Tunable polarization control with self-assembled arrays of anisotropic plasmonic coaxial nanocavities.

Polarization control with nanostructures having a tunable design and allowing inexpensive large-scale fabrication is important for many nanophotonic applications. For this purpose, we developed and experimentally demonstrated nanostructured plasmonic surfaces based on hexagonal arrays of anisotropic coaxial nanocavities, which can be fabricated by a low-cost self-assembled nanosphere lithography method. Their high polarization sensitivity is achieved by engineering anisotropy of the coaxial nanocavities, while the optical response is enhanced by the excitation of surface plasmon resonances. Particularly, varying the geometrical parameters of the coaxial nanocavities, namely the height and tilt angle of their central core nanoellipsoids, the plasmonic resonance wavelengths as well as the polarization-selective behavior can be individually tuned in the entire visible and near-infrared spectral regions, which makes such nanostructures good candidates for the implementation of polarization-controlled optical switches and polarization-tunable filters. Moreover, the developed nanostructures demonstrate sensitivity up to 1335 nm/RIU in refractive index sensing.

Molecular Plasmonics with Metamaterials.

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Broadband Transient Response and Wavelength-Tunable Photoacoustics in Plasmonic Hetero-nanoparticles.

The optically driven acoustic modes and nonlinear response of plasmonic nanoparticles are important in many applications, but are strongly resonant, which restricts their excitation to predefined wavelengths. Here, we demonstrate that multilayered spherical plasmonic hetero-nanoparticles, formed by alternating layers of gold and silica, provide a platform for a broadband nonlinear optical response from visible to near-infrared wavelengths. They also act as a tunable optomechanical system with mechanically decoupled layers in which different acoustic modes can be selectively switched on/off by tuning the excitation wavelength. These observations not only expand the knowledge about the internal structure of composite plasmonic nanoparticles but also allow for an additional degree of freedom for controlling their nonlinear optical and mechanical properties.

Waveguide-Integrated Light-Emitting Metal-Insulator-Graphene Tunnel Junctions.

Ultrafast interfacing of electrical and optical signals at the nanoscale is highly desired for on-chip applications including optical interconnects and data processing devices. Here, we report electrically driven nanoscale optical sources based on metal-insulator-graphene tunnel junctions (MIG-TJs), featuring waveguided output with broadband spectral characteristics. Electrically driven inelastic tunneling in a MIG-TJ, realized by integrating a silver nanowire with graphene, provides broadband excitation of plasmonic modes in the junction with propagation lengths of several micrometers (∼10 times larger than that for metal-insulator-metal junctions), which therefore propagate toward the junction edge with low loss and couple to the nanowire waveguide with an efficiency of ∼70% (∼1000 times higher than that for metal-insulator-metal junctions). Alternatively, lateral coupling of the MIG-TJ to a semiconductor nanowire provides a platform for efficient outcoupling of electrically driven plasmonic signals to low-loss photonic waveguides, showing potential for applications at various integration levels.

Atomically Smooth Single-Crystalline Platform for Low-Loss Plasmonic Nanocavities.

Nanoparticle-on-mirror plasmonic nanocavities, capable of extreme optical confinement and enhancement, have triggered state-of-the-art progress in nanophotonics and development of applications in enhanced spectroscopies. However, the optical quality factor and thus performance of these nanoconstructs are undermined by the granular polycrystalline metal films (especially when they are optically thin) used as a mirror. Here, we report an atomically smooth single-crystalline platform for low-loss nanocavities using chemically synthesized gold microflakes as a mirror. Nanocavities constructed using gold nanorods on such microflakes exhibit a rich structure of plasmonic modes, which are highly sensitive to the thickness of optically thin (down to ∼15 nm) microflakes. The microflakes endow nanocavities with significantly improved quality factor (∼2 times) and scattering intensity (∼3 times) compared with their counterparts based on deposited films. The developed low-loss nanocavities further allow for the integration with a mature platform of fiber optics, opening opportunities for realizing nanocavity-based miniaturized photonic devices for practical applications.

Dual-mode independent detection of pressure and refractive index by miniature grating-coupled surface plasmon sensor.

Multiple parameters need to be monitored to analyze the kinetics of biological progresses. Surface plasmon polariton resonance sensors offer a non-invasive approach to continuously detect the local change of refractive index of molecules with high sensitivity. However, the fabrication of miniaturized, compact, and low-cost sensors is still challenging. In this paper, we propose and demonstrate a grating-coupled SPR sensor platform featuring dual mode operation for simultaneous sensing of pressure and refractive index, which can be fabricated using a highly-efficient low-cost method, allowing large-scale production. Both sensing functionalities are realized by optical means via monitoring the spectral positions of a surface plasmon polariton mode (for refractive index sensing) and Fabry-Perot or metal-insulator-metal modes (for pressure sensing), which are supported by the structure. Simultaneous measurement of refractive index with the sensitivity of 494 nm/RIU and pressure was demonstrated experimentally. The proposed platform is promising for biomonitoring that requires both high refractive index sensitivity and local pressure detection.

Optoelectronic Synapses Based on Hot-Electron-Induced Chemical Processes.

Highly efficient information processing in the brain is based on processing and memory components called synapses, whose output is dependent on the history of the signals passed through them. Here, we have developed an artificial synapse with both electrical and optical memory effects using chemical transformations in plasmonic tunnel junctions. In an electronic implementation, the electrons tunneled into plasmonic nanorods under a low bias voltage are harvested to write information into the tunnel junctions via hot-electron-mediated chemical reactions with the environment. In an optical realization, the information can be written by an external light illumination to excite hot electrons in the plasmonic nanorods. The stored information is nonvolatile and can be read either electrically or optically by measuring the resistance or inelastic-tunneling-induced light emission, respectively. The described architecture provides a high density (∼1010 cm-2) of memristive optoelectronic devices which can be used as multilevel nonvolatile memory, logic units, or artificial synapses in future electronic, optoelectronic, and artificial neural networks.

Plasmonic Metamaterials for Nanochemistry and Sensing.

Plasmonic nanostructures were initially developed for sensing and nanophotonic applications but, recently, have shown great promise in chemistry, optoelectronics, and nonlinear optics. While smooth plasmonic films, supporting surface plasmon polaritons, and individual nanostructures, featuring localized surface plasmons, are easy to fabricate and use, the assemblies of nanostructures in optical antennas and metamaterials provide many additional advantages related to the engineering of the mode structure (and thus, optical resonances in the given spectral range), field enhancement, and local density of optical states required to control electronic and photonic interactions. Focusing on two of the many applications of plasmonic metamaterials, in this Account, we review our work on the sensing and nanochemistry applications of metamaterials based on the assemblies of plasmonic nanorods under optical, as well as electronic interrogation. Sensors are widely employed in modern technology for the detection of events or changes in their local environment. Compared to their electronic counterparts, optical sensors offer a combination of high sensitivity, fast response, immunity to electromagnetic interference, and provide additional options for signal retrieval, such as optical intensity, spectrum, phase, and polarization. Owing to the ability to confine and enhance electromagnetic fields on subwavelength scales, plasmonics has been attracting increasing attention for the development of optical sensors with advantages including both nanometer-scale spatial resolution and single-molecule sensitivity. Inherent hot-electron generation in plasmonic nanostructures under illumination or during electron tunneling in the electrically biased nanostructures provides further opportunities for sensing and stimulation of chemical reactions, which would otherwise not be energetically possible. We first provide a brief introduction to a metamaterial sensing platform based on arrays of strongly coupled plasmonic nanorods. Several prototypical sensing examples based on this versatile metamaterial platform are presented. Record-high refractive index sensitivity of gold nanorod arrays in biosensing based on the functionalization of the nanorod surface for selective absorption arises because of the modification of the electromagnetic coupling between the nanorods in the array. The capabilities of nanorod metamaterials for ultrasound and hydrogen sensing were demonstrated by precision coating of the nanorods with functional materials to create core-shell nanostructures. The extension of this metamaterial platform to nanotube and nanocavity arrays, and metaparticles provides additional flexibility and removes restrictions on the illumination configurations for the optical interrogation. We then discuss a nanochemical platform based on the electrically driven metamaterials to stimulate and detect chemical reactions in the tunnel junctions constructed with the nanorods by exploiting elastic tunneling for the activation of chemical reactions via generated hot-electrons and inelastic tunneling for the excitation of plasmons facilitating optical monitoring of the process. This represents a new paradigm merging electronics, plasmonics, photonics and chemistry at the nanoscale, and creates opportunities for a variety of practical applications, such as hot-electron-driven nanoreactors and high-sensitivity sensors, as well as nanoscale light sources and modulators. With a combination of merits, such as the ability to simultaneously support both localized and propagating modes, nanoporous texture, rapid and facile functionalization, and low cost and scalability, plasmonic nanorod metamaterials provide an attractive and versatile platform for the development of optical sensors and nanochemical platforms using hot-electrons with high performance for applications in fundamental research and chemical and pharmaceutical industries.

Optimizing hot carrier effects in Pt-decorated plasmonic heterostructures.

Hot carrier generation by light in various semiconductors and metallic nanostructures is important for many photocatalytic and photochemical processes, including water and hydrogen splitting. Here, we report on investigations of hot electron generation and extraction from Pt decorated SiO2-Au nanoparticles using the degradation of methylene blue dye as a test-bed. Enhanced catalytic activity was found with an increase of Pt loading on the surface of the heterostructures. The small size of the Au nanoparticles (∼12 nm) decorating the silica nanoparticles reduces hot electron collisions and related thermalization processes, since charge carriers have short paths to the surface where reactions take place and where Pt is situated. The heterostructures exhibit a broad plasmonic resonance in the visible wavelength range from 500 to 700 nm and hot carrier generation predominately takes place under resonant excitation. Electron-microscopy characterization and numerical modelling have allowed the optimization of Pt coverage for hot-electron transfer, consisting of a thin Pt shell covering the Au nanoparticle with Pt nanoparticles additionally placed on top. This geometry provides an increased number of active sites for methylene blue degradation and promotes separation of charge carriers generated by plasmonic excitations in Au. Such SiO2-Au-Pt nanoparticles are attractive for hot-electron production due to the tunability of their plasmonic resonance and enhanced catalytic activity.

Nanocone-based plasmonic metamaterials.

Metamaterials and metasurfaces provide unprecedented opportunities for designing light-matter interactions. Optical properties of hyperbolic metamaterials with meta-atoms based on plasmonic nanorods, important in nonlinear optics, sensing and spontaneous emission control, can be tuned by varying geometrical sizes and arrangement of the meta-atoms. At the same time the role of the shape of the meta-atoms forming the array has not been studied. We present the fabrication and optical characterization of metamaterials based on arrays of plasmonic nanocones closely packed at the subwavelength scale. The plasmonic mode structure of the individual nanocones and pronounced coupling effects between them provide multiple degrees of freedom to engineer both the field enhancement and the optical properties of the resulting metamaterials. The metamaterials are fabricated using a scalable manufacturing procedure, allowing mass-production at the centimeter scale. The ultra-sharp cone apex ([Formula: see text]2 nm) and the associated field enhancement provide an extremely high density of electromagnetic hot-spots (∼1010 cm-2). These properties of nanocone-based metamaterials are important for the development of gradient-index metamaterials and in numerous applications in fluorescence enhancement, surface enhanced Raman spectroscopy as well as hot-carrier plasmonics and photocatalysis.

Hyperbolic metamaterial antenna for second-harmonic generation tomography.

The detection and processing of information carried by evanescent field components are key elements for subwavelength optical microscopy as well as single molecule sensing applications. Here, we numerically demonstrate the potential of a hyperbolic medium in the design of an efficient metamaterial antenna enabling detection and tracking of a nonlinear object, with an otherwise hidden second-harmonic signature. The presence of the antenna provides 103-fold intensity enhancement of the second harmonic generation (SHG) from a nanoparticle through a metamaterial-assisted access to evanescent second-harmonic fields. Alternatively, the observation of SHG from the metamaterial itself can be used to detect and track a nanoparticle without a nonlinear response. The antenna allows an optical resolution of several nanometers in tracking the nanoparticle's location via observations of the far-field second-harmonic radiation pattern.

Looking into meta-atoms of plasmonic nanowire metamaterial.

Nanowire-based plasmonic metamaterials exhibit many intriguing properties related to the hyperbolic dispersion, negative refraction, epsilon-near-zero behavior, strong Purcell effect, and nonlinearities. We have experimentally and numerically studied the electromagnetic modes of individual nanowires (meta-atoms) forming the metamaterial. High-resolution, scattering-type near-field optical microscopy has been used to visualize the intensity and phase of the modes. Numerical and analytical modeling of the mode structure is in agreement with the experimental observations and indicates the presence of the nonlocal response associated with cylindrical surface plasmons of nanowires.

Gradient-induced long-range optical pulling force based on photonic band gap.

Optical pulling provides a new degree of freedom in optical manipulation. It is generally believed that long-range optical pulling forces cannot be generated by the gradient of the incident field. Here, we theoretically propose and numerically demonstrate the realization of a long-range optical pulling force stemming from a self-induced gradient field in the manipulated object. In analogy to potential barriers in quantum tunnelling, we use a photonic band gap design in order to obtain the intensity gradients inside a manipulated object placed in a photonic crystal waveguide, thereby achieving a pulling force. Unlike the usual scattering-type optical pulling forces, the proposed gradient-field approach does not require precise elimination of the reflection from the manipulated objects. In particular, the Einstein-Laub formalism is applied to design this unconventional gradient force. The magnitude of the force can be enhanced by a factor of up to 50 at the optical resonance of the manipulated object in the waveguide, making it insensitive to absorption. The developed approach helps to break the limitation of scattering forces to obtain long-range optical pulling for manipulation and sorting of nanoparticles and other nano-objects. The developed principle of using the band gap to obtain a pulling force may also be applied to other types of waves, such as acoustic or water waves, which are important for numerous applications.

Large area single crystal gold of single nanometer thickness for nanophotonics.

Two-dimensional single crystal metals, in which the behavior of highly confined optical modes is intertwined with quantum phenomena, are highly sought after for next-generation technologies. Here, we report large area (>104 µm2), single crystal two-dimensional gold flakes (2DGFs) with thicknesses down to a single nanometer level, employing an atomic-level precision chemical etching approach. The decrease of the thickness down to such scales leads to the quantization of the electronic states, endowing 2DGFs with quantum-confinement-augmented optical nonlinearity, particularly leading to more than two orders of magnitude enhancement in harmonic generation compared with their thick polycrystalline counterparts. The nanometer-scale thickness and single crystal quality makes 2DGFs a promising platform for realizing plasmonic nanostructures with nanoscale optical confinement. This is demonstrated by patterning 2DGFs into nanoribbon arrays, exhibiting strongly confined near infrared plasmonic resonances with high quality factors. The developed 2DGFs provide an emerging platform for nanophotonic research and open up opportunities for applications in ultrathin plasmonic, optoelectronic and quantum devices.

Self-induced torque in hyperbolic metamaterials.

Optical forces constitute a fundamental phenomenon important in various fields of science, from astronomy to biology. Generally, intense external radiation sources are required to achieve measurable effects suitable for applications. Here we demonstrate that quantum emitters placed in a homogeneous anisotropic medium induce self-torques, aligning themselves in the well-defined direction determined by an anisotropy, in order to maximize their radiation efficiency. We develop a universal quantum-mechanical theory of self-induced torques acting on an emitter placed in a material environment. The theoretical framework is based on the radiation reaction approach utilizing the rigorous Langevin local quantization of electromagnetic excitations. We show more than 2 orders of magnitude enhancement of the self-torque by an anisotropic metamaterial with hyperbolic dispersion, having negative ratio of permittivity tensor components, in comparison with conventional anisotropic crystals with the highest naturally available anisotropy.

Surface plasmon polariton amplification upon electrical injection in highly integrated plasmonic circuits.

We propose a very efficient approach for amplification of surface plasmon polaritons (SPPs) in a nanoscale waveguiding geometry with strong (∼λ/10) mode confinement. The implemented scheme of electric pumping is based on a single-heterostructure Schottky-barrier diode and has been numerically shown to ensure full compensation of the SPP propagation losses at wavelengths around 3 µm and, moreover, to provide net SPP gain. The presented concept creates the backbone for the implementation of highly integrated large-scale hybrid electronic-plasmonic circuits operating at extremely high speeds and opens the prospects for the realization of integrated coherent SPP sources.

Multimode hybrid gold-silicon nanoantennas for tailored nanoscale optical confinement.

High-index dielectric nanoantennas, which provide an interplay between electric and magnetic modes, have been widely used as building blocks for a variety of devices and metasurfaces, both in linear and nonlinear regimes. Here, we investigate hybrid metal-semiconductor nanoantennas, consisting of a multimode silicon nanopillar core coated with a gold layer, that offer an enhanced degree of control over the mode selection and confinement, and emission of light on the nanoscale exploiting high-order electric and magnetic resonances. Cathodoluminescence spectra revealed a multitude of resonant modes supported by the nanoantennas due to hybridization of the Mie resonances of the core and the plasmonic resonances of the shell. Eigenmode analysis revealed the modes that exhibit enhanced field localization at the gold interface, together with high confinement within the nanopillar volume. Consequently, this architecture provides a flexible means of engineering nanoscale components with tailored optical modes and field confinement for a plethora of applications, including sensing, hot-electron photodetection and nanophotonics with cylindrical vector beams.

All-plasmonic modulation via stimulated emission of copropagating surface plasmon polaritons on a substrate with gain.

We experimentally demonstrate suppressed absorption and stimulated emission of surface plasmon polaritons (SPP) leading to all-plasmonic modulation of an SPP signal propagating at the interface between a metal and a gain medium; these observations are supported by the developed theory. The use of copropagating signal and control waves can provide more than 10 times more efficient SPP stimulated emission compared to out-of-plane pumping and opens up the possibility to realize integratable plasmonic components for active nanophotonic circuitry.

Optically-programmable nonlinear photonic component for dielectric-loaded plasmonic circuitry.

We demonstrate both experimentally and numerically a compact and efficient, optically tuneable plasmonic component utilizing a surface plasmon polariton ring resonator with nonlinearity based on trans-cis isomerization in a polymer material. We observe more than 3-fold change between high and low transmission states of the device at milliwatt control powers (∼100 W/cm2 by intensity), with the performance limited by switching speed of the material. Such plasmonic components can be employed in optically programmable and reconfigurable integrated photonic circuitry.

Guiding light at the nanoscale: numerical optimization of ultrasubwavelength metallic wire plasmonic waveguides.

We present a comprehensive numerical analysis of the guiding of a photonic signal in the form of a strongly confined asymmetric surface plasmon polariton (SPP) mode along metallic nanowire waveguides. The proposed approach provides extremely high localization of the SPP mode, nanoscale integration density, and a feasible technological platform. The waveguide performance was studied over a broad range of subwavelength cross sections at a telecommunication wavelength. It was optimized using a conventional figure of merit for data transfer along a straight waveguide and an all-inclusive figure of merit has been introduced.