Optical Nanoantennas Research Articles

Broadband Enhancement of the Spontaneous Emission by a Split-Ring Nanoantenna: Impact of Azimuthally Propagating Surface Plasmon Polaritons

We propose a split-ring optical nanoantenna which can achieve enhancement factors of total and radiative emission rates up to high values of 7411 and 1941, respectively, with a tunable bandwidth of enhanced emission (Purcell factor over 1000) up to 221 nm at visible wavelengths. To clarify the underlying physical mechanism, we build up a semi-analytical model by considering an intuitive multiple-scattering process of surface plasmon polaritons (SPPs) that propagate azimuthally along the curved antenna arms. The SPP model provides an intuitive picture that under two phase matching conditions, the SPPs form paired and tunable whispering-gallery-like resonances and are then coupled to the emitter in the nano-gap or scattered into free space, which results in an enhancement of the total or radiative emission rate, respectively. Based on the SPP model, we further build up a semi-analytical model for the quasi-normal modes (QNMs) supported by the antenna, which provides a frequency analyticity and indicates that the broadband enhancement of the spontaneous emission rate results from an excitation of paired symmetric and anti-symmetric QNMs with different eigenfrequencies. The proposed models clarify the impact of azimuthally propagating SPPs on the antenna radiation and may be extended to other optical antennas with curved arms.

Turning a hot spot into a cold spot: polarization-controlled Fano-shaped local-field responses probed by a quantum dot

Optical nanoantennas can convert propagating light to local fields. The local-field responses can be engineered to exhibit nontrivial features in spatial, spectral and temporal domains, where local-field interferences play a key role. Here, we design nearly fully controllable local-field interferences in the nanogap of a nanoantenna, and experimentally demonstrate that in the nanogap, the spectral dispersion of the local-field response can exhibit tuneable Fano lineshapes with nearly vanishing Fano dips. A single quantum dot is precisely positioned in the nanogap to probe the spectral dispersions of the local-field responses. By controlling the excitation polarization, the asymmetry parameter q of the probed Fano lineshapes can be tuned from negative to positive values, and correspondingly, the Fano dips can be tuned across a broad spectral range. Notably, at the Fano dips, the local-field intensity is strongly suppressed by up to ~50-fold, implying that the hot spot in the nanogap can be turned into a cold spot. The results may inspire diverse designs of local-field responses with novel spatial distributions, spectral dispersions and temporal dynamics, and expand the available toolbox for nanoscopy, spectroscopy, nano-optical quantum control and nanolithography.

Active plasmonic nanoantenna: an emerging toolbox from photonics to neuroscience

Abstract Concepts adapted from radio frequency devices have brought forth subwavelength scale optical nanoantenna, enabling light localization below the diffraction limit. Beyond enhanced light–matter interactions, plasmonic nanostructures conjugated with active materials offer strong and tunable coupling between localized electric/electrochemical/mechanical phenomena and far-field radiation. During the last two decades, great strides have been made in development of active plasmonic nanoantenna (PNA) systems with unconventional and versatile optical functionalities that can be engineered with remarkable flexibility. In this review, we discuss fundamental characteristics of active PNAs and summarize recent progress in this burgeoning and challenging subfield of nano-optics. We introduce the underlying physical mechanisms underpinning dynamic reconfigurability and outline several promising approaches in realization of active PNAs with novel characteristics. We envision that this review will provide unambiguous insights and guidelines in building high-performance active PNAs for a plethora of emerging applications, including ultrabroadband sensors and detectors, dynamic switches, and large-scale electrophysiological recordings for neuroscience applications.

Dipolar analysis of substrated particles using a far-field response method

The presence of a substrate, when working with nanoparticles, is essential in many applications, like optical nanoantennas, solar cells, and sensing. Understanding the effects of substrates upon the nanoparticles is, therefore, important, as the substrates typically affect the resonance behaviors of particles, as well as the interactions between their electric and magnetic resonances. In order to better understand the impacts of substrates in practical applications with nanoparticles, this paper presents a semianalytical method to calculate the polarizability tensors of individual nanoparticles located on dielectric substrates. This approach is based on a sampling of the scattered far-field responses to plane-wave illuminations from structures. By using scattered far fields, the induced electric and magnetic dipole moments are calculated at the geometrical center of a particle. Then, using these dipole moments, the individual polarizability tensors of the substrated particle are calculated. To show the accuracy of the proposed method, the numerical results of different particles on a substrate are compared to two other approaches, and the results are shown to be in good agreement with these approaches. Moreover, the effect of the refractive index of the substrate and the geometric characteristics of the particle on the substrate-induced bianisotropy are also investigated. The proposed method clearly demonstrates how a particle without any bianisotropic interaction in free space can possess this property in the presence of a dielectric substrate.

Cold and Hot Spots: From Inhibition to Enhancement by Nanoscale Phase Tuning of Optical Nanoantennas.

Optical nanoantennas are well-known for the confinement of light into nanoscale hot spots, suitable for emission enhancement and sensing applications. Here, we show how control of the antenna dimensions allows tuning the local optical phase, hence turning a hot spot into a cold spot. We manipulate the local intensity exploiting the interference between driving and scattered field. Using single molecules as local detectors, we experimentally show the creation of subwavelength pockets with full suppression of the driving field. Remarkably, together with the cold excitation spots, we observe inhibition of emission by the phase-tuned nanoantenna. The fluorescence lifetime of a molecule scanned in such volumes becomes longer, showing slow down of spontaneous decay. In conclusion, the spatial phase of a nanoantenna is a powerful knob to tune between enhancement and inhibition in a 3-dimensional subwavelength volume.

Cold and Hot Spots: From Inhibition to Enhancement by Nanoscale Phase Tuning of Optical Nanoantennas

Cold and Hot Spots: From Inhibition to Enhancement by Nanoscale Phase Tuning of Optical Nanoantennas

Far-Field Control of Nanoscale Hotspots by Near-Field Interference

Optical nanoantennas coherently scatter incident optical radiation, creating a localized electromagnetic near-field imprinted with subwavelength phase distribution, rapidly changing in space. The n...

Forward and Backward Unidirectional Scattering by the Core-Shell Nanocube Dimer with Balanced Gain and Loss.

An optical nanoantenna consisting of a Au-dielectric core-shell nanocube dimer with switchable directionality was designed and described. Our theoretical model and numerical simulation showed that switching between forward and backward directions can be achieved with balanced gain and loss, using a single element by changing the coefficient κ in the core, which can be defined by the relative phase of the polarizability. The optical response indicated a remarkable dependence on the coefficient κ in the core as well as frequency. The location of the electric field enhancement was specified by the different coefficient κ and, furthermore, the chained optical nanoantenna and coupled electric dipole emitted to the optical nanoantenna played significant roles in unidirectional scattering. This simple method to calculate the feasibility of unidirectional and switchable scattering provides an effective strategy to explore the functionalities of nanophotonic devices.

Optical nanoantenna for beamed and surface‐enhanced Raman spectroscopy

AbstractOptical nanoantenna is a key component in plasmonic circuitry, which can receive or transmit an electromagnetic field in the near field of an object (nanomaterials). It has wide range of applications including surface‐enhanced Raman scattering (SERS), photocatalytic reactions, remote SERS, solar cell, plasmonic nanoscopy, and quantum plasmonics. The main challenges in optical nanoantenna research include both fundamental understanding of the underlying physics and issues related to fabrication of low cost, high throughput nanostructures beyond the diffraction limit. The nanoscale feature size of optical antennas limits our ability to design, manufacture, and characterize their resonant behavior. This review investigates the fabrication methods and SERS applications of optical nanoantenna.

Silicon Nanowire on Mirror Nanoantennas: Engineering Hybrid Gap Mode for Light Sources and Sensing Platforms

It has been demonstrated recently that a nanowire (NW) of high refractive index dielectric materials works as a nanoantenna because of the excitation of the Mie-type resonances. Here, we explore the capability of a silicon (Si) NW as an optical nanoantenna by combining it with an external metallic component. We investigate the light scattering property of a single Si NW placed on a gold (Au) mirror via a very thin dielectric spacer and demonstrate strong confinement of electromagnetic fields in the gap because of the coupling of the resonance modes with a Au mirror. We demonstrate that the resonance wavelength of the hybrid mode can be tuned by the gap length, and the hybrid gap mode strongly modifies the spectral shape of the emission from a quantum dot (QD) monolayer incorporated in the gap. Quantitative analyses of the data in combination with numerical simulations reveal that the enhancement and the polarization control of the QD emission are achieved by the coupling.

The role of morphology in all-dielectric SERS: A comparison between conformal (T-rex) and non conformal TiO2 shells

All-dielectric optical nanoantennas and resonators are attracting an ever increasing interest for enhanced Raman spectroscopy. These systems can take advantage of many factors that concur to determine their Raman response, including the type of material, refractive index contrast between interfaces, size, shape, roughness and geometrical configuration. However, the relative impact of each one of those terms on the Raman enhancement is still unclear.Here we report the Raman characterization of SiO2@TiO2 semishell microspheres obtained by rf-magnetron sputtering deposition of a TiO2 thin layer on SiO2 cores. The samples were able to significantly amplify Raman scattering in comparison to planar counterparts, however the overall enhancement was very limited with reference to that observed in conformal, smooth and compact TiO2 shell layers obtained by atomic layer deposition. These results demonstrated that geometry is a key factor to maximize the Raman response of these systems and, in general, it is more important than other parameters, such as the surface roughness or even refractive index of the shell layer, which are usually considered crucial for achieving Raman enhancement.

A Review on the Development of Tunable Graphene Nanoantennas for Terahertz Optoelectronic and Plasmonic Applications.

Exceptional advancement has been made in the development of graphene optical nanoantennas. They are incorporated with optoelectronic devices for plasmonics application and have been an active research area across the globe. The interest in graphene plasmonic devices is driven by the different applications they have empowered, such as ultrafast nanodevices, photodetection, energy harvesting, biosensing, biomedical imaging and high-speed terahertz communications. In this article, the aim is to provide a detailed review of the essential explanation behind graphene nanoantennas experimental proofs for the developments of graphene-based plasmonics antennas, achieving enhanced light–matter interaction by exploiting graphene material conductivity and optical properties. First, the fundamental graphene nanoantennas and their tunable resonant behavior over THz frequencies are summarized. Furthermore, incorporating graphene–metal hybrid antennas with optoelectronic devices can prompt the acknowledgment of multi-platforms for photonics. More interestingly, various technical methods are critically studied for frequency tuning and active modulation of optical characteristics, through in situ modulations by applying an external electric field. Second, the various methods for radiation beam scanning and beam reconfigurability are discussed through reflectarray and leaky-wave graphene antennas. In particular, numerous graphene antenna photodetectors and graphene rectennas for energy harvesting are studied by giving a critical evaluation of antenna performances, enhanced photodetection, energy conversion efficiency and the significant problems that remain to be addressed. Finally, the potential developments in the synthesis of graphene material and technological methods involved in the fabrication of graphene–metal nanoantennas are discussed.

Collective magnetic modes excitation in GaAs nanoclusters by azimuthally polarized vector beams

We investigate optical response of oligomers of dielectric nanodisks made of GaAs – anisotropic material with bulk quadratic nonlinearity. By using numerical approach, we analyse linear response of nanoparticle clusters in the vicinity of the magnetic dipole resonance for two types of incident radiation: linearly polarized plane wave and azimuthally polarized vector beams. Achieved results may be used to design highly efficient nonlinear optical nanoantennas with controllable radiation characteristics.

Shaping Field Gradients for Nanolocalization

Deep subwavelength localization and displacement sensing of optical nanoantennas have emerged as extensively pursued objectives in nanometrology, where focused beams serve as high-precision optical...

Unidirectional, Ultrafast, and Bright Spontaneous Emission Source Enabled By a Hybrid Plasmonic Nanoantenna

AbstractInefficient and wide‐angle emission as well as low emission rate of optical nanoemitters (such as quantum dots) have been strongly limiting their practical applications in next‐generation nanophotonic devices, such as nanoscale light‐emitting diodes (LEDs) and on‐chip single photon sources. Optical nanoantennas provide a promising way to deal with these challenges. Yet, there has been no solution that can overcome these drawbacks simultaneously on a single device. Here, a hybrid plasmonic nanoantenna consisting of a silver nanocube positioned at the center of a gold concentric‐ring structure is proposed, which can simultaneously enhance the emission directionality and decay rate of quantum dots embedded in the nanogap beneath the nanocube while maintaining high quantum efficiency. Coupling quantum dots to this nanoantenna can result in 60% of the emitted photons collected by the first lens with a numerical aperture (NA) of 0.5 and 21% for a NA of 0.12. The total emission intensity and decay rate are enhanced by 121‐fold and 424‐fold, respectively, compared with quantum dots on a glass substrate. A high quantum efficiency above 50% is obtained in simulation. This novel platform can be applied to enhance various types of optical nanoemitters and to develop high‐speed directional nano‐LEDs and single photon sources.

Nonlinear Nanophotonic Circuitry: Tristable and Astable Multivibrators and Chaos Generator

AbstractThe concept of lumped optical nanoelements (or metactronics), wherein nanometer‐scale structures act as nanoinductors, nanocapacitors, and nanoresistors, has attracted a great deal of attention as a simple toolbox for engineering different nanophotonic devices in analogy with microelectronics. While recent studies of the topic have been predominantly focused on linear functionalities, nonlinear dynamics in microelectronic devices plays a crucial role and provides a majority of functions, employed in modern applications. Here, the metactronics paradigm is extended and nonlinear dynamical modalities are added to those nanophotonic devices that have never been associated with optical nanoantennas. Specifically, it is shown that nonlinear dimer nanoantennae can operate in the regimes of tristable and astable multivibrators as well as chaos generators. The physical mechanism behind these modalities relies on the Kerr‐type nonlinearity of nanoparticles in the dimer enhanced by a dipolar localized surface plasmon resonance. This allows one to provide a positive nonlinear feedback at moderate optical intensities, leading to the desired dynamical behavior via tuning the driving field parameters. The findings shed light on a novel class of nonlinear nanophotonic devices with a tunable nonlinear dynamical response.

Isosbestic light absorption by metallic dimers: effect of interparticle electromagnetic coupling.

Isosbestic plasmonic nanostructures, which feature an invariance of optical absorption and heat generation upon varying the incident light polarization, have broad application in many fields such as nanochemistry, optical nanoantennas, and microbubble formation. In this study, we focus on the isosbestic optical absorption by metallic dimers and systematically investigate the coupling between two interacting particles by using both the superposition T-matrix method and dipole approximation model. We observe that the interparticle coupling effects on particle absorption can be both positive and negative, compared to an isolated particle. Meanwhile, the optical absorption properties of spheres with small size parameters can realize more flexible control through changing the sphere size, interparticle distance, and incident light wavelength. For illuminations with incident light propagating perpendicularly to the line joining the centers of the two spheres, isosbestic conditions will be satisfied as long as the absorption efficiencies for transverse and longitudinal illuminations are equal. For transverse illuminations along the dimer axis, the ratio of absorption efficiency of the two metallic spheres presents the fluctuation change with the interparticle distance. Owing to the strong interparticle coupling effects, it even leads to the absorption efficiency of the far sphere being higher than that of the near sphere. Our results are aimed at expanding our understanding of the interparticle electromagnetic coupling effects on isosbestic light absorption in plasmonic nanoparticle systems.

Enhancing optical nonlinearity

Optics Intense pulses of light interacting with a dielectric material can induce optical nonlinear behavior, whereby the frequency of the output light can be doubled or tripled or excited to even higher harmonics of the input light. Usually this interaction is weak and occurs over many thousands of wavelengths, typically requiring the combination of bulk volumes of material with a confining cavity. Using a mechanism of light confinement called bound states in the continuum, Koshelev et al. show that enhanced second-harmonic generation can be obtained in nanoscale subwavelength cylinders of a dielectric material. The results on these optical nanoantennas offer a platform for developing integrated nonlinear nanophotonic devices. Science , this issue p. [288][1] [1]: /lookup/doi/10.1126/science.aaz3985

Selectively accessing the hotspots of optical nanoantennas by self-aligned dry laser ablation.

Plasmonic nanostructures serve as optical antennas for concentrating the energy of incoming light in localized hotspots close to their surface. By positioning nanoemitters in the antenna hotspots, energy transfer is enabled, leading to novel hybrid antenna-emitter-systems, where the antenna can be used to manipulate the optical properties of the nano-objects. The challenge remains how to precisely position emitters within the hotspots. We report a self-aligned process based on dry laser ablation of a calixarene that enables the attachment of molecules within the electromagnetic hotspots at the tips of gold nanocones. Within the laser focus, the ablation threshold is exceeded in nanoscale volumes, leading to selective access of the hotspot areas. A first indication of the site-selective functionalization process is given by attaching fluorescently labelled proteins to the nanocones. In a second example, Raman-active molecules are selectively attached only to nanocones that were previously exposed in the laser focus, which is verified by surface enhanced Raman spectroscopy. Enabling selective functionalization is an important prerequisite e.g. for preparing single photon sources for quantum optical technologies, or multiplexed Raman sensing platforms.

非线性超构表面：谐波产生与超快调控

Metasurfaces refer to the optical nanoantenna arrays composed of subwavelength structures. Nanoantennas can have resonances under appropriate excitation conditions, to achieve near-field enhancement, and thus enhancing nonlinear optical effects. Compared with conventional nonlinear optical crystals, metasurfaces are more compact, with the possibility for on-chip integration. Due to the subwavelength light-matter interaction length, for applications such as nonlinear harmonic generation, metasurfaces does not suffer from the limitation of phase-matching. In addition, metasurfaces own the subwavelength spatial resolution. By designing and arranging the meta-atoms, metasurfaces can realize flexible control of the phase, polarization, and amplitude of the harmonic wave. Recent works on nonlinear metasurfaces for applications in optical frequency conversion, nonlinear wavefront control and ultrafast all-optical modulation were reviewed, the challenges and perspectives for the practical application of metasurfaces were presented.