Multiple Fano resonances in plasmonic heptamer clusters composed of split nanorings

Enhancing Fano resonances through coupling of dark modes in a dual-ring nanostructure

In this paper we investigate the Fano resonances of a ring-disc nanostructure that consists of two nanodiscs and two concentric nanorings. The dark modes of both nanorings can couple to the bright mode of the nanodiscs, leading to separate Fano resonances from the outer and the inner nanoring. The concentric arrangement of the two nanorings allows for a coupling between the dark modes of the outer and the inner nanoring, thus creating an additional interaction that influences the Fano resonances of the dual-ring nanostructure. This interaction is investigated by comparing the Fano resonances of the complete dual-ring structure with the isolated Fano resonances of the individual single-ring structures. The effect of the coupling between dark modes on the Fano resonances is verified using a model of coupled harmonic oscillators that describe the Fano resonances of this system in a classical analogy. Lastly we compare the sensitivity of a single-ring nanostructure with that of a dual-ring nanostructure to investigate the effects of a coupling between dark modes on the sensing performance.

Multi-Wavelength Selective and Broadband Near-Infrared Plasmonic Switches in Anisotropic Plasmonic Metasurfaces

Anisotropic plasmonic metasurfaces have attracted broad research interest since they possess novel optical properties superior to natural materials and their tremendous design flexibility. However, the realization of multi-wavelength selective plasmonic metasurfaces that have emerged as promising candidates to uncover multichannel optical devices remains a challenge associated with weak modulation depths and narrow operation bandwidth. Herein, we propose and numerically demonstrate near-infrared multi-wavelength selective passive plasmonic switching (PPS) that encompasses high ON/OFF ratios and strong modulation depths via multiple Fano resonances (FRs) in anisotropic plasmonic metasurfaces. Specifically, the double FRs can be fulfilled and dedicated to establishing tailorable near-infrared dual-wavelength PPS. The multiple FRs mediated by in-plane mirror asymmetries cause the emergence of triple-wavelength PPS, whereas the multiple FRs governed by in-plane rotational asymmetries avail the implementation of the quasi-bound states in the continuum-endowed multi-wavelength PPS with the ability to unfold a tunable broad bandwidth. In addition, the strong polarization effects with in-plane anisotropic properties further validate the existence of the polarization-resolved multi-wavelength PPS. Our results provide an alternative approach to foster the achievement of multifunctional meta-devices in optical communication and information processing.

Digital coding Fano resonance based on active plasmonic metamaterials

A novel approach that employs active plasmonic metamaterials to create a digital coding Fano resonator is proposed, to the best of our knowledge. The meta-device consists of three concentric spoof localized surface plasmon (LSP) resonators and three positive-intrinsic-negative (PIN) diodes positioned at three slits located in the middle and inner LSP resonators. Four Fano resonant modes can be independently switched by controlling the biased voltage applied to the three diodes. This provides a means for encoded modulation of multiple Fano resonances in metamaterials, which could have broad applications in fields such as multiway sensing, plasmonic circuits, and switching. We experimentally demonstrate the effectiveness of the proposed approach, which offers promising potential for practical implementation.

Plasmonic Sensing and Switches Enriched by Tailorable Multiple Fano Resonances in Rotational Misalignment Metasurfaces

Fano resonances that feature strong field enhancement in the narrowband range have motivated extensive studies of light-matter interactions in plasmonic nanomaterials. Optical metasurfaces that are subject to different mirror symmetries have been dedicated to achieving nanoscale light manipulation via plasmonic Fano resonances, thus enabling advantages for high-sensitivity optical sensing and optical switches. Here, we investigate the plasmonic sensing and switches enriched by tailorable multiple Fano resonances that undergo in-plane mirror symmetry or asymmetry in a hybrid rotational misalignment metasurface, which consists of periodic metallic arrays with concentric C-shaped- and circular-ring-aperture unit cells. We found that the plasmonic double Fano resonances can be realized by undergoing mirror symmetry along the X-axis. The plasmonic multiple Fano resonances can be tailored by adjusting the level of the mirror asymmetry along the Z-axis. Moreover, the Fano-resonance-based plasmonic sensing that suffer from mirror symmetry or asymmetry can be implemented by changing the related structural parameters of the unit cells. The passive dual-wavelength plasmonic switches of specific polarization can be achieved within mirror symmetry and asymmetry. These results could entail benefits for metasurface-based devices, which are also used in sensing, beam-splitter, and optical communication systems.

Thermo-optically tunable slot waveguide-based dual mode-splitting resonators with enhanced sharp lineshapes

Slot waveguide plays an essential role in achieving high-performance on-chip photonic sensors and nonlinear devices. Ideally, slot waveguide features a large evanescent field ratio and strong electric field intensity in the slot, leading to a high waveguide sensitivity. Unfortunately, the microring resonator (MRR) based on the slot waveguide suffers the less steep spectral slope due to the low quality factor induced by the huge optical propagation loss of the slot waveguide. In this work, a novel dual mode-splitting resonator based on the slot waveguide is proposed and demonstrated to steepen the slope of lineshapes. The device is implemented by two racetrack resonators based on a slot waveguide and a feedback waveguide to introduce coherent optical mode interference, which could induce mode-splitting resonance (MR) with sharp asymmetry line shape and large extinction ratio (ER). The proposed device is fabricated by the standard complementary metal-oxide-semiconductor (CMOS) technologies on silicon-on-insulator (SOI) platform, and the characterization results show dual MRs with an ER of 45.0 dB and a slope rate (SR) of 58.3 dB/nm, exhibiting a much steeper lineshape than that of the conventional MRR with slot waveguide. And the resonance can be tuned efficiently by applying various voltages of the TiN microheater. Investigations in dual MRs devices promote many potential applications in the field of optical switching, optical modulating, and on-chip optical sensing.

Multiple Sharp Fano Resonances in a Deep-Subwavelength Spherical Hyperbolic Metamaterial Cavity

We theoretically study the multiple sharp Fano resonances produced by the near-field coupling between the multipolar narrow plasmonic whispering-gallery modes (WGMs) and the broad-sphere plasmon modes supported by a deep-subwavelength spherical hyperbolic metamaterial (HMM) cavity, which is constructed by five alternating silver/dielectric layers wrapping a dielectric nanosphere core. We find that the linewidths of WGMs-induced Fano resonances are as narrow as 7.4–21.7 nm due to the highly localized feature of the electric fields. The near-field coupling strength determined by the resonant energy difference between WGMs and corresponding sphere plasmon modes can lead to the formation of the symmetric-, asymmetric-, and typical Fano lineshapes in the far-field extinction efficiency spectrum. The deep-subwavelength feature of the proposed HMM cavity is verified by the large ratio (~5.5) of the longest resonant wavelength of WGM_1,1 (1202.1 nm) to the cavity size (diameter: 220 nm). In addition, the resonant wavelengths of multiple Fano resonances can be easily tuned by adjusting the structural/material parameters (the dielectric core radius, the thickness and refractive index of the dielectric layers) of the HMM cavity. The narrow linewidth, multiple, and tunability of the observed Fano resonances, together with the deep-subwavelength feature of the proposed HMM cavity may create potential applications in nanosensors and nanolasers.

Engineered nano-sphere array of gold-DNA core-shells and junctions as opto-plasmonic sensors for biodetection

In this paper, we design opto-plasmonic sensors by the engineered arrangement of gold-nanospheres. We use DNA-gold nanoparticle (GNP) core-shells and DNA rods as junctions between GNPs with a fishnet ground layer for controlling and improving the absorbance and reflection in the range of 100-300 THz. Based on available data, we check the effects of healthy and cancerous cells on the reflection parameter. Here, we demonstrate how the DNA junctions and distance between the nanospheres can be considered to modify the reflection. These structures can be utilized as opto-plasmonic sensors with high sensitivity to distinguish materials in terms of refractive indices. We can use an array of these sensors for both spectroscopy and optical imaging on a real scale. The proposed structures with different topologies are analyzed and their figure of merits (FOM) and sensitivities are obtained. The structure based on the DNA rods as junctions between GNPs shows the best FOM value of 340 RIU-1 and the core-shell heptamer structure has the best sensitivity of about 1287 nm RIU-1.

Dual polarized engineering the extinction cross-section of a dielectric wire using graphene-based oligomers

In this paper, graphene-coated spherical nanoparticles are arranged around an infinite length dielectric cylinder to enhance its extinction cross-section. Initially, a single longitudinal one-dimensional periodic array is considered in different loci concerning the transverse electric (TE) incident plane wave. It is observed that regardless of the position of the particles, the extinction cross-section of the dielectric cylinder is considerably enhanced with respect to the bare one. Later, by increasing the number of longitudinal plasmonic arrays around the cylinder, each residing in a different azimuthal direction, the extinction cross-section is further manipulated to observe double pronounced Fano resonances. The origin of the Fano resonances is described by considering their planar counterparts constructed by the periodic assembly of plasmonic oligomers. Finally, the hexamer configuration is considered as the prototype, and the effect of various optical, geometrical, and material parameters on the optical response is investigated in detail. Interestingly, due to the spherical symmetry of the cells, the extinction cross-section is also enhanced for the transverse magnetic (TM) incident wave, which is unattainable using a continuous plasmonic cover made of metal or graphene. The potential application of our proposed structure is in the design of reconfigurable conformal optical absorbers and sensors.

Hollow micro and nanostructures for therapeutic and imaging applications

Hollow particles have been extensively used in bioanalytical and biomedical applications for almost two decades due to their unique and tunable optoelectronic properties as well as their significantly high loading capacities. These intrinsic properties led them to be used in various bioimaging applications as contrast agents, controlled delivery (i.e. drugs, nucleic acids and other biomolecules) platforms and photon-triggered therapies (e.g. photothermal and photodynamic therapies). Since recent studies showed that imaging-guided targeted therapeutics have higher success rates, multimodal theranostic platforms (combination of one or more therapy and diagnosis modality) have been employed more often and hollow particles (i.e. nanoshells) have been one of the most efficient candidates to be used in multiple-purpose platforms, owing to their intrinsic properties that enable synergistic multimodal performance. In this review, recent advances in the applications of such hollow particles fabricated with various routes (either inorganic or organic based) were summarized to delineate strategies for tuning their properties for more efficient biomedical performance by overcoming common biological barriers. This review will pave the ways for expedited progress in design of next generation of hollow particles for clinical applications.

Strongly coupled evenly divided disks: a new compact and tunable platform for plasmonic Fano resonances

Plasmonic artificial molecules are promising platforms for linear and nonlinear optical modulation at various regimes including the visible, infrared and terahertz bands. Fano resonances in plasmonic artificial structures are widely used for controlling spectral lineshapes and tailoring of near-field and far-field optical response. Generation of a strong Fano resonance usually relies on strong plasmon coupling in densely packed plasmonic structures. Challenges in reproducible fabrication using conventional lithography significantly hinders the exploration of novel plasmonic nanostructures for strong Fano resonance. In this work, we propose a new class of plasmonic molecules with symmetric structure for Fano resonances, named evenly divided disk, which shows a strong Fano resonance due to the interference between a subradiant anti-bonding mode and a superradiant bonding mode. We successfully fabricated evenly divided disk structures with high reproducibility and with sub-20 nm gaps, using our recently developed sketch and peel lithography technique. The experimental spectra agree well with the calculated response, indicating the robustness of the Fano resonance for the evenly divided disk geometry. Control experiments reveal that the strength of the Fano resonance gradually increases when increasing the number of split parts on the disk from three to eight individual segments. The Fano-resonant plasmonic molecules that can also be reliably defined by our unique fabrication approach open up new avenues for application and provide insight into the design of artificial molecules for controlling light-matter interactions.

Double Fano resonances in hybrid disk/rod artificial plasmonic molecules based on dipole-quadrupole coupling

Fano resonance can be achieved by the destructive interference between a superradiant bright mode and a subradiant dark mode. A variety of artificial plasmonic oligomers have been fabricated to generate Fano resonance for its extensive applications. However, the Fano resonance in plasmonic oligomer systems comes from the interaction of all metal particles, which greatly limits the tunability of the Fano resonance. Besides, only a single Fano resonance is supported by many existing plasmonic oligomers, while multiple Fano resonances mostly occur in complex and multilayer structures, whose fabrication is greatly challenging. Here, a simple asymmetric plasmonic molecule consisting of a central metal disk and two side-coupled parallel metal rods is demonstrated. The simulation and experimental results clearly show that double Fano resonances appear in the transmission spectrum. In addition, the two Fano peaks can be independently tuned and single/double Fano peak switching can be achieved by changing one rod length or the gap distances between the rods and the disk. The modulation method is simple and effective, which greatly increases the tunability of the structure. The proposed asymmetric artificial plasmonic molecule can have applications in multi-channel optical switches, filters and biosensors. Moreover, the controllable plasmonic field intensity in the gap between the disk and rods also provides a new control means for plasmon-induced photocatalytic reactions and biosynthesis.

Multiple Fano Resonances with Tunable Electromagnetic Properties in Graphene Plasmonic Metamolecules

Multiple Fano resonances (FRs) can be produced by destroying the symmetry of structure or adding additional nanoparticles without changing the spatial symmetry, which has been proved in noble metal structures. However, due to the disadvantages of low modulation depth, large damping rate, and broadband spectral responses, many resonance applications are limited. In this research paper, we propose a graphene plasmonic metamolecule (PMM) by adding an additional 12 nanodiscs around a graphene heptamer, where two Fano resonance modes with different wavelengths are observed in the extinction spectrum. The competition between the two FRs as well as the modulation depth of each FR is investigated by varying the materials and the geometrical parameters of the nanostructure. A simple trimer model, which emulates the radical distribution of the PMM, is employed to understand the electromagnetic field behaviors during the variation of the parameters. Our proposed graphene nanostructures might find significant applications in the fields of single molecule detection, chemical or biochemical sensing, and nanoantenna.

Independently tunable double Fano resonances based on waveguide-coupled cavities

In this Letter, we first demonstrate periodically and independently tunable double Fano resonances (DFRs) using waveguide-coupled cavities consisting of two silicon microring resonators (MRRs) and a feedback-coupled waveguide. The proposed device is fabricated on the silicon-on-insulator substrate using the standard complementary metal-oxide-semiconductor fabrication process. The DFR can be tuned independently by changing the resonant wavelengths of two MRRs using the thermo-optic effect. The highest extinction ratio of the Fano resonances is measured to be as high as 29.20 dB, which enables this device to be a promising candidate for high-performance multi-wavelength optical switches and high-sensitivity biochemical sensors.

Tunable multiple Fano resonance employing polarization-selective excitation of coupled surface-mode and nanoslit antenna resonance in plasmonic nanostructures

Modeling and tailoring of multispectral Fano resonance in plasmonic system employing nanoslit-antenna array is demonstrated and investigated. Efficient control of the multiple Fano profile can be manipulated, where the overall spectral is achieved by the separate contributions from the fundamental subgroups plasmonic resonance eigenstates. A polarization-selective strategy on nano-antennas resonance is proposed to shed light on the efficient manipulation of the multiple Fano resonances. Theory prediction of TM₋₁ surface mode excited in the system and thorough dispersion analysis of the supported Bloch modes provides evidence for understanding the origin of the transmission spectra. Compact nanophotonics planar optical linear-polarizer in the proposed nanostructure is investigated and demonstrated, where flexible Fano resonance control over the profile, linewidth and spectral contrast is appealing for applications such as sensing, switches and multifunctional nanophotonics devices.

Tailoring Fano lineshapes using plasmonic nanobars for highly sensitive sensing and directional emission

Plasmonic oligomers are one class of the most promising nanoclusters for generating Fano resonances. This study reveals that a nanobar-based heptamer concurrently sustains triple polarization-dependent Fano resonances, in sharp contrast to traditional nanodisk or nanosphere-based counterparts. Benefiting from the enhanced near field and reduced spectral linewidth, the gold heptamer exhibits a high refractive index sensitivity (940 nm per RIU) together with a figure of merit (FoM) value as large as 20.9, which outperforms that of most other gold oligomers. On the other hand, it is found that the spectral positions of hybridized eigenmodes depend strongly on the spatial configurations of the constituent nanobars. As a proof of concept, we design a simple heterodimer comprising a nanocross and a nanobar, where plasmonic modes with opposite radiative decay characteristics are excellently overlapped both spectrally and spatially by elaborate tailoring. Double strong Fano resonances appear on opposite sides of the spectrum as expected. More interestingly, the radiation main lobes all point to one direction at these two Fano resonances due to the spatial charge distributions and mode interferences with the maximal directivity ratio (DR) as high as 22.4, in a similar manner to the radio frequency (RF) Yagi-Uda antenna. Furthermore, the emission directions can also be easily switched by adjusting the orientations of the individual nanobar in the heterodimer. Our study demonstrates that the nanobar-based oligomers with tailored Fano lineshapes could serve as versatile and delicate platforms for the label-free biochemical sensing and directional transmission of optical information at the nanometre scale.

Five-fold plasmonic Fano resonances with giant bisignate circular dichroism

Chiral metamaterials with versatile designs can exhibit orders of magnitude enhancement in chiroptical responses compared with that of the natural chiral media. Here, we propose an ease-of-fabrication three-dimensional (3D) chiral metamaterial consisting of vertical asymmetric plate-shape resonators along a planar air hole array with extraordinary optical transmission. It is theoretically shown that such chiral metamaterials simultaneously support five-fold plasmonic Fano resonance states and exhibit significant bisignate circular dichroism (CD) with amplitude as large as 0.8 due to the distinctive local electric field distributions. More interestingly, a "bridge" in the proposed double-plate-based architectures can act as a flipped ruler that is able to continuously manipulate optical chirality including the handedness-selective enhancement and the switching of CD signals. Importantly, the proposed designs have been readily fabricated by using a focused-ion-beam irradiation-induced folding technique and they consistently exhibited five-fold Fano resonances with strong CD effects in experiments. The studies are helpful for the understanding, designing and improvement of chiral optical systems towards potential applications such as ultrasensitive biosensing, polarimetric imaging, quantum information processing, etc.

Tunable multiband plasmonic response of indium antimonide touching microrings in the terahertz range

In this paper, we propose a terahertz (THz) plasmonic structure that supports three resonance modes, including the charge transfer plasmon (CTP), the bonding dipole-dipole plasmon, and the antibonding dipole-dipole plasmon, which can be strongly tuned by geometrical parameters, passively, and the temperature, actively. The structure exhibits a considerable thermal sensitivity of more than 0.01 THz/K. The introduced multiband and tunable THz plasmonic structures offer important applications in thermal switches, thermo-optical modulators, broadband filters, design of multifunctional molecules originating from the multiband specification of the proposed structure, and improvement in plasmonic sensor applications stemming from a detailed study of the CTP mode.

Polydopamine-based concentric nanoshells with programmable architectures and plasmonic properties

Nanoshells, classically comprising gold as the metallic component and silica as the dielectric material, are important for fundamental studies in nanoplasmonics. They also empower a myriad of applications, including sensing, energy harvesting, and cancer therapy. Yet, laborious preparation precludes the development of next-generation nanoshells with structural complexity, compositional diversity, and tailorable plasmonic behaviors. This work presents an efficient approach to the bottom-up assembly of concentric nanoshells. By employing polydopamine as the dielectric material and exploiting its intrinsic adhesiveness and pH-tunable surface charge, the growth of each shell only takes 3-4 hours at room temperature. A series of polydopamine-based concentric nanoshells with programmable nanogap thickness, elemental composition (gold and silver), and geometrical configuration (number of layers) is prepared, followed by extensive structural characterization. Four of the silver-containing nanostructures are newly reported. Systematic investigations into the plasmonic properties of concentric nanoshells as a function of their structural parameters further reveal multiple Fano resonances and local-field "hot spots", infrequently reported plasmonic features for individual nanostructures fabricated using bottom-up wet chemistry. These results establish materials design rules for engineering complex plasmon-based systems originating from the integration of multiple plasmonic elements into defined locations within a compact nanostructure.

Magnetic Fano resonance of heterodimer nanostructure by azimuthally polarized excitation

The optical properties of a Si-Au heterodimer nanostructure, which is composed of an Au split nanoring surrounded by a Si nanoring with a larger diameter, are investigated both theoretically and numerically. It is found that a pure magnetic plasmon Fano resonance can be achieved in the Si-Au heterodimer nanostructure when it is excited by an azimuthally polarized beam. It is revealed that the pure magnetic Fano resonance is generated by the destructive interference between the magnetic dipole resonance of the Si nanoring and the magnetic dipole resonance of the Au split nanoring. A coupled oscillator model is employed to analyze the Fano resonance of the Si-Au heterodimer nanostructure. The pure magnetic response of the Si-Au heterodimer nanostructure is verified by the current density distributions and the scattering powers of the electric and magnetic multipoles. The Fano resonance in the Si-Au heterodimer nanostructure exhibits potential applications of low-loss magnetic plasmon resonance in the construction of artificial magnetic metamaterials.

Fano resonance with high local field enhancement under azimuthally polarized excitation

Being an enabling technology for applications such as ultrasensitive biosensing and surface enhanced spectroscopy, enormous research interests have been focused on further boosting the local field enhancement at Fano resonance. Here, we demonstrate a plasmonic Fano resonance resulting from the interference between a narrow magnetic dipole mode and a broad electric dipole mode in a split-ring resonator (SRR) coupled to a nanoarc structure. Strikingly, when subjected to an azimuthally polarized beam (APB) excitation, the intensity enhancement becomes more than 60 times larger than that for a linearly polarized beam (LPB). We attribute this intensity enhancement to the improved conversion efficiency between the excitation and magnetic dipole mode along with improved near-field coupling. The APB excited Fano structure is further used as a nanoruler and beam misalignment sensor, due to the high sensitivity of intensity enhancement and scattering spectra to structure irregularities and excitation beam misalignment. Interestingly, we find that, regardless of the presence of structural translations, the proposed structure still maintains over 60 times better intensity enhancement under APB excitation compared to LPB excitation. Moreover, even if the APB excitation is somewhat misaligned, our Fano structure still manages to give a larger intensity enhancement than its counterpart excited by LPB.

Fano Transparency in Rounded Nanocube Dimers Induced by Gap Plasmon Coupling

Homodimers of noble metal nanocubes form model plasmonic systems where the localized plasmon resonances sustained by each particle not only hybridize but also coexist with excitations of a different nature: surface plasmon polaritons confined within the Fabry-Perot cavity delimited by facing cube surfaces (i.e., gap plasmons). Destructive interference in the strong coupling between one of these highly localized modes and the highly radiating longitudinal dipolar plasmon of the dimer is responsible for the formation of a Fano resonance profile and the opening of a spectral window of anomalous transparency for the exciting light. We report on the clear experimental evidence of this effect in the case of 50 nm silver and 160 nm gold nanocube dimers studied by spatial modulation spectroscopy at the single particle level. A numerical study based on a plasmon mode analysis leads us to unambiguously identify the main cavity mode involved in this process and especially the major role played by its symmetry. The Fano depletion dip is red-shifted when the gap size is decreasing. It is also blue-shifted and all the more pronounced that the cube edge rounding is large. Combining nanopatch antenna and plasmon hybridization descriptions, we quantify the key role of the face-to-face distance and the cube edge morphology on the spectral profile of the transparency dip.

Strong plasmon coupling in self-assembled superparamagnetic nanoshell chains

Construction of ordered patterns of plasmonic nanoparticles is greatly important for nanophotonics relevant applications. We have reported a facile and low-cost magnetic field induced self-assembly approach to construct plasmonic superparamagnetic nanoshell (SN) chains up to several hundred micrometers in a few seconds in a large area without templates or other assistance processes. Experimental and theoretical investigations of the near- and far-field optical properties indicate that the super- and sub-radiant modes of the SN chains continuously redshift with the increase of SN number and the Fano resonance emerges in the infinite double- and triple-line SN chains. Strong plasmon coupling effects in the SN chains result in great electric field enhancements at visible and infrared wavelengths, which indicates that these chain structures potentially can be used as a common substrate for both surface enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA) application. This fabrication method also offers a general strategy alternative to top-down processing that enables the construction of nanostructures for metamaterials, electromagnetic energy transport, and optical waveguide.

Quantitative and Direct Near-Field Analysis of Plasmonic-Induced Transparency and the Observation of a Plasmonic Breathing Mode

We investigated experimentally and numerically in the optical near-field a plasmonic model system similar to a dolmen-type structure for phenomena such as plasmon-induced transparency. Through engineering of coupling strength, structure orientation, and incident angle and phase of the excitation source it was possible to control near-field excitation of the dark modes. We showed that quantitative analysis of near-field amplitude and excitation strength provided essential information that allowed identifying the interaction between the bright and the dark mode and how it causes the formation of plasmon-induced transparency features and a Fano resonance. In addition, we introduced a mechanism to excite field distributions in plasmonic structures that cannot be accessed directly using far-field illumination and demonstrated the excitation of a dark mode akin to a symmetry-forbidden plasmonic breathing mode using a linearly polarized far-field source.

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.

A sensitive 2D plasmon ruler based on Fano resonance

In this paper, we designed a 2D distance and rotation angle plasmon ruler based on Fano resonance of a trimer nanostructure, which consists of a concentric square nanoring–disk and an outside nanorod (CSRDR).

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.

Polarization state-based refractive index sensing with plasmonic nanostructures

Spectral-based methods are often used for label-free biosensing. However, practical implementations with plasmonic nanostructures suffer from a broad line width caused by strong radiative and nonradiative losses, and the sensing performance characterized by figure of merit is poor for these spectral-based methods. This study provides a polarization state-based method using plasmonic nanostructures to improve the sensing performance. Instead of the intensity spectrum, the polarization state of the transmitted field is monitored to analyze variations of the surrounding medium. The polarization state of incidence is strongly modified due to the excitation of surface plasmons, and the ellipticity of the transmitted field changes dramatically around plasmon resonances. Sharp resonances with line widths down to sub-nanometer are achieved by plotting the spectra of the reciprocal of ellipticity. Therefore, the sensing performance can be significantly improved, and a theoretical value of the figure of merit exceeding 1700 is achieved by using the polarization state-based sensing approach.

Polarization-selective dynamically tunable multispectral Fano resonances: decomposing of subgroup plasmonic resonances

We analyze the design of near infrared all-optical controllable and dynamically tunable multispectral Fano resonances based on subgroup decomposition of plasmonic resonances in hybrid nanoslits antenna plasmonic system. The theoretical investigation complemented with numerical simulations show that the Fano resonance lines shape can be tailored efficiently and continuously with the nanoslits geometry and the variation of the polarization states of the incident light. The subgroup decomposition of the spectral profile and the modification of plasmonic resonances lineshape that leads to the Fano-type profile of transmission is investigated and revealed. The separate contribution from individual spectral of single-slit array subgroup is attributed to the resulting overall multispectral Fano lineshape of the proposed T-shaped slits array at their corresponding spectral peaks zone. The polarization-selective tunability of the multispectral Fano resonances in the planar hybrid plasmonic system creates new avenues for designing multi-channel multi-wavelength tunable Fano effect.

Magnetic plasmonic Fano resonance at optical frequency

Plasmonic Fano resonances are typically understood and investigated assuming electrical mode hybridization. Here we demonstrate that a purely magnetic plasmon Fano resonance can be realized at optical frequency with Au split ring hexamer nanostructure excited by an azimuthally polarized incident light. Collective magnetic plasmon modes induced by the circular electric field within the hexamer and each of the split ring can be controlled and effectively hybridized by designing the size and orientation of each ring unit. With simulated results reproducing the experiment, our suggested configuration with narrow line-shape magnetic Fano resonance has significant potential applications in low-loss sensing and may serves as suitable elementary building blocks for optical metamaterials.

Fano-like resonances in split concentric nanoshell dimers in designing negative-index metamaterials for biological-chemical sensing and spectroscopic purposes

In this study, we investigated numerically the plasmon response of a dimer configuration composed of a couple of split and concentric Au nanoshells in a complex orientation. We showed that an isolated composition of two concentric split nanoshells could be tailored to support strong plasmon resonant modes in the visible wavelengths. After determining the accurate geometric dimensions for the presented antisymmetric nanostructure, we designed a dimer array that shows complex behavior during exposure to different incident polarizations. We verified that the examined dimer was able to support destructive interference between dark and bright plasmon modes, which resulted in a pronounced Fano-like dip. Observation of a Fano minimum in such a simple molecular orientation of subwavelength particles opens new avenues for employing this structure in designing various practical plasmonic devices. Depositing the final dimer in a strong coupling condition on a semiconductor metasurface and measuring the effective refractive index at certain wavelengths, we demonstrate that each one of dimer units can be considered a meta-atom due to the high aspect ratio in the geometric parameters. Using this method, by extending the number of dimers periodically and illuminating the structure, we examined the isotropic, polarization-dependent, and transmission behavior of the metamaterial configuration. Using numerical methods and calculating the effective refractive indices, we computed and sketched corresponding figure of merit over the transmission window, where the maximum value obtained was 42.3 for Si and 54.6 for gallium phosphide (GaP) substrates.

Templated fabrication of periodic arrays of metallic and silicon nanorings with complex nanostructures

Here we report a scalable colloidal templating approach for fabricating periodic arrays of metallic and silicon nanorings with complex nanostructures. Non-close-packed monolayer silica colloidal crystal prepared by a simple spin-coating technology is first used as template for making periodic arrays of mushroom-like composite nanostructures consisting of silica spherical caps and polymer stems. Subsequent metal sputtering and reactive ion etching lead to the formation of ordered asymmetric nickel nanorings which can be further utilized as etching masks for patterning periodic arrays of symmetric silicon nanorings. Moreover, periodic arrays of metallic and silicon concentric double nanorings can be fabricated by using the asymmetric nickel nanorings as templates. We have also demonstrated that gold concentric double nanorings show strong surface-enhanced Raman scattering (SERS) with a SERS enhancement factor of ∼9.5 × 10(7) from adsorbed benzenethiol molecules. The SERS enhancement and the electric field amplitude distribution surrounding gold concentric double nanorings have been calculated by using finite element electromagnetic modeling. This new colloidal templating technique is compatible with standard microfabrication and enables wafer-scale production of a variety of periodic nanorings with hierarchical structures that could find important technological applications in plasmonic and magnetic devices.

Magnetic-based Fano resonance of hybrid silicon-gold nanocavities in the near-infrared region

Direct interference between the orthogonal electric and magnetic modes in a hybrid silicon-gold nanocavity is demonstrated to induce a pronounced asymmetric magnetic-based Fano resonance in the total scattering spectrum at near-infrared frequencies. Differing from the previously reported magnetic-based Fano resonances in metal nanoparticle clusters, the narrow discrete mode provided by the silicon magnetic dipole resonance can be directly excited by external illumination, and greatly enhanced electric and magnetic fields are simultaneously obtained at the Fano dip.

Tuneable complementary metamaterial structures based on graphene for single and multiple transparency windows

Novel graphene-based tunable plasmonic metamaterials featuring single and multiple transparency windows are numerically studied in this paper. The designed structures consist of a graphene layer perforated with quadrupole slot structures and dolmen-like slot structures printed on a substrate. Specifically, the graphene-based quadrupole slot structure can realize a single transparency window, which is achieved without breaking the structure symmetry. Further investigations have shown that the single transparency window in the proposed quadrupole slot structure is more likely originated from the quantum effect of Autler-Townes splitting. Then, by introducing a dipole slot to the quadrupole slot structure to form the dolmen-like slot structure, an additional transmission dip could occur in the transmission spectrum, thus, a multiple-transparency-window system can be achieved (for the first time for graphene-based devices). More importantly, the transparency windows for both the quadrupole slot and the dolmen-like slot structures can be dynamically controlled over a broad frequency range by varying the Fermi energy levels of the graphene layer (through electrostatic gating). The proposed slot metamaterial structures with tunable single and multiple transparency windows could find potential applications in many areas such as multiple-wavelength slow-light devices, active plasmonic switching, and optical sensing.

From Fano-like interference to superscattering with a single metallic nanodisk

Superscattering was theoretically proposed to significantly enhance the scattering cross-section of a subwavelength nanostructure, far exceeding its single-resonance limit by employing resonances of multiple plasmonic modes. By numerical simulation, we design a subwavelength nanodisk as a simple candidate to achieve superscattering. Due to the phase retardation, the subradiant mode can be excited and interact with the superradiant mode in both spatial and frequency domains. By changing the height and diameter of the nanodisk, we show high tunability of the mode interaction and evolution of the resulting spectral features from Fano-like resonance to superscattering. A model of two-driven coupled oscillators is proposed to quantitatively analyze the spectral evolution. We find that the evolution is caused by not only alignment of the resonant wavelengths of related plasmonic modes, but also reasonably high loss. We show that superscattering doubles the near-field intensity, potentially enhancing the signal 16 times for SERS and 4 times for SEIRS, and doubles the far-field intensity and decreases the peak linewidth, improving the figure of merit for plasmonic refractometric sensing. Our study provides quantitative physical insight into understanding superscattering and Fano-like resonances in a single nanoparticle.

Multiple Fano resonances in spoof localized surface plasmons

We present the occurrence of bright modes and dark modes in spoof localized surface plasmons (LSPs) generated by ultrathin corrugated metallic disks. As two such disks with asymmetric geometries are placed in close proximity, we find that dark modes (in multipoles) of one disk emerge by coupling with the bright modes (in dipoles) of the other disk. Then we further observe multiple Fano resonances due to destructive interferences of dark modes with the overlapping and broadened bright modes. These Fano line-shapes clearly exhibit the strong polarization dependence. We design and fabricate the ultrathin corrugated bi-disk structure in the microwave frequency, and the measurement results show reasonable agreement with theoretical predictions and numerical simulations. Such multiple Fano resonances could be exploited for the plasmonic devices at lower frequencies.

Independently tunable double Fano resonances in asymmetric MIM waveguide structure

In this paper, an asymmetric plasmonic structure composed of a MIM (metal-insulator-metal) waveguide and a rectangular cavity is reported, which can support double Fano resonances originating from two different mechanisms. One of Fano resonance originates from the interference between a horizontal and a vertical resonance in the rectangular cavity. And the other is induced by the asymmetry of the plasmonic structure. Just because the double Fano resonances originate from two different mechanisms, each Fano resonance can be well tuned independently by changing the parameters of the rectangular cavity. And during the tuning process, the FOMs (figure of merit) of both the Fano resonances can keep unchanged almost with large values, both larger than 650. Such, the transmission spectra of the plasmonic structure can be well modulated to form transmission window with the position and the full width at half maximum (FWHM) can be tuned freely, which is useful for the applications in sensors, nonlinear and slow-light devices.

Multiple magnetic mode-based Fano resonance in split-ring resonator/disk nanocavities

Plasmonic Fano resonance, enabled by the weak interaction between a bright super-radiant and a subradiant resonance mode, not only is fundamentally interesting, but also exhibits potential applications ranging from extraordinary optical transmission to biosensing. Here, we demonstrate strong Fano resonances in split-ring resonators/disk (SRR/D) nanocavities. The high-order magnetic modes are observed in SRRs by polarization-resolved transmission spectroscopy. When a disk is centered within the SRRs, multiple high-order magnetic modes are coupled to a broad electric dipole mode of SRR/D, leading to significant Fano resonance spectral features in near-IR regime. The strength and line shape of the Fano resonances are tuned through varying the SRR split-angle and interparticle distance between SRR and disk. Finite-difference-time-domain (FDTD) simulations are conducted to understand the coupling mechanism, and the results show good agreement with experimental data. Furthermore, the coupled structure gives a sensitivity of ∼282 nm/RIU with a figure of merit ∼4.

Substrate-mediated charge transfer plasmons in simple and complex nanoparticle clusters

A conductive substrate can provide a simple and straightforward way to induce charge-transfer plasmon modes in Au nanoparticle clusters. For a simple dimer structure, a remarkably narrow charge transfer plasmon, which differs dramatically from the dipolar plasmon mode of the electrically isolated nanostructure, is clearly observed. For a more complex nonamer cluster that supports a strong Fano resonance on an insulating substrate, a mixed charge transfer-dipole mode is observed, where charge transfer is induced on the outer nanoparticles, establishing an opposing dipole on the intervening central particles, resulting in a strongly damped far field response.

Dipole plasmon resonance induced large third-order optical nonlinearity of Au triangular nanoprism in infrared region

Au triangular nanoprisms with strong dipole plasmon absorption peak at 1240 nm were prepared by wet chemical methods. Both numerical calculations and experiments were carried out to investigate the optical properties of the samples. Finite difference time domain (FDTD) and Local Density of States (LDOS) calculations demonstrate that strong electric field enhancement and large LDOS can be obtained at tip areas of the Au triangular nanoprisms. Z scan techniques were used to characterize the nonlinear absorption, nonlinear refraction, as well as one- and two-photon figures of merit (W and T, respectively) of the sample. The results show that maximum nonlinear refractive index can be obtained around the resonance absorption wavelength of 1240 nm, detuning the wavelength from the absorption peak will lead to the decrease of the nonlinear refractive index n(2), while the nonlinear absorption coefficient β doesn't change much with the wavelength. This large wavelength dependence of n(2) and small change of β enable the sample to satisfy the all-optical switching demand of W> 1 and T< 1 easily in a large wavelength range of 1200-1300 nm. These significant nonlinear properties of the sample imply that Au triangular nanoprism is a good candidate for future optical switches in infrared optical communication wavelength region.

Periodic arrays of metal nanorings and nanocrescents fabricated by a scalable colloidal templating approach

Here, we report a scalable bottom-up approach for fabricating periodic arrays of metal nanorings and nanocrescents. Wafer-scale monolayer silica colloidal crystals with an unusual non-close-packed structure prepared by a simple and rapid spin-coating technology are used as both etching and shadowing masks to create nanoring-shaped trenches in between templated polymer posts and sacrificial nanoholes. Directional deposition of metals in the trenches followed by liftoff of the polymer posts and the sacrificial nanoholes results in forming ordered metal nanorings. The inner and outer radii of the final nanorings are determined by the sizes of the templated polymer posts and the silica microspheres which can be easily adjusted by tuning the spin-coating and templating conditions. Most importantly, by simply controlling the tilt angle of the substrate toward the directional metal beams, continuous geometric transition from concentric nanorings to eccentric nanorings to nanocrescents can be achieved. This new colloidal templating approach is compatible with standard semiconductor microfabrication, promising for mass-production and on-chip integration of periodic nanorings and nanocrescents for a wide spectrum of technological applications ranging from nanooptical devices and ultrasensitive biosensing to magnetic memories and logic circuits.

Au nanocrystal array/silicon nanoantennas as wavelength-selective photoswitches

Au nanocrystal array/silicon nanoantennas exhibiting wavelength-selective photocurrent enhancement were successfully fabricated by a facile and inexpensive method combining colloidal lithography (CL) and a metal-assisted chemical etching (MaCE) process. The localized surface plasmon resonance (LSPR) response and wavelength-selective photocurrent enhancement characteristics were achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through a MaCE process. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. In addition, it can be integrated to well-developed Si-based manufacturing process. These characteristics make Au nanocrystal array/Si nanoantennas promising as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices.

Mechanisms of Fano resonances in coupled plasmonic systems

Fano resonances in hybridized systems formed from the interaction of bright modes only are reported. Despite precedent works, we demonstrate theoretically and experimentally that Fano resonances can be obtained by destructive interference between two bright dipolar modes out of phase. A simple oscillator model is provided to predict and fit the far-field scattering. The predictions are verified with numerical calculations using a surface integral equation method for a wide range of geometrical parameters. The validity of the model is then further demonstrated with experimental dark-field scattering measurements on actual nanostructures in the visible range. A remarkable set of properties like crossings, avoided crossings, inversion of subradiant and superradiant modes and a plasmonic equivalent of a bound state in the continuum are presented. The nanostructure, that takes advantage of the combination of Fano resonance and nanogap effects, also shows high tunability and strong near-field enhancement. Our study provides a general understanding of Fano resonances as well as a simple tool for engineering their spectral features.

Geometric Dependence of the Line Width of Localized Surface Plasmon Resonances

For the same number of electrons and plasmon frequencies, longitudinal plasmon resonances in metallic nanorods exhibit narrower line widths than plasmon modes in spherical particles. We show that this property is a general feature of high aspect ratio nanostructures and can be explained very simply by incorporating retardation effects into a harmonic oscillator model. The origin of the effect is dynamic depolarization, which renormalizes the mass of the electrons and the oscillating electron liquid. The scattering spectrum derived from our model agrees very well with FDTD simulations. Because plasmon damping determines many important features and applications of LSPR, such as the Q factor of plasmonics devices and the magnitude of the induced field enhancements, our study will play an important role for the design of nanostructures with narrow plasmon resonances.

Multiple Fano resonances in single-layer nonconcentric core-shell nanostructures

Multiple plasmonic Fano resonances are generally considered to require complex nanostructures, such as multilayer structure, to provide several dark modes that can couple with the bright mode. In this paper, we show the existence of multiple Fano resonances in single layer core-shell nanostructures where the multiple dark modes appear due to the geometrical symmetry breaking induced by axial offset of the core. Both dielectric-core-metal-shell (DCMS) and metal-core-dielectric-shell (MCDS) configurations have been studied. Compared to the MCDS structure, the DCMS configuration provides higher modulation depth. Analytical studies based on transformation optics and numerical simulations have been performed to investigate the role of geometrical and material parameters on the optical properties of the proposed nanostructures. Refractive index sensing with higher-order Fano resonances has also been described, providing opportunity for multiwavelength sensing with high figure of merit.

Double Fano resonances due to interplay of electric and magnetic plasmon modes in planar plasmonic structure with high sensing sensitivity

Double Fano resonant characteristics are investigated in planar plasmonic structure by embedding a metallic nanorod in symmetric U-shaped split ring resonators, which are caused by a strong interplay between a broad bright mode and narrow dark modes. The bright mode is resulted from the nanorod electric dipole resonance while the dark modes originate from the magnetic dipole induced by LC resonances. The overlapped dual Fano resonances can be decomposed to two separate ones by adjusting the coupling length between the nanorod and U-shaped split ring resonators. Fano resonances in the designed structure exhibit high refractive-index sensing sensitivity and figure of merit, which have potential applications in single or double-wavelength sensing in the near-infrared region.

Plasmonic oligomers in cylindrical vector light beams

We investigate the excitation as well as propagation of magnetic modes in plasmonic nanostructures. Such structures are particularly suited for excitation with cylindrical vector beams. We study magneto-inductive coupling between adjacent nanostructures. We utilize high-resolution lithographic techniques for the preparation of complex nanostructures consisting of gold as well as aluminium. These structures are subsequently characterized by linear optical spectroscopy. The well characterized and designed structures are afterwards studied in depth by exciting them with radial and azimuthally polarized light and simultaneously measuring their plasmonic near-field behavior. Additionally, we attempt to model and simulate our results, a project which has, to the best of our knowledge, not been attempted so far.

Detailed mechanism for the orthogonal polarization switching of gold nanorod plasmons

In this work, we describe an electro-optic material capable of orthogonally switching the polarization of the localized surface plasmon resonance scattering of single gold nanorods, independent of their orientation. Liquid crystal samples are prepared in a sandwich configuration with electrodes arranged so that an applied voltage induces alignment-switching of the liquid crystal molecules covering individual gold nanorods. Due to the birefringence of the nematic liquid crystal, the reorientation in the nematic director alignment causes a change in the output polarization of the scattered light. We propose the underlying mechanism to be based on a homogeneous nematic to twisted nematic phase transition and provide support for it via Jones calculus by modelling the effect of ideally aligned homogeneous nematic and twisted nematic phases on polarized light transmitted through the sample. In the model, we include the effects of sample thickness and surface plasmon resonance wavelength, expressed in terms of the phase retardation, χ, on the observed output polarization. We find four distinctively different trends for the output polarization as a function of the incident polarization as χ is varied. Two of these cases provide reproducible orthogonal polarization switching of the surface plasmon resonance while maintaining a high degree of polarization. These results are verified experimentally with liquid crystal cells of different thicknesses. The deviation of the experimental samples from ideal behaviour can be explained by the inherent variations in the surface plasmon resonance maximum and local cell thickness.