Showing papers on "Near and far field published in 2001"
TL;DR: In this paper, a point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners and tee structures is possible at high efficiencies.
Abstract: The further integration of optical devices will require the fabrication of waveguides for electromagnetic energy below the diffraction limit of light. We investigate the possibility of using arrays of closely spaced metal nanoparticles for this purpose. Coupling between adjacent particles sets up coupled plasmon modes that give rise to coherent propagation of energy along the array. A point dipole analysis predicts group velocities of energy transport that exceed 0.1c along straight arrays and shows that energy transmission and switching through chain networks such as corners (see Figure) and tee structures is possible at high efficiencies. Radiation losses into the far field are expected to be negligible due to the near-field nature of the coupling, and resistive heating leads to transmission losses of about 6 dB/lm for gold and silver particles. We analyze macroscopic analogues operating in the microwave regime consisting of closely spaced metal rods by experiments and full field electrodynamic simulations. The guiding structures show a high confinement of the electromagnetic energy and allow for highly variable geometries and switching. Also, we have fabricated gold nanoparticle arrays using electron beam lithography and atomic force microscopy manipulation. These plasmon waveguides and switches could be the smallest devices with optical functionality.
1,650 citations
Patent•
12 Oct 2001
TL;DR: In this paper, an adaptive antenna array is configured to receive a transmission signal from a transmitter and in response transmit corresponding outgoing multi-beam electromagnetic signals exhibiting a plurality of selectively placed transmission peaks and transmission nulls within a far field region of a coverage area.
Abstract: Methods, apparatuses and systems are provided for use in a wireless routing network. One apparatus, for example, includes an adaptive antenna that is configurable to receive a transmission signal from a transmitter and in response transmit corresponding outgoing multi-beam electromagnetic signals exhibiting a plurality of selectively placed transmission peaks and transmission nulls within a far field region of a coverage area. The adaptive antenna may also be configured to selectively receive at least one incoming electromagnetic signal directed through the coverage area. The adaptive antenna includes at least one antenna array and logic. The antenna array has a plurality of antenna elements. The logic is operatively coupled to the antenna array and configured to selectively control the placement of the transmission peaks and transmission nulls within the outgoing multi-beam electromagnetic signals. The logic may also be configured to selectively control the reception of at least one incoming electromagnetic signal. The logic is configured to be responsive to routing information. Such routing information may be dynamically maintained in one or more routing tables.
404 citations
TL;DR: Optical transmission of a two-dimensional array of subwavelength holes in a metal film has been numerically studied using a differential method and reveals an intensity enhancement greater than 2 orders of magnitude in localized spots resulting from the interference of the surface polariton Bragg scattered by the holes in an array.
Abstract: Recent experimental discovery of the enhanced optical transmission through metal films with periodic subwavelength holes has given rise to a considerable interest in the optical properties of such structures due to their possible numerous applications in optics and optoelectronics as well as rich physics behind the phenomenon of the transmission enhancement [1–4]. The transmission of a subwavelength aperture is very low and proportional to the fourth power of the ratio of its diameter and light wavelength. However, if a metal film is perforated with a periodic array of such holes, the optical transmission can be significantly enhanced [1]. Being normalized to the total area of the illuminated holes, the transmission coefficient corresponds to an enhancement up to 3 orders of magnitude compared to the transmission of the same number of individual holes. This enhancement depends on the array geometry (hole diameter and periodicity), light wavelength, angle of incidence, as well as material of a film.
315 citations
TL;DR: In this paper, a photon scanning tunneling microscope was used to probe the field of surface plasmon polariton modes excited on finite-width thin metal films (metal stripes).
Abstract: We use a photon scanning tunneling microscope to probe the field of surface plasmon polariton modes excited on finite-width thin metal films (metal stripes). We first investigate the coupling between surface plasmons launched by a focused beam on a homogeneous thin film and the modes sustained by metal stripes of different widths. We show that, if the width of the metal stripe is about a few micrometers, a strong coupling with the stripe modes can be achieved at visible frequencies. A sharp transverse confinement of the field associated with the surface plasmon modes propagating on the metal stripe is unambiguously observed on the constant height photon scanning tunneling microscope images. The back-reflection of these modes at the end of the stripe leads to a surface-wave interference pattern from which the wavelength of the stripe surface plasmon modes is directly measured. We finally demonstrate that metal stripes could be used for optical addressing purposes at the micrometer scale since a stripe with a triangularly shaped termination performs the focusing of the stripe surface plasmon field.
310 citations
TL;DR: The reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions.
Abstract: Enhancement γ of the electrical field at the end of a tip relative to the incident field in a focused radiation beam is calculated by the finite-element time-domain (FETD) method. First, the reliability of the FETD method is established by calculating the electric field on simple structures like thin cylinders, spheres, and ellipsoids, and comparing the results with analytical solutions. The calculations on these test structures also reveal that phase retardation effects substantially modify γ when the size of the structure is larger than approximately λ/4, λ being the radiation wavelength. For plasmon resonance, in particular, phase retardation severely reduces the resonance and the expected field enhancement for a gold tip. The small value of γ=4 calculated by FETD is about an order of magnitude smaller than the value found in recent published work. Resonance effects can be recovered for special tips, which have a discontinuity or a different material composition at the end of the tip. Some tuning of the discontinuity dimension is needed to maximize the resonance. Under optimal conditions for plasmon resonance, an enhancement in the electric field of about 50 is calculated at the end of a small gold protrusion mounted on a wider silicon or glass tip.
229 citations
TL;DR: In this article, the equations of electromagnetism can be directly obtained in a finite form without going throught the differential formulation, which is a natural extension of the network theory to electromagnetic field.
Abstract: This paper shows that the equations of electromagnetism can be directly obtained in a finite (=discrete) form, i.e. without going throught the differential formulation. This finite formulation is a natural extension of the network theory to electromagnetic field and it is suitable for computational electromagnetics.
189 citations
TL;DR: In this article, a high frequency electromagnetic pump wave transmitted into ionospheric plasma from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave, and the numerous spectral features of these stimulated electromagnetic emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and related theoretical, numerical, and simulation results are discussed.
Abstract: A high frequency electromagnetic pump wave transmitted into the ionospheric plasma from the ground can stimulate electromagnetic radiation with frequencies around that of the ionospherically reflected pump wave. The numerous spectral features of these stimulated electromagnetic emissions (SEE) and their temporal evolution on a wide range of time scales are reviewed and related theoretical, numerical, and simulation results are discussed. On long (thermal) time scales the SEE constitutes a self-organization of the ionospheric plasma which depends on the interaction of nonlinear processes in a hierarchy of time scales in response to the electromagnetic pumping. Particularly, the appearance of the rich SEE spectrum is associated with the slow self-structuring of the plasma density into a spectrum of magnetic field-aligned density striations. The dependence of the SEE on electron gyroharmonic effects and the presence of density striations suggests that the existence of a magnetic field in the plasma is important for plasma turbulence to dissipate into non-thermal electromagnetic radiation during the long time quasi-stationary state of the turbulence evolution.
188 citations
TL;DR: The apertureless scattering‐type scanning near‐field optical microscope is enhanced by two improvements which together achieve a recording of the true near field without any height‐induced artefact.
Abstract: We have enhanced the apertureless scattering-type scanning near-field optical microscope by two improvements which together achieve a recording of the true near field without any height-induced artefact. These are the use of interferometric detection of the scattered light on one hand, and the use of higher-harmonic dither demodulation of the scattered signal on the other. Here we present the basic rationale for these techniques, and give examples measured with two different experiments, one in the infrared (10 microm wavelength), the other in the visible (633 nm). The latter operates in a fully heterodyne mode and displays simultaneous images of optical near-field phase and amplitude, at below 10 nm resolution.
185 citations
Patent•
17 Apr 2001TL;DR: In this article, a space-fed active array lens antenna system has been proposed, which consists of a front antenna aperture, a rear antenna aperture and an array of transmit/receive modules sandwiched between the front aperture and rear aperture.
Abstract: A space-fed active array lens antenna system has an active array lens with a first array of radiating elements defining a front antenna aperture which transmits and receives RF energy from free space, a second array of radiating elements defining a rear antenna aperture which transmits and receives RF energy from a feed aperture, and an array of transmit/receive (T/R) modules sandwiched between the front aperture and rear aperture. The T/R modules include a phase control circuit and an amplitude control circuit which provide phase and amplitude control for RF signals passing through the modules. The feed aperture includes a wide band CTS aperture which produces a plane wave in the near field.
184 citations
TL;DR: A passive, matched two-time-derivative Lorentz material medium is designed to have its equivalent permittivity and permeability smaller than their values in free space over a large range of frequencies.
Abstract: A passive, matched two-time-derivative Lorentz material medium is designed to have its equivalent permittivity and permeability smaller than their values in free space over a large range of frequencies. Superluminal pulse propagation in this medium and consequent superluminal information exchange without a violation in causality are demonstrated. Additional properties of this medium are developed including the energy in it and the force characteristics induced on it by electromagnetic field interactions. It is shown that the force on the medium can be made to be attractive or repulsive using a change in frequency or a change in the material characteristics. Potential applications are discussed.
167 citations
TL;DR: In this article, a model based on the pupil approach and the Fourier modal method is presented for numerical simulation of high numerical aperture polarization microscopy images and the near-fields producing them.
TL;DR: In this article, a new method for predicting the far-field radiated emissions and for finding the radiation sources of a device from near-field measurements is presented, based on the substitution of the original device by an equivalent set of elemental dipoles, placed over the main radiating sources, which radiate the same near field (and therefore, farfield).
Abstract: A new method for predicting the far-field radiated emissions and for finding the radiation sources of a device from near-field measurements is presented. It is based on the substitution of the original device by an equivalent set of elemental dipoles, placed over the main radiating sources, which radiate the same near-field (and therefore, far-field). This equivalent set of elemental dipoles is generated using a genetic algorithm. From the position and type of the equivalent elemental dipoles, the position of the actual radiating sources is determined. Since the field produced by an elemental dipole is known, the far-field radiation of the actual radiating source can be calculated. The new method has been tested using synthetic data and real measurements from the radiation generated by a modem PCB demonstrating its viability and usefulness.
TL;DR: In this paper, a refined discussion of the near-field scattering of spherical nanoparticles and the electromagnetic fields close to the particle surface is given New results for the dependence on the distance from the surface and the angular distribution of the scattered light in the near field are given.
Abstract: A refined discussion of the near-field scattering of spherical nanoparticles and the electromagnetic fields close to the particle surface is given New results for the dependence on the distance from the surface and the angular distribution of the scattered light in the near-field are given It will be shown that the radial component of the electric field leads to striking differences in the phase functions in the near-field and the far-field Exemplary computations are presented for Ag and Au particles with different size In a second part the discussion is extended to assemblies of spherical Ag and Au nanoparticles It will be shown that large near-fields at wavelengths commonly used in SERS experiments are obtained for aggregates In the near-field scattering intensity “hot spots” mark regions between particles in the aggregate where the near-field is particularly high
TL;DR: A plasmon waveguide was designed and fabricated using a metal-coated silicon wedge structure that converted propagating far-field light to the near field illumination as discussed by the authors, and the waveguide's beam width and propagation length were determined using a near-field microscope.
Abstract: A plasmon waveguide was designed and fabricated using a metal-coated silicon wedge structure that converts propagating far-field light to the near field Illumination (λ=830 nm) of the waveguide (plateau width 150 nm) caused transverse magnetic plasmon-mode excitation Use of a near-field microscope allowed us to determine its beam width and propagation length as 150 nm and 25 μm, respectively
TL;DR: In this paper, an extended field (EF) model formulation is proposed for inductively coupled plasma, where vector potential equations are solved with corresponding continuity, momentum, and energy transfer equations using the commercial code ''FLUENT''.
Abstract: An extended-field (EF), two dimensional (2D) model formulation is proposed for inductively coupled plasma. By extending the calculating domain of the electromagnetic (EM) field outside of the plasma discharge region, the boundary conditions of vector potential used by the standard (ST) 2D model are replaced by simpler far field boundary conditions. The extended model converges faster than the standard formulation and gives rise to consistent solutions throughout the computational domain. Vector potential equations are solved with corresponding continuity, momentum, and energy transfer equations using the commercial code `FLUENT'. The computational domain for vector potential equations are extended well beyond the induction coil region, while for all the other equations, computations are limited to the discharge region inside the plasma confinement tube. The computational results are compared with those obtained using the ST 2D model. The difference between the results of the two models is noted mostly in the entrance regions of the flow, and close to the induction coil. To validate the EF model, a load with constant electric conductivity is placed centrally in the coil region and the calculated radial profile of the axial magnetic field is compared with existing analytical solutions. The results are in good agreement within an uncertainty of 1%.
Patent•
27 Jul 2001
TL;DR: In this paper, the authors proposed an optical assist magnetic head and optical assist disk device, where high-density recording is carried out, and light utilization efficiency and reliability are improved, and a distance between the position of a near field light and a magnetic field formed by the thin-film magnetic transducers is the shortest.
Abstract: PROBLEM TO BE SOLVED: To provide an optical assist magnetic head and an optical assist magnetic disk device, wherein high-density recording is carried out, and light utilization efficiency and reliability are improved. SOLUTION: The optical assist magnetic head 1 includes an optical waveguide 5 for emitting a laser beam from a semiconductor laser, a magneto-resistive sensor 3, and a thin-film magnetic transducer 4 which are integrated in the rear end surface 2a of a floating slider 2, and a magnetic gap 34 formed in the exiting end 5c of the optical waveguide 5. A distance between the position of a near field light and a magnetic field formed by the thin-film magnetic transducer is the shortest. Thus, since recording is carried out simultaneously with or immediately after heating by the near field light, the effect of heat diffusion is ignored, and laser beam utilization efficiency is increased. Since a heated part is not expanded by the heat diffusion, a recorded part is narrowed to achieve a high density. COPYRIGHT: (C)2007,JPO&INPIT
TL;DR: A representation of the general solution of the Maxwell equations is proposed in terms of the plane-wave spectrum of the electromagnetic field and the concept of the so-called closest field to a given beam is introduced.
Abstract: A representation of the general solution of the Maxwell equations is proposed in terms of the plane-wave spectrum of the electromagnetic field. In this representation the electric field solution is written as a sum of two terms that are orthogonal to each other at the far field: One is transverse to the propagation axis, and the magnetic field associated with the other is also transverse. The concept of the so-called closest field to a given beam is introduced and applied to the well-known linearly polarized Gaussian beam.
TL;DR: In this paper, the surface plasmon oscillations in percolation films are localized in small nanometer-scale areas, where the local fields are much larger than the field of an incident electromagnetic wave.
Abstract: Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy (SNOM). The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale areas, ``hot spots,'' where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot's large local fields may result in local, frequency and spatially selective photomodification of percolation films.
TL;DR: In this article, the authors derived expressions for the smallest achievable radiation quality factor (Q) of an electrically small antenna in front of a conducting plane using a low-frequency approximation to the source region involving an electric or a magnetic point dipole plus their images behind the plane.
Abstract: Expressions are derived for the smallest achievable radiation quality factor (Q) of an electrically small antenna in front of a conducting plane. Applying the low-frequency approximation to the source region involving an electric or a magnetic point dipole plus their images behind the plane, an expression is formed for the field in the radiation zone. The contribution of non-propagating energy in the near field is obtained explicitly using a spherical harmonics decomposition. The radiation Q is found to depend on the radius (relative to wavelength) of the smallest sphere that encloses the antenna and its image, the ratio of the vertical and horizontal dipole moments, as well as the positions of the dipoles relative to each other and to the plane. A number of simple wire structures are analysed with NEC (based on the method of moments), and the approximate Q obtained from their fractional bandwidth are compared to the corresponding theoretical minima.
TL;DR: In this article, the super-resolution near-field structure of AgOx type has been studied by the tapping-mode tuning-fork near field scanning optical microscope as well as the transmission electron microscope, and numerical calculations confirm that localized surface plasmon and the nonuniform material structures are the main causes for these unusual characteristics.
Abstract: As a promising disk structure for ultrahigh density optical data storage, super-resolution near-field structure of AgOx type has been studied by the tapping-mode tuning-fork near-field scanning optical microscope as well as the transmission electron microscope. This structure presents strong near-field intensity enhancement and nonlinear optical effect, compared with the ordinary material. Numerical calculations confirm that the localized surface plasmon and the nonuniform material structures are the main causes for these unusual characteristics.
TL;DR: In this article, the near field of an apertureless near-field scanning optical microscopy probe is investigated with a multiple-multipole technique to obtain optical fields in the vicinity of a silicon probe tip and a glass substrate.
Abstract: The near field of an apertureless near-field scanning optical microscopy probe is investigated with a multiple-multipole technique to obtain optical fields in the vicinity of a silicon probe tip and a glass substrate. The results demonstrate that electric field enhancements of >15 relative to the incident fields can be achieved near a silicon tip, implying intensity enhancements of several orders of magnitude. This enhancement arises both from the antenna effect of the elongated probe and from a proximity effect when the probe is near the substrate surface and its image dipoles play a role.
TL;DR: It is shown that the intensity in a grating's near field can be enhanced significantly compared with conventional interferometric lithography, and predicted that high-intensity, high-contrast patterns with 135-nm period can be formed in photoresists more than 50 nm thick.
Abstract: Simulation results are presented to illustrate the main features of what we believe is a new photolithographic technique, evanescent interferometric lithography (EIL). The technique exploits interference between resonantly enhanced, evanescently decaying diffracted orders to create a frequency-doubled intensity pattern in the near field of a metallic diffraction grating. It is shown that the intensity in a grating’s near field can be enhanced significantly compared with conventional interferometric lithography. Contrast in the interference pattern is also increased, owing to a reduction in the zeroth-order transmission near resonance. The pattern’s depth of field reduces as the wavelength is increased beyond cutoff of the first-order diffracted components, and results are presented showing the trade-offs that can be made between depth of field and intensity enhancement. Examples are given for a 270-nm-period grating embedded in material with refractive index n = 1.6 and illuminated with wavelengths near 450 nm. Under these conditions it is predicted that high-intensity, high-contrast patterns with 135-nm period can be formed in photoresists more than 50 nm thick.
TL;DR: In this article, the authors present a theoretical model for the effects induced in near field optical microscope (SNOM) images by modulation of the probe height, which introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components.
Abstract: Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection.
TL;DR: In this article, a 3D modified boundary layer analysis is performed using the finite element method to study the crack-front constraint for an elastic-plastic thin plate, and the results show that the in-plane-stress fields at the crack front for various K I − T loads possess the plane-strain nature throughout the thickness except for the region near the free surface, and can be characterized by the J-A 2 three-term solution under the small scale yielding condition.
TL;DR: The conversion of static electric fields to electromagnetic radiation by the incidence of a superluminous ionization front on plasma is investigated and has important implications in astrophysical plasmas and to laboratory development of compact, coherent, tunable radiation sources in the THz range.
Abstract: The conversion of static electric fields to electromagnetic radiation by the incidence of a superluminous ionization front on plasma is investigated. For extremely superluminous fronts, the radiation is close to the plasma frequency and is converted with efficiency of order unity. A proof-of-principle experiment was conducted using semiconductor plasma containing an alternately charged capacitor array. The process has important implications in astrophysical plasmas, such as supernova emission, and to laboratory development of compact, coherent, tunable radiation sources in the THz range.
TL;DR: Scattering of a Hermite-Gaussian beam field by a chiral sphere is analyzed and the effects of the chirality and the radius of the Chiral sphere on the fields are examined.
Abstract: Scattering of a Hermite–Gaussian beam field by a chiral sphere is analyzed. A Hermite–Gaussian beam field is expressed as a superposition of multipole fields at complex-source points. Electromagnetic fields are expanded in terms of the spherical vector wave functions. The unknown expansion coefficients for the scattered field and the internal field are determined by the boundary conditions. As numerical examples, the scattered near fields of the beam incidence are calculated, and the effects of the chirality and the radius of the chiral sphere on the fields are examined. The results for a Gaussian beam incidence are also compared with those of a plane-wave incidence.
TL;DR: It is shown that the two-point correlation function in the near field of scattered light is simply related to the scattered intensity distribution.
Abstract: We show that the two-point correlation function in the near field of scattered light is simply related to the scattered intensity distribution. We present a new, to our knowledge, optical scheme to measure the correlation function in the near field, and we describe a processing technique that permits the subtraction of stray light on a statistical basis. We present experimental data for solutions of latex spheres, and we show that this novel technique is a powerful alternative to static light scattering.
TL;DR: In this article, electric field-induced modulation of the near-field photoluminescence of thin films of the conjugated polymer MEH−PPV was measured, and experiments performed with a blocking layer indicate that carriers are generated by photoexcitation in the bulk of the polymer film.
Abstract: Electric field-induced modulation of the near-field photoluminescence of thin films of the conjugated polymer MEH−PPV was measured. A voltage bias applied between the near-field probe and the substrate results in a highly spatially confined electric field. The near-field photoluminescence intensity decreases when the sample bias is positive with respect to the probe while the intensity increases when the sample bias is negative. The modulation of photoluminescence intensity provides a sensitive measure of changes in the local carrier density induced by the applied electric field. Experiments performed with a blocking layer indicate that carriers are generated by photoexcitation in the bulk of the polymer film. On the basis of the modulation results, we estimate the concentration of holes under optical and electrical excitation to be 1017 cm-3. The temporal response of the fluorescence upon the application of voltage is described in terms of the carrier mobility and electric field.
TL;DR: The experiments reveal a strong influence of the tip–surface interaction as well as polarization of the excitation light on images obtained with apertureless second‐harmonic SNOM.
Abstract: We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip-surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.
TL;DR: In this paper, a near-field terahertz wave imaging system with a dynamic aperture created on a GaAs wafer is described, which achieves a sub-wavelength spatial resolution of /spl sim/spl lambda/10.
Abstract: We describe and discuss near-field terahertz wave imaging with a dynamic aperture created on a semiconductor wafer. The spatial resolution of a near-field terahertz wave imaging system with a dynamic aperture created on a GaAs wafer is determined by the diameter of the gating-beam-induced thin photocarrier layer. With a dynamic aperture created on a GaAs wafer, we have achieved a subwavelength spatial resolution of /spl sim//spl lambda//10. In addition, near-field terahertz wave imaging with a dynamic aperture overcomes the limitations in the reported methods of near-field terahertz wave imaging. Particularly, it is free of the high-pass filtering effect. Various terahertz images are presented to show the possible applications.