Dynamic tuning and symmetry lowering of Fano resonance in plasmonic nanostructure

Terahertz stretchable metamaterials with deformable dolmen resonators for uniaxial strain measurement

In this study, we propose a terahertz stretchable metamaterial that can measure uniaxial strain. Gold dolmen resonators formed on a sheet of polydimethylsiloxane (PDMS) is deformed by strain, and its resonance peak exhibits the gradual decrease in reflectance without a frequency shift, which is suitable for imaging applications at a single frequency. The metamaterial was designed by mechanical and electromagnetic simulations and fabricated by microfabrication including a transfer process of gold structures from a glass substrate to a PDMS sheet. By measuring the reflectance and observing the deformation under different strains, the reflectance decrease was obtained at 0.292 THz despite the appearance of wrinkles on gold structures. Linear response and repeatability up to 20% strain were also confirmed. Furthermore, the strain measurement through a sheet of paper was demonstrated, suggesting that our method can be applied even in situations where opaque obstacles in the visible region exist.

Magnetoplasmonics beyond Metals: Ultrahigh Sensing Performance in Transparent Conductive Oxide Nanocrystals

Active modulation of the plasmonic response is at the forefront of today's research in nano-optics. For a fast and reversible modulation, external magnetic fields are among the most promising approaches. However, fundamental limitations of metals hamper the applicability of magnetoplasmonics in real-life active devices. While improved magnetic modulation is achievable using ferromagnetic or ferromagnetic-noble metal hybrid nanostructures, these suffer from severely broadened plasmonic response, ultimately decreasing their performance. Here we propose a paradigm shift in the choice of materials, demonstrating for the first time the outstanding magnetoplasmonic performance of transparent conductive oxide nanocrystals with plasmon resonance in the near-infrared. We report the highest magneto-optical response for a nonmagnetic plasmonic material employing F- and In-codoped CdO nanocrystals, due to the low carrier effective mass and the reduced plasmon line width. The performance of state-of-the-art ferromagnetic nanostructures in magnetoplasmonic refractometric sensing experiments are exceeded, challenging current best-in-class localized plasmon-based approaches.

Ultranarrow and Tunable Fano Resonance in Ag Nanoshells and a Simple Ag Nanomatryushka

We study theoretically the Fano resonances (FRs) produced by the near-field coupling between the lowest-order (dipolar) sphere plasmon resonance and the dipolar cavity plasmon mode supported by an Ag nanoshell or the hybrid mode in a simple three-layered Ag nanomatryushka constructed by incorporating a solid Ag nanosphere into the center of Ag nanoshell. We find that the linewidth of dipolar cavity plasmon resonance or hybrid mode induced FR is as narrow as 6.8 nm (corresponding to a high Q-factor of ~160 and a long dephasing time of ~200 fs) due to the highly localized feature of the electric-fields. In addition, we attribute the formation mechanisms of typical asymmetrical Fano line profiles in the extinction spectra to the constructive (Fano peak) and the destructive interferences (Fano dip) arising from the symmetric and asymmetric charge distributions between the dipolar sphere and cavity plasmon or hybrid modes. Interestingly, by simply adjusting the structural parameters, the dielectric refractive index required for the strongest FR in the Ag nanomatryushka can be reduced to be as small as 1.4, which largely reduces the restriction on materials, and the positions of FR can also be easily tuned across a broad spectral range. The ultranarrow linewidth, highly tunability together with the huge enhancement of electric fields at the FR may find important applications in sensing, slow light, and plasmon rulers.

Nanophotonics for light detection and ranging technology

Light detection and ranging (LiDAR) technology, a laser-based imaging technique for accurate distance measurement, is considered one of the most crucial sensor technologies for autonomous vehicles, artificially intelligent robots and unmanned aerial vehicle reconnaissance. Until recently, LiDAR has relied on light sources and detectors mounted on multiple mechanically rotating optical transmitters and receivers to cover an entire scene. Such an architecture gives rise to limitations in terms of the imaging frame rate and resolution. In this Review, we examine how novel nanophotonic platforms could overcome the hardware restrictions of existing LiDAR technologies. After briefly introducing the basic principles of LiDAR, we present the device specifications required by the industrial sector. We then review a variety of LiDAR-relevant nanophotonic approaches such as integrated photonic circuits, optical phased antenna arrays and flat optical devices based on metasurfaces. The latter have already demonstrated exceptional functional beam manipulation properties, such as active beam deflection, point-cloud generation and device integration using scalable manufacturing methods, and are expected to disrupt modern optical technologies. In the outlook, we address the upcoming physics and engineering challenges that must be overcome from the viewpoint of incorporating nanophotonic technologies into commercially viable, fast, ultrathin and lightweight LiDAR systems.

Strain Sensor via Wood Anomalies in 2D Dielectric Array

Optical sensing is one of many promising applications for all-dielectric photonic materials. Herein, we present an analytical and numerical study on the strain-responsive spectral properties of a bioinspired sensor. The sensor structure contains a two-dimensional periodic array of dielectric nanodisks to mimic the optical behavior of grana lamellae inside chloroplasts. To accumulate a noticeable response, we exploit the collective optical mode in grana ensemble. In higher plants, such a mode appears as Wood's anomaly near the chlorophyll absorption line to control the photosynthesis rate. The resonance is shown persistent against moderate biological disorder and deformation. Under the stretching or compression of a symmetric structure, the mode splits into a couple of polarized modes. The frequency difference is accurately detected. It depends on the stretch coefficient almost linearly providing easy calibration of the strain-sensing device. The sensitivity of the considered structure remains at 5 nm/% in a wide range of strain. The influence of the stretching coefficient on the length of the reciprocal lattice vectors, as well as on the angle between them, is taken into account. This adaptive phenomenon is suggested for sensing applications in biomimetic optical nanomaterials.

Role of electric currents in the Fano resonances of connected plasmonic structures

In this work, we use finite elements simulations to study the far field properties of two plasmonic structures, namely a dipole antenna and a cylinder dimer, connected to a pair of nanorods. We show that electrical, rather than near field, coupling between the modes of these structures results in a characteristic Fano lineshape in the far field spectra. This insight provides a way of tailoring the far field properties of such systems to fit specific applications, especially maintaining the optical properties of plasmonic antennas once they are connected to nanoelectrodes. This work extends the previous understanding of Fano resonances as generated by a simple near field coupling and provides a route to an efficient design of functional plasmonic electrodes.

Periodic arrays of plasmonic crossed-bowtie nanostructures interspaced with plasmonic nanocrosses for highly sensitive LSPR based chemical and biological sensing

In this paper, we present novel localized surface plasmon resonance (LSPR) sensors based on periodic arrays of gold crossed-bowtie nanostructures interspaced with gold nanocross pillars. Finite difference time domain (FDTD) numerical simulations were carried out to model bulk sensors as well as localized sensors based on the plasmonic nanostructures being proposed. The geometrical parameters of the plasmonic nanostructures are varied to obtain the best possible sensing performance in terms of sensitivity and figure of merit. A very high bulk sensitivity of 1753 nm per unit change in refractive index (nm RIU-1), with a figure of merit for bulk sensing (FOMbulk) of 3.65 RIU-1, is obtained for these plasmonic nanostructures. This value of bulk sensitivity is higher in comparison to previously proposed LSPR sensors based on plasmonic nanopillars and nanocrosses. Moreover, the optimized LSPR sensors being proposed in this paper provide maximum sensitivity of localized refractive index sensing of 70 nm/nm with a FOMlocalized of 0.33 nm-1. This sensitivity of localized refractive index sensing is the highest reported thus far in comparison with previously reported LSPR sensors. It is also demonstrated that the operating resonance wavelengths of these LSPR sensors can be controllably tuned for specific applications by changing the dimensions of the plasmonic nanostructures.

Wearable plasmonic-metasurface sensor for noninvasive and universal molecular fingerprint detection on biointerfaces

Wearable sensing technology is an essential link to future personalized medicine. However, to obtain a complete picture of human health, it is necessary but challenging to track multiple analytes inside the body simultaneously. Here, we present a wearable plasmonic-electronic sensor with "universal" molecular recognition ability. Flexible plasmonic metasurface with surface-enhanced Raman scattering (SERS)-activity is introduced as the fundamental sensing component in a wearable sensor since we solved the technical challenge of maintaining the plasmonic activities of their brittle nanostructures under various deformations. Together with a flexible electronic sweat extraction system, our sensor can noninvasively extract and "fingerprint" analytes inside the body based on their unique SERS spectra. As a proof-of-concept example, we successfully monitored the variation of trace-amounts drugs inside the body and obtained an individual's drug metabolic profile. Our sensor bridges the existing gap in wearable sensing technology by providing a universal, sensitive molecular tracking means to assess human health.

Surface-plasmon-coupled optical force sensors based on metal-insulator-metal metamaterials with movable air gap

We proposed surface-plasmon-coupled optical force sensors based on metal–insulator–metal (MIM) metamaterials with a movable air gap as an insulator layer. The MIM metamaterial was composed of an air gap sandwiched by a metal nanodot array and a metal diaphragm, the resonant wavelength of which was red-shifted when the air gap was narrowed by applying a normal force. We designed and fabricated a prototype of the proposed sensor and confirmed that the MIM metamaterial could be used as a force sensor with larger sensitivity than a force sensor based on Fabry-Pérot interferometer (FPI).

Generation and Annihilation of Topologically Protected Bound States in the Continuum and Circularly Polarized States by Symmetry Breaking

We demonstrate by breaking the C_{6} symmetry for higher-order at-Γ bound states in the continuum (BICs) with topological charge -2 in photonic crystals (i) deterministic generation of off-Γ BICs from the at-Γ BIC and (ii) a variety of pair-creation and annihilation processes of circularly polarized states with opposite topological charges and the same handedness. To explain these phenomena, we introduce the handedness-wise topological charge quantized to a half-integer. The handedness-wise charge gives a unified picture of various phenomena involving BICs and circularly polarized states.

Multifunctional Nonlocal Metasurfaces

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wave front at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. Here, we introduce a rational design paradigm using quasibound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrow band spatially tailored wave fronts at multiple selected wavelengths and yet are otherwise transparent.

Broadband continuous/discrete spectrum optical absorber using graphene-wrapped fractal oligomers

In this paper, a second-order fractal oligomer constructed by graphene-coated cylindrical nano-rods is proposed as the unit cell of a wideband optical absorber. Nano-rods have resided on a dielectric substrate with a thick metallic mirror. The fractional bandwidth of the designed structure is 88.67% for the absorption above 90%. Broadband absorption originates from the cooperative excitation of localized surface plasmon resonances (LSPRs) of the bottom, top, and lateral surfaces of the rods, engineered by the geometrical parameters through the fractal concept. Designed full absorber has an acceptable performance concerning the incident angles up to around 35° and it is polarization insensitive. Moreover, broadband absorption can be altered to multi-band performance in the same spectrum with the desired number of frequency bands. This feature is obtained by manipulating the substrate thickness to excite multiple orders of Fabry-Perot cavity resonances. Our proposed structure has potential applications in various optical devices such as filters, sensors, and modulators.

Soft optical metamaterials

Optical metamaterials consist of artificially engineered structures exhibiting unprecedented optical properties beyond natural materials. Optical metamaterials offer many novel functionalities, such as super-resolution imaging, negative refraction and invisibility cloaking. However, most optical metamaterials are comprised of rigid materials that lack tunability and flexibility, which hinder their practical applications. This limitation can be overcome by integrating soft matters within the metamaterials or designing responsive metamaterial structures. In addition, soft metamaterials can be reconfigured via optical, electrical, thermal and mechanical stimuli, thus enabling new optical properties and functionalities. This paper reviews different types of soft and reconfigurable optical metamaterials and their fabrication methods, highlighting their exotic properties. Future directions to employ soft optical metamaterials in next-generation metamaterial devices are identified.

Stimuli-Responsive DNA-Linked Nanoparticle Arrays as Programmable Surfaces

Self- and directed-assembly approaches have enabled precise control over the composition and geometry of 2D and 3D nanoparticle constructs. However, the resulting structures are typically static, providing only a single structural arrangement of the nanoparticle building blocks. In this work, the power of DNA-linked nanoparticle assembly is coupled to a grayscale patterning technique to create programmable surfaces for assembly and thermally activated reorganization of gold nanoparticle arrays. Direct grayscale patterning of DNA monolayers by electron-beam lithography (DNA-EBL) enables the production of surfaces with nanometer-scale control over the density of functional DNA. This enables tuning of the particle-surface interactions with single-nanoparticle resolution and without the need for a physical template as employed in most directed assembly methods. This technique is applied on suspended membrane structures to achieve high-resolution assembly of 2D nanoparticle arrays with highly mutable architectures. Gold nanorods assembled on grayscale-patterned surfaces exhibit temperature-dependent configurations and ordering behavior that result in tunable polarization-dependent optical properties. In addition, spherical gold particles assembled from a bimodal suspension produce arrays with temperature-dependent configurations of small and large particles. These results have important implications for the design and fabrication of reconfigurable nanoparticle arrays for application as structurally tunable optical metasurfaces.

Controllable Formation of Luminescent Carbon Quantum Dots Mediated by the Fano Resonances Formed in Oligomers of Gold Nanoparticles

Rapid and controllable formation of fluorescent carbon quantum dots (CQDs) is highly desirable in the fields of nanophotonics and biophotonics. Here, a novel strategy for creating CQDs, which emit white light efficiently under the excitation of either laser light or a mercury lamp, is proposed and demonstrated. The luminescent CQDs are generated by irradiating a poly(vinyl alcohol) (PVA) film doped with dense gold nanoparticles (AuNPs) with femtosecond laser pulses. The creation of CQDs from PVA is a two-step dehydration process mediated by AuNPs which act not only as heat sources but also as catalytic agents. The formation of CC, CC, and CO bonds is confirmed by infrared Fourier transformation spectroscopy and X-ray photoelectron spectroscopy. It is revealed both numerically and experimentally that a spatially localized temperature distribution at the deep subwavelength scale can be achieved in oligomers of AuNPs by resonantly exciting the Fano resonances formed in the oligomers of AuNPs, enabling the generation of CQDs with small diameters. As one of the potential applications, it is demonstrated that optical display and optical data storage with ultralow energy can be realized by selectively introducing luminescent CQDs in the AuNP/PVA film.

Tunable, polarization-sensitive, dual guided-resonance modes in photonic crystals

We present a photonic-crystal design which supports multiple guided-resonance modes in a narrow spectral range. Introduction of mutually-orthogonal slots within a conventional lattice allows us to create polarization-sensitive guided modes with distinct near-field periodicities and tunable resonance wavelengths. The device can potentially be used as a reconfigurable optical trap, multiband tunable filter, or differential sensor.

Nanoscale uniformity in the active tuning of a plasmonic array by polymer gel volume change

Active plasmonic tuning is an attractive but challenging research subject, leading to various promising applications. As one of the approaches, nanostructures are placed in or on soft matter, such as elastomers and gels, and their gap distances are tuned by the mechanical extension or volume change of the supporting matrices. As hydrogels possess various types of stimuli-responsiveness with large volume change and biocompatibility, they are good candidates as supporting materials for active nanostructure tuning. However, it remains unclear how accurately we can control their nanogap distance changes using polymer gels with a low deviation due to major difficulties in the precise observation of nanostructures on the gels. Here, we prepared gold arrays with sub-100 nm dots on silicon substrates by electron beam lithography and transferred them onto the hydrogel surface. Then, their nanopattern was actively tuned by the changes in gel size in water and their structural changes were confirmed by optical microscopy, microspectroscopy, and atomic force microscopy (AFM). Further, we successfully prepared ionic liquid (IL) gels with various degrees of swelling via solvent exchange. Scanning electron microscopy (SEM) observation of the IL gels provided clear pictures at nanoscale resolution. Finally, we calculated the plasmonic spectra using a finite difference time domain (FDTD) simulation based on the SEM images and compared them with the measured spectra. The results in this study totally support the notion that active changes in plasmonic nanodot patterns via volume changes in the hydrogel are quite homogenous on a several nanometer scale, making them ideal for precise active surface plasmon tuning.

Engineering surface lattice resonance of elliptical gold nanodisk array for enhanced strain sensing

We demonstrate an elliptical gold nanodisk array (GNA) for engineering the spectral profile of surface lattice resonance (SLR). The nanodisk's shape has a great impact on SLR. Small linewidth of 20 nm at an aspect ratio of 1.17, as well as large wavelength tuning of 64 nm within 4% strain via different orientations and polarizations, are achieved experimentally. The enhanced wavelength response of 6.93 nm per 1% strain variation for elliptical GNA is 2.4 times better than that for general circular GNA. Furthermore, the strain sensing for elliptical GNA approaches is 5.7 times greater than that for circular GNA.

Stretchable Nanolasing from Hybrid Quadrupole Plasmons

This paper reports a robust and stretchable nanolaser platform that can preserve its high mode quality by exploiting hybrid quadrupole plasmons as an optical feedback mechanism. Increasing the size of metal nanoparticles in an array can introduce ultrasharp lattice plasmon resonances with out-of-plane charge oscillations that are tolerant to lateral strain. By patterning these nanoparticles onto an elastomeric slab surrounded by liquid gain, we realized reversible, tunable nanolasing with high strain sensitivity and no hysteresis. Our semiquantum modeling demonstrates that lasing build-up occurs at the hybrid quadrupole electromagnetic hot spots, which provides a route toward mechanical modulation of light-matter interactions on the nanoscale.

Electrically tunable polarizer based on graphene-loaded plasmonic cross antenna

The unique gate-voltage dependent optical properties of graphene make it a promising electrically-tunable plasmonic material. In this work, we proposed in situ control of the polarization of nanoantennas by combining plasmonic structures with an electrostatically tunable graphene monolayer. The tunable polarizer is designed based on an asymmetric cross nanoantenna comprising two orthogonal metallic dipoles sharing the same feed gap. Graphene monolayer is deposited on a Si/SiO₂ substrate, and inserted beneath the nanoantenna. Our modelling demonstrates that as the chemical potential is incremented up to 1 eV by electrostatic doping, resonant wavelength for the longer graphene-loaded dipole is blue shifted for 500 nm (~10% of the resonance) in the mid-infrared range, whereas the shorter dipole experiences much smaller influences due to the unique wavelength-dependent optical properties of graphene. In this way, the relative field amplitude and phase between the two dipole nanoantennas are electrically adjusted, and the polarization state of the reflected wave can be electrically tuned from the circular into near-linear states with the axial ratio changing over 8 dB. Our study thus confirms the strong light-graphene interaction with metallic nanostructures, and illuminates promises for high-speed electrically controllable optoelectronic devices.

Building superlattices from individual nanoparticles via template-confined DNA-mediated assembly

DNA programmable assembly has been combined with top-down lithography to construct superlattices of discrete, reconfigurable nanoparticle architectures on a gold surface over large areas. Specifically, the assembly of individual colloidal plasmonic nanoparticles with different shapes and sizes is controlled by oligonucleotides containing “locked” nucleic acids and confined environments provided by polymer pores to yield oriented architectures that feature tunable arrangements and independently controllable distances at both nanometer- and micrometer-length scales. These structures, which would be difficult to construct by other common assembly methods, provide a platform to systematically study and control light-matter interactions in nanoparticle-based optical materials. The generality and potential of this approach are explored by identifying a broadband absorber with a solvent polarity response that allows dynamic tuning of visible light absorption.

A flexible pressure responsive device based on the interaction between silver nanoparticles and an aluminum reflector

The unique and tunable optical properties of metal nanoparticles have attracted intense and sustained academic attention in recent years. In tandem with the demand for low-cost responsive materials, one particular topic of interest is the development of mechanically responsive device structures. This work describes the design, fabrication, and testing of a mechanically responsive plasmonic device structure that has been integrated onto a standard commercial plastic substrate. With a low actuation force and a visually perceivable color shift, this device would be attractive for applications requiring responsive features that can be activated by the human hand.

Electrically Tunable Fano Resonance from the Coupling between Interband Transition in Monolayer Graphene and Magnetic Dipole in Metamaterials

Fano resonance modulated effectively by external perturbations can find more flexible and important applications in practice. We theoretically study electrically tunable Fano resonance with asymmetric line shape over an extremely narrow frequency range in the reflection spectra of metamaterials. The metamaterials are composed of a metal nanodisk array on graphene, a dielectric spacer, and a metal substrate. The near-field plasmon hybridization between individual metal nanodisks and the metal substrate results into the excitation of a broad magnetic dipole. There exists a narrow interband transition dependent of Fermi energy E_f, which manifests itself as a sharp spectral feature in the effective permittivity ε_g of graphene. The coupling of the narrow interband transition to the broad magnetic dipole leads to the appearance of Fano resonance, which can be electrically tuned by applying a bias voltage to graphene to change E_f. The Fano resonance will shift obviously and its asymmetric line shape will become more pronounced, when E_f is changed for the narrow interband transition to progressively approach the broad magnetic dipole.

Two-Dimensional Active Tuning of an Aluminum Plasmonic Array for Full-Spectrum Response

Color pixels composed of plasmonic nanostructures provide a highly promising approach for new display technologies, capable of vivid, robust coloration and incorporating the use of low-cost plasmonic materials, such as aluminum. Here we report a plasmonic device that can be tuned continuously across the entire visible spectrum, based on integrating a square array of aluminum nanostructures into an elastomeric substrate. By stretching the substrate in either of its two dimensions, the period and therefore the scattering color can be modified to the blue or the red of the at-rest structure, spanning the entire visible spectrum. The unique two-dimensional design of this structure enables active mechanical color tuning, under gentle elastic modulation with no more than 35% strain. We also demonstrate active image switching with this structure. This design strategy has the potential to open the door for next-generation flexible photonic devices for a wide variety of visible-light applications.

Room-Temperature Strong Light-Matter Interaction with Active Control in Single Plasmonic Nanorod Coupled with Two-Dimensional Atomic Crystals

Strong light-matter coupling manifested by Rabi splitting has attracted tremendous attention due to its fundamental importance in cavity quantum-electrodynamics research and great potentials in quantum information applications. A prerequisite for practical applications of the strong coupling in future optoelectronic devices is an all-solid-state system exhibiting room-temperature Rabi splitting with active control. Here we realized such a system in heterostructure consisted of monolayer WS₂ and an individual plasmonic gold nanorod. By taking advantages of the small mode volume of the nanorod and large transition dipole moment of the WS₂ exciton, giant Rabi splitting energies of 91-133 meV can be achieved at ambient conditions, which only involve a small number of excitons. The strong light-matter coupling can be dynamically tuned either by electrostatic gating or temperature scanning. These findings can pave the way toward active nanophotonic devices operating at room temperature.

Fano resonances in plasmonic heptamer nano-hole arrays

The optical properties of gold heptamer nanohole arrays have been investigated theoretically and numerically. This structure support pronounced Fano resonances with high transmittance (~50%) and narrow bandwidths (down to 12 nm). The Fano features arise from the interference between light directly transmitted through the holes, and light indirectly scattered through the excitation of localized surface plasmon polaritons (LSPPs), propagating surface plasmon polaritons (SPPs), or/and waves related to Wood's anomaly (WA). The mechanisms behind the generation of these resonances are revealed by observing near-field distributions, altering the structural parameters and applying the Bloch wave model. Furthermore, it is shown that Fano resonances associated with LSPPs exhibit high surface (2 nm/nm) and bulk sensitivities (400 nm/RIU). However, the highest figure of merit (~24 RIU^-1) occurs for a Fano resonance involving a WA and SPP mode.

A Mie resonant antenna with high sensitivity for force and strain measurement

We demonstrate the experimental and simulated performances of a Mie resonant antenna for force sensing with high sensitivity for compressive force and strain measurements inside soft materials. The proposed sensor is compatible with biological specimens and has small dimensions. It comprises a pair of dielectric cubes and an elastic layer of silicone rubber. The applied force is determined by measuring the redshift of the operating frequency when a mechanical load applied. Both simulated and experimental results demonstrate that the relationship between the frequency shift of the sensor and the applied compressive load could be fitted well using a quadratic equation with a maximum fitting error less than 17%, which enables highly sensitive telemetric force measurements to be performed inside structures by observing the operating frequency shift. The proposed design provides a simple and low-cost approach to realizing highly-efficient telemetric measurement inside soft materials such as biological tissues.

Hybrid Resonators and Highly Tunable Terahertz Metamaterials Enabled by Vanadium Dioxide (VO2)

Hybrid metamaterials that exhibit reconfigurable responses under external stimulus, such as electric fields and light radiation, have only recently been demonstrated by combining active media with patterned metallic structures. Nevertheless, hybrid terahertz (THz) metamaterials whose spectral performance can be dynamically tuned over a large scale remain rare. Compared with most active media (for instance, silicon) that provide limited activity, vanadium dioxide (VO2), which exhibits an insulator-to-metal transition, has been recently explored to facilitate dynamically tunable metamaterials. More importantly, the phase transition yields a three orders of magnitude increase in THz electrical conductivity, which suggests the potential for creating VO2 based hybrid resonators that operate at THz frequencies. Here, we show that an integration of VO2 structures and conventional metallic resonating components can enable a class of highly tunable THz metamaterials. Considering the widely studied phase-transition dynamics in VO2, the proposed hybrid metamaterials are capable of offering ultrafast modulation of THz radiation.

SERS-enhanced piezoplasmonic graphene composite for biological and structural strain mapping

Thin-film optical strain sensors have the ability to map small deformations with spatial and temporal resolution and do not require electrical interrogation. This paper describes the use of graphene decorated with metallic nanoislands for sensing of tensile deformations of less than 0.04% with a resolution of less than 0.002%. The nanoisland-graphene composite films contain gaps between the nanoislands, which when functionalized with benzenethiolate behave as hot spots for surface-enhanced Raman scattering (SERS). Mechanical strain increases the sizes of the gaps; this increase attenuates the electric field, and thus attenuates the SERS signal. This compounded, SERS-enhanced "piezoplasmonic" effect can be quantified using a plasmonic gauge factor, and is among the most sensitive mechanical sensors of any type. Since the graphene-nanoisland films are both conductive and optically active, they permit simultaneous electrical stimulation of myoblast cells and optical detection of the strains produced by the cellular contractions.

Programmable and reversible plasmon mode engineering

Plasmonic nanostructures with enhanced localized optical fields as well as narrow linewidths have driven advances in numerous applications. However, the active engineering of ultranarrow resonances across the visible regime-and within a single system-has not yet been demonstrated. This paper describes how aluminum nanoparticle arrays embedded in an elastomeric slab may exhibit high-quality resonances with linewidths as narrow as 3 nm at wavelengths not accessible by conventional plasmonic materials. We exploited stretching to improve and tune simultaneously the optical response of as-fabricated nanoparticle arrays by shifting the diffraction mode relative to single-particle dipolar or quadrupolar resonances. This dynamic modulation of particle-particle spacing enabled either dipolar or quadrupolar lattice modes to be selectively accessed and individually optimized. Programmable plasmon modes offer a robust way to achieve real-time tunable materials for plasmon-enhanced molecular sensing and plasmonic nanolasers and opens new possibilities for integrating with flexible electronics.

Directed self-assembly of sub-10 nm particle clusters using topographical templates

Directed self-assembly of nanoparticles (DSA-n) is an approach that creates suitable conditions to capture nanoparticles randomly dispersed in a liquid and position them into predefined locations on a solid template. Although DSA-n is emerging as a potential bottom-up patterning technique to build nanostructures using nanoparticles of various sizes, geometries and material compositions, there are still several outstanding challenges. In this paper, we focus on the DSA-n of sub-10 nm particles using topographical templates to guide them into 1D and 2D ordered arrays. The process mechanism leading DSA-n at sub-10 nm size scale has been reviewed and experimental evidence of the impact of the template on the positioning both individual and clusters of particles with low level of structure defects have also been demonstrated. Furthermore, by controlling the drying direction of the liquid within polygonal traps, we are also able to tune the spacing between the trapped nanoparticle clusters. This self-structuring phenomenon is of crucial importance for various applications such as plasmonics and charge transport within quantum circuits, whereby the coupling effects are highly dependent on the size of the nanoparticles and their separation.

Light-tunable Fano resonance in metal-dielectric multilayer structures

High-Q optical Fano resonances realized in a variety of plasmonic nanostructures and metamaterials are very much promising for the development of new potent photonic devices, such as optical sensors and switches. One of the key issues in the development is to establish ways to effectively modulate the Fano resonance by external perturbations. Dynamic tuning of the Fano resonance applying the mechanical stress and electric fields has already been demonstrated. Here, we demonstrate another way of tuning, i.e., photo-tuning of the Fano resonance. We use a simple metal-dielectric multilayer structure that exhibits a sharp Fano resonance originating from coupling between a surface plasmon polariton mode and a planar waveguide mode. Using a dielectric waveguide doped with azo dye molecules that undergo photoisomerization, we succeeded in shifting the Fano resonance thorough photo-modulation of the propagation constant of the waveguide mode. The present work demonstrates the feasibility of photo-tuning of the Fano resonance and opens a new avenue towards potential applications of the Fano resonance.

Flexible Plasmonic Sensors

Mechanical flexibility and the advent of scalable, low-cost, and high-throughput fabrication techniques have enabled numerous potential applications for plasmonic sensors. Sensitive and sophisticated biochemical measurements can now be performed through the use of flexible plasmonic sensors integrated into existing medical and industrial devices or sample collection units. More robust sensing schemes and practical techniques must be further investigated to fully realize the potentials of flexible plasmonics as a framework for designing low-cost, embedded and integrated sensors for medical, environmental, and industrial applications.

Tunable Metasurface and Flat Optical Zoom Lens on a Stretchable Substrate

A mechanically reconfigurable metasurface that can continuously tune the wavefront is demonstrated in the visible frequency range by changing the lattice constant of a complex Au nanorod array fabricated on a stretchable polydimethylsiloxane substrate. It is shown that the anomalous refraction angle of visible light at 632.8 nm interacting with the tunable metasurface can be adjusted from 11.4° to 14.9° by stretching the substrate by ∼30%. An ultrathin flat 1.7× zoom lens whose focal length can continuously be changed from 150 to 250 μm is realized, which also demonstrates the potential of utilizing metasurfaces for reconfigurable flat optics.

Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Reconfigurable nanomechanical photonic metamaterials

The changing balance of forces at the nanoscale offers the opportunity to develop a new generation of spatially reconfigurable nanomembrane metamaterials in which electromagnetic Coulomb, Lorentz and Ampère forces, as well as thermal stimulation and optical signals, can be engaged to dynamically change their optical properties. Individual building blocks of such metamaterials, the metamolecules, and their arrays fabricated on elastic dielectric membranes can be reconfigured to achieve optical modulation at high frequencies, potentially reaching the gigahertz range. Mechanical and optical resonances enhance the magnitude of actuation and optical response within these nanostructures, which can be driven by electric signals of only a few volts or optical signals with power of only a few milliwatts. We envisage switchable, electro-optical, magneto-optical and nonlinear metamaterials that are compact and silicon-nanofabrication-technology compatible with functionalities surpassing those of natural media by orders of magnitude in some key design parameters.

Single protein sensing with asymmetric plasmonic hexamer via Fano resonance enhanced two-photon luminescence

Fano resonances in plasmonic systems have been proved to facilitate various sensing applications in the nanoscale. In this work, we propose an experimental scheme to realize a single protein sensing by utilizing its two-photon luminescence enhanced by a plasmonic Fano resonance system. The asymmetric gold hexamer supporting polarization-dependent Fano resonances and plasmonic modes without in-plane rotational symmetry is used as a referenced spatial coordinate for bio-sensing. We demonstrate via the full-vectorial three-dimensional simulation that the moving direction and the spatial location of a protein can be detected via its two-photon luminescence, which benefits from the resonant near-field interaction with the electromagnetic hot-spots. The sensitivity to changes in position of our method is substantially better compared with the conventional linear sensing approach. Our strategy would facilitate the sensing, tracking and imaging of a single biomolecule in deep sub-wavelength scale and with a small optical extinction cross-section.

Double Fano resonances in plasmonic nanocross molecules and magnetic plasmon propagation

Double Fano resonances in optical frequency are investigated in an artificial plasmonic molecule consisting of seven identical nanocrosses. These two Fano resonances are found to originate from different physical mechanisms. One is caused by the excitation of the inherent quadrupole dark mode supported by a single nanocross, and the other is attributed to the magnetic plasmon mode due to the generation of antiphase ring currents in adjacent fused tetramers. The two Fano resonances can either be tuned simultaneously or independently within a wide spectral range by adjusting the geometrical parameters of the nanocrosses. The excitation of the magnetic plasmon in a chain made of coupled nanoparticles allows for subwavelength guiding of optical energy with low radiative losses. The field decay length is as long as 2.608 μm, which is comparable to that of the magnetic plasmon waveguides and far surpasses the value achieved in electric plasmon counterparts. Because of the special shape of the nanocross, a Mach-Zehnder interferometer can be built to steer optical beams. These results show that the proposed plasmonic nanostructures have potential applications in biochemical sensing, narrow line-shape engineering and on-chip optical signal propagation in nanoscale integrated optics.

Switching plasmon coupling through the formation of dimers from polyaniline-coated gold nanospheres

Active modulation of the plasmon coupling in homodimers of polyaniline (PANI)-coated Au nanospheres is achieved by changing the proton-doping state of the PANI shell. Such a PANI-enabled modulation of the plasmon coupling in the dimers gives rise to remarkable spectral shifts, which show an exponential dependence on the interparticle gap distance. For the dimer with a 10 nm PANI shell thickness and a 0.5 nm gap distance, the shift of the stronger scattering peak in response to the active modulation reaches 231 nm. Electrodynamic simulations reveal that the shift of the dipolar bonding plasmon mode dominates the position variation of the stronger scattering peak for the dimers with different gap distances. Moreover, the quadrupolar bonding plasmon mode can be turned on and off by controlling the proton-doping state of the dimers with gap distances of less than ∼3 nm. These results are of high importance for fundamentally understanding the sensitivity of coupled plasmon resonance modes to the dielectric environment, as well as for designing active plasmonic devices with high modulation performances.

Actively tunable Fano resonances based on colossal magneto-resistant metamaterials

In this Letter, a periodic structure in which each unit cell consists of one manganese oxide (La(0.7)Ca(0.3)MnO(3)) strip and two gold strips is designed. By simulating the electromagnetic responses of the structure, we confirm that Fano resonances can be actively controlled in the infrared region by modulating the intensity of the external magnetic field applied to the structure. This is due to the colossal magneto-resistance of the La(0.7)Ca(0.3)MnO(3) material. Furthermore, a transmission phase can also be effectively tuned. The phase has a shift of ΔΦ=1.05  rad at a frequency of 130 THz when the intensity of the external magnetic field varies from 5083 to 5193  kA/m. Such a tunable method has potential applications in controllable photoelectric elements.

A flexible pressure sensitive colour changing device using plasmonic nanoparticles

We report on the fabrication of a flexible pressure sensitive device based on near-field coupling between silver nanoparticles and an underlying conductor. Visually apparent colour changes can be realized with minimal change in separation owing to the high fields localized to the particle's surface. The use of soft and compliant materials enables actuation of the device at low strain.

Plexcitonic crystals: a tunable platform for light-matter interactions

Coupled states of surface plasmon polaritons (SPPs) and excitons are collectively called plexcitons [Nano Lett.8, 3481 (2008)]. Plexcitonics is an emerging field of research aiming to control light-matter interaction at the nanometer length scale using coupled pairs of surface-plasmons and excitons. Ability to control the interaction between localized excitons and propagating surface-plasmons is important for realization of new photonic devices. In this letter, we report plexcitonic crystals that yield direction-dependent plasmon-exciton coupling. We have fabricated one- and two-dimensional plexcitonic crystals on periodically corrugated silver surfaces, which are loaded with J-aggregate complexes. We show that plasmon-exciton coupling is blocked for some crystal directions when exciton energy falls inside the plasmonic band gap of the periodically corrugated metallic surface.

Plasmonic Fano resonances in metallic nanorod complexes

Plasmonic Fano resonances (FRs) in nanostructures have been extensively studied in recent years. Nanorod-based complexes for FRs have also attracted much attention. The basic optical properties and fabrication technology of different kinds of plasmonic nanorods have been greatly developed over the last several years. The mutipole plasmon resonances and their flexible adjustment ranges on nanorods make them promising for FR modifications and structure diversity. In this paper, we review some recently studied plasmonic nanorod based nanostructures for FRs, including single nanorods, dimers, mutipole rods and nanorod-nanoparticle hybrids. The corresponding applications of the FRs are also briefly discussed.

Plasmonic Fano resonances in nanohole quadrumers for ultra-sensitive refractive index sensing

Plasmonic Fano resonances arising from electromagnetic interactions in metallic nanostructures exhibit spectral characteristics analogous to those from the electron waves in oligomer molecules. Though a great deal of research interest has been attracted to study the optical properties and explore the associated applications of metallic nanoparticle oligomers, the plasmonic response of their complementary structures--nanohole clusters--remains largely unexplored. Here we show numerically by a full-wave finite element method that a nanohole quadrumer can sustain two Fano resonances when the incident electric field is oriented along the long-axis of the quadrumer system. The underlying physical mechanisms responsible for the Fano resonance formation are revealed explicitly by spectrally deconstructing the Fano lineshape, spatially decomposing the structure configuration and mapping the electric field profile and charge distribution, which collectively demonstrate a strong mode coupling between either two antiparallel dipolar modes or dipole-quadruple modes in the nanohole quadrumer. We further show that the spectral profile of the Fano resonance including the resonance linewidth and spectral contrast can be engineered flexibly by adjusting the geometrical parameters of the nanohole cluster, including the nanohole diameter, film thickness and interhole distance. With an optimized and realistic geometrical configuration, the nanohole quadrumer system exhibits an overall sensing figure of merit up to 14.25, far surpassing the value reported for conventional nanoparticle oligomers.

Tunable SERS using gold nanoaggregates on an elastomeric substrate

We report on the self-assembly of colloidal gold nanoparticles on a stretchable, elastomeric membrane, and the use of this membrane as a base substrate for far-field confocal Raman measurements. Surface-enhanced Raman scattering (SERS) enhancement for such a substrate was estimated as 10(6) to 10(7). Atomic force microscopy has been used to study the changes in nanoparticle topography when the membrane is stretched. The homogeneous strain defined by average relative motion of nanoparticles is approximately half the macroscopically-applied biaxial strain. The SERS intensity was maximized when the membrane was at rest (i.e. without stretch), and reduced as stretching was increased. Our measurements are consistent with theoretical and numerical SERS enhancements for the interstitial gap between two spheres. The data indicate that the resting gap between the spheres is 11 nm or 16 nm, using two theoretical models. This work represents progress towards particularly facile sample fabrication and in situ tuning techniques for SERS.

Plasmonics in nanostructures

Plasmonics has developed into one of the rapidly growing research topics for nanophotonics. With advanced nanofabrication techniques, a broad variety of nanostructures can be designed and fabricated for plasmonic devices at nanoscale. Fundamental properties for both surface plasmon polaritons (SPP) and localized surface plasmons (LSP) arise a new insight and understanding for the electro-optical device investigations, such as plasmonic nanofocusing, low-loss plasmon waveguide and active plasmonic detectors for energy harvesting. Here, we review some typical functional plasmonic nanostructures and nanosmart devices emerging from our individual and collaborative research works.

Hybrid phase-change plasmonic crystals for active tuning of lattice resonances

Tunable lattice resonances are demonstrated in a hybrid plasmonic crystal incorporating the phase-change material Ge2Sb2Te5 (GST) as a 20-nm-thick layer sandwiched between a gold nanodisk array and a quartz substrate. Non-volatile tuning of lattice resonances over a range Δλ of about 500 nm (1.89 µm to 2.27 µm) is achieved experimentally via intermediate phase states of the GST layer. This work demonstrates the efficacy and ease of resonance tuning via GST in the near infrared, suggesting the possibility to design broadband non-volatile tunable devices for optical modulation, switching, sensing and nonlinear optical devices.