Far-field Pattern Research Articles

Inverse Random Potential Scattering for Elastic Waves

This paper is concerned with the inverse elastic scattering problem for a random potential in three dimensions. Interpreted as a distribution, the potential is assumed to be a microlocally isotropic Gaussian random field whose covariance operator is a classical pseudodifferential operator. Given the potential, the direct scattering problem is shown to be well-posed in the sense of distributions by studying the equivalent Lippmann–Schwinger integral equation. For the inverse scattering problem, we demonstrate that the microlocal strength of the random potential can be uniquely determined with probability one by a single realization of the high frequency limit of the averaged compressional or shear backscattered far-field pattern of the scattered wave. The analysis employs the integral operator theory, the Born approximation in the high frequency regime, the microlocal analysis for the Fourier integral operators, and the ergodicity of the wave field.

Highly efficient waveguide-coupled output in asymmetrically deformed square cavity microlasers.

We propose and demonstrate deformed square cavity microlasers for realizing highly efficient output from a connected waveguide. The square cavities are deformed asymmetrically by replacing two adjacent flat sides with circular arcs to manipulate the ray dynamics and couple the light to the connected waveguide. The numerical simulations show that the resonant light can efficiently couple to the fundamental mode of the multi-mode waveguide by carefully designing the deformation parameter utilizing global chaos ray dynamics and internal mode coupling. An enhancement of approximately six times in the output power is realized in the experiment compared to the non-deformed square cavity microlasers, while the lasing thresholds are reduced by about 20%. The measured far-field pattern shows highly unidirectional emission agreeing well with the simulation, which confirms the feasibility of the deformed square cavity microlasers for practical applications.

Optical properties of circular Bragg gratings with labyrinth geometry to enable electrical contacts

We present an optical study of various device designs for electrically contactable circular Bragg grating cavities in labyrinth geometries. To create an electrical connection between the central disk and the surrounding membrane, which are separated through air gaps, we introduce connections between the adjacent rings. We propose to rotate these connections, creating a labyrinth-like structure, to disable waveguiding and keep the mode confinement. To investigate how different arrangements and sizes of the connections affect the optical properties and to find the optimal design, six different layouts with either threefold or fourfold symmetry and one with twofold symmetry are investigated experimentally and by numerical simulations. Reflectivity measurements and simulations show that rotating the connections improves the mode confinement, far-field pattern, and Purcell factor compared to layouts with connections arranged in straight lines. We compare results between different layouts for different connection widths and perform polarization resolved measurements to investigate whether the connections create asymmetries in the photonic confinement that would impede the performance of the device.

Speckle-learned convolutional neural network for the recognition of intensity degenerate orbital angular momentum modes

Intensity degenerate orbital angular momentum (OAM) modes are impossible to recognize by direct visual inspection even using available machine learning techniques. We are reporting speckle-learned convolutional neural network (CNN) for the recognition of intensity degenerate Laguerre–Gaussian (LGp , l) modes, intensity degenerate LG superposition modes, and intensity degenerate perfect optical vortices. The CNN is trained on the simulated one-dimensional far-field intensity speckle patterns of the corresponding intensity degenerate OAM modes. The trained CNN recognizes intensity degenerate OAM modes with an accuracy >99 % . Speckle-learned CNNs are also capable of recognizing intensity degenerate OAM modes even under the presence of high Gaussian white noise and atmospheric turbulence with an accuracy >97 % .

Tailoring 3D Speckle Statistics.

We experimentally generate three-dimensional speckles with customized intensity statistics. By appropriately modulating the phase front of a laser beam, the far-field speckles can maintain a desired intensity probability density function upon axial propagation: while evolving into different spatial patterns. We also demonstrate how to design speckle patterns that obtain distinct tailored intensity statistics on multiple designated axial planes. The ability to design 3D speckle statistics opens many possibilities: three-dimensional imaging and sensing, optical trapping, and manipulation.

Single-Cut Far-Field Pattern Determination of Polarized Linear Antenna Array via Quasi-Far-Field Mathematical Absorber Reflection Suppression

In this work, a method for rapidly and accurately determining the single-cut far-field (FF) patterns of any polarized linear antenna array, termed as quasi-far-field mathematical absorber reflection suppression (QFF-MARS), is introduced. The proposed QFF-MARS method takes advantage of the cylindrical wave expansion (CWE) theory and is able to exclude interference from scatters at the QFF range. In this method, probe compensation is not needed, owing to the fact that the measurement procedure is the same as that for FF. It is only required that the measurement distance for an antenna under test (AUT) satisfies the FF and QFF conditions for its smaller and larger dimensions, respectively. To validate the proposed method, a 45° polarized antenna array and a phased array have been simulated. The results obtained by the proposed approach are consistent with both theoretical and measured results. This indicates that the proposed approach can directly obtain the single-cut FF patterns of polarized linear antenna arrays at a QFF range, while identifying and extracting range-related multipath effects.

Beam Waist Properties of Spiral Antenna Coupled HEB Mixers at Supra-THz Frequencies

We have simulated and measured the beam properties of lens-antenna coupled hot electron bolometer mixers at a few supra-terahertz frequencies between 1.4 and 5.3 THz. The quasi-optical structures consist of an elliptical lens and a logarithmic spiral antenna. The model used for our simulations consists of a finite-element analysis to simulate the far-field radiation pattern of the antenna, geometrical optics to map the antenna radiation to the lens surface, and physical optics to calculate an arbitrary far field. We perform a thorough study of the beam properties, such as beam waist radius, phase center location and axial ratio by varying the diameter and extension of the lens, and misalignments of the antenna relative to the lens, at different operating frequencies. The simulation results are applied to the design and optimization of three different lenses for mixers to be operated at 1.4, 1.9, and 4.7 THz, respectively, which will be used in the heterodyne array receivers on board of NASA's balloon borne GUSTO observatory. The beam properties were verified experimentally by measuring the beam patterns in amplitude at multiple planes using a heterodyne technique. We found that the experimental results show good agreement with those from the simulations. Our work has delivered the mixers with the required beam characteristics for GUSTO.

Coaxial Beam Rotating HPM Mode Converter Using Dielectrics Plates

The incorporation of the dielectric plates to produce phase delay for propagating waves is a widely employed technique in phase shifters and mode converters. In this article, a compact dielectric plate loaded coaxial beam rotating (CBR) mode converter having high gain and co-polarized (CP) pattern has been proposed for high-power microwave (HPM) applications at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${X}$ </tex-math></inline-formula> -band. Here, the input transverse magnetic (TM), i.e., TM01 mode, is converted to CP transverse electric (TE), i.e., TE11 likeness mode. The proposed design consists of a circular waveguide (WG) loaded with 12 dielectric polytetrafluoroethylene (PTFE) plates in a radial-azimuthal direction. For 5-mm plate thickness, the proposed design yielded a reflection loss of −20.01 dB with a radiation efficiency of 98.34% and a 17.9 dB gain at 10.084 GHz. Also, the power handling capacity of the proposed structure is predicted as 3.20 GW. Furthermore, the mode conversion is verified by measuring the far-field radiation pattern. Besides, the simulation studies of the thermal behavior of the proposed design are also discussed with the aid of CST Studio Suite.

Bidirectional wide-angle waveguide grating antennas with flat-top far-field patterns for optical phased arrays.

To build advanced all solid-state LiDAR, optical phased arrays (OPAs) with a large field of view are highly desirable. As a critical building block, a wide-angle waveguide grating antenna is proposed here. Instead of aiming at the elimination of downward radiation of waveguide grating antennas (WGAs) to improve efficiencies, we in turn utilize the downward radiation and double the range of beam steering. In addition to widened field of views, the steered beams in two directions come from a common set of power splitters, phase shifters and antennas, which greatly reduces chip complexity and power consumption, especially for large-scale OPAs. Beam interference and power fluctuation in the far field due to downward emission can be decreased by specially designed SiO2/Si3N4 antireflection coating. The WGA exhibits balanced emissions in both the upward and downward directions, in which the field of view in each direction is more than 90°. The normalized intensity remains almost the same with a small variation of 10% from -39° to 39° for the upward emission and from -42° to 42° for the downward emission. This WGA is featured by a flat-top radiation pattern in far field, high emission efficiency and good tolerance to device fabrication errors. It holds good potential to achieve wide-angle optical phased arrays.

A Novel Low-Cost Compact High-Performance Flower-Shaped Radiator Design for Modern Smartphone Applications

This manuscript examines the design principle and real-world validation of a new miniaturized high-performance flower-shaped radiator (FSR). The antenna prototype consists of an ultracompact square metallic patch of 0.116λ0 × 0.116λ0 (λ0 is the free space wavelength at 3.67 GHz), a rectangular microstrip feed network, and a partial metal ground plane. A novel, effective, and efficient approach based on open circuit loaded stubs is employed to achieve the antenna's optimal performance features. Rectangular, triangular, and circular disc stubs were added to the simple structure of the square radiator, and hence, the FSR configuration was formed. The proposed antenna was imprinted on a low-cost F4B laminate with low profile thickness of 0.018λ0, relative permittivity εr = 2.55, and dielectric loss tangent δ = 0.0018. The designed radiator has an overall small size of 0.256λ0 × 0.354λ0. The parameter study of multiple variables and their influence on the performance results has been extensively studied. Moreover, the impact of different substrate materials, impedance bandwidths, resonance tuning, and impedance matching has also been analyzed. The proposed antenna model has been designed, simulated, and fabricated. The designed antenna exhibits a wide bandwidth of 5.33 GHz ranging from 3.67 to 9.0 GHz at 10 dB return loss, which resulted in an 83.6% fractional impedance bandwidth; a maximum gain of 7.3 dBi at 8.625 GHz; optimal radiation efficiency of 89% at 4.5 GHz; strong intensity current flow across the radiator; and stable monopole-like far-field radiation patterns. Finally, a comparison between the scientific results and newly published research has been provided. The antenna's high-performance simulated and measured results are in a good agreement; hence, they make the proposed antenna an excellent choice for modern smartphones' connectivity with the sub-6 GHz frequency spectrum of modern fifth-generation (5G) mobile communication application.

Thermally Triggered Multilevel Diffractive Optical Elements Tailored by Shape-Memory Polymers for Temperature History Sensors.

The morphological transitions induced by external stimuli in shape-memory polymers (SMPs) can be exploited with the real-time response of far-field diffraction patterns in diffractive optical elements (DOEs). In this paper, we combine the temperature characteristics of SMPs and the display characteristics of DOEs to obtain an optical temperature sensing film where the temperature information is taken as a change of far-field diffraction images. This process was achieved by imprinting the micropatterns of the DOEs on the epoxy-based SMP film, which can be programmed to hold a temporary optical image and revert to its original image upon exposure to a specific temperature. Furthermore, the specific temperature at which the image transformation occurs can be customized by varying the chain flexibility of the SMP. Based on a range of transition points, by imprinting the desired combination of SMP-DOEs on a film, a sensor that can record and inform the temperature history is demonstrated. As for the feasible application of this technique, it can be used for the compact and reliable optical temperature indicators, which can be applied in temperature-sensitive industries such as food and pharmaceuticals.

Including Radiation-Pattern Effects in Ground-Motion Models for Taiwan

ABSTRACT Most ground-motion models (GMMs) parameterize the earthquake source by magnitude, style of faulting, and source depth. GMMs are isotropic in terms of the source scaling for these three parameters. The radiation pattern, which leads to azimuthally varying source effects, is not included in most GMMs. As a result, any systematic radiation-pattern effects are treated as aleatory variability in the path terms in the GMMs, which does not make physical sense. We incorporate the far-field radiation pattern into GMMs for pseudospectral acceleration and Fourier amplitude spectra for the Taiwan region. A key issue is how to combine the radiation pattern from the SH and SV components for predicting the RotD50 amplitudes of ground motions used in GMMs. A suite of point-source simulations was generated for a range of focal mechanisms and site azimuths. Empirical models for the phase differences between the radial and transverse components were developed to constrain the between-component phase differences in simulations. The results show that the vector sum of the SH and SV far-field radiation patterns, FS, has a higher correlation with the RotD50 amplitudes of the ground motion than the arithmetic mean or the geometric mean. The radiation-pattern term is modeled by S0(M)+S1(M)ln(Fs+S2), in which S0(M) and S1(M) are linear functions of magnitude between M 4 and 6; they equal 0 for M &amp;gt; 6. The magnitude dependence reflects that, for large magnitudes and short distances, ground motions are more affected by the slip distribution and rupture timing than by the average radiation pattern from the subevents along the rupture, but the radiation pattern has a significant effect on the long-period ground-motion amplitude for small-magnitude events. Including radiation patterns into GMMs will improve estimates of 3D path effects from small-magnitude events by removing radiation-pattern effects from combined radiation-pattern and path terms used in current GMMs.

Optimizing Sensing Matrices for Spherical Near-Field Antenna Measurements

In this article, we address the problem of reducing the number of required samples for spherical near-field (SNF) antenna measurements by using compressed sensing (CS). A condition to ensure the numerical performance of sparse recovery algorithms is the design of a sensing matrix with low mutual coherence. Without fixing any part of the sampling pattern, we directly find sampling points that minimize the mutual coherence of the respective sensing matrix. Numerical experiments show that the proposed sampling scheme yields a higher recovery success in terms of phase transition diagram when compared to other known sampling patterns, such as the spiral and Hammersley sampling schemes. Furthermore, we also demonstrate that the application of CS with an optimized sensing matrix requires fewer samples than classical approaches to reconstruct the spherical mode coefficients (SMCs) and far-field pattern.

Experimental evaluation of Luneburg lens using radial holes for relative permittivity distribution

In this paper, we describe a novel 3-D Luneburg lens using radial holes. A Luneburg lens can suppress the side lobe of the antenna and increase the gain. The prototype Luneburg lens was fabricated using a 30mm radius dielectric sphere with uniform permittivity by 2.1 and many holes by 1mm radius, called radial holes drilled vertically on the sphere surface. The relative permittivity distribution of the Luneburg lens is realized by changing the number and depth of five types of holes. To confirm the effectiveness of the fabricated Luneburg lens, the far-field radiation pattern at 25GHz was measured using a flanged rectangular waveguide as a wave source. It achieved a high gain of 22.4dBi. The measured radiation pattern generally agreed well with the theoretical radiation pattern of the Luneburg lens, and the validity of the Luneburg lens with radial holes at the quasi-millimeter wave was confirmed. It can be used for beam steering in 5G/6G base stations.

Transverse Photonic Crystal Mode Engineering for Broad-Area Semiconductor Lasers With Narrow-Divergence Angles

Broad-area semiconductor lasers, coupled with asymmetric transverse photonic crystals (TPCs), are designed by the effective index method and the transfer matrix method. Due to the propagation constant of the fundamental mode lying in the forbidden band of the TPCs, but those of the high-order modes lying in the allowed band of the TPCs, the fundamental mode is mainly concentrated in the active ridge waveguide, while all the high-order modes extend into the lossy TPCs on both sides of the active ridge waveguide. Therefore, the fundamental mode possesses larger optical confinement factor than those of the high-order modes, and reaches the lasing threshold more easily, which is demonstrated by the single-lobe horizontal far-field pattern of the TPC broad-area laser under an injection current of 0.3 A near the threshold. The proposed TPC broad-area laser provides a new strategy of designing broad-area semiconductor lasers with narrow divergence angles.

Generation of orbital angular momentum multiplexing millimeter waves based on a circular traveling wave antenna.

Orbital angular momentum (OAM) has recently attracted extensive attention in the radio frequency domain due to its potential applications in various areas. In the OAM-based communication system, the development of the OAM-generating antennas lies at the heart of the matter to generate and receive vortex beams. In this work, a multiplexing/demultiplexing millimeter-wave OAM antenna based on the traveling-wave circular loop structure is proposed and experimentally demonstrated. The feeding networks are implemented using waveguide ports which are inherent integration in millimeter wave communication systems. A prototype with OAM states l = ±3 carried by the z polarization and l = ±2 for the x and y polarizations at 60 GHz is fabricated and measured. Measured near-field distributions and far-field radiation patterns show excellent agreement with the simulated ones. Furthermore, based on the designer strategy, four coaxially propagating waves with OAM modes l = ±3 and ±5 for the z polarization component and l = ±2 and ±4 for the x, y polarization components are investigated, respectively. The antenna will have a positive effect on the application potential of OAM-based wireless communication.

An inverse nano-cavity/-inhomogeneity problem with surface/interface effects in terahertz frequency range

In nondestructive evaluation, the maximum wavelength of the incident wave must be smaller than twice the perceptible obstacle size. Hence, in order to be able to detect a nanosize fiber of radius about a few nanometer, the frequency of the incident elastic wave should be in the order of 1012 Hz, i.e., in the terahertz frequency range. The present work addresses the possibility of application of elastic waves in terahertz frequency range to detect embedded circular nanosize inhomogeneity or nanocavity for which, respectively, the interface and surface effects are remarkable. Traditionally, singular sources method (SSM) and linear sampling method (LSM) have been utilized to treat inverse problems in the context of classical elasticity theory, where the surface/interface effects are not important. In the present paper these methods are extended to operate in the mathematical framework of surface/interface elasticity theory, suitable for capturing the surface/interface effects. In this framework the boundary conditions in contrast to the common Dirichlet and Neumann types involve the second derivative of the displacement field. The validity and uniqueness of the extended LSM and SSM in this framework will be established. An exact analysis emphasizing the importance of the consideration of surface/interface effects will also be given for the direct problem of SH waves scattering. In this work, the scatterer will be reconstructed solely based on data from the incident waves and the far-field pattern. Using the proposed theory, the size and location of the embedded nano-size cavity and nano-size inhomogeneity are detected. The surface effects of a nano-size cavity within a silver plate are more significant than the copper–silver interface effects in the case of a nano-size copper inhomogeneity embedded in a silver plate.

Active tuning of directional forward and backward scattering with metallic core/phase change shell nanoparticles

Phase change materials, such as GeTe that exhibits reversible phase transitions between amorphous and crystalline states, are promising for reconfigurable photonic devices. Here we propose a nanoparticle composed of Au core and GeTe shell that can realize active tuning of directional scattering by adjusting the crystallinities of the GeTe shell to manipulate the electric and magnetic Mie responses of the nanoparticle. The scattering, the far-field radiation patterns and the ratio of the forward (backward) to backward (forward) scattering cross section of the hybridized nanoparticle are calculated analytically with Mie theory. The theoretical results show that when the GeTe shell is switched from amorphous phase to crystalline phase, the phase differences between the overlapped electric and magnetic modes can be changed from 0 to π. As a result, the Kerker’s first and second conditions are satisfied simultaneously, leading to tunable forward and backward scattering in near-infrared wavelength. Our results are useful for designing reconfigurable optical nanoantennas that may find applications in compact photonic circuits and all-optical wireless communications.

Low-profile electronic beam-scanning metasurface antenna for Ka-band applications

This article proposes a low-profile real-time electronic beam-scanning metasurface antenna (EBSMA) with hybrid (active and passive) meta-cells in the Ka-band. The EBSMA includes a radiating source, single-layered metasurface with dedicated microcontroller unit, and a power source. The proposed metasurface comprises 4 active subarrays and 11 passive meta-cells, where each subarray comprises 2 × 3 active meta-cells. The metasurface was using a source patch antenna. The EBSMA was realized by integrating one PIN-diode onto each slotted square-ring element in each subarray. Two-dimensional beam scanning was realized by controlling PIN-diode states in real-time for each active subarray using microcontroller hardware. The proposed EBSMA configuration produced a boresight beam at θ = 0° or ±10° tilted beam in the φ = 0° and 90° planes. The antenna achieved 14.18% 3 dB fractional impedance bandwidth with measured realized gains ≈ 10.7 dBi for all states. We fabricated an EBSMA device as proof-of-concept and measured its far-field radiation patterns. Two-dimensional beam-scanning for the proposed metasurface antenna was verified numerically and experimentally.

Tomographic imaging of perfectly conducting objects.

In this paper, a new algorithm for tomographic imaging of perfectly conducting scatterers, with boundary conditions of the Dirichlet or Neumann type, is proposed. The boundary value problem is converted into a volume integral equation with a singular double-layer potential. Then, the resulting far-field pattern is expressed in the form of an impact parameter model, i.e.,as a true Fourier transform of the profile function. No approximations are made in the construction of the forward model and derivation of the inversion algorithm. Instead, some elementary facts of the microlocal analysis, in particular the pseudo-locality of the corresponding operator, are used to recover the support of the scattering potential and, therefore, the shape of the obstacle. Mathematically, the problem is reduced to the Radon inversion of a classical computed tomography. It is shown that the algorithm is also capable of classifying the type (Dirichlet or Neumann) of the boundary condition imposed. A relation of the proposed algorithm to a previously known solution based on the physical optics approximation is discussed.