Showing papers on "Near and far field published in 2008"
TL;DR: The sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.
Abstract: The optical properties of plasmonic dipole and bowtie nanoantennas are investigated in detail using the Green’s tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.
454 citations
TL;DR: 3D numerical calculations demonstrate that the interaction of a single quantum emitter with the electromagnetic field is both enhanced and directed by a nano-optical Yagi-Uda antenna, making such antennas a promising candidate for compact easy-to-address planar sensors.
Abstract: We demonstrate by 3D numerical calculations that the interaction of a single quantum emitter with the electromagnetic field is both enhanced and directed by a nano-optical Yagi-Uda antenna. The single emitter is coupled in the near field to the resonant plasmon mode of the feed element, enhancing both excitation and emission rates. The angular emission of the coupled system is highly directed and determined by the antenna mode. Arbitrary control over the main direction of emission is obtained, regardless of the orientation of the emitter. The directivity is even more increased by the presence of a dielectric substrate, making such antennas a promising candidate for compact easy-to-address planar sensors.
289 citations
TL;DR: In this article, photocurrents of silicon pn junctions patterned with arrays of elliptical Au nanodisks were experimentally and theoretically investigated near the particle plasmon resonance wavelengths, for varying light polarizations and angles of incidence.
Abstract: Photocurrents of silicon pn junctions patterned with arrays of elliptical Au nanodisks were experimentally and theoretically investigated near the particle plasmon resonance wavelengths, for varying light polarizations and angles of incidence. At plasmon resonance wavelengths, overall backscattering and dissipation were strongly enhanced compared to an unpatterned junction, resulting in lower photocurrents. In contrast, enhanced photocurrents were observed for wavelengths slightly off resonance. Measurements and finite element calculations show that the photocurrent changes occur via plasmon-induced far field effects, rather than by near field enhancement close to the particles. The far field effects are strongly dependent on the particle proximity and coupling to the Si substrate.
271 citations
TL;DR: The principles of near-field optics are outlined, the mechanisms contributing to local field enhancement and how it can be used to enhance optical signals are discussed and several recent examples of Raman and fluorescence microscopy with 10 nm spatial resolution of single molecules are reviewed.
Abstract: Spectroscopic methods with high spatial resolution are essential for understanding the physical and chemical properties of nanoscale materials, including quantum structures and biological surfaces An optical technique is reviewed that relies on the enhanced electric fields in the proximity of a sharp, laser-irradiated metal tip These fields are utilized for spatially confined probing of various optical signals, thus allowing for a detailed sample characterization far below the diffraction limit In addition, tip-enhanced fields also provide the sensitivity crucial for the detection of nanoscale volumes After outlining the principles of near-field optics, the mechanisms contributing to local field enhancement and how it can be used to enhance optical signals are discussed Different experimental methods are presented and several recent examples of Raman and fluorescence microscopy with 10 nm spatial resolution of single molecules are reviewed
262 citations
TL;DR: Using a patterned, grating-like plate to control the electromagnetic near field, this work demonstrates focusing well beyond the diffraction limit at ∼ 1 gigahertz.
Abstract: Using a patterned, grating-like plate to control the electromagnetic near field, we demonstrate focusing well beyond the diffraction limit at ∼ 1 gigahertz. The near-field plate consists of only capacitive elements and focuses microwaves emanating from a cylindrical source to a spot of size ≈ λ/20 (half-power beamwidth), where λ is the free-space wavelength. These plates will find application in antennas, beam-shaping devices, nonradiative wireless power-transfer systems, microscopy, and lithography.
253 citations
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18 Jul 2008TL;DR: In this article, the electromagnetic field propagates in a dielectric-filled transverse electromagnetic waveguide, and at least one slice of an electro-optic material is disposed in the DME for modulation by the incident electromagnetic field.
Abstract: A method of and apparatus for modulating an optical carrier by an incident electromagnetic field. The electromagnetic field propagates in a dielectric-filled transverse electromagnetic waveguide, At least one slice of an electro-optic material is disposed in the dielectric-filled transverse electromagnetic waveguide, the electro-optic material in the dielectric-filled transverse electromagnetic waveguide having at least one optical waveguide therein which has at least a major portion thereof guiding light in a direction orthogonal with respect to a direction in which the dielectric-filled transverse electromagnetic waveguide guides the incident electromagnetic field. Light is caused to propagate in the at least one optical waveguide in the at least one slice of an electro-optic material in the dielectric-filled transverse electromagnetic waveguide for modulation by the incident electromagnetic field.
160 citations
TL;DR: This paper presents a detailed modeling and characterization of a microfabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes, using finite-element analysis.
Abstract: We present a detailed modeling and characterization of our scalable microwave nanoprobe, which is a micro-fabricated cantilever-based scanning microwave probe with separated excitation and sensing electrodes. Using finite-element analysis, the tip-sample interaction is modeled as small impedance changes between the tip electrode and the ground at our working frequencies near 1GHz. The equivalent lumped elements of the cantilever can be determined by transmission line simulation of the matching network, which routes the cantilever signals to 50 Ohm feed lines. In the microwave electronics, the background common-mode signal is cancelled before the amplifier stage so that high sensitivity (below 1 atto-Farad capacitance changes) is obtained. Experimental characterization of the microwave probes was performed on ion-implanted Si wafers and patterned semiconductor samples. Pure electrical or topographical signals can be realized using different reflection modes of the probe.
147 citations
TL;DR: A comprehensive parametric study has been carried out to understand the effects of various dimensional parameters and to optimize the performance of the designed antenna, which exhibits stable far-field radiation characteristics in the entire operating bandwidth, relatively high gain, and low cross polarization.
Abstract: The design and analysis of a novel printed wide-slot antenna, fed by a microstrip line, for wideband communication systems is presented. Detailed simulation and experimental investigations are conducted to understand its behavior and optimize for broadband operation. The designed antenna has a wide operating bandwidth of over 120% (2.8-11.4 GHz) for S11 <-10 dB. In addition to being small in size, the antenna exhibits stable far-field radiation characteristics in the entire operating bandwidth, relatively high gain, and low cross polarization. By properly choosing the suitable slot shape, selecting similar feed shape and tuning their dimensions, the design with very wide operating bandwidth, relatively small size and improved radiation pattern is obtained. A comprehensive parametric study has been carried out to understand the effects of various dimensional parameters and to optimize the performance of the designed antenna. Results show that the impedance matching of this kind of antenna is greatly affected by the feed-slot combination and feed gap width, with the slot shape being a main contributor of the radiation characteristics. The simulated and measured Results for return loss, far-field E and H-plane radiation patterns, and gain of designed antenna are presented and discussed.
135 citations
TL;DR: A scattering-type scanning near-field optical microscope (s-SNOM) with broadband THz illumination with cantilevered W tip used in tapping AFM mode and scattering spectra of differently doped Si are presented.
Abstract: We demonstrate a scattering-type scanning near-field optical microscope (s-SNOM) with broadband THz illumination. A cantilevered W tip is used in tapping AFM mode. The direct scattering spectrum is obtained and optimized by asynchronous optical sampling (ASOPS), while near-field scattering is observed by using a space-domain delay stage and harmonic demodulation of the detector signal. True near-field interaction is determined from the approach behavior of the tip to Au samples. Scattering spectra of differently doped Si are presented.
117 citations
TL;DR: In this article, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near field measurements, which exploits the benefits of diagonalized translation operators, known from fast multipole methods.
Abstract: Near-field antenna measurements combined with a near-field far-field transformation are an established antenna characterization technique. The approach avoids far-field measurements and offers a wide area of post-processing possibilities including radiation pattern determination and diagnostic methods. In this paper, a near-field far-field transformation algorithm employing plane wave expansion is presented and applied to the case of spherical near-field measurements. Compared to existing algorithms, this approach exploits the benefits of diagonalized translation operators, known from fast multipole methods. Due to the plane wave based field representation, a probe correction, using directly the probe's far-field pattern can easily be integrated into the transformation. Hence, it is possible to perform a full probe correction for arbitrary field probes with almost no additional effort. In contrast to other plane wave techniques, like holographic projections, which are suitable for highly directive antennas, the presented approach is applicable for arbitrary radiating structures. Major advantages are low computational effort with respect to the coupling matrix elements owing to the use of diagonalized translation operators and the efficient correction of arbitrary field probes. Also, irregular measurement grids can be handled with little additional effort.
116 citations
TL;DR: Based on the rectangular equal frequency contour of this metamaterial, magnifying lenses for acoustic waves, analogous to electromagnetic hyperlenses demonstrated recently in the optical regime, can be realized.
Abstract: A kind of two-dimensional acoustic metamaterial is designed so that it exhibits strong anisotropy along two orthogonal directions. Based on the rectangular equal frequency contour of this metamaterial, magnifying lenses for acoustic waves, analogous to electromagnetic hyperlenses demonstrated recently in the optical regime, can be realized. Such metamaterial may offer applications in imaging for systems that obey scalar wave equations.
TL;DR: Analytical formulas for the elements of the 2x2 cross-spectral density matrix of a kind of stochastic electromagnetic array beam propagating through the turbulent atmosphere are derived with the help of vector integration and it is shown that for the array beam composed by isotropic Gaussian-Schell model sources, the spectral degree of polarization in the sufficiently far field returns to the value of the array source.
Abstract: Analytical formulas for the elements of the 2×2 cross-spectral density matrix of a kind of stochastic electromagnetic array beam propagating through the turbulent atmosphere are derived with the help of vector integration. Two types of superposition (i.e. the correlated superposition and the uncorrelated superposition) are considered. The changes in the spectral density and in the spectral degree of polarization of such an array beam generated by isotropic or anisotropic electromagnetic Gaussian Schell-model sources on propagation are determined by the use of the analytical formulas. It is shown by numerical calculations that for the array beam composed by isotropic Gaussian-Schell model sources, the spectral degree of polarization in the sufficiently far field returns to the value of the array source; for the array beam composed by anisotropic sources, the spectral degree of polarization in the far field approaches a fixed value that is different from the source.
TL;DR: It is demonstrated that near-field coupling can be expressed in a simple form verifying the optical theorem for each particle modes and the electromagnetic coupling mechanisms between the emitter and the particle.
Abstract: We analytically and numerically analyze the fluorescence decay rate of a quantum emitter placed in the vicinity of a spherical metallic particle of mesoscopic size (i.e with dimensions comparable to the emission wavelength). We discuss the efficiency of the radiative decay rate and non-radiative coupling to the particle as well as their distance dependence. The electromagnetic coupling mechanisms between the emitter and the particle are investigated by analyzing the role of the plasmon modes and their nature (dipole, multipole or interface mode). We demonstrate that near-field coupling can be expressed in a simple form verifying the optical theorem for each particle modes.
TL;DR: In this article, it was shown that, in spite of the limited spatial coherence of synchrotron radiation, speckles with remarkable properties can be observed when the sensor is placed in the near field.
Abstract: Whenever coherent radiation impinges on a scattering object, a speckled intensity pattern is produced. In the far field the speckle size and shape do not mirror any properties of the object. Here we show that, in spite of the limited spatial coherence of synchrotron radiation, speckles with remarkable properties can be observed when the sensor is placed in the near field. The statistical analysis of these speckles generates static and dynamic X-ray-scattering data, and the results from two typical scattering samples are given. When compared with conventional far-field techniques, the method enables a substantial increase of around four orders of magnitude in the beam size and power and opens the way to a previously inaccessible region of scattering angles. It also offers the possibility of tracking the spatio-temporal evolution of complex fluids and other inhomogeneous systems.
TL;DR: A dramatic enhancement of the signal amplitude and spectroscopic contrast of the sample layer, together with a change of the spectral line shape is obtained, opening the possibility to perform ultra-sensitive near field infrared spectroscopy of monolayers and biomolecules.
Abstract: We study the amplitude and phase signals detected in infrared scattering-type near field optical microscopy (s-SNOM) when probing a thin sample layer on a substrate. We theoretically describe this situation by solving the electromagnetic scattering of a dipole near a planar sample consisting of a substrate covered by thin layers. We perform calculations to describe the effect of both weakly (Si and SiO2) and strongly (Au) reflecting substrates on the spectral s-SNOM signal of a thin PMMA layer. We theoretically predict, and experimentally confirm an enhancement effect in the polymer vibrational spectrum when placed on strongly reflecting substrates. We also calculate the scattered fields for a resonant tip-substrate interaction, obtaining a dramatic enhancement of the signal amplitude and spectroscopic contrast of the sample layer, together with a change of the spectral line shape. The enhanced contrast opens the possibility to perform ultra-sensitive near field infrared spectroscopy of monolayers and biomolecules.
TL;DR: In this paper, a theoretical investigation was carried out for predicting radiation characteristics of single-walled carbon nanotube (SWCNT) bundle dipole antennas based on the distributed circuit parameters and the model of an SWCNT, where the cross section of bundles can be in a circular and a rectangular geometry, respectively.
Abstract: A theoretical investigation is carried out for predicting radiation characteristics of single-walled carbon nanotube (SWCNT) bundle dipole antennas based on the distributed circuit parameters and the model of an SWCNT, where the cross section of bundles can be in a circular and a rectangular geometry, respectively. The current distributions in such novel antennas are predicted to investigate the effects of bundle cross-sectional size, tube diameter, tube length, and operating frequency. Furthermore, comparative studies are performed to show the geometry- and frequency-dependent radiation resistance, far-field pattern, and radiation efficiency of some typical bundle dipole antennas, which are numerically confirmed to outperform an SWCNT antenna by 30-40 dB in radiation efficiency.
TL;DR: In this article, the authors describe the design of near-field focusing plates, which are grating-like structures that can focus electromagnetic radiation to spots or lines of arbitrarily small subwavelength dimension.
Abstract: We describe the design of near-field focusing plates, which are grating-like structures that can focus electromagnetic radiation to spots or lines of arbitrarily small subwavelength dimension. A general procedure is outlined for designing a near-field plate to achieve a desired focus, and its implementation at microwave frequencies is discussed in detail. Full-wave (method of moments) simulations clearly demonstrate a passive near-field plate's ability to overcome the diffraction limit. Finally, it is shown that performance of the plates is weakly affected by losses..
TL;DR: It is observed, that holes can act as differential operators whose near-field output is the time-derivative of the incident electric field.
Abstract: Using terahertz-light excitation, we have measured with sub-wavelength spatial, and sub-cycle temporal resolution the time- and frequency-dependent electric-field and surface-charge density in the vicinity of small metallic holes. In addition to a singularity like concentration of the electric field near the hole edges, we observe, that holes can act as differential operators whose near-field output is the time-derivative of the incident electric field. Our results confirm the well-known predictions made by Bouwkamp, Philips Res. Rep. 5, 321-332 (1950), and reveal, with unprecedented detail, what physically happens when light passes through a small hole.
TL;DR: In this paper, exact electrodynamic theory was used to study the radiative electromagnetic coupling between spherical silver nanoparticles equidistantly arranged in a linear chain, and two distinct coupling regimes were observed depending on the distance between the particles in the chain and on the polarization of the incident light.
Abstract: The extinction spectra of arrays of nanoparticles provides a potentially powerful platform for chemical and biological sensing based on variation in plasmon wavelength excitation with analyte index of refraction. In this paper we use exact electrodynamic theory to study the radiative electromagnetic coupling between spherical silver nanoparticles equidistantly arranged in a linear chain. Two distinct coupling regimes are observed depending on the distance between the particles in the chain, and on the polarization of the incident light. Near-field coupling of the particles occurs when the interparticle distance is smaller than twice the particle diameter. This leads to pronounced red shifts in the plasmon resonance wavelength and increasing widths as the particle separation decreases for p-polarization. Far-field coupling leads to non-monotonic shift and broadening at larger distances, with more important effects being associated with s-polarization. A region where the plasmon width is significantly smaller than the isolated particle width is identified in the far-field regime that could be important in biological sensing applications. We also develop quasi-static analytical models to study the transition from near to far-field coupling.
TL;DR: In this paper, the optical near-field distribution and enhancement near the apex of model scanning probe tips are calculated within the quasistatic approximation, and a pronounced structural plasmon resonant behavior is found for gold tips, which redshifts upon tip−sample approach on the length scale given by the tip radius.
Abstract: The optical near-field distribution and enhancement near the apex of model scanning probe tips are calculated within the quasistatic approximation. The optical tip−sample coupling sensitively depends on both the tip and sample material. This, in addition to the tip−sample distance and apex geometry, is found to affect the spatial resolution that can be obtained in scattering near-field microscopy (s-SNOM). A pronounced structural plasmon resonant behavior is found for gold tips, which redshifts upon tip−sample approach on the length scale given by the tip radius. This near-field tip−sample coupling also allows for surface plasmon excitation in the sample. With the critical dimensions of the tip apex in the range of 10 to several 10s of nanometers, the results are found to be in good agreement with experiment and more rigorous theoretical treatments.
TL;DR: In this paper, a near-field scanning thermal microscope is used to detect the local density of states of the thermally excited electromagnetic modes at nanometer distances from some material, which can be employed for nanoscale imaging of structures on that material's surface.
Abstract: We show that a near-field scanning thermal microscope, which essentially detects the local density of states of the thermally excited electromagnetic modes at nanometer distances from some material, can be employed for nanoscale imaging of structures on that material's surface. This finding is explained theoretically by an approach which treats the surface structure perturbatively.
03 Jul 2008
TL;DR: In this paper, the potential of one-dimensional and two-dimensional deterministic aperiodic arrays for the design of electromagnetic coupling and plasmon-enhanced, sub-wavelength optical fields on chip-scale devices was explored.
Abstract: In this paper we explore the potential of one-dimensional and two-dimensional deterministic aperiodic plasmonic arrays for the design of electromagnetic coupling and plasmon-enhanced, sub-wavelength optical fields on chip-scale devices. In particular, we investigate the spectral, far-field and near-field optical properties of metal nanoparticle arrays generated according to simple deterministic sequences characterized by fractal Fourier spectra. Additionally, we will consider the case of flat Fourier-transform sequences, which reproduce the behavior of purely random systems to an arbitrary degree of accuracy. Based on the coupled dipole approach (CDA) and finite difference time domain (FDTD) simulations, we study the radiative (long-range) and quasi-static (short-range) electromagnetic coupling in deterministic aperiodic plasmon arrays of metal nanoparticles. In addition, we investigate the local field enhancement and the enhancement scaling in periodic and aperiodic arrays with increasing degree of complexity. We believe that the accurate control of electromagnetic coupling and sub-wavelength field enhancement in deterministic aperiodic environments will enable novel nanodevice applications in areas such as field-enhanced nanosensors, engineered SERS substrates and optical nano-antenna arrays.
TL;DR: In this paper, a model for guided wave scattering from non-symmetric blind holes in isotropic plates using Poisson and Mindlin plate wave theories for in-plane and flexural wave modes, respectively, is presented.
Patent•
TDK1
TL;DR: In this paper, a surface plasmon antenna comprises an edge having a portion for coupling with a light in a surface-plasmon mode and extends from the portion to a near-field light generating end surface that emits near field light.
Abstract: Provided is a surface plasmon antenna that can be set so that the emitting position on the end surface of the plasmon antenna where near-field light is emitted is located sufficiently close to the end of a magnetic pole. The surface plasmon antenna comprises an edge having a portion for coupling with a light in a surface plasmon mode. The edge is provided for propagating surface plasmon excited by the light and extends from the portion to a near-field light generating end surface that emits near-field light. The edge for propagating surface plasmon is a very narrow propagation region. Therefore, the near-field light generating end surface, which appears as a polished surface processed through polishing in the manufacturing of the plasmon antenna, can be made a shape with a very small size, and further can be set so that surface plasmon propagates to reach the end surface reliably.
TL;DR: The here-introduced phase-shifting interferometry method is considerably simpler than heterodyne methods and less sensitive to errors than the two-step homodyne method, and demonstrates the ability to measure, with nanoscale resolution, amplitude and phase distributions of optical fields on sample surfaces.
Abstract: Scattering-type scanning near-field optical microscopy has allowed for investigation of light-matter interaction of a large variety of samples with excellent spatial resolution. Light incident on a metallic probe experiences an amplitude and phase change on scattering, which is dependent on optical sample properties. We implement phase-shifting interferometry to extract amplitude and phase information from an interferometric near-field scattering system, and compare recorded optical images with theoretical predictions. The results demonstrate our ability to measure, with nanoscale resolution, amplitude and phase distributions of optical fields on sample surfaces. The here-introduced phase-shifting method is considerably simpler than heterodyne methods and less sensitive to errors than the two-step homodyne method.
TL;DR: In this article, the linear sampling method used to retrieve obstacles in a 2D or 3D acoustic waveguide has been studied, and it is shown that the numerical reconstruction of obstacles by using the far field is feasible, even by using a few incident waves.
Abstract: This paper concerns the linear sampling method used to retrieve obstacles in a 2D or 3D acoustic waveguide The classical mathematical results concerning the identifiability of the obstacle and the justification of the inverse method are established for this particular geometry Our main concern is to derive a modal formulation of the linear sampling method that is well adapted to the waveguide configuration In particular, thanks to such formulation, we highlight the fact that finding some obstacles from remote scattering data is more delicate in a waveguide than in free space Indeed, the presence of evanescent modes increases the ill posedness of the inverse problem However, we show that the numerical reconstruction of obstacles by using the far field is feasible, even by using a few incident waves
TL;DR: Combining experimental near- field optical studies with near- and far-field optical simulations provides a detailed understanding of resonance mechanisms in subwavelength structures and will facilitate an efficient approach to improved designs.
Abstract: We investigate the local optical response of split-ring resonator-(SRR)-based metamaterials with an apertureless scanning near-field optical microscope. By mapping the near fields of suitably resonant micrometer-sized SRRs in the near-infrared spectral region with an uncoated silicon tip, we obtain a spatial resolution of better than lambda/50. The experimental results confirm numerical predictions of the near-field excitations of SRRs. Combining experimental near-field optical studies with near- and far-field optical simulations provides a detailed understanding of resonance mechanisms in subwavelength structures and will facilitate an efficient approach to improved designs.
TL;DR: The paper derives the Shannon information capacity of canonical wireless and antenna systems in free space, for a given additive Gaussian noise level, as well as an associated number of degrees of freedom resulting from such capacity calculations.
Abstract: This paper investigates information-theoretic characterization, via Shannon's information capacity and number of degrees of freedom, of wave radiation (antenna) and wireless propagation systems. Specifically, the paper derives, from the fundamental physical point of view of Maxwell's equations describing electromagnetic fields, the Shannon information capacity of space-time wireless channels formed by electromagnetic sources and receivers in a known background medium. The theory is developed first for the case of sources working at a fixed frequency (time-harmonic case) and is expanded later to the more general case of temporally bandlimited systems (time-domain fields). In the bandlimited case we consider separately the two cases of time-limited and essentially bandlimited systems and of purely bandlimited systems. The developments take into account the physical radiated power constraint in addition to a constraint in the source L 2 norm which acts to avoid antenna superdirectivity. Based on such radiated power and current L 2 norm constraints we derive the Shannon information capacity of canonical wireless and antenna systems in free space, for a given additive Gaussian noise level, as well as an associated number of degrees of freedom resulting from such capacity calculations. The derived results also illustrate, from a new information-theoretic point of view, the transition from near to far fields.
TL;DR: Optical vortices of integer and fractional charge were successfully generated at a wavelength of 633 nm and observed in the far field and the obtained intensity patterns proved to be in agreement with the theoretical predictions on integer and fractionsal charge optical vortice.
Abstract: We present a method for the creation of optical vortices by using a deformable mirror. Optical vortices of integer and fractional charge were successfully generated at a wavelength of 633 nm and observed in the far field (2000 mm). The obtained intensity patterns proved to be in agreement with the theoretical predictions on integer and fractional charge optical vortices. Interference patterns between the created optical vortex carrying beams and a reference plane wave were also produced to verify and confirm the existence of the phase singularities.