Electric Field Intensity Profiles Research Articles

Theoretical Study on Generation of Multidimensional Focused and Vector Vortex Beams via All-Dielectric Spin-Multiplexed Metasurface.

The optical vortex (OV) beams characterized by orbital angular momentum (OAM) possess ubiquitous applications in optical communication and nanoparticle manipulation. Particularly, the vortex vector beams are important in classical physics and quantum sciences. Here, based on an all-dielectric transmission metasurface platform, we demonstrate a spin-multiplexed metadevice combining propagation phase and Pancharatnam–Berry (PB) phase. By utilizing a phase-only modulation method, the metadevice can generate spin-dependent and multidimensional focused optical vortex (FOV) under the orthogonally circularly polarized incident light, and it can successfully realize the multiplexed of the above-mentioned FOVs for linearly polarized light. Meanwhile, the superposition of multiple OAM states can also produce vector vortex beams with different modes. Additionally, the evolution process of the electric field intensity profile is presented after the resultant vector vortex beams through a horizontal linear polarization. This work paves an innovative way for generating structured beams, and it provides promising opportunities for advanced applications in optical data storage, optical micromanipulation, and data communication.

(Invited) Plasmon Resonant Amplification of Hot Electron-Driven Photocatalysis

We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (~mA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1µA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO2 produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO2 semiconductor where they then reduce hydrogen ions in solution forming H2 (i.e., 2H+ + 2e- → H2). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal–liquid interface are smaller than those of the metal–semiconductor system, mainly because, in the metal–semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.1 2, 3 We also report plasmon resonant excitation of hot electrons in a metal-based photocatalyst in contact with ferrocene redox couple in acetonitrile solution. Here the photocatalyst consists of a 50nm thick Au film and a top thin (5nm) film of Al2O3 deposited using atomic layer deposition. In this configuration, hot electrons, photo-excited in the metal, jump over the oxide barrier ultimately reducing ferrocenium in solution (i.e., Fe(C5H5)2 + + e- → Fe(C5H5)2) producing a photocurrent. In order to amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633nm wavelength light. We observe peaks in the photocurrent at incident angles of ±10o from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.3X in the charge transfer rate. At these same angles, we also observe sharp dips in the photo-reflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain (FDTD) simulations also predict sharp dips in the photoreflectance at ±10o, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.

Plasmon resonant amplification of hot electron-driven photocatalysis

We report plasmon resonant excitation of hot electrons in a metal based photocatalyst in the oxygen evolution half reaction in aqueous solution. Here, the photocatalyst consists of a 100-nm thick Au film deposited on a corrugated silicon substrate. In this configuration, hot electrons photoexcited in the metal are injected into the solution, ultimately reversing the water oxidation reaction (O2 + 4H+ + 4e− ⇋ 2H2O) and producing a photocurrent. In order to amplify this process, the gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633 nm wavelength light. We observe peaks in the photocurrent at incident angles of ±9° from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.1× in the charge transfer rate. At these same angles, we also observe sharp dips in the photoreflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain simulations also predict sharp dips in the photoreflectance at ±9°, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.

Angle dependent localized surface plasmon resonance from near surface implanted silver nanoparticles in SiO2 thin film

Near surface silver nanoparticles embedded in silicon oxide were obtained by 40 keV silver negative ion implantation without the requirement of an annealing step. Ion beam induced local heating within the film leads to an exodiffusion of the silver ions towards the film surface, resulting in the protrusion of larger nanoparticles. Cross-sectional transmission electron microscopy reveals the presence of poly-disperse nanoparticles (NPs), ranging between 2 nm and 20 nm, at different depths of the SiO2 film. The normal incidence reflectance spectrum shows a double kink feature in the vicinity of 400 nm, indicating a strong localized surface plasmon resonance (LSPR) from the embedded NPs. However, due to overlap of the bilayer interference and LSPR, the related features are difficult to separate. The ambiguity in associating the correct kink with the LSPR related absorption is cleared with the use of transfer matrix simulations in combination with an effective medium approximation. The simulations are further verified with angle dependent reflectance measurements. Additionally, transfer matrix simulation is also used to calculate the electric field intensity profile through the depth of the film, wherein an enhanced electric field intensity is predicted at the surface of the implanted films.

Plasmon resonant amplification of a hot electron-driven photodiode

We report plasmon resonant excitation of hot electrons in a photodetector based on a metal/oxide/metal (Au/Al2O3/graphene) heterostructure. In this device, hot electrons, excited optically in the gold layer, jump over the oxide barrier and are injected into the graphene layer, producing a photocurrent. To amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500 nm. The photocurrent produced in this device is measured using 633-nm-wavelength light as a function of incident angle. We observe the maximum photocurrent at ±10° from normal incidence under irra-diation with light polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating, and a constant (angle-independent) photocurrent under irradiation with light polarized perpendicular to the incident plane (s-polarization) and parallel to the grating. These data show an amplification factor of 4.6× under resonant conditions. At the same angle (±10°), we also observe sharp dips in the photoreflectance corresponding to waveve-ctor matching between the incident light and the plasmon mode in the grating. In addition, finite-difference time-domain simulations predict sharp dips in the photoreflectance at ±10°, and the electric field intensity profiles show clear excitation of a plasmon resonant mode when illuminated with p-polarized light at this angle.

High-efficiency photon capturing in ultrathin silicon solar cells with double-sided skewed nanopyramid arrays

Light trapping is essential to improve the performance of thin film solar cells. In this paper, we performed a parametric optimization of double-sided skewed nanopyramid arrays that act as a light trapping scheme to increase light absorption in thin-film c-Si solar cells. Our theoretical optimization reveals that the short-circuit current density in a solar cell, employing only 1 μm silicon could reach as high as 38.57 mA cm−2, which is 17% and 245% higher than that of the Yablonovitch limit and planar-film counterparts, respectively. Furthermore, we analyzed the underlying physics of the light absorption enhancement through electric field intensity profiles.

Focused azimuthally polarized vector beam and spatial magnetic resolution below the diffraction limit

An azimuthally electric-polarized vector beam (APB), with a polarization vortex, has a salient feature that it contains a magnetic-dominant region within which electric field ideally has a null while longitudinal magnetic field is maximum. Fresnel diffraction theory and plane-wave spectral (PWS) calculations are applied to quantify field features of such a beam upon focusing through a lens. The diffraction-limited full width at half maximum (FWHM) of the beam's longitudinal magnetic field intensity profile and complementary FWHM (CFWHM) of the beam's annular-shaped total electric field intensity profile are examined at the lens's focal plane as a function of the lens's paraxial focal distance. Then, we place a subwavelength dense dielectric Mie scatterer in the minimum-waist plane of a self-standing converging APB and demonstrate for the first time that a very high resolution magnetic field at optical frequency is achieved with total magnetic field FWHM of 0.23{\lambda} (i.e., magnetic field spot area of 0.04{\lambda}^2) within a magnetic-dominant region. The theory shown here is valuable for development of optical microscopy and spectroscopy systems based on magnetic dipolar transitions which are in general much weaker than their electric counterparts.

Fault detection of antenna arrays using infrared thermography

A fast and easy method for fault detection in antenna arrays using infrared thermography is presented. A thin, minimally perturbing, microwave absorption screen made of carbon loaded polymer is kept close in front of the faulty array. Electromagnetic waves falling on the screen increase its temperature. This temperature profile on the screen is identical to electric field intensity profile at the screen location. There is no temperature rise observed on the screen corresponding to non-radiating (faulty) elements and hence can be easily detected by IR thermography. The array input power is modulated at a low frequency which permits thermography to detect even weak fields. It also improves the resolution of thermal images. The power fed to the array is only 30dBm. In order to show the utility of this technique, an example of 14GHz 4×4 patch antenna array is given. The simulations are carried in CST Microwave Studio 2013. A good agreement between simulation and experimental results is observed.

A simple and exact method to analyze optical waveguide with graded index profile

A simple analytical method is proposed to obtain the exact propagation constant and distribution of electric field intensity of optical waveguides with graded refractive index profile. The method is based on the Wenzel-Kramers-Brillouin (WKB) solution, variational method, modified eigen-equations and discretized scalar wave equation for planar optical waveguide. The expressions of the distribution of electric field intensity based on the conventional WKB method, which diverge around the turning point, have been demonstrated to be very exact in the region beyond the turning point where the refractive index profile varies slowly. The proposed method uses the conventional WKB method to calculate the values of electric field intensity at two adjacent positions beyond the turning point and then the electric field intensity profile for the whole region is obtained by making use of the two calculated values. Two simple and explicit formulas are deduced from the discretized scalar wave equation, which provide a relationship among the values of electric field intensity at three adjacent positions. If the effective refractive index of optical waveguide and the refractive index profile for the whole region are known, we can obtain the value of electric field intensity at any position according to the corresponding values at the adjacent positions by using the two formulas aforementioned. By using the two values calculated by WKB method, the electric field intensity profile for the whole region can be determined through the iterative use of the two formulas. The accuracy of the electric field intensity profile determined by the proposed method is greatly dependent on the accuracy of the applied value of the effective refractive index. To achieve exact propagation constant and distribution of electric field intensity, the variational method and modified eigen-equations are employed in the proposed method. Variational method is a very useful method to improve the accuracy of the propagation constant in the analysis of optical waveguide with step-asymmetrical graded refractive index profile. By combining the traditional variational method and calculation of electric field intensity profile by the proposed method, the improved variational method is presented to obtain the exact propagation constant of optical waveguide. The value of propagation constant calculated by WKB method and the corresponding electric intensity field profile determined by the proposed method are chosen as the initial trial value and trial function in the variational method. Propagation constant and the corresponding electric field intensity profile with better accuracy can be achieved by the variational calculation and then are regarded as the new trial value and trial function. By the iterative use of the variational method and calculation of electric field intensity profile by the proposed method at finite times, quite accurate results are obtained. The modified eigen-equations in combination with the proposed method is another approach to calculating accurate propagation constants of optical waveguides with both the step-asymmetrical and symmetrical graded index profile. In comparison with other published methods, the proposed method has the advantages of the simplicity and considerable accuracy.

Analytical electric field intensity profile in diffused channel waveguides

A novel analytic solution to field distribution is used to calculate the intensity distribution for channel waveguides with a two-dimensional diffusion-type refractive index profile. Composition profiles for the waveguides are obtained by applying the two-dimensional diffusion theory to treat the case of isotropic sideways diffusion. The dependence of the field distributions on the width/depth ratio is studied as an aid to the design of modulators and switches for integrated optics.

Light-absorption enhancement in thin-film silicon solar cells with front grating and rear-located nanoparticle grating

A new light-trapping structure combining front grating and rear-located metal nanoparticle grating for thin-film crystalline silicon solar cells is theoretically investigated. Over a wide range of the solar spectrum, light absorption has been improved. By analyzing light-absorption enhancement and the electric-field intensity profiles, it is demonstrated that light-absorption improvement lies in the superposition of the grating diffraction and the particle scattering effects. Furthermore, optimizations of front grating and rear metal NPs are performed, and an impressive short-circuit current density within a 1 µm c-Si layer is obtained.

Nanopyramids and rear-located Ag nanoparticles for broad spectrum absorption enhancement in thin-film solar cells.

Light trapping is essential to improve the performance of thin-film solar cells. In this paper, we performed a parametric optimization of nanopyramids and rear-located Ag nanoparticles that act as light trapping scheme to increase light absorption in thin-film c-Si solar cells. Our optimization reveals that the short-circuit current density in a solar cell employing only 5 μm silicon could exceed that of a standard 300 μm c-silicon wafer-based cell. Furthermore, we analyzed the underlying physics of the light absorption enhancement through the electric field intensity profiles.

Effect of guided resonance modes on emission from GaN core–shell nanorod arrays

We model the process of incoherent emission from \(\hbox {In}_{x}\hbox {Ga}_{1-x}\hbox {N/GaN}\) quantum wells in GaN core–shell nanorod arrays using finite-difference time-domain simulations. We find that high-intensity features in the emitted field correspond to guided resonance modes near the \(\varGamma \)-point of the photonic band structure. We identify one \(\varGamma \)-point mode whose electric field intensity profile is ideal for core–shell nanorod array geometries. Using this mode, we are able to simultaneously enhance the radiative recombination rate and extraction efficiency relative to an in-filled slab. We determine the conditions on radiative and nonradiative recombination rates for which the nanorod array has a higher internal and external quantum efficiency than a reference slab. We present one nanorod array geometry where the external quantum efficiency is enhanced up to a factor of 25.

Extraordinary optical absorption based on diffraction grating and rear-located bilayer silver nanoparticles

A new light-trapping-structure combined of diffraction grating and rear-located bilayer silver nanoparticles for thin-film silicon solar cells are theoretically investigated. By analyzing the absorption spectra and electric field intensity profiles, we find that the grating suppresses the reflection and results in the incident light angular redistribution, and that the rear-located bilayer nanoparticles fold light into the active layer. Furthermore, the grating and metal NPs are optimized and a short-circuit current density as high as 29.7 mA/cm2 is obtained within a 1-µm-thick cell. In addition this design is applicable to silicon solar cells of various thicknesses.

Effect of dipole location on profile properties of symmetric surface plasmon polariton mode in Au/Al2O3/Au waveguide

This study uses a dipole embedded in Al2O3 layer to excite a symmetric surface plasmon polariton (SPP) mode in Au/Al2O3/Au waveguide to investigate its profile properties by using finite-difference time-domain (FDTD) method. The excited dipole decay radiatively direct near-field coupling to SPP mode owing to thin Al2O3 layer of 100 nm. The effects of electric and magnetic field intensity profiles and decay length have been considered and characterized. It is found that dipole location is an important factor to influence the horizontal and vertical profile properties of symmetric SPP mode in Au/Al2O3/Au waveguide. The amplitudes of electric and magnetic field intensity and the wavelengths of metal-insulatormetal (MIM) SPP resonance mode can be tuned by varying dipole location. The horizontal and vertical decay lengths are 19 and 24 nm, respectively. It is expected that the Au/Al2O3/Au waveguide structure is very useful for the practical applications of designing a SPP source.

Needleless electrospinning. I. A comparison of cylinder and disk nozzles

AbstractIn this study, we demonstrated the needleless electrospinning of poly(vinyl alcohol) (PVA) nanofibers with two nozzles, a rotating disk and a cylinder, and examined the effect of the nozzle shape on the electrospinning process and resultant fiber morphology. The disk nozzle needed a relatively low applied voltage to initiate fiber formation, and the fibers were mainly formed on the top disk edge. Also, the PVA concentration had little influence on the disk electrospinning process (up to 11 wt %). In comparison, the cylinder electrospinning showed a higher dependence on the applied voltage and polymer concentration. The fibers were initiated from the cylinder ends first and then from the entire cylinder surface only if the applied voltage were increased to a certain level. With the same polymer solution, the critical voltage needed to generate nanofibers from the disk nozzle was lower than that needed to generate nanofibers from the cylinder. Both electrospinning systems could produce uniform nanofibers, but the fibers produced from the disk nozzle were finer than those from the cylinder when the operating conditions were the same. A thin disk (8 cm in diameter and 2 mm thick) could produce nanofibers at a rate similar to that of a cylinder of the same diameter but 100 times wider (i.e., 20 cm long). Finite element analysis of electric field profiles of the nozzles revealed a concentrated electric field on the disk edge. For the cylinder nozzle, an uneven distribution of the electric field intensity profile along the nozzle surface was observed. The field lines were mainly concentrated on the cylinder ends, with a much lower electric field intensity formed in the middle surface area. At the same applied voltage, the electric field intensity on the disk edge was much higher than that on the cylinder end. These differences in the electric field intensity profiles could explain the differences in the fiber fineness and rate of the nanofibers produced from these two nozzles. These findings will benefit the design and further development of large‐scale electrospinning systems for the mass production of nanofibers for advanced applications. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Needleless electrospinning of nanofibers with a conical wire coil

AbstractIn this article, we have demonstrated a novel needleless electrospinning of PVA nanofibers by using a conical metal wire‐coil as spinneret. Multiple polymer jets were observed to generate on the coil surface. Up to 70 kV electric voltage can be applied to this needleless electrospinning nozzle without causing “corona discharge.” Compared with conventional needle electrospinning, this needleless electrospinning system produced finer nanofibers on a much larger scale, and the fiber processing ability showed a much greater dependence on the applied voltage. Finite element calculation indicates that the electric field intensity profiles for the two systems are also quite different. This novel concept of using wire coil as the electrospinning nozzle will contribute to the further development of new large‐scale needleless electrospinning system for nanofiber production. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Localization of Electromagnetic Wave in 3D Periodic and Fractal Structures

Abstract Localization or confinement of electromagnetic or optical waves is reviewed briefly in respect to the wave interaction with the specific structures of periodic, random, and fractal (self-similar) geometries, which can enhance strong interference, scattering, and resonance of waves. The localization of microwave in photonic crystals with diamond structure and in photonic fractals with Menger sponge structure of dielectric media is reported and discussed about the difference of localization behaviors. These 3D structures are fabricated easily from liquid photosensitive resins or ceramic dispersed ones by a CAD/CAM stereolithography. The localization in photonic fractals is supported with several measurements of transmission, 90° wave scattering, electric field intensity profile, and FDTD (finite-difference time-domain) simulation for localized modes. An empirical equation to predict the localization frequencies shows good agreements with measured results as well. The localization in a photonic crystal can be explained by Bragg scattering, while that in a photonic fractal by resonance in the fractal cavity.

Scanning probe microscope

Graded-index optical channel waveguides like Ti-indiffused LiNbO3 waveguides have been modelled by effective refractive index profile along the lateral direction, discretising the profile and then using the matrix method. This effective-index-based matrix method, which was earlier used to compute the modal propagation constants of Ti:LiNbO3 channel waveguides and directional couplers [Fib. Integrat. Opt. 17 (1998) 139] and the bending loss [J. Opt. Commun. 155 (1998) 125], has been established in this paper as a computationally fast, analytical method to compute electric field and modal intensity profiles in single and coupled waveguides. In order to handle coupled waveguides consisting of three waveguides, the concept of overlap integral has been used to analyse power coupling to the middle waveguide for applications in power splitting. Some of the computed intensity profiles have been compared with experimental data obtained by the authors. Although computation has been performed only for TE and TM polarisations, the method has the inherent capability of handling arbitrary polarisation.

Analysis of spatiotemporal coupling in a femtosecond pulse shaper by the Wigner distribution function

Masahiko Mori National Institute of Advanced Industrial Science and Technology 1-1-4, Umezono, Tsukuba, Ibaraki, 305-8568 Japan Abstract. The spatiotemporal coupling effect in ultrafast light pulses is described by the Wigner distribution function. This approach enables us to calculate: the output electric field intensity profile by defocusing lenses, grating pairs with different grating constants, lenses with different focal lengths, and combinations of these in pulse shapers. We also gain a clear understanding of the spatiotemporal properties of pulse shaping. © 2001 Society of Photo-Optical Instrumentation Engineers. [DOI: 10.1117/1.1387439]