Local Electric Field Enhancement Research Articles

Enhancement of a whispering gallery mode microtoroid resonator by plasmonic triangular gold nanoprism for label-free biosensor applications

Whispering gallery mode microresonators coupled to plasmonic nanoparticles have shown great promise for ultra-sensitive and label-free biological sensing. We analyze a whispering gallery mode microtoroid biosensor with a triangular gold nanoprism bound to its surface. We choose triangular nanoprism because of its capability of considerable enhancement of electromagnetic field at the tips and because its localized surface plasmon resonance band position is readily tunable. The local electric field enhancement at different points of plasmonic triangular gold nanoprism is calculated and the effect of rounded tip is investigated. This field enhancement permits the detection and characterization of some large protein molecules. We have studied the detection of single bovine serum albumin protein using our hybrid microcavity.

Intrinsic space charge layers and field enhancement in ferroelectric nanojunctions

Conducting characteristics of topological defects in ferroelectric materials, such as charged domain walls, engendered a broad interest on their scientific merit and the possibility of novel applications utilizing domain engineering. At the same time, the problem of electron transport in ferroelectrics still remains full of unanswered questions and becomes yet more relevant over the growing interest in ferroelectric semiconductors and new improper ferroelectric materials. We have employed self-consistent phase-field modeling to investigate the physical properties of a local metal-ferroelectric (Pb(Zr0.2Ti0.8)O3) junction in applied electric field. We revealed an up to 10-fold local enhancement of electric field realized by large polarization gradient and over-polarization effects due to inherent non-linear dielectric properties of Pb(Zr0.2Ti0.8)O3. The effect is independent of bias polarity and maintains its strength prior, during and after ferroelectric switching. The observed field enhancement can be considered on similar grounds as increased doping level, giving rise to reduced switching bias and threshold voltages for charge injection, electrochemical and photoelectrochemical processes.

Pure magnetic resonances controlled by the relative azimuth angle between meta-atoms.

A plasmonic molecule showing strong magnetic resonance modes and flexible tunability is proposed. The molecule is composed of two elements, a crescent shaped metallic disk and a smaller one embedded in the cavity of the larger one. The cavity and gap formed by these two elements enable the molecule to support magnetic resonances in the visible and near infrared spectral region, while electric resonances are much weaker to be detected. We show that by changing the relative orientation angle of these two meta-atoms, the resonance wavelength can be changed from the visible to near infrared without modification of the size of the molecule. Anti-crossings and crossings of resonance energy levels, which stem from the coupling effect, are analyzed. When resonating magnetically, the local electric field enhancement at the crescent tip can reach up to hundreds of times with high spatial confinement, which renders the molecule promising applications in many fields.

The enhancement of local electric field and farfield optical response of nanometallic shells

The optical response of various geometrical metallic nanoshell with silica are investigated by CST Microwave Studio® simulation. Farfield optical properties such as extinction cross section and enhancement of electric field on different geometrical surface are presented. The positions of plasmon absorptions near shorter and longer wavelength are also altered as changing the dimension parameters. Distorted irregular shapes NPs enormously enhance electric field as compared to regular NPs. Some particular simulation result is also compared to previously published work done by the method of Finite Difference Time Domain (FDTD) and Mie Theory. Assembles of NPs with different geometry size and shape are helpful in modeling multiple detected nano-antenna within optical frequency range.

Spatiotemporal properties of nanoshell plasmonic response for strong-field experiments

Field enhancement behavior of a ${\mathrm{SiO}}_{2}$/Au nanoshell is studied in the framework of strong-field physics. Localized plasmonic fields induce local electric field enhancement with the potential to lead to the strong-field regime without the use of costly amplified lasers. In this framework, electrons are tunnel ionized from the nanoshell and accelerated by the local field being spatially inhomogeneous in terms of spectral and polarization properties. These processes are happening within a single laser shot, and thermal effects are therefore neglected. We show that the localized response to ultrashort femtosecond pulses can be investigated by extending Mie theory to multilayer spherical particles. Nanoshell plasmonic resonances can be easily tuned depending on the volume ratio between core and whole particle. Optimum geometric parameters of the nanoshell are selected for use with ultrashort pulses centered at 800 nm where most femtosecond oscillators operate. It is shown that phase and amplitude reshaping of the incident pulse by the plasmonic resonance can be partially corrected using active shaping leading to ultrashort response of the medium. Finally, free electron classical trajectories are calculated to highlight the inhomogeneous nature of the local enhancement in a strong-field picture.

Analysis of Asymmetric Dipoles Interacting in Heterogeneous Metal Nanorod Dimers

The localized electric field enhancement of Au-Ag nanorods dimer is theoretically investigated based on finite element method (FEM). An analytical mutual-interference model between asymmetric dipole-dimer has been built to describe the dipoles coupling with different resonance modes in heterogeneous dimer. Three prominent enhancement peaks could be observed from ultraviolet to visible spectrum, in which the ultraviolet resonance especially corresponds to a higher mode in both Ag and Au nanorods. The results reveal the strong coupling mechanism among different dipoles existing in the asymmetric dipole system, which could support the design of plasmonic nanodevices in larger resonance wavelength range.

Size-dependence of plasmonic Au nanoparticles in photocatalytic behavior of Au/TiO2 and Au@SiO2/TiO2

Au nanoparticles (NPs) with sizes of 3, 7 and 17nm and their corresponding Au@SiO2 core–shell structures have been selectively synthesized and loaded on the TiO2 surface to obtain six different plasmonic photocatalysts. In decomposing salicylic acid (SA) under UV–visible light, all photocatalysts reveal significantly enhanced catalytic activity than the bare TiO2. Interestingly, the 3nm-sized Au NP-loaded TiO2 (Au3/TiO2) exhibits the highest activity among the bare Au-loaded TiO2, whereas the Au17@SiO2/TiO2 shows the highest value among the Au@SiO2-loaded TiO2 systems. In the Au/TiO2 systems, it is derived that the space-charge separation is dominant factor in enhancing photocatalytic activity and the gradual decrease of catalytic activity with increase of Au NP size is caused by the increased localized surface plasmon resonance (LSPR) sensitization effect. In the Au@SiO2/TiO2 systems, however, space-charge separation or LSPR sensitization by Au NP is fundamentally blocked by the insulating SiO2 shell. Thus the local electric field enhancement is the major factor in enhancing the photocatalytic activity, explaining why the photocatalytic activity of Au@SiO2/TiO2 increases rapidly as the size of Au NP increases.

Ultrafast dynamics of near-field enhancements at an off-resonance nano-dimer via femtosecond laser excitations

Giant electric-field enhancements localized on nano-antennas are important for the optical near-field applications in fields such as super-resolution imaging, near-field optical tweezers, and photothermal therapy. Physically, the field enhancement requires plasmon resonance with respect to structure matching. We report a tunable near-field effect, including localized electric-field enhancement and resistive heating at an off-resonance Au nano-sphere dimer via femtosecond laser irradiation. The near field was strongly modified (up to 81 times) with respect to time evolution at a laser fluence of . The results are explained as thermal dynamics manipulation of the Au nano-sphere dimer plasmon resonances. This study provides a new alternative route to tailoring the near-field enhancement for wide applications in nano-antennas.

Thin film micro-scaled cold cathode structures of undoped and Si-doped AlN grown on SiC substrate with low turn-on voltage**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005, and 61176126), the Open Project of the Key Laboratory of Nano-devices and Applications, China (Grant No. 13ZS04), and the National Science Fund for Distinguished Young Scholars,

The field emission characteristics of the AlN thin films with micro-scaled cold cathode structures are tested in the high vacuum system. The aluminum nitride (AlN) thin films with a thickness of about 100 nm are prepared on the n-type 6H-SiC (0001) substrate at 1100 °C by metal organic chemical vapor deposition (MOCVD) under low pressure. The I–V curves and surface micro-images of undoped and Si-doped AlN films are investigated. From the I–V and Fowler–Nordheim plots, it can be seen that the Si-doped AlN shows better field emission characteristics compared with the undoped AlN sample. The obtained turn-on field is 6.7 V/μm and the maximum emission current density is 154 mA/cm2 at 69.3 V for the Si-doped AlN film cathode after proper surface treatment. It is proposed that the relatively low electric resistivity of Si-doped AlN films is significant for electron migration to the surface region, and their rougher surface morphology is beneficial to a higher local electric field enhancement for the field emission.

Graphitization wave in diamond bulk induced by ultrashort laser pulses

Multi-pulse laser irradiation of diamond bulk after the optical breakdown causes extension of continuous graphitized region toward the laser beam that can be described as propagation of a “graphitization wave.” Velocity of the graphitization wave in single-crystal diamond is measured experimentally as a function of local laser fluence for a few numerical apertures (NA = 0.36–0.09), pulsewidths (140 fs–5 ps), and beam orientations (along [110] or [100] diamond axes). The experimental results are used to develop the model of the crack-assisted thermal graphitization of diamond at the boundary of the laser-modified region. Velocity of the graphitization wave is determined in general case by diffusion of heat from the light-absorbing modified region. The revealed rise in the graphitization wave velocity for the [110] beam orientation can be explained by the local electric field enhancement near the crack tip, which facilitates diamond ionization and plasma-assisted energy absorption. The proposed model predicts a specific internal structure of the laser-modified region: the network of graphitic inclusions with diamond-filled gaps between them.

Interaction between Nanoprisms with Different Coupling Strength

The aggregated noble metal system, consisting of two or more nanoparticles, possesses unique optical properties. The most important is the ability to generate larger local electric field enhancement than a single particle. In this work, we have modeled the system composed of silver nanoprisms with different geometries. For this purpose, the optical properties of the single silver nanoprism and the aggregated nanoprism dimer with adjacent and coplanar bases configuration have been studied by the finite integration technique. The relationship between the geometrical parameters, in particular the radius of the edges of curvature of a single nanoprism, and the position of the extinction peak has been described in a form of a mathematical equation. By moving and rotating one of the nanoprisms relative to the other in the dimer system, the coupling strength between them has been investigated by analyzing the near-field and far-field properties. On the basis of these results, a hybridization model of the nanopri...

Optimization of Fringing Field Effect Using Dielectric Separation and Local Electric Field Enhancement

A novel structure using dielectric separation and local electric field enhancement is proposed to reduce the fringing field effect of high-resolution spatial light modulators (SLMs). In our configuration, photoreactive wall structures are used to suppress the crosslink effect of liquid crystal (LC) molecules between adjacent pixels, and slit-step electrodes can enhance the local electric field, which compensate the disclination caused by the split of fringing electric field. The phase modulation of conventional and novel configurations were simulated, and the effect of LC alignment azimuthal angle was analyzed. The results show that the phase modulation characteristics are greatly improved, which indicates the proposed configuration can suppress the annoying interpixel coupling of SLM sufficiently.

The Effect of Local Electric Field in the Middle Dielectric Wall on the Infrared Plasmonic Shift of the Concentric Gold Double Nanotubes.

The effect of inserted gold nanotube on the localized surface plasmon resonance (LSPR) shifting and local electric field enhancement of concentric gold double nanotubes has been studied by using the quasi-static electricity. Because of the combined effect from inter-surface and inter-tube plasmon coupling, the LSPR in gold double nanotubes could be easily tuned into the infrared wavelength region. The calculation results indicate the plasmon peak shifting is quite sensitive to the thickness of the inner gold tube and the gap between the inner and outer gold tubes. The physical mechanism has been attributed to the local field effect of the separate dielectric layer. Because of the additional attraction from the inner gold tube to the free electrons at outer gold tube, the decreasing electric field in the middle dielectric spacer layer reduces the restoring force and lead to intense red shift of LSPR.

Highly efficient nanoplasmonic SERS on cardboard packaging substrates

This work reports on highly efficient surface enhanced Raman spectroscopy (SERS) constructed on low-cost, fully recyclable and highly reproducible cardboard plates, which are commonly used as disposable packaging material. The active optical component is based on plasmonic silver nanoparticle structures separated from the metal surface of the cardboard by a nanoscale dielectric gap. The SERS response of the silver (Ag) nanoparticles of various shapes and sizes were systematically investigated, and a Raman enhancement factor higher than 106 for rhodamine 6G detection was achieved. The spectral matching of the plasmonic resonance for maximum Raman enhancement with the optimal local electric field enhancement produced by 60 nm-sized Ag NPs predicted by the electromagnetic simulations reinforces the outstanding results achieved. Furthermore, the nanoplasmonic SERS substrate exhibited high reproducibility and stability. The SERS signals showed that the intensity variation was less than 5%, and the SERS performance could be maintained for up to at least 6 months.

Enhanced spectral response of an AlGaN-based solar-blind ultraviolet photodetector with Al nanoparticles.

An enhanced spectral response was realized in an AlGaN-based solar-blind ultraviolet (SB-UV) detector using aluminum (Al) nanoparticles (NPs) of 20-60 nm. The peak responsivity of the detector (about 288 nm) with 60 nm Al NPs is more than two times greater than that of a detector without Al NPs under a 5-V bias, reaching 0.288 A/W. To confirm the enhancement mechanism of the Al NPs, extinction spectra were simulated using time-domain and frequency-domain finite-element methods. The calculation results show that the dipole surface plasmon resonance wavelength of the Al NPs is localized near the peak responsivity position of AlGaN-based SB-UV detectors. Thus, the improvement in the detectors can be ascribed to the localized surface plasmon resonance effect of the Al NPs. The localized electric field enhancement and related scattering effect result in the generation of more electron-hole pairs and thus a higher responsivity. In addition, the dark current of AlGaN-based SB-UV detectors does not increase after the deposition of Al nanoparticles. The results presented here is promising for applications of AlGaN-based SB-UV detectors.

Spatial profiles of electron and metastable atom densities in positive polarity fast ionization waves sustained in helium

Fast ionization waves (FIWs), often generated with high voltage pulses over nanosecond timescales, are able to produce large volumes of ions and excited states at moderate pressures. The mechanisms of FIW propagation were experimentally and computationally investigated to provide insights into the manner in which these large volumes are excited. The two-dimensional structure of electron and metastable densities produced by short-pulse FIWs sustained in helium were measured using laser-induced fluorescence and laser collision-induced fluorescence diagnostics for times of 100–120 ns after the pulse, as the pressure was varied from 1 to 20 Torr. A trend of center-peaked to volume-filling to wall-peaked electron density profiles was observed as the pressure was increased. Instantaneous FIW velocities, obtained from plasma-induced emission, ranged from 0.1 to 3 × 109 cm s−1, depending on distance from the high voltage electrode and pressure. Predictions from two-dimensional modeling of the propagation of a single FIW correlated well with the experimental trends in electron density profiles and wave velocity. Results from the model show that the maximum ionization rate occurs in the wavefront, and the discharge continues to propagate forward after the removal of high voltage from the powered electrode due to the potential energy stored in the space charge. As the pressure is varied, the radial distribution of the ionization rate is shaped by changes in the electron mean free path, and subsequent localized electric field enhancement at the walls or on the centerline of the discharge.

Memristive behavior of Al2O3 film with bottom electrode surface modified by Ag nanoparticles

The memristive behavior of Al2O3-based device is significantly improved by introducing Ag nanoparticles (NPs). Inserting Ag NPs can effectively reduce the switching voltages, increase the resistance ratio (about 104) and enhance the sweep endurance (300 cycles). In particular, the stable switching properties are obtained by inserting an Ag NPs layer with an average diameter of 14 nm on the surface of bottom electrode, and the devices show a long retention time (more than 106 s) compared with the devices without Ag NPs. The switching mechanism is related to the oxygen-vacancy-based conducting filaments and the interfacial effect. The local enhancement and nonuniform distribution of electric field have the benefits to promote, induce and modulate the growth of conducting filaments, such as shape, location and orientation, which are responsible for the improvement performance of the devices.

Pulsed laser-deposited MoS₂ thin films on W and Si: field emission and photoresponse studies.

We report field electron emission investigations on pulsed laser-deposited molybdenum disulfide (MoS2) thin films on W-tip and Si substrates. In both cases, under the chosen growth conditions, the dry process of pulsed laser deposition (PLD) is seen to render a dense nanostructured morphology of MoS2, which is important for local electric field enhancement in field emission application. In the case of the MoS2 film on silicon (Si), the turn-on field required to draw an emission current density of 10 μA/cm(2) is found to be 2.8 V/μm. Interestingly, the MoS2 film on a tungsten (W) tip emitter delivers a large emission current density of ∼30 mA/cm(2) at a relatively lower applied voltage of ∼3.8 kV. Thus, the PLD-MoS2 can be utilized for various field emission-based applications. We also report our results of photodiode-like behavior in (n- and p- type) Si/PLD-MoS2 heterostructures. Finally we show that MoS2 films deposited on flexible kapton substrate show a good photoresponse and recovery. Our investigations thus hold great promise for the development of PLD MoS2 films in application domains such as field emitters and heterostructures for novel nanoelectronic devices.

Au nanoparticles embedded at the interface of Al/4H-SiC Schottky contacts for current density enhancement

Nanostructured contacts, comprised of nanoparticles (NPs) embedded at the interface of contact/semiconductor, offer a viable solution in modification of Schottky barrier height (SBH) in Schottky contacts. The successful performance of devices with such nanostructured contacts requires a feasible selection of NPs/contact material based on theoretical calculations and a cost effective and reproducible route for NPs deposition. Acidification of commercially available colloidal Au NPs solution by HF has been selected here as a simple bench-top technique for deposition of Au NPs on n- and p-type 4H-SiC substrates. Theoretical calculations based on the model of inhomogeneity in SBH (ISBH) were used to make a more appropriate selection of NPs type (Au) and size (5 and 10 nm, diameter) with respect to contact metal (Al). Al/Au NPs/SiC Schottky barrier diodes were then fabricated, and their electrical characteristics exhibited current density enhancement due to the SBH lowering. The source of SBH lowering was determined to be the local electric field enhancement due to NPs effect, which was further investigated using the models of ISBH and tunneling enhancement at triple interface.

Structural control of metamaterial oscillator strength and electric field enhancement at terahertz frequencies

The design of artificial nonlinear materials requires control over internal resonant charge densities and local electric field distributions. We present a MM design with a structurally controllable oscillator strength and local electric field enhancement at terahertz frequencies. The MM consists of a split ring resonator (SRR) array stacked above an array of closed conducting rings. An in-plane, lateral shift of a half unit cell between the SRR and closed ring arrays results in an increase of the MM oscillator strength by a factor of 4 and a 40% change in the amplitude of the resonant electric field enhancement in the SRR capacitive gap. We use terahertz time-domain spectroscopy and numerical simulations to confirm our results. We show that the observed electromagnetic response in this MM is the result of image charges and currents induced in the closed rings by the SRR.