Metal-dielectric Nanostructures Research Articles

Numerical investigation of high-order harmonic generation by gold nanorod arrays in hybrid nanostructures: effect of number of gold nanorod

Hybrid metal-dielectric nanostructures are highly effective and helpful in generating and enhancing linear and nonlinear optical phenomena due to their unique optical properties. In this study, the linear optical responses and second- and third-order harmonics have been calculated numerically for this structure. Nonlinear optical response efficiency has been studied at the resonance wavelengths of the nanostructure by varying its parameters (radius and length). The desired efficiency can be achieved by adjusting the radius and length of the nanorods and matching the resonance wavelengths with the fundamental and generation frequencies of nonlinear optical phenomena in specific applications. Moreover, the effect of the number of gold nanorods on harmonic generation has been investigated. By increasing the number of nanorods in pairs, it is possible to generate high-order harmonics. The acquired results can be used to improve optical devices such as biomedical equipment, optical integrated circuits, and sensors.

Ultralow switching threshold optical bistable devices based on epsilon-near-zero Ga<sub>2</sub>O<sub>3</sub>-SiC-Ag multilayer structures

Optical bistability has attracted much attention due to its enormous potential applications in all-optical operation and signal processing. However, the weak nonlinear responses typically require huge pump power to reach the threshold of the optical bistability, thus hindering the real applications. In this study, we propose an efficient optical bistable metamaterial, which is composed of multilayer Ga<sub>2</sub>O<sub>3</sub>-SiC-Ag metal-dielectric nanostructures. We not only use the epsilon-near-zero (ENZ) with SiC-Ag thin layers to enhance the substantial field, but also incorporate the SiC material to increase its significant optical nonlinear coefficient. In the structural design, the introduction of Ga<sub>2</sub>O<sub>3</sub> layer facilitates the light field concentration, contributing to the further reduction in threshold power for optical bistability, and also conducing to the improvement of the physical and chemical stability of the device. The influences of the thickness and length of the ENZ layer on the optical bistability are systematically investigated by using the finite element method. The results demonstrate that optical bistability becomes more pronounced with the increase of the thickness and length of ENZ layer, exhibiting a bistability switching threshold as low as ~10<sup>–6</sup> W/cm<sup>2</sup> in the telecommunication band. Comparing with the previously reported optical bistability based on ENZ mechanism, the threshold shows a significant reduction by 9 orders of magnitude, demonstrating great application potential in the fields of semiconductor devices and photonic integrated circuits.

Near-Perfect Infrared Transmission Based on Metallic Hole and Disk Coupling Array for Mid-Infrared Refractive Index Sensing

Nanostructured color filters, particularly those generated by the extraordinary optical transmission (EOT) resonance of metal–dielectric nanostructures, have been intensively studied over the past few decades. In this work, we propose a hybrid array composed of a hole array and a disk array with the same working period within the 3–14 μm mid-infrared band. Through numerical simulations, near-perfect transmission (more than 99%) and a narrower linewidth at some resonance wavelengths were achieved, which is vital for highly sensitive sensing applications. This superior performance is attributed to the surface plasmon coupling resonance between the hole and disk arrays. A high tunability of the near-perfect transmission peak with varying structural parameters, characteristics of sensitivity to the background refractive index, and angle independence were observed. We expect that this metallic hole and disk coupling array is promising for use in various applications, such as in plasmon biosensors for the high-sensitivity detection of biochemical substances.

Optical anapole modes in hybrid metal–dielectric nanoantenna for near-field enhancement and optical sensing

Metal–dielectric nanostructures in the optical anapole modes are essential for light–matter interactions due to the low material loss and high near-field enhancement. Herein, a hybrid metal–dielectric nanoantenna composed of six wedge-shaped gold (Au) nanoblocks as well as silica (SiO2) and silicon (Si) nanodiscs is designed and analyzed by the finite element method (FEM). The nanoantenna exhibits flexibility in excitation and manipulation of the anapole mode through the strong coupling between the metal and dielectrics, consequently improving the near-field enhancement at the gap. By systematically optimizing the structural parameters, the electric field enhancement factors at wavelengths corresponding to the anapole modes (AM1 and AM2) can be increased to 518 and 1482, respectively. Moreover, the nanoantenna delivers great performance in optical sensing such as a sensitivity of 550 nm/RIU. The results provide guidance and insights into enhancing the coupling between metals and dielectrics for applications such as surface-enhanced Raman scattering and optical sensing.

Self-assembled photonic structure: a Ga optical antenna on GaP nanowires.

Semiconductor nanowires are the perfect platform for nanophotonic applications owing to their resonant, waveguiding optical properties and technological capabilities providing control over their crystalline and chemical compositions. The vapor-liquid-solid growth mechanism allows the formation of hybrid metal-dielectric nanostructures promoting sub-wavelength light manipulation. In this work, we explore both experimentally and numerically the plasmonic effects promoted by a gallium (Ga) nanoparticle optical antenna decorating the facet of gallium phosphide (GaP) nanowires. Raman, photoluminescence and near-field mapping techniques are used to study the effects. We demonstrate several phenomena including field enhancement, antenna effect and increase in internal reflection. We show that the observed effects have to be considered when nanowires with a plasmonic particle are used in nanophotonic circuits and discuss the ways for utilization of these effects for efficient coupling of light into nanowire waveguide and field tailoring. The results open up promising pathways for the development of both passive and active nanophotonic elements, light harvesting and sensorics.

Manipulation of optical bound states in the continuum in a metal-dielectric hybrid nanostructure

Optical bound states in the continuum (BICs) are spatially localized states with vanishing radiation, despite their energy embedded in the continuum spectrum of the environment. They are expected to greatly enhance light–matter interaction due to their long lifetime and high quality factor. However, the BICs in all-dielectric structures generally exhibit large mode volumes and their properties are difficult to manipulate. In this paper, we propose a metal–dielectric hybrid nanostructure where a silver film is inserted into the silicon (Si) substrate under the Si nanopillar array. We show that symmetry-protected BIC in this system can couple with surface plasmon polaritons (SPPs) to form a hybridized mode. Compared with previous symmetry-protected BICs in all-dielectric structures, the SPP-coupled BIC has a significantly decreased mode volume, and its corresponding electric field is strongly localized below the Si nanopillars. We also show that the SPP mode makes the original polarization-independent symmetry-protected BIC become polarization-dependent. In addition, we demonstrate that the silver film in the considered structure can induce a metal mirror effect. The destructive interference between the magnetic dipole inside the Si nanopillars and the mirror magnetic dipole in the silver film can lead to the formation of accidental BICs. Our hybrid structure provides a versatile platform for the manipulation of light–matter interaction in the nanoscale.

Full color metasurface with high-transmission and omnidirectional characteristics

Nanostructured color filters particularly generated by surface plasmonic resonance of metal-dielectric nanostructures have been intensively studied in the past decades. However, the excitation of high-order scattering of surface plasmons and the intrinsic ohmic loss of metals hinder the generation of high efficiency and high color purity. In this study, we present a full color metasurface based on the concept of surface plasmons to produce the structural colors with high monochromaticity, low-loss, and wide gamut. Owing to the suppression of high-order scattering, the transmission spectra with narrow bandwidth and high efficiency could be formed to exhibit omnidirectional characteristic. Meanwhile, the dual- and multiple-resonance could be switched by changing the polarization angle of incident light. The proposed full color metasurface exhibits the enhancement of transmittance and increment of color monochromaticity. We expect that the proposed full color metasurface is promising for applications in real industries, such as information coding, optical data storage with high density, 2D/3D display, and so on.

Resonant Hybrid Metal–Dielectric Nanostructures for Local Color Generation

Here, we experimentally and theoretically demonstrate a laser-induced change in local color based on the reshaping of gold–silicon asymmetric nanostructures. The evolution of scattering properties enabled by laser reshaping shows the potential of hybrid metal–dielectric nanostructures for color printing applications. The reshaping process can tune the resonance of the nanostructure in the wavelength range between 500 and 800 nm resulting in different colors of illuminated nanostructures. Moreover, the modeling of the scattering diagram of hybrid nanoparticles before and after femtosecond laser reshaping shows that color tuning is simultaneously accompanied by substantial reconfiguration of the distribution pattern for both peaks in the scattering spectrum.

Numerical simulations of the surface plasmon-assisted photo-magnetic effect in metal-dielectric nanostructures [Invited

We study numerically plasmonic localization and enhancement of the photo-magnetic excitation in metal-garnet nanostructures with surface plasmons excited at the opposite interfaces of the metallic layer. Contrary to intuitive expectations, the resonance at the metal/air interface results in the highest photo-magnetic excitation efficiency. Our findings highlight the key role of the phase shift between the electric field components inside the garnet layer. A comprehensive analysis of the optical losses reveals the importance of absorption in the photo-magnetic dielectric. These results provide a fruitful perspective for future experiments on metal-dielectric photo-magnetic systems towards all-optical magnetization switching at the nanoscale.

Quantum surface effects in strong coupling dynamics

Plasmons in nanostructured metals are widely utilized to trigger strong light--matter interactions with quantum light sources. While the nonclassical behavior of such quantum emitters (QEs) is well-understood in this context, the role of quantum and surface effects in the plasmonic resonator is usually neglected. Here, we combine the Green's tensor approach with the Feibelman $d$-parameter formalism to theoretically explore the influence of quantum surface effects in metal-dielectric layered nanostructures on the relaxation dynamics of a proximal two-level QE. Having identified electron spill-out as the dominant source of quantum effects in jellium-like metals, we focus our study on sodium. Our results reveal a clear splitting in the emission spectrum, indicative of having reached the strong-coupling regime, and, more importantly, non-Markovian relaxation dynamics of the emitter. Our findings establish that strong light--matter coupling is not suppressed by the emergence of nonclassical surface effects in the optical response of the metal.

Dual Nanoresonators for Ultrasensitive Chiral Detection.

The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal–dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.

Imaging Strongly Coupled Plasmon–Phonon Modes in Mid-Infrared Double Antennas

Strong-coupling effects alter the polaritonic states of metal–dielectric nanostructures and, therefore, can be used to tailor the response of infrared (IR) photonic elements. We report on the devel...

Engineering metal-dielectric nanostructures involving silver decorated Halloysite for augmented surface plasmon-coupled directional emission

Metal-dielectric hybrid nanostructures have recently drawn interest because of their direct coupling between metal surface plasmon resonance and dielectric electric dipoles leading to hybridized modes resulting in low losses and high field enhancements. Surface plasmon-coupled emission is a similar phenomenon that exploits the surface plasmon polaritons generated at the junction of dielectric and metal planar boundary. In this regard, we propose and demonstrate the use of dielectric nanotubes – Halloysite (HS) as spacer materials on metallic silver thin film. In this work, we selectively modified HS by acid or base treatment. Further they were loaded with silver nanoparticles (AgNPs) by solution deposition (in-situ method), thereby creating a metal-dielectric hybrid material. Selective acid and base treatment of HS nanotubes affected the surface charge distribution and aggregation of the nanotubes. Better dispersion ability and higher electronegativity behaviour was observed in base-treated HS that aided in higher loading of AgNPs. Enhanced interparticle plasmon coupling and better transmission of plasmon mediating energy was observed in base-treated HS-Ag that augmented dipole emission. The enhanced plasmon-coupled emission of HS-Ag nanostructures were also explored using a low-cost mobile phone as a detector to simplify the process of fluorescence detection.

Plasmonic refractive index sensing enhanced by anapole modes in metal-dielectric nanostructure array

Periodic nanostructures consisting of metal-dielectric nanostructures array are studied for refractive index (RI) sensing. The anapole mode of metal-dielectric structure is used to enhance plasmonic resonance and reduce energy losses. High sensitivity RI sensing can be achieved by the nanostructure with circular polarization illumination. Numerical results show that the sensor exhibit almost perfect linearity of resonance wavelength shift with RI ranging from 1.3 to 1.39 with sensitivity of 593 nm RIU−1. The proposed nanostructure could find applications in biomolecules manipulation and optical sensors.

Hybrid Metal-Dielectric Metasurfaces for Refractive Index Sensing.

Hybrid metal-dielectric nanostructures have recently gained prominence because they combine strong field enhancement of plasmonic metals and the several low-loss radiation channels of dielectric resonators, which are qualities pertaining to the best of both worlds. In this work, an array of such hybrid nanoantennas is successfully fabricated over a large area and utilized for bulk refractive index sensing with a sensitivity of 208 nm/RIU. Each nanoantenna combines a Si cylinder with an Al disk, separated by a SiO2 spacer. Its optical response is analyzed in detail using the multipoles supported by its subparts and their mutual coupling. The nanoantenna is further modified experimentally with an undercut in the SiO2 region to increase the interaction of the electric field with the background medium, which augments the sensitivity to 245 nm/RIU. A detailed multipole analysis of the hybrid nanoantenna supports our experimental findings.

Multiband enhanced second-harmonic generation via plasmon hybridization

Boosting nonlinear frequency-conversion efficiencies in hybrid metal-dielectric nanostructures generally requires the enhancement of optical fields that interact constructively with nonlinear dielectrics. Inevitably for localized surface plasmons, spectra subject to this enhancement tend to span narrowly. As a result, because of the spectral mismatch of resonant modes at frequencies participating in nonlinear optical processes, strong nonlinear signal generations endure the disadvantage of rapid degradations. Here, we experimentally design a multiband enhanced second-harmonic generation platform of three-dimensional metal-dielectric-metal nanocavities that consist of thin ZnO films integrated with silver mushroom arrays. Varying geometric parameters, we demonstrate that the introduction of ZnO materials in intracavity regions enables us to modulate fundamental-frequency-related resonant modes, resulting in strong coupling induced plasmon hybridization between localized and propagating surface plasmons. Meanwhile, ZnO materials can also serve as an efficient nonlinear dielectric, which provides a potential to obtain a well-defined coherent interplay between hybridized resonant modes and nonlinear susceptibilities of dielectric materials at multi-frequency. Finally, not only is the conversion efficiency of ZnO materials increased by almost two orders of magnitude with respect to hybrid un-pattered systems at several wavelengths over a 100-nm spectral range but also a hybrid plasmon-light coupling scheme in three-dimensional nanostructures can be developed.

Enhancing third-harmonic generation by mirror-induced electric quadrupole resonance in a metal-dielectric nanostructure.

Electric quadrupole resonance (EQR), a commonly available high-order Mie-type resonance in all-dielectric nanoparticles, suffers from weak field enhancement and thus inferior third-harmonic generation (THG). In this work, according to the intrinsic centrosymmetry of current distribution, mirror-induced EQR in a silicon disk is effectively generated by introducing a bottom metal film with the perfect electric conductor (PEC) mirror effect, manifesting preeminent capabilities of tailoring far-field scattering and enhancing near-field intensity. The beneficial THG by mirror-induced EQR is enhanced by more than 50-fold as compared to that of the typical EQR without the PEC mirror effect. Furthermore, the influence of the silicon Kerr effect on THG is investigated under increasing pump intensity, achieving maximal efficiency of 2.2×10-4 under pump intensity I0=3GW/cm2. This work opens possibilities of exploring new mirror-induced Mie-type resonances in hybrid nanostructures, finding important applications in frequency conversion, spectroscopy, and sensing at the nanoscale.

Computation of scattering resonances in absorptive and dispersive media with applications to metal-dielectric nano-structures

In this paper we consider scattering resonance computations in optics when the resonators consist of frequency dependent and lossy materials, such as metals at optical frequencies. The proposed computational approach combines a novel hp-FEM strategy, based on dispersion analysis for complex frequencies, with a fast implementation of the nonlinear eigenvalue solver NLEIGS. Numerical computations illustrate that the pre-asymptotic phase is significantly reduced compared to standard uniform h and p strategies. Moreover, the efficiency grows with the refractive index contrast, which makes the new strategy highly attractive for metal-dielectric structures. The hp-refinement strategy together with the efficient parallel code result in highly accurate approximations and short runtimes on multi processor platforms.

Strong Coupling between Dark Plasmon and Anapole Modes.

Plasmonic nanocavities enable extreme light-matter interaction by pushing light down to the nanoscale. The dipolar feature of bright modes allows coupling with the external excitation from free space but results in a radiating background, whereas nonradiating dark plasmon modes can hardly be excited. Here, we report for the first time on strong coupling between dark plasmon and anapole modes in a hybrid metal-dielectric nanostructure. With the aid of vanishing dipole characteristics of the anapole and dark plasmons, the hybrid modes exhibit minimum far-field scattering and maximum near-field enhancement. The dark mode coupling in the metal-dielectric nanostructure offers a nonradiating air cavity with greatly improved field enhancement in a broadened band, thus providing a background-free experimental platform for spectroscopic applications. The proposed approach to dark plasmon excitation, i.e., via anapole, may boost practical exploitation of dark plasmons by allowing linearly polarized light illumination and scalable arrays of individual nanostructure units.

New Trends in the Simulation of Nanosplasmonic Optical D-Type Fiber Sensors.

This article presents a review of the numerical techniques employed in simulating plasmonic optical sensors based on metal-dielectric nanostructures, including examples, ranging from conventional D-type fiber sensors, to those based on photonic crystal D-type fibers and incorporating metamaterials, nanowires, among other new materials and components, results and applications. We start from the fundamental physical processes, such as optical and plasmonic mode coupling, and discuss the implementation of the numerical model, optical response customization and their impact in sensor performance. Finally, we examine future perspectives.