in the vertical-MIM-TJs the MIM-SPP mode preferentially out-couples to single-interface SPPs along the strip waveguides while in the lateral-MIM-TJs photon out-coupling to the far-field is more efficient.

Unidirectional propagation of electrically driven surface plasmon polaritons: a numerical study

Electrically excited surface plasmon polaritons (SPPs) created, for instance, through the energy loss from inelastic electron tunneling, have received great interest recently because of their potential for facilitating the integration of on-chip information processing elements. Consequently, finding a convenient method of realizing the propagation control of such a kind of SPPs becomes a valuable research topic. Optical antennas array, with a relatively large footprint, are commonly used for steering SPPs. In this paper, with the aid of two plasmonic strip-type antennas that are placed next to each other, we present a feasible method of propagation control of electrically driven SPPs in a scanning tunneling microscope (STM) system. Results show that STM-induced SPPs propagate along a preferred direction due to the in-plane SPP interference effects, which can be easily engineered by the widths of the antennas. This result is very useful for compact plasmonic circuits design.

Localized surface plasmon mode-enhanced spectrum-tunable radiation in electrically driven plasmonic antennas

A spectrum-tunable source with ultra-small volume is highly desired by on-chip information processing technologies. As a promising candidate, light emission from electrically driven tunnel junctions has gained much interest. In this Letter, using a gap bowtie antenna-based metal-insulator-metal junction as the source, multiple peaks are found in the electroluminescence spectrum of the antenna system. We attribute the peaks observed in the experimental emission spectrum to resonant plasmon modes that are supported by the antennas. This explanation is confirmed numerically by finite difference time domain calculations and analytically by using a theory imitated from scanning tunneling microscopy. Our results show that the localized surface plasmon modes can be finely tuned by varying the gap distances and the geometries of the antennas, which eventually contribute to a spectrum-tunable light source. This Letter may provide a path for spectrum-tunable electrically driven light sources on photonic 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.

Electrically-driven Yagi-Uda antennas for light

Yagi-Uda antennas are a key technology for efficiently transmitting information from point to point using radio waves. Since higher frequencies allow higher bandwidths and smaller footprints, a strong incentive exists to shrink Yagi-Uda antennas down to the optical regime. Here we demonstrate electrically-driven Yagi-Uda antennas for light with wavelength-scale footprints that exhibit large directionalities with forward-to-backward ratios of up to 9.1 dB. Light generation is achieved via antenna-enhanced inelastic tunneling of electrons over the antenna feed gap. We obtain reproducible tunnel gaps by means of feedback-controlled dielectrophoresis, which precisely places single surface-passivated gold nanoparticles in the antenna gap. The resulting antennas perform equivalent to radio-frequency antennas and combined with waveguiding layers even outperform RF designs. This work paves the way for optical on-chip data communication that is not restricted by Joule heating but also for advanced light management in nanoscale sensing and metrology as well as light emitting devices.

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.

Optical antennas driven by quantum tunneling: a key issues review

Analogous to radio- and microwave antennas, optical nanoantennas are devices that receive and emit radiation at optical frequencies. Until recently, the realization of electrically driven optical antennas was an outstanding challenge in nanophotonics. In this review we discuss and analyze recent reports in which quantum tunneling-specifically inelastic electron tunneling-is harnessed as a means to convert electrical energy into photons, mediated by optical antennas. To aid this analysis we introduce the fundamentals of optical antennas and inelastic electron tunneling. Our discussion is focused on recent progress in the field and on future directions and opportunities.

Antenna surface plasmon emission by inelastic tunneling

Surface plasmons polaritons are mixed electronic and electromagnetic waves. They have become a workhorse of nanophotonics because plasmonic modes can be confined in space at the nanometer scale and in time at the 10 fs scale. However, in practice, plasmonic modes are often excited using diffraction-limited beams. In order to take full advantage of their potential for sensing and information technology, it is necessary to develop a microscale ultrafast electrical source of surface plasmons. Here, we report the design, fabrication and characterization of nanoantennas to emit surface plasmons by inelastic electron tunneling. The antenna controls the emission spectrum, the emission polarization, and enhances the emission efficiency by more than three orders of magnitude. We introduce a theoretical model of the antenna in good agreement with the results.

Critical coupling and extreme confinement in nanogap antennas

Nanogap antennas are compelling structures for squeezing light into ultrasmall volumes. However, when gaps are shrunk to the nanometer scale, the mode losses dramatically increase. In this Letter, we report the conditions of critical coupling between the arrays of nanogap resonant metal-insulator-metal (MIM) antennas and free space. Adapting the antenna density, critical coupling is achievable for any thickness of insulator, from 100 down to 0.1 nm. The fundamental optical mode can be described as continuous transitions through three types of modes: a perfect MIM mode, coupling between the MIM mode and surface plasmon polariton, and a gap plasmon mode. We found that the space between adjacent antennas is an essential parameter to perform critical coupling for thinner gaps. These results pave the way towards understanding extreme confinement in nanogap antenna structures such as MIM or nanoparticle arrays.

Design of plasmonic directional antennas via evolutionary optimization

We demonstrate inverse design of plasmonic nanoantennas for directional light scattering. Our method is based on a combination of full-field electrodynamical simulations via the Green dyadic method and evolutionary optimization (EO). Without any initial bias, we find that the geometries reproducibly found by EO work on the same principles as radio-frequency antennas. We demonstrate the versatility of our approach by designing various directional optical antennas for different scattering problems. EO-based nanoantenna design has tremendous potential for a multitude of applications like nano-scale information routing and processing or single-molecule spectroscopy. Furthermore, EO can help to derive general design rules and to identify inherent physical limitations for photonic nanoparticles and metasurfaces.

Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH₂)_nBPh (BPh = biphenyl, n = 1-7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle of the alkyl chain is fixed at 30° and independent of n. For values of n = 1-3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.

Nano-antenna enhanced waveguide integrated light source based on an MIS tunnel junction

Ultrafast electro-optical conversion at nanoscale is of fundamental interest for information transfer and optical interconnects. Light emission from a quantum tunnel junction provides an opportunity owing to its unique capability of ultrafast response and small footprint. However, the main challenge to the wide adoption of the tunnel junction is its low emission efficiency caused by the low inelastic electron tunneling proportion and radiation efficiency. In this Letter, an electrically driven silicon light source with its efficiency enhanced by using a nano-antenna in a metal-insulator-semiconductor junction is proposed. Strong plasmon confinement in the nano-antenna provides large local density of optical states and bridges the wave vector mismatch between nanoscale volume field confinement and far-field radiation. Two orders of magnitude of emission enhancement are achieved over typical planar MIS junctions.

Light from van der Waals quantum tunneling devices

The understanding of and control over light emission from quantum tunneling has challenged researchers for more than four decades due to the intricate interplay of electrical and optical properties in atomic scale volumes. Here we introduce a device architecture that allows for the disentanglement of electronic and photonic pathways—van der Waals quantum tunneling devices. The electronic properties are defined by a stack of two-dimensional atomic crystals whereas the optical properties are controlled via an external photonic architecture. In van der Waals heterostructures made of gold, hexagonal boron nitride and graphene we find that inelastic tunneling results in the emission of photons and surface plasmon polaritons. By coupling these heterostructures to optical nanocube antennas we achieve resonant enhancement of the photon emission rate in narrow frequency bands by four orders of magnitude. Our results lead the way towards a new generation of nanophotonic devices that are driven by quantum tunneling.

Electrical and optical properties of light emitting devices driven by quantum tunneling are closely intertwined. Parzefall et al. show that these properties can be individually controlled by cointegrating van der Waals heterostructures with optical antennas.

Electromigrated electrical optical antennas for transducing electrons and photons at the nanoscale

Background: Electrically controlled optical metal antennas are an emerging class of nanodevices enabling a bilateral transduction between electrons and photons. At the heart of the device is a tunnel junction that may either emit light upon injection of electrons or generate an electrical current when excited by a light wave. The current study explores a technological route for producing these functional units based upon the electromigration of metal constrictions. Results: We combine multiple nanofabrication steps to realize in-plane tunneling junctions made of two gold electrodes, separated by a sub-nanometer gap acting as the feedgap of an optical antenna. We electrically characterize the transport properties of the junctions in the light of the Fowler-Nordheim representation and the Simmons model for electron tunneling. We demonstrate light emission from the feedgap upon electron injection and show examples of how this nanoscale light source can be coupled to waveguiding structures. Conclusion: Electromigrated in-plane tunneling optical antennas feature interesting properties with their unique functionality enabling interfacing electrons and photons at the atomic scale and with the same device. This technology may open new routes for device-to-device communication and for interconnecting an electronic control layer to a photonic architecture.

Generalized Kerker effects in nanophotonics and meta-optics [Invited]

The original Kerker effect was introduced for a hypothetical magnetic sphere, and initially it did not attract much attention due to a lack of magnetic materials required. Rejuvenated by the recent explosive development of the field of metamaterials and especially its core concept of optically-induced artificial magnetism, the Kerker effect has gained an unprecedented impetus and rapidly pervaded different branches of nanophotonics. At the same time, the concept behind the effect itself has also been significantly expanded and generalized. Here we review the physics and various manifestations of the generalized Kerker effects, including the progress in the emerging field of meta-optics that focuses on interferences of electromagnetic multipoles of different orders and origins. We discuss not only the scattering by individual particles and particle clusters, but also the manipulation of reflection, transmission, diffraction, and absorption for metalattices and metasurfaces, revealing how various optical phenomena observed recently are all ubiquitously related to the Kerker's concept.

Controlled electromigration protocol revised

Electromigration has evolved from an important cause of failure in electronic devices to an appealing method, capable of modifying the material properties and geometry of nanodevices. Although this technique has been successfully used by researchers to investigate low dimensional systems and nanoscale objects, its low controllability remains a serious limitation. This is in part due to the inherent stochastic nature of the process, but also due to the inappropriate identification of the relevant control parameters. In this study, we identify a suitable process variable and propose a novel control algorithm that enhances the controllability and, at the same time, minimizes the intervention of an operator. As a consequence, the algorithm facilitates the application of electromigration to systems that require exceptional control of, for example, the width of a narrow junction. It is demonstrated that the electromigration rate can be stabilized on pre-set values, which eventually defines the final geometry of the electromigrated structures.