Electric Field Intensity Enhancement Research Articles

Simulation and Fabrication of Nanoscale Spirals Based on Dual-Scale Self-Assemblies.

Archimedean spirals in nanometer scale have shown remarkable plasmonic responses derived from their linear and rotational asymmetry. Despite the unique optical properties of nanoscale spirals, their applications have been limited due to the difficulty in fabricating large-scale arrays with uniform and systematic control of the morphology. Here, we report simulation results of spiral morphologies, which are used to design a scalable fabrication process for nanoscale spirals and predict their plasmonic responses. First, self-consistent field theory (SCFT) simulations were performed to design optimal templates to guide self-assembly into spiral morphologies. Using the SCFT results, we developed a scalable fabrication process, which is based on the micron-scale assembly of microspheres combined with glancing angle deposition and nanoscale assembly of block copolymers, to induce the formation of uniform nanospirals with diverse size, handedness, orientation, and winding number. Finally, finite-difference time-domain simulation results show linear dichroism and electric field intensity enhancement effects of these nanospirals, which are highly dependent on the winding number of the spirals, indicating the importance of precise control of the structural parameters.

Optical trapping of single nano-size particles using a plasmonic nanocavity

Trapping and manipulating micro-size particles using optical tweezers has contributed to many breakthroughs in biology, materials science, and colloidal physics. However, it remains challenging to extend this technique to a few nanometers particles owing to the diffraction limit and the considerable Brownian motion of trapped nanoparticles. In this work, a nanometric optical tweezer is proposed by using a plasmonic nanocavity composed of the closely spaced silver coated fiber tip and gold film. It is found that the radial vector mode can produce a nano-sized near field with the electric-field intensity enhancement factor over 103 through exciting the plasmon gap mode in the nanocavity. By employing the Maxwell stress tensor formalism, we theoretically demonstrate that this nano-sized near field results in a sharp quasi-harmonic potential well, capable of stably trapping 2 nm quantum dots beneath the tip apex with the laser power as low as 3.7 mW. Further analysis reveals that our nanotweezers can stably work in a wide range of particle-to-tip distances, gap sizes, and operation wavelengths. We envision that our proposed nanometric optical tweezers could be compatible with the tip-enhanced Raman spectroscopy to allow simultaneously manipulating and characterizing single nanoparticles as well as nanoparticle interactions with high sensitivity.

Efficiency enhancement in polymer solar cells using combined plasmonic effects of multi-positional silver nanostructures

Abstract Achieving high absorption enhancement in a broad wavelength range without increasing the thickness of active layer is essential for improving the performance of polymer solar cells. In this work, we report efficiency enhancement in poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno [3,4-b]thiophenediyl}): [6,6]-phenyl C71-butyric acid methyl ester active layer based polymer solar cells by incorporating silver nanostructures simultaneously in cathode buffer layer and active layer. Efficiency of the device was increased from 6.53% to 7.53% after incorporating the nanostructures. The plasmonic devices exhibited superior performance in a broad wavelength range from 450 nm to 750 nm. Photoluminescence measurements were carried out to confirm the improvement in light harvesting capability of the devices. Finite-difference time-domain simulations and Raman spectroscopy were employed to verify the electric field intensity enhancement around the nanostructures. The performance enhancement mechanism was realized based on theoretical simulations and optical measurements.

A comparative study among WS2, MoS2 and graphene based surface plasmon resonance (SPR) sensor

In the present paper, we propose a new design of surface plasmon resonance (SPR) sensor having better performance parameters by employing the well-known transfer matrix method. The proposed SPR sensor is comprised of BK-7 prism-ZnO-Ag-BaTiO3-Transition metal dichalcogenides (TMDCs, mainly WS2 and MoS2)/Graphene. A comparison is made among their performance parameters i.e. sensitivity, detection accuracy, quality parameter with other traditional reported SPR sensor. We have also analysed full width at half maxima (FWHM) and electric field intensity enhancement factor (EFIEF) for the proposed SPR sensor. On comparison, it is found that the proposed SPR sensor with monolayer WS2 configuration has better sensitivity of 180°/RIU as compared to monolayer MoS2 and graphene which are 174°/RIU and 157°/RIU respectively. However, the proposed SPR sensor with graphene layer shows small FWHM, higher quality parameter and detection accuracy. We have found that maximum sensitivity of 2350/RIU corresponds to the two-layer WS2 configuration. Our proposed SPR sensor shows better performance as compared to the other reported works.

Atomic Layer Engineering of Epsilon‐Near‐Zero Ultrathin Films with Controllable Field Enhancement

AbstractEnhanced and controlled light absorption, as well as field confinement in optically thin materials, are pivotal for energy‐efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films can support the so‐called epsilon near zero (ENZ) modes in a frequency region of near‐zero permittivity, which can lead to the perfect light absorption and ultrastrong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, engineered absorption and FIE are experimentally demonstrated in aluminum‐doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and the number of macrocycles. It is also demonstrated that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next‐generation tailorable photonic devices based on flat, zero‐index optics.

Impact of metal silicide nanocrystals on the resistance ratio in resistive switching of epitaxial Fe3O4 films on Si substrates

Fe3O4 films on Si substrates have been intensively studied for the realization of resistance random access memory composed of only ubiquitous elements. The biggest issue for the application of Fe3O4 film/Si in small-scaled devices is the low Off/On resistance ratio. For the enhancement of the Off/On resistance ratio, we propose epitaxial Fe3O4 films including hemispherical small metal α-FeSi2 nanocrystals on Si substrates, where an electric field is concentrated at the interface between Fe3O4/α-FeSi2. The concentrated electric field largely promotes the movement of oxygen ions, contributing to resistive switching. As a result, the Fe3O4 films including hemispherical small α-FeSi2 nanocrystals exhibit the largest Off/On resistance ratio (∼200) in Fe3O4-based nanomaterials. Finite element method simulations proved that the introduction of metal nanocrystals into films caused the enhancement of electric field intensity near the interface between nanocrystals and films. This significant enhancement method will open an avenue for realizing high-performance ubiquitous-element resistive switching materials in the next-generation information society.

Lab on D-shaped fiber excited via azimuthally polarized vector beam for surface-enhanced Raman spectroscopy.

We present a method for Raman examination using a silver-nanoparticles (Ag-NPs) coated D-shaped fiber (DSF) internally excited via an in-fiber azimuthally polarized beam (APB) generated by an acoustically induced fiber grating. Simulation results show that an electric-field intensity enhancement factor can be effectively improved under APB excitation compared with the linear polarization beam (LPB) excitation, because the strong gap-mode is uniformly generated between two adjacent Ag NPs on the surface of the DSF planar side. Experimental results show that the Raman signal intensity of the methylene blue (MB) detected by DSF in the case of APB excitation is ∼4.5 times as strong as that of LPB excitation, and the Raman detection sensitivity is ∼10-9 M. The time stability of this method is also tested to be guaranteed.

Structures for surface-enhanced nonplasmonic or hybrid spectroscopy

Abstract Absorption, scattering, and fluorescence are processes that increase with electric field intensity. The most prominent way to enhance electric field intensity is to use localized or propagating surface plasmon polaritons (SPPs) based on metallic particles and nanostructures. In addition, several other, much less well-known, photonic structures that increase electric field intensity exist. Interference enhancement provided by thin dielectric coatings on reflective substrates is able to provide electric field intensity enhancement over the whole substrate and not only at certain hotspots, thereby being in particular suitable for the spectroscopy of thin surface layers. The same coatings on high refractive index substrates may be used for interference-enhanced total internal reflection-based spectroscopy in much the same way as Kretschmann or Otto configuration for exciting propagating SPPs. The latter configurations can also be used to launch Bloch surface waves on 1D photonic crystal structures for the enhancement of electric field intensity and thereby absorption, scattering, and fluorescence-based spectroscopies. High refractive index substrates alone can also, when nanostructured, enhance infrared absorption or Raman scattering via Mie-type resonances. As a further method, this review will cover recent developments to employ phonon polaritons in the reststrahlen region.

Optical Properties of Au-Ag Bimetallic Nanoparticles of Different Shapes for Making Efficient Bimetallic-Photonic Whispering Gallery Mode Hybrid Microresonators

For the first time, we propose a bimetallic-photonic whispering gallery mode (WGM) hybrid microresonator for the detection and sizing of single protein molecules in real time. To optimize the sensitivity of hybrid microresonators, theoretical simulations have been carried out to understand the optical properties of different types of Au-Ag bimetallic nanoparticles such as solid nanospheres, dielectric core-bimetallic nanoshells, nanostars, and nanoprisms using Lorenz-Mie theory, Aden-Kerker theory, and finite element method (FEM). The role of size and Au-Ag ratio on the optical properties of the nanospheres and nanoshells has been studied. In addition, the effect of the Au-Ag ratio on the optical properties of nanostars and nanoprisms has been investigated. The local electric field intensity enhancement values of bimetallic nanostructures of different shapes have been estimated using the FEM. Finally, the advantages of the bimetallic-WGM hybrid microresonator as compared to the monometallic-WGM hybrid microresonator have been explained in detail.

Second-order surface optical nonlinear response of plasmonic tip axially excited via ultrafast vector beams

We present the second-order surface optical nonlinear response of a plasmonic tip under axial excitation of ultrafast vector beams. Theoretical calculations show that the tip has an optimized nanofocusing characteristic under axial excitation of the radial vector beam (RVB), and the electric-field intensity enhancement factor is higher than that of linear polarization beam (LPB) and azimuthal vector beam (AVB) excitations. In the experiment, the second harmonic spectra are clearly measured under three excitation beams. The intensity of the second harmonic obtained via RVB excitation is one order of magnitude higher than that of LPB and AVB excitations.

Tight focus and field enhancement of terahertz waves using a probe based on spoof surface plasmons

In order to improve the resolution of terahertz near-field microscopic imaging technology, an ultra-thin thickness-graded silver-plated strip probe with the same duty cycle is designed to realize the excitation of spoof surface plasmons. By comparing with two other probes with different structures, it can be found that the thickness-graded silver-plated strip probe can produce a strong electric field enhancement effect. Thereafter, the influence of the polarization direction of the incident electric field and the number of periodic metal stripes on the electric field which are generated at the tip of the probe is investigated. It is found that this case is highly consistent with the electric field distribution in Richards-Wolf vector diffraction theory when the incident light is linearly polarized. The electric field intensity generated at the tip of the thickness-graded silver-plated strip probe can be flexibly and effectively manipulated by changing the polarization direction of the incident electric field. When the number of thickness-graded silver-plated strips is 12, the minimum size of the focal spot is 20 μm, which is λ/150. When the number of thickness-graded silver-plated strips is 4, the electric field intensity enhancement factor at the focal spot is 849. The electric field intensity enhancement factor at the focal spot increases continuously as the number of periodic metal stripes increases, and the size of focal spot decreases continuously as the number of periodic metal stripes decreases. This result shows that the tight focusing and electric field enhancement of terahertz waves can be achieved by using an ultra-thin thickness-graded silver-plated strip probe. The research results in this paper have important guiding significance for manipulating the electric field in the terahertz band.

Spectral and Angle-Resolved Magneto-Optical Characterization of Photonic Nanostructures.

Photonic crystals are periodic nanostructures that can support a variety of confined electromagnetic modes. Such confined modes are usually accompanied by local enhancement of electric field intensity that strengthens light-matter interactions, enabling applications such as surface-enhanced Raman scattering (SERS) and surface plasmon enhanced sensing. In the presence of magneto-optically active materials, the local field enhancement gives rise to anomalous magneto-optical activity. Typically, the confined modes of a given photonic crystal depend strongly on the wavelength and incidence angle of the incident electromagnetic radiation. Thus, spectral and angular-resolved measurements are needed to fully identify them as well as to establish their relationship with the magneto-optical activity of the crystal. In this article, we describe how to use a Fourier-plane (back focal plane) microscope to characterize magneto-optically active samples. As a model system, here we use a plasmonic grating built out of magneto-optically active Au/Co/Au multilayer. In the experiments, we apply a magnetic field on the grating in situ and measure its reciprocal space response, obtaining the magneto-optical response of the grating over a range of wavelengths and incident angles. This information enables us to build a complete map of the plasmonic band structure of the grating and the angle and wavelength dependent magneto-optical activity. These two images allow us to pinpoint the effect that the plasmon resonances have on the magneto-optical response of the grating. The relatively small magnitude of magneto-optical effects requires a careful treatment of the acquired optical signals. To this end, an image processing protocol for obtaining magneto-optical response from the acquired raw data is laid out.

Plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles fabricated using a femtosecond pulse

We present the plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles (Ag NPs) fabricated using a femtosecond pulse. Theoretical analysis indicates Ag NPs with a diameter of ∼100 nm have excellent linear response within the visible band, and the electric field intensity enhancement factor reaches ∼105 under excitation of continuous light of 632.8 nm. Meanwhile, the simulation result of second-order surface nonlinear optical response shows that the second harmonic conversion efficiency of the Ag NPs dimer is two orders of magnitude higher than that of a single Ag NP, under excitation of a femtosecond pulse. In experiment, the linear response of Ag NPs is examined using surface-enhanced Raman spectroscopy (SERS) with a Raman enhancement factor of ∼1.7 × 1010, revealing the excellent linear optical response of Ag NPs. Moreover, the spectra of the second harmonic can be measured clearly under conditions of an average pump power of 40 μW, revealing the excellent second-order surface nonlinear optical response of Ag NPs.

Surface-Enhanced Raman Spectroscopy Based on a Silver-Film Semi-Coated Nanosphere Array.

In this paper, we present a convenient and economical method to fabricate a silver (Ag)-film semi-coated polystyrene (PS) nanosphere array substrate for surface-enhanced Raman spectroscopy (SERS). The SERS substrate was fabricated using the modified self-assembled method combined with the vacuum thermal evaporation method. By changing the thickness of the Ag film, the surface morphology of the Ag film coated on the PS nanospheres can be adjusted to obtain the optimized localized surface plasmonic resonance (LSPR) effect. The 3D-finite-difference time-domain simulation results show that the SERS substrate with an Ag film thickness of 10 nm has tens of times the electric field intensity enhancement. The Raman examination results show that the SERS substrate has excellent reliability and sensitivity using rhodamine-6G (R6G) and rhodamine-B (RB) as target analytes, and the Raman sensitivity can reach 10−10 M. Meanwhile, the SERS substrate has excellent uniformity based on the Raman mapping result. The Raman enhancement factor of the SERS substrate was estimated to be 5.1 × 106. This kind of fabrication method for the SERS substrate may be used in some applications of Raman examination.

Dark mode plasmonic optical microcavity biochemical sensor

Whispering gallery mode (WGM) microtoroid optical resonators have been effectively used to sense low concentrations of biomolecules down to the single molecule limit. Optical WGM biochemical sensors such as the microtoroid operate by tracking changes in resonant frequency as particles enter the evanescent near field of the resonator. Previously, gold nanoparticles have been coupled to WGM resonators to increase the magnitude of resonance shifts via plasmonic enhancement of the electric field. However, this approach results in increased scattering from the WGM, which degrades its quality (Q) factor, making it less sensitive to extremely small frequency shifts caused by small molecules or protein conformational changes. Here, we show using simulation that precisely positioned trimer gold nanostructures generate dark modes that suppress radiation loss and can achieve high (>106) Q with an electric-field intensity enhancement of 4300, which far exceeds that of a single rod (∼2500 times). Through an overall evaluation of a combined enhancement factor, which includes the Q factor of the system, the sensitivity of the trimer system was improved 105× versus 84× for a single rod. Further simulations demonstrate that unlike a single rod system, the trimer is robust to orientation changes and has increased capture area. We also conduct stability tests to show that small positioning errors do not greatly impact the result.

Nanofocusing of Surface Plasmon Polaritons on Metal-Coated Fiber Tip Under Internal Excitation of Radial Vector Beam

We theoretically present the nanofocusing of the metal-coated fiber tip under internal excitation of the radial vector beam within visible band based on the finite difference time domain (FDTD) analysis. The electric field intensity enhancement factor of the localized surface plasmons (LSP) mode at the tip apex is quantitatively shown in relation with incident wavelength, coating material, conical angle of tip, and coating film thickness/length. Specially, the evolution of fiber radial vector mode to surface mode with respect to the radius of metal-coated fiber tip is calculated under typical excitation wavelengths of 633 nm and 785 nm. Furthermore, the reason of the tip eliminating far-field background signal is explained, and the transverse electric field distributions of LSP mode and the tip-substrate coupling are also given at the optimal excitation wavelength. These calculation results will be a good reference for the fabrication of metal-coated fiber tips and for the experimental design of the tip-enhanced spectroscopy (TES) system.

Highly efficient plasmonic nanofocusing on a metallized fiber tip with internal illumination of the radial vector mode using an acousto-optic coupling approach

Abstract Tip-based plasmonic nanofocusing, which delivers light into a nanoscale region and achieves localized electromagnetic (EM) field enhancement beyond the diffraction limit, is highly desired for light-matter interaction-based super-resolution imaging. Here, we present the plasmonic nanofocusing at the apex of a silver (Ag)-coated fiber tip with the internal illumination of a radial vector mode (RVM) generated directly in an optical fiber based on an acoustically-induced fiber grating (AIFG). As illustrated by theoretical calculation, a picture of the nanofocusing plasmonic tip given by analyzing the mode conversion process that the surface plasmon polariton (SPP) mode excited via the radial polarization optical mode can propagate to the apex of the plasmonic tip for nanofocusing because it is not cut off as the tip radius decreases; while the SPP mode which transited from the linear polarization optical mode cannot propagate to the tip apex for nanofocusing because it is cut off as the tip radius decreases. The electric field intensity enhancement factor | E apex 2 | / | E input 2 | $|{\rm{E}}_{{\rm{apex}}}^{\rm{2}}|/|{\rm{E}}_{{\rm{input}}}^{\rm{2}}|$ of a plasmonic tip with a tip radius of 20 nm was calculated to be ~2 × 103. Furthermore, the electric field enhancement characteristic at the tip apex was also experimentally verified by using surface-enhanced Raman spectroscopy (SERS). The Raman scattering intensity was observed to be ~15 times as strong as that with internal illumination using the linear polarization mode (LPM), revealing their significantly different nanofocusing characteristics. A Raman sensitivity of 10−14 m was achieved for the target analyte of malachite green (MG), denoting significant electric field enhancement and effective plasmonic nanofocusing. The energy conversion efficiency of the radial polarization optical mode to the corresponding SPP mode at the tip apex was measured to be ~17%. This light delivery technique can be potentially further exploited in near-field microscopy with improved resolution and conversion efficiency.

Electric and Magnetic Hotspots via Hollow InSb Microspheres for Enhanced Terahertz Spectroscopy

We study electric and magnetic hotspots in the gap between hollow InSb microspheres forming dimers and trimers. The outer radius, core volume fraction, distance, and temperature of the microspheres can be chosen to achieve field enhancement at a certain frequency corresponding to the transition between energy levels of a molecule placed in the gap. For example, utilizing 80 μm radius spheres at a gap of 2 μm held at a temperature of 295 K, allow electric field intensity enhancements of 10–2880 and magnetic field intensity enhancements of 3–61 in the frequency window 0.35–1.50 THz. The core volume fraction and the ambient temperature affect the enhancements, particularly in the frequency window 1.5–2 THz. Electric and magnetic hotspots are promising for THz absorption and circular dichroism spectroscopy.

Theory and method for large electric field intensity enhancement in the nanoantenna gap.

We first investigate the field intensity in the nanoantenna gap as a function of common antenna properties including polarization, input resistance, and gain. This function provides us a method on how to effectively enhance the field intensity. In the case of polarization matched to the incident wave, the nanoantenna should have both large input resistance and high gain in the arrival direction. To meet these demands, the flat feed gap is modified to a bowtie form, and a hemispherical lens is attached to the nanoantenna. Consequently, the relative field intensity in the gap is found to be 2.6×103 a.u., which is about 8 times larger than the original value, and they all agree well with the simulations. This research is expected to be used as guidelines for the design of nanoantennas and to promote them in plasmonic applications such as spectroscopy and photodetection.

Tip-assisted electrohydrodynamic jet printing for high-resolution microdroplet deposition

The controlled patterning of microdroplets or microwires at precise positions plays an essential role in micro/nano device applications. In this context, we propose a novel and efficient method called tip-assisted electrohydrodynamic (EHD) printing that combines the tip-assisted electric field intensity enhancement effect and the high-resolution capability of EHD printing. The tip-assisted electric field enhancement effect is first systematically investigated by simulation analysis. Under optimized experimental conditions, we obtain microdroplet arrays and microwires down to sizes of 2.3 μm and controlled micropatterns composed of droplet arrays or microwires with sizes smaller than the inner diameter of nozzle by means of the tip-assisted technique. We find that with our designed tip-assisted nozzle, solutions with a high viscosity of >2000 cps can be extracted from the nozzle, and the printing resolution can increase by nearly five times the current resolution. Our results demonstrate that tip-assisted EHD printing provides a potential and flexible method for precision and controlled manufacturing of micropatterns.