Dielectric Cone Research Articles

Taking advantage of an axisymmetric plasmonic structure and grooves to nanofocus and ultraenhance a radially polarized electric field

This study investigates the enhancement and nanofocusing of a radially polarized electric field by a conical plasmonic structure (CPS). The CPS is a dielectric cone with nanometer metal cladding on a dielectric substrate. Concentric circular slanted grooves are etched on the surface of the dielectric substrate. These grooves converge the incident field on the structure. Angled periodic gratings are engraved on the CPS metal surface near the tip, creating a plasmonic momentum and contributing to the field enhancement above the apex. The symmetry of the incident radially polarized light and the structure significantly boosts nanofocusing and field enhancement. The optimal width of the nanofocusing and the electric field enhancement factor obtained are approximately 9 nm and 30000, respectively. Because of its impressive effects, this scheme is a valuable tool for plasmonic, optics, and laser applications.

Arrangement of Micro Dielectric Particles With Vector Vortex Beam Generated by Dual-Helical Dielectric Cone

Optical vortex is of great value in optical trapping, manipulating, and arranging. Here, we propose a dual-helical dielectric cone to generate a dual-vortex beam which can be used to manipulate and arrange dielectric microparticles in three-dimensional space. With the finite-difference time-domain simulation, we calculate the electromagnetic field intensity distribution, phase, and Poynting vector during the propagation of the dual-vortex beam, the optical force and optical torque on the microparticles in the range of dual-vortex beam is also considered. In the experiment, we use the phase mask projected on spatial light modulator to generate a dual-vortex beam, which can trap, manipulate, and arrange the fluorescent microparticles to a specific and stable shape, which is consistent with the results of our simulation calculations. The dual-vortex beam generated by the dual-helical dielectric cone provides a feasible method three-dimensional optical manipulation, which is significant in biophotonical researches.

Cherenkov radiation of a charge flying through the inverted conical target

Radiation generated by a charge moving through a vacuum channel in a dielectric cone is analyzed. It is assumed that the charge moves through the cone from the apex side to the base side (the case of "inverted" cone). The cone size is supposed to be much larger than the wavelengths under consideration. We calculate the wave field outside the target using the "aperture method" developed in our previous papers. Contrary to the problems considered earlier, here the wave which incidences directly on the aperture is not the main wave, while the wave once reflected from the lateral surface is much more important. The general formulas for the radiation field are obtained, and the particular cases of the ray optics area and the Fraunhofer area are analyzed. Significant physical effects including the phenomenon of "Cherenkov spotlight" are discussed. In particular it is shown that this phenomenon allows reaching essential enhancement of the radiation intensity in the far-field region at certain selection of the problem parameters. Owing to the "inverted" cone geometry, this effect can be realized for arbitrary charge velocity, including the ultra relativistic case, by proper selection of the cone material and the apex angle. Typical radiation patterns in the far-field area are demonstrated.

Wideband Near-Zone Radiative System for Exploring the Existence of Electromagnetic Emission from Biological Samples

This article reports on a wideband near-zone radiative system operating from 1 to 50 GHz for examining the possibility of electromagnetic emission from a community of bacteria in a petri dish or any other distributed sources confined in a small focusing area. The system includes a dual-polarized signal collector, a Dicke switch, broadband low-noise amplifiers (LNAs), and a precision spectrum analyzer. The signal collector is composed of a quadruple-ridged horn-type structure of dimensions ${\lambda }_{L} \times {\lambda }_{L} \times 1.6\,\,\lambda _{L}$ (where ${\lambda }_{L}$ is the wavelength at the lowest frequency) having two orthogonal probes. The provision of a dielectric cone along the horn central axis allows for collimating the waves in the focusing area over the entire bandwidth. Therefore, the signal collector acts as a field concentrator to provide a uniform field distribution across the focusing area (source location). The entire structure is designed to be EM-shielded and free from resonance. It can effectively exclude outside interference while maintaining a uniform collection of the electromagnetic spectrum.

Peculiarities of Cherenkov radiation from a charge moving through a dielectric cone

Radiation generated by a charge moving in a vacuum channel through a dielectric cone is analyzed. The size of the conical target is assumed to be much greater than the wavelengths under consideration. We calculate the wave field outside the target using Stratton-Chu formulas (``aperture integrals''). This work focuses mainly on investigation of the electromagnetic field in the far-field area (Fraunhofer area). We calculate representative radiation patterns and show that typically the maximal radiation is generated in the direction of one of the refracted rays. However, in the specific case where the refracted ray is parallel to the symmetry axis, the maximal radiation is observed at some small (but nonzero) angle with respect to this axis. The maximum radiation in this case is much more intensive compared to all other situations. This phenomenon can be referred to as the ``Cherenkov spotlight.''

Field-enhanced nanofocusing of radially polarized light by a tapered hybrid plasmonic waveguide with periodic grooves.

This study reports the field-enhanced nanofocusing of radially polarized light by tapered hybrid plasmonic waveguide (THPW) with periodic grooves. The THPW consists of a conical high-index dielectric cone, a sandwiched low-index dielectric thin layer, and a metal cladding. The axially symmetric 3D finite element method is used to investigate the nanofocusing effect. Under radially polarized illumination at 632.8nm, strongly enhanced nanofocusing occurs. The hybrid plasmonic structure effectively reduces the energy loss and improves the field enhancement nearly 554 times. Furthermore, periodic grooves are constructed on the metallic surface of the THPW, satisfying the phase-matching condition, and they couple the light energy from the inside to the outside. Finally, an optimized nanofocusing performance with field enhancement of approximately 1810 times is obtained. The results offer an important reference for designing related photonic devices, and the proposed scheme could be potentially exploited in the application of light-matter interactions.

Vector beam tuning with axisymmetric dielectric micro helical cone

Here, we present a novel structure named as axisymmetric dielectric microhelical cone, which combines axisymmetric dielectric microhelical structure and a conic structure to produce localized vector vortex nanofocused optical field. The intensity distribution, Poynting vector, and phase distributions of the electric fields generated by the structure are simulated with the three-dimensional finite-difference time-domain method. The simulation results indicate that special electric field with different localized vortex effects generated when the structure is illuminated by different polarization states and the off-axis focused light spots can be tuned by changing the numbers of symmetry axes N. We also experimentally prove that it can generate a shaped beam similar to the beam generated by the axisymmetric dielectric microhelical cone structure, which can keep well by imposing the phase masks of the structure onto a spatial light modulator. The localized vector vortex beam generated by the structure has potential application in optical tweezers.

Design and Simulation of a New Coaxial Probe for Near Field Scanning Optical Microscopy

Design of the new photonic devices having attractive optical properties such as subwavelength confinement of light has a numerous applications in developing nanoscale sensors, photonic integrated circuits, solar cells and etc. Abbe’s diffraction limit restricts the use of subwavelength scale photonic elements due to their size and therefore new approaches are required in order to achieve desirable nanometer scale properties of these devices. Delivering light energy to nanometer scale regions can be achieved by surface plasmon polariton (SPP) waves that propagate in metal-dielectric interface. The paper presents results in FDTD (finite-difference time domain) simulation of a new coaxial probe of nanometer-scale size.Based on FDTD simulations of the coaxial probe field localization and enhancement in the region around the tip of coaxial probe has been obtained. Configuration of the probe consists of the truncated Au cone as a core, truncated dielectric (SiO2) cone in the middle and outer metallic cladding. Simulations of the probe revealed that increasing the radius of Au core structure could lead to the enhanced field around the tip. Additionally, increasing radius of outer cladding layer doesn’t affect field enhancement around the tip. Different configurations of the probe have been analyzed in the range of visible spectrum of the source and as a result the most promising configuration has been selected

Light localization and SERS in tip-shaped silicon metasurface.

Optical properties of two dimensional periodic system of the silicon micro-cones are investigated. The metasurface, composed of the silicon tips, shows enhancement of the local optical field. Finite element computer simulations as well as real experiment reveal anomalous optical response of the dielectric metasurface due to excitation of the dielectric resonances. Various electromagnetic resonances are considered in the dielectric cone. The metal-dielectric resonances, which are excited between metal nanoparticles and dielectric cones, are also considered. The resonance local electric field can be much larger than the field in the usual surface plasmon resonances. To investigate local electric field the signal molecules are deposited on the metal nanoparticles. We demonstrate enhancement of the electromagnetic field and Raman signal from the complex of DTNB acid molecules and gold nanoparticles, which are distributed over the metasurface. The metasurfaces composed from the dielectric resonators can have quasi-continuous spectrum and serve as an efficient SERS substrates.

A Wideband End-Fire Conformal Vivaldi Antenna Array Mounted on a Dielectric Cone

The characteristics of a novel antipodal Vivaldi antenna array mounted on a dielectric cone are presented. By employing antipodal Vivaldi antenna element, the antenna array shows ultrawide bandwidth and end-fire radiation characteristics. Our simulations show that the cone curvature has an obvious influence on the performance of the conformal antenna, in terms of both the bandwidth and the radiation patterns. The thickness and permittivity of the dielectric cone have an effect on the bandwidth of the conformal antenna. Measurement results of both single antenna and conformal antenna array show a good agreement with the simulated results. The measured conformal antenna can achieve a −10 dBS11with bandwidth of 2.2–12 GHz and demonstrate a typical end-fire radiation beam. These findings provide useful guidelines and insights for the design of wideband end-fire antennas mounted on a dielectric cone.

Axial magnetic resonances of rotationally symmetric high-permittivity dielectric particles of arbitrary shape

We develop a simple semi-analytical approach for calculating the magnetic response of rotationally symmetric high-permittivity dielectric particles to an axially applied magnetic field. By using this approach, magnetic resonances of dielectric rings of arbitrary width and thickness and of dielectric cones with arbitrary height-to-diameter aspect ratio are studied. The approach is validated by a comparison with the results of rigorous numerical simulations using the Maxwell equations.

High Aspect Ratio Plasmonic Nanocones for Enhanced Light Absorption in Ultrathin Amorphous Silicon Films

Strategies for enabling high light absorption in ultrathin solar cell layers may contribute importantly to more viable photovoltatic. To this end, we investigate the effect of the enhanced near-field, associated with nanoparticle plasmon resonances, on the light absorption in ultrathin (20 nm) hydrogenated amorphous silicon (a-Si:H) films. In order to maintain the dipolar plasmon resonance above the a-Si:H optical gap, we employ high aspect ratio Ag nanocones coated with the a-Si:H by chemical vapor deposition. Experiments were performed for Ag/aSi:H nanocomposites on glass and on a spacer-reflector resonant cavity, used to boost and tailor the optical response. Finite element calculations were employed to model and extract the absorption rates of the different components of the samples, and to help explain the origin of the spectral features. The highest intergrated absorption in the a-Si:H film, corresponding to an ideal photocurrent of 12.5 mA/cm(2), was observed for a nanocone/a-Si:H system on a 40 nm thick TiO2 spacer placed on an Al reflector. Due to the rather high (217 nm) structures employed, the a-Si:H absorptance was however, not very sensitive to the spacer thickness. Numerical comparison with systems wherer the core Ag cones were substituted by dielectric cones, demonstrated that the effect of the plasmon resonance added significantly to the benefits of the increased amount of absorber material and enhanced light interaction imposed by other geometrical effects.

Light absorption in conical silicon particles

The problem of the absorption of light by a nanoscale dielectric cone is discussed. A simplified solution based on the analytical Mie theory of scattering and absorption by cylindrical objects is proposed and supported by the experimental observation of sharply localized holes in conical silicon tips after high-fluence irradiation. This study reveals that light couples with tapered objects dominantly at specific locations, where the local radius corresponds to one of the resonant radii of a cylindrical object, as predicted by Mie theory.

Localization of light on a cone: theoretical evidence and experimental demonstration for an optical fiber

The classical rays propagating along a conical surface are bounded on the narrower side of the cone and unbounded on its wider side. In contrast, it is shown here that a dielectric cone with a small half-angle γ can perform as a high Q-factor optical microresonator which completely confines light. The theory of the discovered localized conical states is confirmed by the experimental demonstration, providing a unique approach for accurate local characterization of optical fibers (which usually have γ ~ 10(-5) or less) and a new paradigm in the field of high Q-factor resonators.

DIAGONAL HORN GAUSSIAN EFFICIENCY ENHANCEMENT BY DIELECTRIC LOADING FOR SUBMILLIMETER WAVE APPLICATION AT 150 GHz

The dielectric pyramid loaded and the dielectric cone loaded diagonal horn working at 150GHz are investigated by using Gaussian beam mode analysis. With extremely low cross-polarized and axially symmetrical fleld distribution in the horn aperture, the calculated fundamental Gaussian mode coupling achieves about 98%. The far fleld radiation patterns of the two antennas are analyzed using fundamental Gaussian mode aperture-fleld distribution model whose results agree with high-accuracy CST TM software computations, indicating that the dielectric loaded horns radiate flne Gaussian beams. The dielectric loaded geometry may be used to modify the diagonal horns with distorted beam.

Two-Interval Sturm–Liouville Theory for the Electrostatic Potential of a Point Charge near a Dielectric Cone

This paper provides the electrostatic potential of a point charge near a dielectric cone of circular cross section. Our theory applies to any pair of dielectrics on either side of the conical interface and any cone angle. Separation of variables yields two associated Legendre differential equations which are coupled through boundary conditions on the surface of the cone. We solve this boundary value problem by consideration of a two-interval Sturm–Liouville operator which acts on the direct sum of two $L^2$ spaces, as suggested by Wang, Sun, and Zettl [J. Math. Anal. Appl., 328 (2007), pp. 390–399]. We proceed further to determine the inverse—integral—operator, we prove that it is compact and symmetric, we determine its eigenvectors, which form an orthogonal basis, and we use that basis, along with a classical Fourier series, to formulate the solution of the aforesaid electrostatic problem. Our analytical solution confirms van Bladel's prediction for the field at the tip of the cone, an outcome of his numerical investigation [IEEE Trans. Antennas and Propagation, 33 (1985), pp. 893–895]. As for the far field, we show that it is as if generated by the very same point charge in an unbounded homogenous medium with an effective dielectric constant wherein the media occupying the exterior and interior of the cone contribute according to the cone angle. Finally, we present briefly the formal derivation of our solution via the classical Mellin transform method and discuss its disadvantages over our proposed method.

Near-field optical microscope working on TEM wave

A new type of probe for the near-field optical microscope incorporating metallic strips on the surface of a dielectric cone (microstrip probe) is proposed. The numerical simulation has shown a significant improvement in optical efficiency for this type of probe compared with the conventional probe. It was found theoretically that scanning near-field optics with the microstrip probe is promising for applications in optical information recording and can be employed as optical heating elements in magnetic information recording. Application of the microstrip probe in nonlinear optical research of nanoscaled medium is possible due to strong electric and magnetic field near the aperture and weak dependence of its transmission parameters on the frequency of the incident beam. A model for the near-field strip probe with cleaved apex is proposed. A method is presented to control the distance between the probe apex and sample surface based on mechanical vibrations of the cleaved apex excited by applying voltage pulses. The oscillation amplitude and attenuation can be determined by measuring the amplitude of microwave radiation excited by oscillations of charges with opposite sign at the probe apex. The investigation was based on mathematical models and experiments necessary to confirm the theoretical prediction.

Conical dielectric resonator antennas excited by a coaxial probe

AbstractDielectric resonator antennas of conical shapes are experimentally examined. The dielectric cone resides above a ground plane, and is excited by a coaxial probe. Two configurations of the cone are tested: one with the wide base placed on the ground plane, and the other with the cone inverted on the ground plane. Results show that the inverted cone achieves a wider bandwidth. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 160–161, 2001.

Rawlin's method and the diaphonous cone

Approximate expansions are obtained for the field when an electromagnetic wave is scattered by a dielectric circular cone with refractive index near unity. They indicate that the field near the tip of the cone contains not only powers of the distance but also its logarithm. The analysis entails the singular behaviour of an integral of a product of Hankel functions as a generalized function. Also a uniformly valid asymptotic formula for the hypergeometric function with parameters outside the usual range is derived.

A multicomposite, multilayered cylindrical dielectric resonator for application in MMIC's

A method, based on the mode matching technique, to study various resonant modes of a multicomposite, multilayered cylindrical dielectric resonator is presented. Dielectric sphere and cone resonators placed in practical environments are investigated. The calculated resonant frequencies show very good agreement compared with the numerical results of the finite difference method for the spherical resonator and with the measured values for the conical resonator, respectively. Experimental investigations show that the conical resonator can be coupled to a circular microstrip line, which has small size at high frequencies and may be integrated together with an oscillator. >