Exciton-plasmon-photon conversion in plasmonic nanostructures

Diffractionless transmission of optical beams through a four-level atomic system affected by a plasmonic nanostructure

A nondiffracting propagation of an optical beam through a four-level double-V-type quantum system near a plasmonic nanostructure is investigated. We study the linear absorption and dispersion properties of the quantum system as it interacts with two laser fields. We discuss the effect of the control beam with a Laguerre-Gaussian (LG) profile on the focusing of the probe beam in the presence of a plasmonic nanostructure. An appropriately selected control beam excites one transition of the atomic system and generates a spatially varying refraction index modulation for a weak probe beam that couples to the other transition. We demonstrate that placing a plasmonic nanostructure at a nanometer distance from the atomic system and using a control field with the spatial structure leads to the diffraction-less propagation of the probe beam through the atomic system. Also, it is shown that the optical properties and probe beam focusing can be controlled by adjusting the distance of the plasmonic nanostructure from the atomic system. The proposed all-optical waveguide with high contrast and transmission can be used to implement applications such as image transfer through the medium and image processing.

Single-Molecule Fluorescence Probes Interactions between Photoactive Protein-Silver Nanowire Conjugate and Monolayer Graphene

In this work, we apply single-molecule fluorescence microscopy and spectroscopy to probe plasmon-enhanced fluorescence and Förster resonance energy transfer in a nanoscale assemblies. The structure where the interplay between these two processes was present consists of photoactive proteins conjugated with silver nanowires and deposited on a monolayer graphene. By comparing the results of continuous-wave and time-resolved fluorescence microscopy acquired for this structure with those obtained for the reference samples, where proteins were coupled with either a graphene monolayer or silver nanowires, we find clear indications of the interplay between plasmonic enhancement and the energy transfer to graphene. Namely, fluorescence intensities calculated for the structure, where proteins were coupled to graphene only, are less than for the structure playing the central role in this study, containing both silver nanowires and graphene. Conversely, decay times extracted for the latter are shorter compared to a protein-silver nanowire conjugate, pointing towards emergence of the energy transfer. Overall, the results show that monitoring the optical properties of single emitters in a precisely designed hybrid nanostructure provides an elegant way to probe even complex combination of interactions at the nanoscale.

Recent progress of exciton transport in two-dimensional semiconductors

Spatial manipulation of excitonic quasiparticles, such as neutral excitons, charged excitons, and interlayer excitons, in two-dimensional semiconductors offers unique capabilities for a broad range of optoelectronic applications, encompassing photovoltaics, exciton-integrated circuits, and quantum light-emitting systems. Nonetheless, their practical implementation is significantly restricted by the absence of electrical controllability for neutral excitons, short lifetime of charged excitons, and low exciton funneling efficiency at room temperature, which remain a challenge in exciton transport. In this comprehensive review, we present the latest advancements in controlling exciton currents by harnessing the advanced techniques and the unique properties of various excitonic quasiparticles. We primarily focus on four distinct control parameters inducing the exciton current: electric fields, strain gradients, surface plasmon polaritons, and photonic cavities. For each approach, the underlying principles are introduced in conjunction with its progression through recent studies, gradually expanding their accessibility, efficiency, and functionality. Finally, we outline the prevailing challenges to fully harness the potential of excitonic quasiparticles and implement practical exciton-based optoelectronic devices.

Exploring plasmonic effect on exciton transport: A theoretical insight from macroscopic quantum electrodynamics

Exciton transport in extended molecular systems and how to manipulate such transport in a complex environment are essential to many energy and optical-related applications. We investigate the mechanism of plasmon-coupled exciton transport by using the Pauli master equation approach, combined with kinetic rates derived from macroscopic quantum electrodynamics. Through our theoretical framework, we demonstrate that the presence of a silver nanorod induces significant frequency dependence in the ability of transporting exciton through a molecule chain, indicated by the exciton diffusion coefficient, due to the dispersive nature of the silver dielectric response. Compared with the same system in vacuum, great enhancement (up to a factor of 103) in the diffusion coefficient can be achieved by coupling the resonance energy transfer process to localized surface plasmon polariton modes of the nanorod. Furthermore, our analysis reveals that the diffusion coefficients with the nearest-neighbor coupling approximation are ∼10 times smaller than the results obtained beyond this approximation, emphasizing the significance of long-range coupling in exciton transport influenced by plasmonic nanostructures. This study not only paves the way for exploring practical approaches to study plasmon-coupled exciton transport but also provides crucial insights for the design of innovative plasmon-assisted photovoltaic applications.

Superplastic Nanomolding of Aluminum Waveguides for Subwavelength Light Routing, Splitting, and Encryption

Plasmonic nanowires (NWs) due to their polarization-dependent optics and enhanced light-matter interactions have presented vibrant capabilities in functional nanophotonic devices. However, current demonstrations have largely been based on chemically synthesized Ag NWs, which are extremely unstable and poorly functional. Here we show single-crystalline Al NWs can be fabricated by a superplastic nanomolding (SPNM) technique on a centimeter scale, which are earth-abundant and highly stable. They present robust properties of multimode waveguiding with long-term stability, high efficiency of beam splitting in response to the polarization, and durable thermal optical modulation, which can be readily applied as nanophotonic routers, splitters, and information encryptors. Moreover, this SNPM technique is extendable to other metals, which are highly exploitable for functional nanophotonic devices and integrated optical chips.

Investigating valley-dependent current generation due to asymmetric energy dispersion for charge-transfer from a quantum dot to single-walled carbon nanotube

Single-wall carbon nanotubes (SWCNT), which consist of a two-dimensional hexagonal lattice of carbon atoms, possess unique mechanical, electrical, optical and thermal properties. SWCNT can be synthesized in diverse chiral indexes to determine certain attributes. This work theoretically investigates electron transport in different directions along SWCNT. The electron studied in this research transfers from the quantum dot that can possibly move to the right or left direction in SWCNT with different valley-dependent probability. These results show that valley polarized current is present. The valley current in the right and left directions has a composition of valley degrees of freedom where its components (K and K') are not identical. Such a result can be traced theoretically by certain effects. That firstly is the curvature effect on SWCNT in which the hopping integral between [Formula: see text] electrons from the flat graphene is altered, and another is curvature-inducing [Formula: see text] mixture. Due to these effects, the band structure of SWCNT is asymmetric in certain chiral indexes leading to the asymmetry of valley electron transport. Our results exhibit that the zigzag chiral indexes is the only type making electron transport symmetrical that is different to the result from the other chiral index types which are the armchair and chiral. This work also illustrates the characteristic of the electron wave function propagating from the initial point to the tip of the tube over time, and the current density of the probability in specific times. Additionally, our research simulates the result from the dipole interaction between the electron in QD and the tube that impacts the lifetime of the electron being in QD. The simulation portrays that more dipole interaction encourages the electron transfer to the tube, thereby shortening the lifetime. We as well suggest the reversed electron transfer from the tube to QD that the time duration of such transfer is much less than the opposite transfer owing to the different orbital of the electron's states. Valley polarized current in SWCNTs may also be used in the development of energy storage devices such as batteries and supercapacitors. The performance and effectiveness of nanoscale devices, including transistors, solar cells, artificial antennas, quantum computers, and nano electronic circuits, must be improved in order to achieve a variety of benefits.

Hyperbolic plasmonics with anisotropic gain-loss metasurfaces

In this Letter, a fundamentally new concept of realization of hyperbolic plasmonic metasurfaces by anisotropic gain-loss competition is proposed, and the possibility of highly directional propagation and amplification of surface plasmon polaritons is predicted. A simple realistic configuration of such a metasurface represents the periodic array of lossy metallic slabs embedded in the gain matrix. Our results may pave the way for numerous applications ranging from integrated and highly directional quantum light emitters to nonlinear-optical frequency converters.

Dielectric Nanowire Hybrids for Plasmon-Enhanced Light-Matter Interaction in 2D Semiconductors

Monolayer transition metal dichalcogenides (TMDs) with a direct band gap are suitable for various optoelectronic applications such as ultrathin light emitters and absorbers. However, their weak light absorption caused by the atomically thin layer hinders more versatile applications for high optical gains. Although plasmonic hybridization with metal nanostructures significantly enhances light-matter interactions, the corrosion, instability of the metal nanostructures, and the undesired effects of direct metal-semiconductor contact act as obstacles to its practical application. Herein, we propose a dielectric nanostructure for plasmon-enhanced light-matter interaction of TMDs. TiO₂ nanowires (NWs), as an example, are hybridized with a MoS₂ monolayer on various substrates. The structure is implemented by placing a monolayer MoS₂ between a TiO₂ NW for a photonic scattering effect and metallic substrates with a spacer for the plasmonic Purcell effect. Here, the thin dielectric spacer is aimed at minimizing emission quenching from direct metal contact, while maximizing optical field localization in ultrathin MoS₂ near the TiO₂ NW. An effective emission enhancement factor of ∼22 is attained for MoS₂ near the NW of the hybrid structure compared to the one without NWs. Our work is expected to facilitate a hybridized platform based on 2D semiconductors for high-performance and robust optoelectronics via engineering dielectric nanostructures with plasmonic materials.

Photothermal Imaging of Individual Nano-Objects with Large Scattering Cross Sections

Photothermal (PT) microscopy enables the efficient detection of absorbing nano-objects with high sensitivity and stability. The PT signal in the current PT microscopy usually comes from the interaction of the probe laser beam with the heating laser beam-induced thermal lens, and the contribution of the scattering field from the imaged nano-object is usually not taken into account. Here, in this paper, we systematically studied the influence of the scattering field from the imaged nanoparticles on the obtained PT signal by using Ag nanowires (NWs) on a glass substrate surrounded by glycerol as an example. Under the excitation of a heating laser beam at 532 nm wavelength, the rise of local temperature around the Ag NW results in the intensity variation of the interferometric scattering probe light at 730 nm wavelength which includes the scattering light from the Ag NW and the reflection light from the glass-glycerol interface. We found that the PT signal on the NW are positive and negative for the probe beam polarized parallel and perpendicular to the NW axis, respectively. Numerical simulations confirm that the heat-induced intensity variation of the pure scattering light from the NW and the thermal lens-induced intensity increase of the reflection light both contribute to the obtained PT signal. Our work provides the basic guidance for the analysis of PT signal from nano-objects with large scattering cross sections.

Plasmonics with Metallic Nanowires

The purpose of this review is to introduce and present the concept of metallic nanowires as building-blocks of plasmonically active structures. In addition to concise description of both the basic physical properties associated with the electron oscillations as well as energy propagation in metallic nanostructures, and methods of fabrication of metallic nanowires, we will demonstrate several key ideas that involve interactions between plasmon excitations and electronic states in surrounding molecules or other emitters. Particular emphasis will be placed on the effects that involve not only plasmonic enhancement or quenching of fluorescence, but also propagation of energy on lengths that exceed the wavelength of light.

Silver nanowires with optimized silica coating as versatile plasmonic resonators

Metal nanoparticles are the most frequently used nanostructures in plasmonics. However, besides nanoparticles, metal nanowires feature several advantages for applications. Their elongation offers a larger interaction volume, their resonances can reach higher quality factors, and their mode structure provides better coupling into integrated hybrid dielectric-plasmonic circuits. It is crucial though, to control the distance of the wire to a supporting substrate, to another metal layer or to active materials with sub-nanometer precision. A dielectric coating can be utilized for distance control, but it must not degrade the plasmonic properties. In this paper, we introduce a controlled synthesis and coating approach for silver nanowires to fulfill these demands. We synthesize and characterize silver nanowires of around 70 nm in diameter. These nanowires are coated with nm-sized silica shells using a modified Stöber method to achieve a homogeneous and smooth surface quality. We use transmission electron microscopy, dark-field microscopy and electron-energy loss spectroscopy to study morphology and plasmonic resonances of individual nanowires and quantify the influence of the silica coating. Thorough numerical simulations support the experimental findings showing that the coating does not deteriorate the plasmonic properties and thus introduce silver nanowires as usable building blocks for integrated hybrid plasmonic systems.

Polymer spacer tunable Purcell-enhanced spontaneous emission in perovskite quantum dots coupled to plasmonic nanowire networks

Bright and fast emission of perovskite quantum dots has been demonstrated by using a polymer spacer to regulate the exciton–plasmon coupling.

Mimicking plasmonic nanolaser emission by selective extraction of electromagnetic near-field from photonic microcavity

Plasmonic nanolasers have attracted significant attention owing to their ability to generate a coherent optical field in the deep subwavelength region, and they exhibit promising applications in integrated photonics, bioimaging and sensing. However, the demonstration of lasing in individual metallic nanoparticles with 3D subwavelength confinement represents a significant challenge and is yet to be realized. Herein, we propose to mimic a plasmonic nanolaser via selective scattering off the evanescent tail of a lasing photonic nanobelt using a single silver nanorod (24 nm × 223 nm). The nanorod acts as an optical antenna that selectively extracts the near-field component along the rod axis. The light output from the silver nanorod mimics the emission of a plasmonic nanolaser in its localized near-field and polarization dependence, except for the lasing wavelength and linewidth, which are inherited from the photonic laser. The realization of localized coherent light sources provides promising nanoscale lighting that shows potential in background-suppressed illumination, biosensing and imaging.

Plasmon-Assisted Selective and Super-Resolving Excitation of Individual Quantum Emitters on a Metal Nanowire

Hybrid systems composed of multiple quantum emitters coupled with plasmonic waveguides are promising building blocks for future integrated quantum nanophotonic circuits. The techniques that can super-resolve and selectively excite contiguous quantum emitters in a diffraction-limited area are of great importance for studying the plasmon-mediated interaction between quantum emitters and manipulating the single plasmon generation and propagation in plasmonic circuits. Here we show that multiple quantum dots coupled with a silver nanowire can be controllably excited by tuning the interference field of surface plasmons on the nanowire. Because of the period of the interference pattern is much smaller than the diffraction limit, we demonstrate the selective excitation of two quantum dots separated by a distance as short as 100 nm. We also numerically demonstrate a new kind of super-resolution imaging method that combines the tunable surface plasmon interference pattern on the NW with the structured illumination microscopy technique. Our work provides a novel high-resolution optical excitation and imaging method for the coupled systems of multiple quantum emitters and plasmonic waveguides, which adds a new tool for studying and manipulating single quantum emitters and single plasmons for quantum plasmonic circuitry applications.

Subwavelength core/shell cylindrical nanostructures for novel plasmonic and metamaterial devices

In this review, we introduce novel plasmonic and metamaterial devices based on one-dimensional subwavelength nanostructures with cylindrical symmetry. Individual single devices with semiconductor/metal core/shell or dielectric/metal core/multi-shell structures experience strong light-matter interaction and yield unique optical properties with a variety of functions, e.g., invisibility cloaking, super-scattering/super-absorption, enhanced luminescence and nonlinear optical activities, and deep subwavelength-scale optical waveguiding. We describe the rational design of core/shell cylindrical nanostructures and the proper choice of appropriate constituent materials, which allow the efficient manipulation of electromagnetic waves and help to overcome the limitations of conventional homogeneous nanostructures. The recent developments of bottom-up synthesis combined with the top-down fabrication technologies for the practical applications and the experimental realizations of 1D subwavelength core/shell nanostructure devices are briefly discussed.

Orientation-Dependent Exciton-Plasmon Coupling in Embedded Organic/Metal Nanowire Heterostructures

The excitation of surface plasmons by optical emitters based on exciton-plasmon coupling is important for plasmonic devices with active optical properties. It has been theoretically demonstrated that the orientation of exciton dipole can significantly influence the coupling strength, yet systematic study of the coupling process in nanostructures is still hindered by the lack of proper material systems. In this work, we have experimentally investigated the orientation-dependent exciton-plasmon coupling in a rationally designed organic/metal nanowire heterostructure system. The heterostructures were prepared by inserting silver nanowires into crystalline organic waveguides during the self-assembly of dye molecules. Structures with different exciton orientations exhibited varying coupling efficiencies. The near-field exciton-plasmon coupling facilitates the design of nanophotonic devices based on the directional surface plasmon polariton propagations.

Active Mediation of Plasmon Enhanced Localized Exciton Generation, Carrier Diffusion and Enhanced Photon Emission

Understanding the enhancement of charge carrier generation and their diffusion is imperative for improving the efficiency of optoelectronic devices particularly infrared photodetectors that are less developed than their visible counterpart. Here, using gold nanorods as model plasmonic systems, InAs quantum dots (QDs) embedded in an InGaAs quantum well as an emitter, and GaAs as an active mediator of surface plasmons for enhancing carrier generation and photon emission, the distance dependence of energy transfer and carrier diffusion have been investigated both experimentally and theoretically. Analysis of the QD emission enhancement as a function of distance reveals a Förster radius of 3.85 ± 0.15 nm, a near-field decay length of 4.8 ± 0.1 nm and an effective carrier diffusion length of 64.0 ± 3.0 nm. Theoretical study of the temporal-evolution of the electron-hole occupation number of the excited states of the QDs indicates that the emission enhancement trend is determined by the carrier diffusion and capture rates.

Tailoring MoS2 Exciton-Plasmon Interaction by Optical Spin-Orbit Coupling

Molybdenum disulfide (MoS₂) monolayer as one of the atomic thickness two-dimensional materials has remarkable electronic and optical properties, which is an ideal candidate for a wide range of optoelectronic applications. However, the atomic monolayer thickness poses a significant challenge in MoS₂ photoluminescence emission due to weak light-matter interaction. Here, we investigate the MoS₂ exciton-plasmon interaction with spin-orbit coupling of light. The plasmonic spiral rings with subwavelength dimensions are designed and fabricated on hybrid substrates. MoS₂ photoluminescence enhancement can be actively controlled by changing the incident optical spin states, laser powers, and the nanospiral geometries, which is arising from the change of field enhancement at near-field region. Planar light-emitting devices based on spin-orbit coupling (SOC) effect were further realized and flexibly controlled by changing the polarization of light. The SOC effect is discussed by the accumulation of geometric and dynamic phases, which can be demonstrated and elaborated by the Majorana sphere model. Our results provide a way to manipulate MoS₂ light-matter interaction actively and can be further applied in the spin-dependent light-emitting devices at the nanoscale.

Direction-resolved radiation from polarization-controlled surface plasmon modes on silver nanowire antennas

Metallic nanowires (NWs) support multiple surface plasmon (SP) modes, which lead to extraordinary SP propagation behaviors. The leaky SP modes in metallic NWs connect the guiding and radiation of light at the nanometer scale. Understanding and controlling these modes are of vital importance for various nanophotonic applications. Here, we investigate the radiation from two polarization-controlled SP modes on supported silver NWs by using leakage radiation imaging and Fourier imaging techniques. The radiation directions from these modes can be clearly resolved from the Fourier images. The radiation polarization of the SP modes is related to the polarization of the excitation light. By depositing thin Al₂O₃ films onto silver NWs or decreasing the excitation wavelength, the radiation angles and wave vectors of the two modes are increased, and the longitudinal mode is more sensitive to Al₂O₃ thickness. Moreover, the propagation length of the longitudinal mode is obtained by analyzing the leakage radiation images, which is decreased with the decrease of the excitation wavelength and the increase of the Al₂O₃ layer thickness. These results show that leakage radiation from different SP modes on silver NWs can be resolved directly and controlled effectively. The supported silver NWs can thus be applied to designing plasmonic circuits, nanoantennas and nanosensors.

Scattering of nanowire surface plasmons coupled to quantum dots with azimuthal angle difference

Coherent scatterings of surface plasmons coupled to quantun dots have attracted great attention in plasmonics. Recently, an experiment has shown that the quantum dots located nearby a nanowire can be separated not only in distance, but also an angle ϕ along the cylindrical direction. Here, by using the real-space Hamiltonian and the transfer matrix method, we analytically obtain the transmission/reflection spectra of nanowire surface plasmons coupled to quantum dots with an azimuthal angle difference. We find that the scattering spectra can show completely different features due to different positions and azimuthal angles of the quantum dots. When additionally coupling a cavity to the dots, we obtain the Fano-like line shape in the transmission and reflection spectra due to the interference between the localized and delocalized modes.

Control of the two-photon fluorescence of quantum dots coupled to silver nanowires

Plasmon-based fluorescence modulation has led to important advances in various fields and has paved the way toward promising scientific research aimed at enabling new applications. However, the modulation of fluorescence properties based on both localized surface plasmon (LSP) and cavity modes of propagating surface plasmon polaritons (SPPs) are rarely reported. Here, we raster scanned a hybrid nanowire (HNW) with quantum dots (QDs) adsorbed onto a Ag nanowire (NW) and obtained two-photon fluorescence (TPF) maps of the intensity and decay rate. The spatial distributions of the intensity and decay rate strongly depend on the Fabry-Pérot (FP) cavity modes of the SPPs, the LSP mode launched by the incident laser and the excitation energy of the QDs. A double exponential decay process was observed, which is attributed to different decay channels through the LSP and cavity modes. The experimental results are explained using numerical simulations. This work shows that many physical parameters, such as the polarization of the incident beam and the geometry of the Ag NW, can modulate the fluorescence properties of the QDs, which has potential applications in many important fields.

Giant Faraday Rotation of High-Order Plasmonic Modes in Graphene-Covered Nanowires

Plasmonic Faraday rotation in nanowires manifests itself in the rotation of the spatial intensity distribution of high-order surface plasmon polariton (SPP) modes around the nanowire axis. Here we predict theoretically the giant Faraday rotation for SPPs propagating on graphene-coated magneto-optically active nanowires. Upon the reversal of the external magnetic field pointing along the nanowire axis some high-order plasmonic modes may be rotated by up to ∼100° on the length scale of about 500 nm at mid-infrared frequencies. Tuning the carrier concentration in graphene by chemical doping or gate voltage allows for controlling SPP-properties and notably the rotation angle of high-order azimuthal modes. Our results open the door to novel plasmonic applications ranging from nanowire-based Faraday isolators to the magnetic control in quantum-optical applications.

Photonic Applications of Metal-Dielectric Heterostructured Nanomaterials

Metal materials, supporting plasmon modes on their surface, can confine the optical field at deep subwavelength scale, which is desired for photonic integration. However, their intrinsic high Ohmic losses make it impossible to construct the whole circuit solely with the metal materials. Integrating the plasmonic components with dielectric materials may offer a solution to this dilemma. With outstanding active optical performance, these dielectric components not only can greatly reduce the optical losses of the entire circuits but also offer an efficient way to launch the surface plasmon polaritons through the evanescent field coupling or the direct exciton-plasmon conversion. Furthermore, the cooperative interaction between metal and dielectric materials would bring vast novel optical phenomena and functional photonic devices. In this review, the synergistic effects among metal and dielectric materials in various heterostructures as well as their related applications are highlighted. Comprehensive understanding on their synergistic interactions would offer useful guidance for the design and fabrication of the ultracompact novel optical devices.

Localised excitation of a single photon source by a nanowaveguide

Nowadays, integrated photonics is a key technology in quantum information processing (QIP) but achieving all-optical buses for quantum networks with efficient integration of single photon emitters remains a challenge. Photonic crystals and cavities are good candidates but do not tackle how to effectively address a nanoscale emitter. Using a nanowire nanowaveguide, we realise an hybrid nanodevice which locally excites a single photon source (SPS). The nanowire acts as a passive or active sub-wavelength waveguide to excite the quantum emitter. Our results show that localised excitation of a SPS is possible and is compared with free-space excitation. Our proof of principle experiment presents an absolute addressing efficiency ηa ~ 10(-4) only ~50% lower than the one using free-space optics. This important step demonstrates that sufficient guided light in a nanowaveguide made of a semiconductor nanowire is achievable to excite a single photon source. We accomplish a hybrid system offering great potentials for electrically driven SPSs and efficient single photon collection and detection, opening the way for optimum absorption/emission of nanoscale emitters. We also discuss how to improve the addressing efficiency of a dipolar nanoscale emitter with our system.

Spatio-temporal second-order quantum correlations of surface plasmon polaritons

We present an experimental methodology to observe spatio-temporal second-order quantum coherence of surface plasmon polaritons which are emitted by nitrogen vacancy color centers attached at the apex of an optical tip. The approach relies on leakage radiation microscopy in the Fourier space, and we use this approach to test wave-particle duality for surface plasmon polaritons.

Quantum Yield of Single Surface Plasmons Generated by a Quantum Dot Coupled with a Silver Nanowire

The interactions between surface plasmons (SPs) in metal nanostructures and excitons in quantum emitters (QEs) lead to many interesting phenomena and potential applications that are strongly dependent on the quantum yield of SPs. The difficulty in distinguishing all the possible exciton recombination channels hinders the experimental determination of SP quantum yield. Here, we experimentally measured for the first time the quantum yield of single SPs generated by the exciton-plasmon coupling in a system composed of a single quantum dot and a silver nanowire (NW). By utilizing the SP guiding property of the NW, the decay rates of all the exciton recombination channels, i.e., direct free space radiation channel, SP generation channel, and nonradiative damping channel, are quantitatively obtained. It is determined that the optimum emitter-NW coupling distance for the largest SP quantum yield is about 10 nm, resulting from the different distance-dependent decay rates of the three channels. These results are important for manipulating the coupling between plasmonic nanostructures and QEs and developing on-chip quantum plasmonic devices for potential nanophotonic and quantum information applications.

Routing of surface plasmons in silver nanowire networks controlled by polarization and coating

Controllable propagation of electromagnetic energy in plasmonic nanowaveguides is of great importance for building nanophotonic circuits. Here, we studied the routing of surface plasmons in silver nanowire structures by combining experiments and electromagnetic simulations. The superposition of different plasmon modes results in the tunable near field patterns of surface plasmons on the nanowire. Using the quantum dot fluorescence imaging technique, we experimentally demonstrate that the near field distribution on the nanowire controls the surface plasmon transmission in the nanowire networks. By controlling the polarization of the input light or by controlling the dielectric coating on the nanowire to modulate the plasmon field distribution and guarantee the strong local field intensity at the connecting junction, the surface plasmons can be efficiently routed to the connected nanowires. Depositing a thin layer of Al2O3 film onto the nanowires can reverse the polarization dependence of the output intensity at the nanowire terminals. These results are instructive for designing functional plasmonic nanowire networks and metal-nanowire-based nanophotonic devices.

Selective Amplification of the Primary Exciton in a MoS_{2} Monolayer

Optoelectronics applications for transition-metal dichalcogenides are still limited by weak light absorption and their complex exciton modes are easily perturbed by varying excitation conditions because they are inherent in atomically thin layers. Here, we propose a method of selectively amplifying the primary exciton (A^{0}) among the exciton complexes in monolayer MoS_{2} via cyclic reexcitation of cavity-free exciton-coupled plasmon propagation. This was implemented by partially overlapping a Ag nanowire on a MoS_{2} monolayer separated by a thin SiO_{2} spacer. Exciton-coupled plasmons in the nanowire enhance the A^{0} radiation in MoS_{2}. The cumulative amplification of emission enhancement by cyclic plasmon traveling reaches approximately twentyfold selectively for the A^{0}, while excluding other B exciton and multiexciton by significantly reduced band filling, without oscillatory spectra implying plasmonic cavity effects.

A study of energy absorption rate in a quantum dot and metallic nanosphere hybrid system

We have studied energy absorption rate in a quantum dot-metallic nanosphere system embedded on a dielectric substrate. We applied a control field to induce dipole moments in the quantum dot and the metal nanosphere, and monitored the energy absorption using a probe field. These external fields induce dipole moments in the metal nanosphere and the quantum dot, and these two structures interact with one another via the dipole-dipole interaction. The density matrix method was used to evaluate the absorption, indicating that it can be shifted by moving the metal nanosphere close to the quantum dot. Also, absorption efficiency can either be quenched or enhanced by the addition of a metal nanosphere. This hybrid system can be used to create ultrafast switching and sensing nanodevices.

One-Dimensional Dielectric/Metallic Hybrid Materials for Photonic Applications

Explorations of 1D nanostructures have led to great progress in the area of nanophotonics in the past decades. Based on either dielectric or metallic materials, a variety of 1D photonic devices have been developed, such as nanolasers, waveguides, optical switches, and routers. What's interesting is that these dielectric systems enjoy low propagation losses and usually possess active optical performance, but they have a diffraction-limited field confinement. Alternatively, metallic systems can guide light on deep subwavelength scales, but they suffer from high metallic absorption and can work as passive devices only. Thus, the idea to construct a hybrid system that combines the merits of both dielectric and metallic materials was proposed. To date, unprecedented optical properties have been achieved in various 1D hybrid systems, which manifest great potential for functional nanophotonic devices. Here, the focus is on recent advances in 1D dielectric/metallic hybrid systems, with a special emphasis on novel structure design, rational fabrication techniques, unique performance, as well as their wide application in photonic components. Gaining a better understanding of hybrid systems would benefit the design of nanophotonic components aimed at optical information processing.

Multiple hybridized resonances of IR-806 chromonic molecules strongly coupled to Au nanorods

Strong coupling of plasmons and molecules generates intriguingly hybridized resonance. The IR-806 molecule is a near-infrared cyanine liquid crystal dye with multiple molecular bands and its tunable absorption spectrum varies dramatically with concentration. In this article, we investigate multiple hybridized resonances of the Au nanorods (AuNRs) strongly coupled to IR-806 molecules. Five hybridized resonance peaks are observed in the extinction spectra of the AuNR@IR-806 hybrids. Two resonance peaks at approximately 840 and 912 nm in the hybrids are reported for the first time. The dependence of the multiple hybridized peaks on the bare plasmon resonance wavelength of AuNRs and the molecular concentration is also demonstrated. The observations presented herein provide a plasmon-molecule coupling route for tuning optical responses of liquid crystal molecules.

Single photon transport along a one-dimensional waveguide with a side manipulated cavity QED system

An external mirror coupling to a cavity with a two-level atom inside is put forward to control the photon transport along a one-dimensional waveguide. Using a full quantum theory of photon transport in real space, it is shown that the Rabi splittings of the photonic transmission spectra can be controlled by the cavity-mirror couplings; the splittings could still be observed even when the cavity-atom system works in the weak coupling regime, and the transmission probability of the resonant photon can be modulated from 0 to 100%. Additionally, our numerical results show that the appearance of Fano resonance is related to the strengths of the cavity-mirror coupling and the dissipations of the system. An experimental demonstration of the proposal with the current photonic crystal waveguide technique is suggested.

The polar side of polyphenylene dendrimers

The site-specific functionalization of poly(phenylene) dendrimers can produce macromolecules with a range of different polarities.

Four-wave parametric amplification in semiconductor quantum dot-metallic nanoparticle hybrid molecules

We study theoretically four-wave parametric amplification arising from the nonlinear optical response of hybrid molecules composed of semiconductor quantum dots and metallic nanoparticles. It is shown that highly efficient four-wave parametric amplification can be achieved by adjusting the frequency and intensity of the pump field and the distance between the quantum dot and the metallic nanoparticle. Specifically, the induced probe-wave gain is tunable in a large range from 1 to 1.43 × 10⁵. This gain reaches its maximum at the position of three-photon resonance. Our findings hold great promise for developing four-wave parametric oscillators.

Control of optical properties of hybrid materials with chirped femtosecond laser pulses under strong coupling conditions

The interaction of chirped femtosecond laser pulses with hybrid materials--materials comprised of plasmon sustaining structures and resonant molecules--is scrutinized using a self-consistent model of coupled Maxwell-Bloch equations. The optical properties of such systems are examined with the example of periodic sinusoidal gratings. It is shown that under strong coupling conditions one can control light transmission using chirped pulses in a spatiotemporal manner. The temporal origin of control relies on chirps non-symmetric in time while the space control is achieved via spatial localization of electromagnetic energy due to plasmon resonances.

Tunable plasmon modes in single silver nanowire optical antennas characterized by far-field microscope polarization spectroscopy

Performing far-field microscope polarization spectroscopy and finite element method simulations, we investigated experimentally and theoretically the surface plasmon modes in single Ag nanowire antennas. Our results show that the surface plasmon resonances in the single Ag nanowire antenna can be tuned from the dipole plasmon mode to a higher order plasmon mode, which would result in the emission with different intensities and polarization states, for the semiconductor quantum dots coupled to the nanowire antenna. The fluorescence polarization is changed with different polarized excitation of the 800 nm light beam, while it remains parallel to the Ag nanowire axis at the 400 nm excitation. The 800 nm incident light interacts nonresonantly with the dipole plasmon mode with the polarized excitation parallel to the Ag nanowire axis, while it excites a higher order plasmon mode with the perpendicular excitation. Under excitation of 400 nm, either the parallel or perpendicular excitation can only result in a dipole plasmon mode. In addition, we demonstrate that the single Ag nanowire antenna can work as an energy concentrator for enhancing the two-photon excited fluorescence of semiconductor quantum dots.

Hybrid material based on plasmonic nanodisks decorated ZnO and its application on nanoscale lasers

Plasmonic noble metal nanodisks with regular (triangular or hexagonal) shapes have been epitaxially formed on ZnO nanorods' (0002) surfaces. The composite material's crystal structures, epitaxial relationships between metal nanodisks, and ZnO host crystals were fully investigated. The effects from metal nanodisks on lasing characteristics of two types of ZnO nanoscale cavities (Fabry-Perot and Whispering Gallery Mode cavity) were studied. The results suggest that metal nanodisks can effectively enhance the lasing performance by lowering the lasing threshold in the ZnO Whispering Gallery Mode nanoplate laser, whereas the Fabry-Perot ZnO nanorods lasers were much less affected by the metal decoration. The plasmonic enhancement mechanism for the ZnO nanoplate cavities was further studied using numerical simulations as well as spatially resolved photoluminescence measurement.

Imaging and steering an optical wireless nanoantenna link

Optical nanoantennas tailor the transmission and reception of optical signals. Owing to their capacity to control the direction and angular distribution of optical radiation over a broad spectral range, nanoantennas are promising components for optical communication in nanocircuits. Here we measure wireless optical power transfer between plasmonic nanoantennas in the far-field and demonstrate changeable signal routing to different nanoscopic receivers via beamsteering. We image the radiation pattern of single-optical nanoantennas using a photoluminescence technique, which allows mapping of the unperturbed intensity distribution around plasmonic structures. We quantify the distance dependence of the power transmission between transmitter and receiver by deterministically positioning nanoscopic fluorescent receivers around the transmitting nanoantenna. By adjusting the wavefront of the optical field incident on the transmitter, we achieve directional control of the transmitted radiation over a broad range of 29°. This enables wireless power transfer from one transmitter to different receivers.

Like conventional antennas, optical nanoantennas can transmit and receive signals but on much smaller length scales. Dregely et al. measure the optical power transmitted and received in the far-field by plasmonic nanoantennas and show that they can control the direction of transmission over a broad range.

Resolving single plasmons generated by multiquantum-emitters on a silver nanowire

Surface plasmons, the collective oscillations of electrons at metal surface, provide the ability to enhance the weak interaction between individual quantum emitters and photons for quantum information applications. The generation of single plasmons by coupling silver nanowire with single quantum emitters opens the prospects of using quantum optical techniques to control single surface plasmons and designing novel quantum plasmonic devices. However, the real applications will deal with multiple plasmons generated from multiple quantum emitters. Here we report the first experimental demonstration of resolving single plasmons generated by a pair of quantum dots (QDs) on a silver nanowire waveguide. The accurate positions of the two QDs with separation ranging from micrometers to 200 nm within the diffraction limit are determined by using super-resolution imaging method. The efficiency of plasmon generation due to the exciton-plasmon coupling is obtained for each QD. Our research takes a crucial step toward the experimental study of coupled systems of multiple quantum emitters and plasmonic waveguides and would shed new light on the study of light-matter interactions for potential quantum optics and quantum information applications.

Quantifying local density of optical states of nanorods by fluorescence lifetime imaging

In this letter, we demonstrate a facile far-field approach to quantify the near-field local density of optical states (LDOS) of a nanorod using CdTe quantum dots (QDs) emitters tethered to the surface of nanorods as beacons for optical read-outs. Radiative decay rate was extracted to quantify the LDOS; our analysis indicates that the LDOS of the nanorod enhance both the radiative and nonradiative decay of QD, particularly radiative decay of QDs at the end of nanorod is enhanced by 1.17 times greater than that at the waist, while the nonradiative decay was uniformly enhanced over the nanorod. To the best of our knowledge, our effort constitutes the first to map the LDOS of a nanostructure via far-field method, to provide clarity on the interaction mechanism between emitters and the nanostructure, and to be potentially employed in the LDOS mapping of high-throughput nanostructures.

Ultrafast energy transfer between molecular assemblies and surface plasmons in the strong coupling regime

The nonlinear optical dynamics of nanomaterials comprised of plasmons interacting with quantum emitters is investigated by a self-consistent model based on the coupled Maxwell-Liouville-von Neumann equations. It is shown that ultrashort resonant laser pulses significantly modify the optical properties of such hybrid systems. It is further demonstrated that the energy transfer between interacting molecules and plasmons occurs on a femtosecond time scale and can be controlled with both material and laser parameters.

Optical properties of excitons in metal-insulator-semiconductor nanowires

The theoretical model for the metal-insulator-semiconductor nanowires is established and the optical properties are investigated. The linear absorption of the hybrid excitons, formed due to the exciton-plasmon interaction, shows obvious red shift on the magnitude of several meVs. The mechanism of the red shift is found to be the joint action of the increased excitonic binding energy attributed to the indirect Coulomb interaction and the decreased effective bandgap caused by the additional self-energy potential. The conclusion is also supported by the evolution of the absorption spectra with the adjustable structural parameters.

Quantum entanglement in plasmonic waveguides with near-zero mode indices

We investigate the quantum entanglement between two quantum dots (QDs) in a plasmonic waveguide with a near-zero mode index, considering the dependence of concurrence on interdot distance, QD waveguide frequency detuning, and coupling strength ratio. High concurrence is achieved for a wide range of interdot distances due to the near-zero mode index, which largely relaxes the strict requirement of interdot distance in conventional dielectric waveguides or metal nanowires. The proposed QD waveguide system with near-zero phase variation along the waveguide near the mode cutoff frequency shows very promising potential in quantum optics and quantum information processing.

Spectral modifications and polarization dependent coupling in tailored assemblies of quantum dots and plasmonic nanowires

The coupling of optical emitters with a nanostructured environment is at the heart of nano- and quantum optics. We control this coupling by the lithographic positioning of a few (1-3) quantum dots (QDs) along plasmonic silver nanowires with nanoscale resolution. The fluorescence emission from the QD-nanowire systems is probed spectroscopically, by microscopic imaging and decay time measurements. We find that the plasmonic modes can strongly modulate the fluorescence emission. For a given QD position, the local plasmon field dictates the coupling efficiency, and thus the relative weight of free space radiation and emission into plasmon modes. Simulations performed with a generic few-level model give very good agreement with experiment. Our data imply that the 2D degenerate emission dipole orientation of the QD can be forced to predominantly emit to one polarization component dictated by the nanowire modes.