Strong Field Enhancement Research Articles

Ordered Array of Metal Particles on Semishell Separated with Ultrathin Oxide: Fabrication and SERS Properties

Metal particles in gap cavities provide an interesting system to achieve hybrid local surface plasmon modes for local field enhancement. Here, we demonstrate a relatively simple method to fabricate Ag nanoparticles positioned on Ag semishells separated by a thin (~5 nm) dielectric layer. The obtained structure can provide strong local electric field enhancement for surface-enhanced Raman scattering (SERS). The fabrication of the ordered array structure was realized by nanosphere self-assembly, atomic layer deposition, and metal thin-film dewetting. Numerical simulation proved that, compared to the conventional metal semishell arrays, the additional Ag particles introduce extra hot spots particularly in the valley regions between adjacent Ag semishells. As a result, the SERS enhancement factor of the metal semishell-based plasmonic structure could be further improved by an order of magnitude. The developed novel plasmonic structure also shows good potential for application in plasmon-enhanced solar water-splitting devices.

Selected Mode Mixing and Interference Visualized within a Single Optical Nanoantenna

Interference-based directional antennas typically consist of multiple dipoles with properly set distances and phases, which cause constructive interferences toward certain directions in radiation or reception. For nano-optical antennas, the directionality can be realized by superposition of multiple eigenmodes in a single structure. Such mode mixing creates locally strong field enhancement, which should be properly controlled for energy conversion or sensing applications. However, experimental verification of the nano-optical field, or especially the hot-spots, created by interference of selected eigenmodes is not trivial. We here visualize how optical fields are distributed when multiple modes interfere within a silver disk nanoantenna. We use angle- and polarization-resolved cathodoluminescence based on scanning transmission electron microscopy to select specific modes and visualize the field distribution at the nanoscale. The interfered field distribution significantly changes depending on the detectio...

Periodic ZnO-Elevated Gold Dimer Nanostructures for Surface-Enhanced Raman Scattering Applications

Electron beam lithography-defined periodic ZnO-elevated gold dimer nanostructures for surface-enhanced Raman spectroscopy with strong electromagnetic field enhancements were fabricated in the present work. The ZnO nanorods (NRs) were grown on the patterned substrate using a hydrothermal process, and Au dimers were deposited on the top surface of ZnO NRs, forming suspended gold dimers and providing plasmonic hot-spots with nanocavity effect. Effects of the dimer radius, gap size, and ZnO NR height on the dark-field scattering spectra and surface-enhanced Raman scattering (SERS) were investigated experimentally and theoretically. The SERS response was enhanced with increasing dimer radius and decreasing gap size. The SERS enhancement factor (EF) due to the localized surface plasmon resonance was calculated from simulation results. The calculated EF as a function of ZnO NR height showed the periodic trend of electric field intensity with the periodicity of about half of the exciting laser wavelength, and was...

Enhanced sensitivity in THz plasmonic sensors with silver nanowires

We developed hybrid slot antenna structures for microbial sensing in the THz frequency range, where silver nanowires (AgNWs) were employed to increase the sensitivity. In order to fabricate the hybrid devices, we partially etched the AgNW in the slot antenna region, where we can expect the field enhancement effect at the AgNW tip. We measured the resonant-frequency shift observed upon the deposition of a polymer layer, and observed that the sensitivity increased upon the introduction of AgNWs, with an enhancement factor of more than four times (approximately six times in terms of figure-of-merit). The sensitivity increased with the AgNW density until saturation. In addition, we tested devices with PRD1 viruses, and obtained an enhancement factor of 3.4 for a slot antenna width of 3 μm. Furthermore, we performed finite-difference time-domain simulations, which confirmed the experimental results. The sensitivity enhancement factor decreased with the decrease of the slot width, consistent with the experimental findings. Two-dimensional mapping of the electric field confirmed the strong field localization and enhancement at the AgNW tips.

Perspective: Strong microwave photon-magnon coupling in multiresonant dielectric antennas

Achieving quantum-level control over electromagnetic waves, magnetisation dynamics, vibrations, and heat is invaluable for many practical applications and possible by exploiting the strong radiation-matter coupling. Most of the modern strong microwave photon-magnon coupling developments rely on the integration of metal-based microwave resonators with a magnetic material. However, it has recently been realised that all-dielectric resonators made of or containing magneto-insulating materials can operate as a standalone strongly coupled system characterised by low dissipation losses and strong local microwave field enhancement. Here, after a brief overview of recent developments in the field, I discuss examples of such dielectric resonant systems and demonstrate their ability to operate as multiresonant antennas for light, microwaves, magnons, sound, vibrations, and heat. This multiphysics behavior opens up novel opportunities for the realisation of multiresonant coupling such as, for example, photon-magnon-phonon coupling. I also propose several novel systems in which strong photon-magnon coupling in dielectric antennas and similar structures is expected to extend the capability of existing devices or may provide an entirely new functionality. Examples of such systems include novel magnetofluidic devices, high-power microwave power generators, and hybrid devices exploiting the unique properties of electrical solitons.

Synthesis and Largely Enhanced Nonlinear Refraction of Au@Cu2O Core-Shell Nanorods

Au@Cu2O core-shell nanorods with tunable thickness of Cu2O shell were synthesized and their linear and nonlinear optical responses were investigated. Two transverse plasmon resonance peaks were observed when the Au nanorods were coated with Cu2O shells, which were adjusted by the Cu2O shell thickness. The nonlinear absorption of the Au@Cu2O nanorods is enhanced by 5 times at the longitudinal plasmon resonance wavelength compared with that of bare Au nanorods. More intriguingly, largely enhanced nonlinear refraction and suppressed nonlinear absorption at the transverse plasmon resonance wavelength were observed in the Au@Cu2O nanorods. Our findings indicate the existence of strong local field enhancement at the interface between the Au core and the Cu2O shell, which would provide a promising strategy in designing plasmonic nonlinear nanodevices with good nonlinear figures of merit.

Electrolyte Solvation Structure at Solid-Liquid Interface Probed by Nanogap Surface-Enhanced Raman Spectroscopy.

Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are critical to the interfacial charge-transfer process across the solid-liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li+ salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode-liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid-liquid interface.

Highly stable silver nanoparticles for SERS applications

Plasmonics sensor configurations utilizing localized plasmon resonances in silver nanostructures typically suffer from rapid degradation of silver in ambient atmospheric conditions. In this work, we report on the fabrication and detailed characterization of ensembles of monocrystalline silver nanoparticles (NPs), which exhibit a long-term stability of optical properties under ambient conditions without any protective treatments. Ensembles with different densities (surface coverages) of size-selected NPs (diameters from 12.5 to 24 nm) on quartz substrates are fabricated using the cluster-beam technique and characterized by linear spectroscopy, surface-enhanced Raman scattering microscopy as well as transmission electron, helium-ion and atomic force microscopies. It is found that the fabricated ensembles of monocrystalline silver NPs preserve their plasmonic properties (monitored with optical spectroscopy) and strong field enhancements (revealed by surface enhanced Raman spectroscopy) at least 5 times longer as compared to chemically synthesized silver NPs with similar sizes. The obtained results are of high practical relevance for the further development of sensors, resonators and metamaterials utilizing plasmonic properties of silver NPs.

Large‐Area Metal Gaps and Their Optical Applications

AbstractRecent technological advances in fabrication methods have allowed researchers to manipulate light–matter interactions in the subwavelength region and develop a wide variety of innovative optical applications from the visible to the microwave region. Metal patterning at a subwavelength scale plays a crucial role in realizing these optical applications. Various standard lithography techniques including laser beam machining, focused ion beam, photolithography, and electron‐beam lithography are used for the subwavelength feature size of the metal patterns. Many recent studies have demonstrated that funneling light into nanometer‐wide gaps in metals gives rise to strong field enhancements and nonlocal electromagnetic effects. However, these standard methods encounter difficulties when one tries to fabricate nanometer feature sizes with macroscopic circumferences, crucial for long‐wavelength applications, over a large area. Here, new lithography techniques that fabricate an array of metal gaps of nanometer‐to‐Ångstrom scale are covered. The corresponding photonic applications in the terahertz and microwave regions are also introduced. These next‐generation metal gaps will have a great impact on the advancement of field enhancement and confinement toward the next level of applications such as metamaterials, quantum tunneling, active switching devices, and ultrasensitive chemical/biological sensors.

High-performance sensor achieved by hybrid guide-mode resonance/surface plasmon resonance platform.

We perform a comprehensive analysis of multiband absorption properties in a metal-dielectric-metal-dielectric (MDMD) nanostructure under TM wave illumination. The multiband absorption can be attributed to the hybridization of the surface plasmon resonance (SPR) and the guide-mode resonance (GMR), and we identify the hybrid GMR/SPR by the dispersion relation equations of the SPR and GMR, respectively. More importantly, the MDMD nanostructure is very sensitive to the change of the dielectric environment for the special hybrid structure; thus, it can function as a good candidate for ultrasensitive biochemical sensing. The highest sensitivity of the MDMD nanostructure reaches 1087nm/RIU with the figure of merit (FoM) of 23 and the new figure of merit (FoM*) of 483; it is performed by the absorption peak at 1796.1nm of the double surface plasmon polariton with the strongest field enhancement at the surface.

Breaking the symmetry to manipulate the magnetic Fano resonance in double split ring/square ring structure

Optical properties of the Double Split Ring/Square Ring nanostructurre are investigated by using finite element method. Electric and magnetic resonances at optical frequency can be induced by this nanostructure. The symmetry of the structure is broken by adjusting the arm length of the double split ring and the offsets of the square ring, which leads the changing of inductance and capacitance. New magnetic resonances can be achieved and enhanced. Fano resonances are produced by the overlapping interference between bright electric mode and dark magnetic mode. The nanostructure allows the generation of multiple magnetic Fano resonances and strong magnetic field enhancement, which has potential applications in plasmons propagation with low-loss magnetic and multi-wavelength surface enhanced spectroscopy.

Efficient biosensing through 1D silver nanostructured devices using plasmonic effect

The current work explores the excitation of surface plasmon polaritons (SPPs) on a one dimensional (1D) silver nano-grating device, simulated on glass substrate, which can sense a very small change in the refractive index of an analyte adjacent to it. The most recent modeling technique finite element analysis is applied in this work by using a COMSOL RF module. The models of 1D grating devices of different slit widths with fixed periodicity and film thickness are simulated. The data is collected and then used to study higher refractive index unit per nanometer (RIU/nm) as well as the effect of the widths of the slits on the RIU. A number of investigations are done by the simulated data, like a dip in the transmission spectra of p-polarized light. This dip is due to SPP resonance with the variation of slit width. Furthermore, the most fascinating part of the research is the COMSOL modeling that provides an opportunity to look into factors affecting higher RIU/nm, while visualizing the cross-sectional view of the grating device and strong electric field enhancement at the surface of the metallic device. When the slit width is almost equal to half of the periodicity of the grating device, SPP resonance increases and it is at maximum for the slit width equal to two-thirds of the periodicity, because the coupling efficiency is at maximum.

Mid-infrared nonlinear silicon hybrid waveguide with high figure of merit

Mid-infrared (MIR) integrated waveguides with strong local field enhancement and relatively low loss have shown their potential in nonlinear optical applications. We present a novel highly nonlinear silicon hybrid waveguide by setting an insulator-metal-insulator structure with [2-[4-(dimethylamino)phenyl]-3-([4-(dimethylamino)phenyl]ethynyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile] (DDMEBT) and silver materials into the silicon slot. The mode field is strongly confined in the DDMEBT layers via the surface plasmon polariton effect. Two rectangular silicon nanobridges are used to further confine the optical field and reduce the loss simultaneously. The waveguide dimensions are optimized to obtain a figure of merit as high as 0.55 at 2.1 μm and a zero-dispersion wavelength of 2.06 μm. In a 6.6-mm-long waveguide, a maximum four-wave mixing efficiency of −6.65 dB and a bandwidth of 512 nm (1.894–2.406 μm) can be simultaneously obtained, pumped by a 2.104-μm source with a power of only 500 mW. Our designed waveguide exhibits excellent overall performance for nonlinear optical applications such as all-optical signal processing in the MIR band.

Launching phase-controlled surface plasmons on Babinet metasurfaces.

Surface plasmons are electromagnetic modes bounded at the metal-dielectric interfaces, which have numerous applications due to the strong field enhancement effect. Therefore, controlling surface plasmons is in high demand for modern photonics and plasmonics. In this Letter, we propose a kind of metasurface for launching surface plasmons with a controlled phase based on the Babinet principle. The phase control is achieved by arranging C-shaped apertures on a metal surface that exhibits highly efficient electric dipole resonance. By properly designing the spatial separation and orientation of the aperture resonators, an arbitrary phase profile is achieved that allows anomalous (directional and focusing/diverging) surface plasmon launching and manipulation of the wavefront.

Photocatalytic Growth of Hierarchical Au Needle Clusters on Highly Active TiO2 Thin Film

AbstractHierarchical gold (Au) structures with sharp edges garner a strong interest for nanoelectronics, nanoenergy harvesting devices, and nano‐biomedical applications due the exceptional strong electric field (hot spot) enhancement at their tips. Herein, a facile method to synthesize hierarchical Au needle clusters (HAuNCs) on highly active titanium oxide (TiO2) thin film is reported. Different from surfactant‐directed photochemical‐assisted synthesis methods, a photocatalytic deposition approach is demonstrated, which allows positioning and patterning of HAuNCs on TiO2 target without using any surfactant or stabilizer. This green synthesis approach enables to control the size and the geometry of deposited HAuNCs by simply altering the photocatalytic activity of TiO2 target, UV light intensity, and irradiation time.

Dispersion engineering with plasmonic nano structures for enhanced surface plasmon resonance sensing

We demonstrate numerically and experimentally the enhancement of Surface Plasmon Resonance (SPR) sensing via dispersion engineering of the plasmonic response using plasmonic nanograting. Following their design and optimization, the plasmonic nanograting structures are fabricated using e-beam lithography and lift-off process and integrated into conventional prism based Kretschmann configuration. The presence of absorptive nanograting near the metal film, provides strong field enhancement with localization and allows to control the dispersion relation which was originally dictated by a conventional SPR structure. This contributes to the enhancement in Q factor which is found to be 3–4 times higher as compared to the conventional Kretschmann configuration. The influence of the incident angle on resonance wavelength is also demonstrated both numerically and experimentally, where, only a negligible wavelength shift is observed with increasing the incident angles for plasmonic nanograting configuration. This surprising feature may be helpful for studying and utilizing light-matter interaction between plasmons and narrow linewidth media (e.g. Rb atom or molecule) having nonlocalities in their susceptibility-momentum relation. Finally, we analyze the role of plasmonic nanograting in enhancing the performance of an SPR sensor. Our results indicate that the integrated SPR-nanograting device shows a great promise as a sensor for various types of analytes.

Roll-to-Roll Nanoforming of Metals Using Laser-Induced Superplasticity.

This Letter describes a low-cost, scalable nanomanufacturing process that enables the continuous forming of thin metallic layers with nanoscale accuracy using roll-to-roll, laser-induced superplasticity (R2RLIS). R2RLIS uses a laser shock to induce the ultrahigh-strain-rate deformation of metallic films at room temperature into low-cost polymeric nanomolds, independently of the original grain size of the metal. This simple and inexpensive nanoforming method does not require access to cleanrooms and associated facilities, and can be easily implemented on conventional CO2 lasers, enabling laser systems commonly used for rapid prototyping or industrial cutting and engraving to fabricate uniform and three-dimensional crystalline metallic nanostructures over large areas. Tuning the laser power during the R2RLIS process enables the control of the aspect ratio and the mechanical and optical properties of the fabricated nanostructures. This roll-to-roll technique successfully fabricates mechanically strengthened gold plasmonic nanostructures with aspect ratios as high as 5 that exhibit high oxidation resistance and strong optical field enhancements. The CO2 laser used in R2RLIS can also integrate the fabricated nanostructures on transparent flexible substrates with robust interfacial contact. The ability to fabricate ultrasmooth metallic nanostructures using roll-to-roll manufacturing enables the large scale production, at a relatively low-cost, of flexible plasmonic devices toward emerging applications.

Giant Field Enhancements in Ultrathin Nanoslots above 1 Terahertz

Strong demand for plasmonic devices with an enormously enhanced electric field and desired resonance frequencies has led to extensive investigations of metallic slot structures. While strong field enhancement can be achieved by reducing the width of the slot, the effect of the gap surface plasmon limits the maximum achievable field enhancement at higher frequencies. Specifically, the effect of the gap surface plasmon becomes stronger as the gap width decreases and strongly suppresses the transmission while causing a red-shift of the resonance. Here, we overcome these issues and realize strong field enhancements at higher frequencies, by managing the metal thickness of the nanoslots. We show that, as the nanoslots become as thin as 10 nm, they show a giant electric field enhancement of up to 7600. Moreover, the resonances are strongly blue-shifted to above 1 THz from 0.33 THz. Our work provides a novel route to achieving high field enhancements at desired frequencies, as well as a means by which to charact...

Giant enhancement of Faraday rotation due to electromagnetically induced transparency in all-dielectric magneto-optical metasurfaces

In this Letter we introduce a new class of Fano-resonant all-dielectric metasurfaces for enhanced, high figure of merit magneto-optical response. The metasurfaces are formed by an array of magneto-optical bismuth-substituted yttrium iron garnet nano-disks embedded into a low-index matrix. The strong field enhancement in the magneto-optical disks, which results in over an order of magnitude enhancement of Faraday rotation, is achieved by engineering two (electric and magnetic) resonances. It is shown that while enhancement of rotation also takes place for spectrally detuned resonances, the resonant excitation inevitably results in stronger reflection and low figure of merit of the device. We demonstrate that this can be circumvented by overlapping electric and magnetic resonances of the nanodisks, yielding a sharp electromagnetically induced transparency peak in the transmission spectrum, which is accompanied by gigantic Faraday rotation. Our results show that one can simultaneously obtain a large Faraday rotation enhancement along with almost 100% transmittance in an all-dielectric metasurface as thin as 300nm. A simple analytical model based on coupled-mode theory is introduced to explain the effects observed in first-principle finite element method simulations.

Tunable nonlinear coherent perfect absorption with epsilon-near-zero plasmonic waveguides.

We propose a scheme to realize nonlinear coherent perfect absorption (CPA) at the nanoscale using epsilon-near-zero (ENZ) plasmonic waveguides. The general conditions to achieve CPA in a linear ENZ plasmonic waveguide are analyzed and presented. The proposed ENZ waveguides support an effective ENZ response at their cutoff frequency, where the CPA effect occurs under the illumination of two counterpropagating plane waves with equal amplitudes and appropriate phase distributions. In addition, the strong and uniform field enhancement inside the nanochannels of the waveguides at the ENZ resonance can efficiently boost Kerr nonlinearities, resulting in a new all-optical switching intensity-dependent CPA phenomenon that can be tunable with ultrafast speed. The proposed free-standing ENZ structures combine third-order nonlinear functionality with standing wave CPA interference effects in a nanoscale plasmonic configuration, thus leading to a novel degree of tunable light-matter interactions achieved in subwavelength regions. Our findings provide a new platform to efficiently excite nonlinear phenomena at the nanoscale and design tunable coherent perfect absorbers.