High Magnitude Spin-Dependent Shift and Field Enhancement in BaTiO3-Based Plasmonics for Quantum Photonic Applications

Photoemission electron microscopy (PEEM) is an imaging method which uses electrons excited through the photoelectric effect to characterize a sample surface with nanometer-level resolution. In PEEM, a high intensity laser excites electrons from the surface of the material and electron optics are used to form an image from the intensity and spatial distribution of the photoemission from the sample. The goal of this research was to study and maximize light confinement, which was accomplished using plasmonic nanostructures. Surface plasmons represent oscillations in the electron density of a material and can occur along the transition interface between a metal and a dielectric material. Under the right conditions, both propagating and localized surface plasmon modes may be excited within the structure. Plasmonic antennas are devices that can convert incident electromagnetic fields into localized areas of high electric field enhancement within specific regions of the structures. These nano- and micron-sized devices can be created through lithography or chemical-synthesis and by varying the materials or geometries of the structures, the antennas can be tailored to a wide range of applications. This research used PEEM to excite surface plasmons in triangular gold nanoplatelets and create areas of high photoemission within the structures. The high photoemission observed in PEEM correlated to localized areas of high electric field enhancement, primarily at the tips of the triangles. Localized field enhancement was demonstrated experimentally within a tip region ~90 nm in diameter and within ~15 nm through numerical calculations, both significantly smaller than the overall size of the platelet, the wavelength of the excitation light, or the wavelength of the plasmon mode. In addition, methods for varying and optimizing the spatial distribution and strength of the field enhancement at the tips of the triangles were demonstrated experimentally through PEEM. Overall, this research involved the chemical-synthesis and deposition of thin, triangular gold nanoplatelets, as well as finite element method (FEM) numerical calculations and PEEM experiments to characterize the plasmonic effects of these structures. The nanoplatelets were evaluated for a range of experimental parameters, geometric characteristics, and surrounding materials. This required a detailed understanding of the role of surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) which may be excited within these structures. The effects of the polarization, wavelength, and angle of incidence of the excitation light on the field enhancement at the tips of the triangles were characterized in-depth. In addition, the roles of platelet size, gold thickness, edge

Two optical antenna designs incorporating structures termed charge and current reservoirs are proposed to realize localized high electric and magnetic field enhancement, respectively. Simulation results show that the fan-rod electric antenna design combines the advantages of the rod antenna and the bowtie antenna, and has higher field enhancement than either. The performance of a loop shaped magnetic antenna consisting of a pair of metallic strips with offsets is also verified numerically, with high magnetic field enhancement being observed in the simulation. In both of the designs, the concepts of charge and current reservoirs contribute to high electric and magnetic field enhancement.

Because of its high information content on chemical structure, Raman scattering is a very promising technique for single molecule spectroscopy, which allows establishing the structural identity of a single molecule based on its vibrational spectrum [1,2]. For experiments performed at room temperature and in solutions, SERS (surface-enhanced Raman scattering) is superior to broad and nonspecific fluorescence spectra obtained under similar conditions. Furthermore, non-fluorescent molecules might be detected and identified at the single-molecule level without need for fluorescence labeling. Raman scattering can be used under electronic "nonresonant" conditions, which avoids photobleaching. Relatively short vibrational lifetimes compared to fluorescence lifetimes allow more excitation emission cycles per time interval and therefore the maximum possible number of emitted Raman photons per time interval will be larger than the number of fluorescence photons [3]. Single molecule Raman spectroscopy is based on the strongly enhanced Raman scattering signal which occurs when the target molecule is attached to silver and gold nanostructures, called surface-enhanced Raman scattering (SERS) [1,2]. It is generally agreed that different effects contribute to the large effective Raman cross section observed in SERS experiments. The enhancement mechanisms are roughly divided into so-called electromagnetic and chemical or first layer effects [4-7]. The basic idea of chemical SERS enhancement is a metal electron mediated resonance Raman effect in the "system" molecule-metal. The magnitude of chemical enhancement has been discussed to reach not more than 102-103. The electromagnetic enhancement factor arises from enhanced local optical fields in the vicinity of the metallic nanostructures due to resonances with their surface plasmons. Particularly high field enhancement seems to exist for ensembles of metallic nanoparticles, such as silver or gold colloidal clusters formed by aggregation of colloidal particles or for island films of those metals. Plasmon resonances in such structures can result in a strong confinement of optical fields in very small areas, so-called "hot spots whose dimensions can be smaller than tenths of the wavelength [8]. The high local optical fields in the hot spots provide a rationale for non-resonant SERS enhancement up to 14 orders of magnitude, where field enhancement can contribute 12 orders of magnitude. This enhancement brings effective Raman cross sections to the level of effective fluorescence cross sections of good laser dyes and allows that a single molecule can be detected by means of its non-resonant surface-enhanced Raman spectrum [9-13]. Moreover, the strong lateral confinement of the field enhancement provides an additional opportunity for spectroscopically selecting a single species [14]. In SERS spectroscopy, the target species "feels" very high local optical field strength and, in particular, also very strong field gradients. These both effects might result in some special effects in single molecule SERS spectra, which do not occur in "normal" Raman spectroscopy [15,16]. The large field gradients on SERS-active substrates can result lowering of the symmetry of the resonance Raman scattering tensor compared to "normal" Raman scattering . In the large field strengths in the hot spots on a SERS-active substrate, the treatment of the molecular vibrations in the harmonic approximation might not be justified and vibrational modes can couple, which can result exchange in scattering power between two phonon modes. Surface-enhanced anti-Stokes Raman scattering originates from vibrational levels, which are populated by the very strong surface-enhanced Raman Stokes process: One photon populates the excited vibrational state, a second photon generates the anti Stokes scattering. Therefore, the anti-Stokes Raman scattering signal depends quadratically on the excitation laser intensity [9,17,18]. The two-photon process inherently confines the volume probed by surface-enhanced anti-Stokes Raman scattering compared to that probed by one-photon "normal" surface-enhanced Stokes scattering. Effective cross sections for two-photon anti-Stokes scattering are on the order of 10-42 cm4s, which is at least seven orders of magnitude larger than typical two-photon fluorescence cross sections.

Because of its high information content on chemical structure, Raman scattering is a very promising technique for single molecule spectroscopy, which allows establishing the structural identity of a single molecule based on its vibrational spectrum [1,2]. For experiments performed at room temperature and in solutions, SERS (surface-enhanced Raman scattering) is superior to broad and nonspecific fluorescence spectra obtained under similar conditions. Furthermore, non-fluorescent molecules might be detected and identified at the single-molecule level without need for fluorescence labeling. Raman scattering can be used under electronic “nonresonant” conditions, which avoids photobleaching. Relatively short vibrational lifetimes compared to fluorescence lifetimes allow more excitation emission cycles per time interval and therefore the maximum possible number of emitted Raman photons per time interval will be larger than the number of fluorescence photons [3]. Single molecule Raman spectroscopy is based on the strongly enhanced Raman scattering signal which occurs when the target molecule is attached to silver and gold nanostructures, called surface-enhanced Raman scattering (SERS) [1,2]. It is generally agreed that different effects contribute to the large effective Raman cross section observed in SERS experiments. The enhancement mechanisms are roughly divided into so-called electromagnetic and chemical or first layer effects [4-7]. The basic idea of chemical SERS enhancement is a metal electron mediated resonance Raman effect in the “system” molecule-metal. The magnitude of chemical enhancement has been discussed to reach not more than 102-103. The electromagnetic enhancement factor arises from enhanced local optical fields in the vicinity of the metallic nanostructures due to resonances with their surface plasmons. Particularly high field enhancement seems to exist for ensembles of metallic nanoparticles, such as silver or gold colloidal clusters formed by aggregation of colloidal particles or for island films of those metals. Plasmon resonances in such structures can result in a strong confinement of optical fields in very small areas, so-called “hot spots whose dimensions can be smaller than tenths of the wavelength [8]. The high local optical fields in the hot spots provide a rationale for non-resonant SERS enhancement up to 14 orders of magnitude, where field enhancement can contribute 12 orders of magnitude. This enhancement brings effective Raman cross sections to the level of effective fluorescence cross sections of good laser dyes and allows that a single molecule can be detected by means of its non-resonant surface-enhanced Raman spectrum [9-13]. Moreover, the strong lateral confinement of the field enhancement provides an additional opportunity for spectroscopically selecting a single species [14]. In SERS spectroscopy, the target species “feels” very high local optical field strength and, in particular, also very strong field gradients. These both effects might result in some special effects in single molecule SERS spectra, which do not occur in “normal” Raman spectroscopy [15,16]. The large field gradients on SERS-active substrates can result lowering of the symmetry of the resonance Raman scattering tensor compared to “normal” Raman scattering . In the large field strengths in the hot spots on a SERS-active substrate, the treatment of the molecular vibrations in the harmonic approximation might not be justified and vibrational modes can couple, which can result exchange in scattering power between two phonon modes. Surface-enhanced anti-Stokes Raman scattering originates from vibrational levels, which are populated by the very strong surface-enhanced Raman Stokes process: One photon populates the excited vibrational state, a second photon generates the anti Stokes scattering. Therefore, the anti-Stokes Raman scattering signal depends quadratically on the excitation laser intensity [9,17,18]. The two-photon process inherently confines the volume probed by surface-enhanced anti-Stokes Raman scattering compared to that probed by one-photon “normal” surface-enhanced Stokes scattering. Effective cross sections for two-photon anti-Stokes scattering are on the order of 10-42 cm4s, which is at least seven orders of magnitude larger than typical two-photon fluorescence cross sections.

Superconducting radiofrequency (SRF) cavities are vital components of particle accelerators nowadays. In order to minimise the energy dissipation, a perfect inner surface of the cavity, hindering the penetration of magnetic field, is required. In this work, we investigated ten planar samples differing in the surface quality of Nb film deposited on Cu substrate, and as a consequence exhibiting various levels of the first entry field, H en, at which the magnetic field starts to enter the film. The observed surface defects are categorised as hills, pits and cracks. For a practical range of dimensions of these features, the factor β, characterising the local magnetic field enhancement, was calculated by the numerical finite-element simulations. It is expected that the local field enhancement causes a premature penetration of the magnetic field, thus lowering H en. Then, for each investigated sample, the range of β values characterising defect type that cause the highest field enhancement, is identified and compared with the H en fields. We have found that the H en of the samples that contain multiple types of the surface features is indeed limited by those defects that cause the highest field enhancement. The H en vs β dependence has shown a good match with linear fit for the set of investigated samples. Thus, the main result is that the local magnetic field enhancement, computed in a straightforward way for the most significant defects, is a strong indicator of the surface quality that is relevant for the superconducting film intended for SRF cavity application.

In this paper, we present a rectification process through electron emission from sharp tips under a high electric field of 109–1010 V/m based on the Fowler–Nordheim theory. A large planar array of 104–105 nanoantennas is proposed to achieve the required electric field for electron emission in the nanoantenna gap. Spatial coherence of the incident wave is a prohibitive issue in using large antenna arrays, and it is overcome by choosing proper size for the array. The design starts with a single-element nanoantenna, for which, five different types are considered, namely dipole, spiral, cross bowtie, sinuses and square spiral. Full-wave simulations show that cross bowtie has the highest field enhancement. Moreover, two array configurations, i.e. series and parallel feed networks, are considered and their field enhancement are compared. Moreover, field enhancement in large array configurations is predicted using proposed equations based on curve fitting.

For electro-optic modulators, traveling wave designs are implemented due to the long interaction area necessary to efficiently modulate a signal with low voltages as a result of LiNbO3’s modest electro-optic coefficient (r33 =30 pm/V). However, with metamaterial inspired, electrically small resonator antennas, high field enhancements can be produced which allow the modulation of optical signals directly from the antennas, bypassing the need for a separate modulator [1]. For this work, Laser Direct Write (LDW) proved to be a useful tool in fabricating various meta-antenna array structures. The additive techniques of laser direct write, coupled with more traditional micromachining, have allowed the quick fabrication of these resonator meta-antennas so that the desired specifications, including resonant frequency and field enhancement, can be tested quickly and appropriate updates to the design can be made. This allows a designer to shift the resonant frequency and increase the field enhancement necessary to get the required device performance. The Laser Induced Forward Transfer (LIFT) technique was used initially for lower frequency designs (X-band); however, Laser Decal Transfer (LDT) was used subsequently for designs with higher frequencies by employing a digital mirror device to modulate the image of the beam. The resulting designs showed good fabrication consistency in terms of resonant frequency and comparable field enhancement to resonators fabricated through traditional lithography.

Infrared (IR) absorbers based on a metal–insulator–metal (MIM) have been widely investigated due to their high absorption performance and simple structure. However, MIM absorbers based on ultrathin spacers suffer from low field enhancement. In this study, we propose a new MIM absorber structure to overcome this drawback. The proposed absorber utilizes a reactive impedance surface (RIS) to boost field enhancement without an ultrathin spacer and maintains near-perfect absorption by impedance matching with the vacuum. The RIS is a metallic patch array on a grounded dielectric substrate that can change its surface impedance, unlike conventional metallic reflectors. The final circular nanodisk array mounted on the optimum RIS offers an electric field enhancement factor of 180 with nearly perfect absorption of 98% at 230 THz. The proposed absorber exhibits robust performance even with a change in polarization of the incident wave. The RIS-integrated MIM absorber can be used to enhance the sensitivity of a local surface plasmon resonance (LSPR) sensor and surface-enhanced IR spectroscopy.

The strong focusing and field enhancement effects of a metal nanofinger surrounded by multiple concentric rings are investigated through both COMSOL Multiphysics and finite-difference time-domain (FDTD) simulations. The aspect ratio of the nanofinger is the main parameter determining the full width at half maximum (FWHM) and the strong local field enhancement. The optimal values of the aspect ratio for the maximal enhancement and minimal FWHM are close to 1.8 and 3.0, respectively. Furthermore, the optimal aspect ratio of maximal field enhancement intensity decreases linearly with the incident wavelength, and the optimal aspect ratio of minimal FWHM increases linearly with the metal film thickness. The nanofinger fabricated with the focused ion beam method has a small conical angle, which results in a higher field enhancement and smaller focal spot size than straight sidewall finger. However, the shorter finger defect deteriorates FWHM and field enhancement because of the bias from the optimal aspect ratio value.

In this work, we put forward an all-dielectric nanotweezer using a quasi-bound state in the continuum (quasi-BIC) mode to trap nanoparticles with a radius of 10 nm. The quasi-BIC mode provides not only a very high electric field enhancement but also a high quality factor ( Q -factor), which gives it potential for the trapping of nanoparticles with low laser power and high stability. The simulation results show that when the input intensity is 1 m W / µ m 2 , the maximum optical trapping force of the 10 nm particles is 2.24 pN, and the maximum trapping potential is 29.08 k B T . Furthermore, the proposed nanotweezer array provides multiple optical hotspots with high field confinement and enhancement, resulting in multiple trapping sites for the parallel trapping of multiple nanoparticles. The high-throughput trapping of nanoparticles provides a good foundation for studying biological cells and protein molecules, especially for the heterogeneity of cells and the large-scale parallel analyses of basic drugs.

In contrast with aperture-<i>limited</i> Scanning Near-field Optical Microscopy, where the focusing of light is achieved only with very high attenuation, in aperture<i>less</i> near-field optics light is both focused <i>and strongly amplified</i> by the surface plasmons of the probe. Although the general feasibility of this idea and the unprecedented in optics lateral resolution of ~ 15-30 nm have already been demonstrated, the actual field enhancement has so far been well below theoretical expectations, and the useful optical signals have been weak. To bridge the gap between the "proof-of-concept" experiments and reliable optical microscopy with molecular-scale resolution, one needs to unify accurate simulation with effective measurements of the optical properties of the tips and with fabrication. We use dark-field microscopy with side collecting optics for measurements of the optical properties of the tip. The side view allows us to observe the radiation of the tip and hence to analyze its optical properties at the apex. In addition, the measured Raman signal provides an estimate of the electric field enhancement by the tip. Our simulation protocol consists of two parts: electrostatics and wave analysis. Electrostatic simulations give good qualitative predictions, are very fast and therefore conducive to multiparametric optimization. Full wave analysis is needed to evaluate the dephasing effects and far-field signals. The Finite Element Method is used for all simulations. Various tip designs with the field enhancement ranging from ~ 50 to over 250 (depending on various parameters), with the commensurate enhancement of the Raman signal by ~ 45⁴ (for gold coating) and ~ 270⁴ (for silver coating), are presented and analyzed.

Enhancement of localized electric field near metal (plasmonic) nanostructures can have various interesting applications in sensing, imaging, photovoltage generation etc., for which significant efforts are aimed towards developing plasmonic systems with well designed and large electromagnetic response. In this paper, we discuss the wafer scale fabrication and optical characterization of a unique three dimensional plasmonic material. The near field enhancement in the visible range of the electromagnetic spectrum obtained in these structures (order of 10⁶), is close to the fundamental limit that can be obtained in this and similar EM field enhancement schemes. The large near field enhancement has been reflected in a huge Raman signal of graphene layer in close proximity to the plasmonic system, which has been validated with FEM simulations. We have integrated graphene photodetectors with this material to obtain record photovoltage generation, with responsivity as high as A/W. As far as we know, this is the highest sensitivity obtained in any plasmonic-graphene hybrid photodetection system till date.

Optical trapping with plasmonic double nanohole (DNH) apertures has proven to be an efficient method for trapping sub-50 nm particles due to their suppressed plasmonic heating effect and very high electric field enhancement in the gap region of the aperture. However, plasmonic tweezers are generally diffusion-limited, requiring particles to diffuse down to a few tens of nanometres from the high field enhancement regions before they can be trapped. The loading of target particles to the plasmonic hotspots can take several minutes for diluted samples. In this work, rapid particle transport and trapping of a 25 nm polystyrene sphere is demonstrated, leveraging an electrothermoplasmonic flow induced upon application of an AC field in the presence of a laser-induced temperature gradient. Using this approach, we demonstrate the rapid transport of a 25 nm polystyrene particle across a distance of 63 μm and trapping at the DNH under 16 s. This platform shows great potential for applications involving simultaneous trapping and plasmon-enhanced spectroscopies, such as Raman enhancement via the intense electric field enhancement in the DNH gap.

A giant electric field on a subwavelength scale is highly beneficial for boosting the light–matter interaction. In this paper, we investigated a hybrid structure consisting of a hemispheric dimer array and a gold film and realized resonant mode coupling of the surface lattice resonance (SLR) and surface plasmon polariton (SPP). Mode coupling is demonstrated by observing anti-crossing in reflection spectra, which corresponds to Rabi splitting. Although the resonance coupling does not enter the strong coupling regime, an improved quality factor (Q~350) and stronger electric field enhancement in the gap region of the dimer (i.e., hot spot) in our hybrid structure are obtained compared to those of the single dimer or dimer array only. Remarkably, the magnitude of electric field enhancement over 500 can be accessible. Such high field enhancement makes our hybridized structure a versatile platform for the realization of ultra-sensitive biosensing, low-threshold nanolasing, low-power nonlinear optical devices, etc.

A high-index fibre-core-based surface plasmon resonance (SPR) sensor using 2D materials such as graphene, graphene oxide (GO) and molybdenum disulphide (MoS2) on a silicon (Si) over-layer in visible and near infrared is reported. Si was used on silver (Ag) followed by different 2D materials of appropriate thickness on a 600 µm core multimode fibre. Here Si is used to tune the resonance to a higher wavelength, and different 2D materials are used to enhance the performance as well as address the oxidation problem of Si. Moreover, a high-index chalcogenide core helps to decrease the full width at half maxima of the SPR spectra, hence increasing the detection accuracy of the sensor. Both the Ag and Si thickness are optimized, and the effects of different 2D materials on the performance of sensor are studied using the transfer matrix method in terms of electric field intensity, sensitivity, figure of merit (FOM) and resolution. We found that the percentage electric field enhancement for the Ag–Si-GO system is 4.65 × 104 in the presence of GO. The GO-based sensor is found to have a sensitivity of 202.2 nm RIU−1 in comparison to the graphene based sensor with a sensitivity of 189.4 nm RIU−1. Moreover, the FOM values are found to be as high as 5.57 RIU−1 for GO, and 5.18 RIU−1 and 2.29 RIU−1 for the MoS2 and graphene respectively. We believe that the proposed 2D-material-based sensors will open a new window for the development of high-performance fibre biosensors by utilizing the biocompatibility aspect of different 2D materials along with its high electric field value in the presence of a dielectric over-layer.