Near-field Patterns Research Articles

Research on near‐field focusing algorithm of reconfigurable reflect‐array

In this study, the near-field beam focusing algorithm of reconfigurable reflect array for security imaging application is proposed. The proposed near-field focusing algorithm is based on classic array method, which is usually adopted for far-field pattern calculation. In order to satisfy the near-field imaging application, the imaging distance is added to array method, and the improved formula can calculate the shifting phase of every array element precisely. In addition, due to discrete phase characteristic of reconfigurable reflect array, a phase choice method based on trigonometric function is used in authors’ design. Based on the proposed algorithm, the shifting phase of every element is calculated and obtained the simulated beam focusing patterns. Meanwhile, the calculated shifting phase is written to whole array by field-programmable gate array, the near-field beam scanning patterns is measured and the measured results are in good agreement with the simulated ones.

Tilted-potential photonic crystal cavities for integrated quantum photonics.

We propose and investigate a new type of photonic crystal (PhC) cavity for integrated quantum photonics, which provides tailored optical modes with both confined and extended spatial components. The structures consist of elongated PhC cavities in which the effective index of refraction is varied quasi-linearly along their axis, implemented by systematic lateral shifts of the PhC holes. The confined modes have approximately Airy-function envelopes, exhibiting single peaks and extended tails, which is useful for optimizing single photon extraction and transmission in integrated quantum photonic devices. The measured spectrally resolved near-field patterns of such devices show the expected spatial and resonance wavelength behavior, in agreement with numerical simulations of the Airy-Bloch modes. The effects of fabrication-induced disorder on the mode features are also analyzed and discussed. Selective excitation of specific Airy-Bloch modes using integrated, site-controlled quantum dots as localized light sources is demonstrated. Based on the tilted-potential cavity, multiple-QD single photon emitters exploiting wavelength division multiplexing are proposed.

Multichannel direct transmissions of near-field information

A digital-coding programmable metasurface (DCPM) is a type of functional system that is composed of subwavelength-scale digital coding elements with opposite phase responses. By configuring the digital coding elements, a DCPM can construct dynamic near-field image patterns in which the intensity of each pixel of the image can be dynamically and independently modulated. Thus, a DCPM can perform both spatial and temporal modulations. Here, this advantage is used to realize multichannel direct transmissions of near-field information. Three points are selected in the near-field region to form three independent channels. By applying various digital phase codes on the DCPM, independent binary digital symbols defined by amplitude codes (namely, weak and strong amplitudes) are transmitted through the three channels. The measured near-field distributions and temporal transmissions of the system agree with numerical calculations. Compared with the conventional multichannel transmission, the proposed mechanism achieves simultaneous spatial and temporal modulations by treating DCPM as an energy radiator and information modulator, thereby enduing DCPM with high potential in near-field information processing and communications.

Scattering of surface modes propagating along metamaterial interface

The problem of surface mode scattering from an iris diaphragm is studied by the integral equation technique. The theory is illustrated by the example of scattering of mode, propagating along metamaterial interface. The characteristics of radiation modes excited by the diaphragm are considered. The surface mode reflection and transmission coefficients, the near-field distribution and far-field radiation pattern are calculated.

Deep learning enabled superfast and accurate M2 evaluation for fiber beams.

We introduce deep learning technique to predict the beam propagation factor M2 of the laser beams emitting from few-mode fiber for the first time, to the best of our knowledge. The deep convolutional neural network (CNN) is trained with paired data of simulated near-field beam patterns and their calculated M2 value, aiming at learning a fast and accurate mapping from the former to the latter. The trained deep CNN can then be utilized to evaluate M2 of the fiber beams from single beam patterns. The results of simulated testing samples have shown that our scheme can achieve an averaged prediction error smaller than 2% even when up to 10 eigenmodes are involved in the fiber. The error becomes slightly larger when heavy noises are added into the input beam patterns but still smaller than 2.5%, which further proves the accuracy and robustness of our method. Furthermore, the M2 estimation takes only about 5 ms for a prepared beam pattern with one forward pass, which can be adopted for real-time M2 determination with only one supporting Charge-Coupled Device (CCD). The experimental results further prove the feasibility of our scheme. Moreover, the method we proposed can be confidently extended to other kinds of beams provided that adequate training samples are accessible. Deep learning paves the way to superfast and accurate M2 evaluation with very low experimental efforts.

Electron-beam excitation of supermodes of a photonic molecule built on twin high refractive index dielectric nanowires

We study the radiation that occurs if a modulated beam of electrons flows between two identical high-index dielectric nanowires, which form a photonic molecule, in the visible wavelength range. The electromagnetic field of such a two-dimensional beam has the shape of the surface wave propagating along its trajectory. This wave induces the polarization and surface currents on the nearby obstacles, and hence, radiation occurs even if the beam does not touch the obstacle. Here, a pair of dielectric nanowires behaves as optically coupled open resonators, thanks to which the diffraction radiation is enhanced near the wavelengths of the natural modes. As known, the latter are so-called supermodes built on the modes of each wire, with the account of two-fold symmetry of the pair. To solve accurately the scattering problem, we use a semianalytical technique based on the Fourier expansions in the local coordinates of each wire and the addition theorems for the cylindrical functions. This leads to the efficient code having mathematically guaranteed convergence. We compute spectral characteristics of the diffraction radiation, analyze their dependences on the electron-beam parameters, and visualize the near-field and far-field patterns. A new form of the optical theorem adapted to the modulated electron-beam excitation is derived and exploited. Possible applications of the studied effects in the design of optical beam position monitors are discussed.

Two-dimensional beam steering analysis in a phase-coupled vertical cavity surface emitting laser array

Electronically controlled optical beam steering is sensitive and reliable. It is significant for phased laser radar applications. We reported the experimental characterization of three-element coherent vertical cavity surface emitting laser arrays and demonstrated two-dimensional electronical beam steering. 2-D far-field patterns, near-field patterns, and the spectra of the beam under different current conditions were measured. It is demonstrated that beam steering is continuous and predicted. Above 17.9% of the total power of the array is concentrated in the central lobe, showing high coupling efficiency. Up to a 3.26° shift angle from normal is demonstrated. The beam steering mechanism of frequency detuning between the three cavities was analyzed via both theory and experiment.

Planar Aperiodic Arrays as Metasurfaces for Optical Near-Field Patterning

Plasmonic metasurfaces have spawned the field of flat optics using nanostructured planar metallic or dielectric surfaces that can replace bulky optical elements and enhance the capabilities of traditional far-field optics. Furthermore, the potential of flat optics can go far beyond far-field modulation and can be exploited for functionality in the near-field itself. Here, we design metasurfaces based on aperiodic arrays of plasmonic Au nanostructures for tailoring the optical near-field in the visible and near-infrared spectral range. The basic element of the arrays is a rhomboid that is modulated in size, orientation, and position to achieve the desired functionality of the micron-size metasurface structure. Using two-photon-photoluminescence as a tool to probe the near-field profiles in the plane of the metasurfaces, we demonstrate the molding of light into different near-field intensity patterns and active pattern control via the far-field illumination. Finite element method simulations reveal that the near-field modulation occurs via a combination of the plasmonic resonances of the rhomboids and field enhancement in the nanoscale gaps in between the elements. This approach enables optical elements that can switch the near-field distribution across the metasurface via wavelength and polarization of the incident far-field light and provides pathways for light matter interaction in integrated devices.

EHF Band Linear Antenna Array Based on Surface Wave Transformation

The experience of using of experimental samples of diffraction radiation antennas shows the need to expand the list of options and modifications of the technical solutions for such antennas. One of variants is the modification of a linear dielectric waveguide for rigidly fixation of the dielectric rod. H-shaped surface-wave waveguides in the form of combination of dielectric rod and rigid metal elements have been proposed for a basic element of diffraction radiation antennas designed for operation in conditions of increased mechanical loads and vibrations at frequencies above 80 GHz. The results of experimental studies of the near-field distribution, radiation patterns, gain and energy losses of the antenna are presented. The obtained results show the effectiveness of the implemented constructive approach. The configuration of the linear antenna grating of diffraction radiation with a modified dielectric waveguide is proposed, which can be applied for the development of scanning antennas in the 80–100 GHz frequency range.

Indium Arsenide-Based Spontaneous Emission Sources (Review: a Decade Later)

The results of investigations of light-emitting diodes based on heterostructures with an InAs active region grown by liquid phase and metalorganic vapor-phase epitaxy over the last decade are reviewed. The near-field pattern, L–I and I–V characteristics, and quantum efficiency of point-contact and flip-chip light-emitting diodes are analyzed in a wide temperature range.

Synthesis Algorithm for Near-Field Power Pattern Control and Its Experimental Verification via Metasurfaces

Antennas and metasurfaces have enabled a number of far-field manipulation functions. According to the addition theorem of multipoles, near fields have richer spatial spectra than far fields; hence, in the near-field region, it is easier to achieve complex manipulations on field distributions. In this paper, an algorithm is presented to synthesize the near fields of source arrays. Given target distributions of near-field intensities and also predefined source magnitudes, the algorithm will find the needed source phases, which can then be applied on active antenna arrays and passive metasurfaces to induce the target distribution of fields. The algorithm adopts the dyadic Green's function as the propagator, and hence, it naturally takes account of the near-field and vector properties of electromagnetic fields. As a typical application of the algorithm, an example is given to obtain the excitation phases of a dipole array, which is then physically imitated by a coding metasurface. Experimental measurement is performed and the result proves the validity of the algorithm. From the perspective of information metasurface, the algorithm finds the phase-coding pattern of metasurface to achieve specific functions.

Comprehensive analysis of photonic-crystal surface-emitting lasers via time-dependent three-dimensional coupled-wave theory

We develop a time-dependent three-dimensional coupled-wave theory (3D-CWT) for the transient analysis of photonic-crystal surface-emitting lasers (PCSELs). Our model takes into account the temporal evolution of both the photon and carrier distribution inside PCSELs, which enable the analysis of various above-threshold lasing characteristics including the relaxation oscillation, spatial hole burning, and multimode lasing. With the developed time-dependent 3D-CWT, we perform transient analysis of the high-power, high-beam-quality PCSELs with double-lattice photonic crystals and reproduce the experimental results of near-field patterns and lasing spectra under high current injection. Our theory enables the comprehensive understanding of the device physics of PCSELs toward the realization of higher-power continuous-wave lasing and short-pulse lasing.

Источники спонтанного излучения на основе арсенида индия (обзор: десять лет спустя)

AbstractThe results of investigations of light-emitting diodes based on heterostructures with an InAs active region grown by liquid phase and metalorganic vapor-phase epitaxy over the last decade are reviewed. The near-field pattern, L – I and I – V characteristics, and quantum efficiency of point-contact and flip-chip light-emitting diodes are analyzed in a wide temperature range.

Generalized Rayleigh-Sommerfeld Diffraction Theory for Metasurface-Modulating Paraxial and Non-Paraxial Near-Field Pattern Estimation

In this paper, different diffraction theories for estimating the diffraction field patterns modulated by metasurfaces are firstly revisited. Further reformulation of these theories is performed to better reveal their inherent mechanisms and differences. To compute the metasurface-modulating paraxial and/or non-paraxial diffraction field patterns within the near-field region, including the evanescent area, a universal pattern-propagation Eigenfactor is introduced to generalize Rayleigh-Sommerfeld diffraction theory. To investigate its applicability and accuracy, a representative monofocal metasurface with an ultrahigh numerical aperture of 0.96, together with two coplanar and non-coplanar multifocal holographic metasurfaces, are constructed as illustrative examples. Their near-field patterns are calculated by the generalized Rayleigh-Sommerfeld (GRS) diffraction integral and compared with those extracted by the finite-different time-domain full wave analysis, generalized Huygens-Fresnel principle, and Huygens's Principle. It is demonstrated that within the near-field region including the non-paraxial and evanescent area, the GRS diffraction integral provides the best and satisfactory agreement with the full wave simulation, and thus offers a more accurate and efficient tool for quantitative analysis and iterative optimization.

Modal selectivity at 850 nm employing standard single-mode couplers: Theory and experimental demonstration

Abstract In this paper, we evaluate experimentally and analyze theoretically and by simulation the demultiplexing of LP01 and LP11 modes in a 850 nm few mode optical link when a standard single-mode coupler theoretically designed for 1550 nm operation is used. The simulation analysis indicates a feasible conversion efficiency higher than 98% for the LP01 mode and 100% efficiency for LP11 mode. These results are experimentally validated considering different off-the-shelf optical couplers configured in 99:1, 90:10, 80:20 and 60:40 coupling ratios. Experimental analysis of the optical couplers demonstrate adequate functionality in correspondence with the theoretical analysis, exhibiting a good near-field pattern and modal profile for the LP01 and LP11 modes. The mode purity observed was 98.4% for the LP01 mode, and higher than 90% for the LP11 mode. The 90:10 optical coupler configuration shows best mode purity with the minimal insertion losses.

Machine Learning Based Source Reconstruction for RF Desense

In radio frequency interference study, equivalent dipole moments are widely used to reconstruct real radiation noise sources. Previous reconstruction methods, such as least square method (LSQ) and optimization method are affected by parameter selections, such as number and locations of dipole moments and choices of initial values. In this paper, a new machine learning based source reconstruction method is developed to extract the equivalent dipole moments more accurately and reliably. Based on the near-field patterns, the proposed method can determine the minimal number of dipole moments and their corresponding locations. Furthermore, the magnitude and phase for each dipole moment can be extracted. The proposed method can extract the dominant dipole moments for the unknown noise sources one by one. The proposed method is applied to a few theoretical examples first. The measurement validation using a test board and a practical cellphone are also given. Compared to the conventional LSQ method, the proposed machine learning based method is believed to have a better accuracy. Also, it is more reliable in handling noise in practical applications.

Light absorption by surface nanoholes and nanobumps

This paper deals with a numerical investigation of the energy deposition induced by ultrafast laser interaction with nanostructures. We calculate and analyze the intensity near-field reactive and radiative patterns resulted from the interference of the incident light with light scattered by individual subwavelength holes and bumps on the surface of metallic and dielectric materials. The role of light polarization, optical material properties, collective effects and nature of the imperfections in localized energy absorption is elucidated. The results open new perspectives in precise light manipulation by surface inhomogeneities and well-controlled surface nanostructuring by ultrashort laser.

Near-Field Pattern Synthesis for Sparse Focusing Antenna Arrays Based on Bayesian Compressive Sensing and Convex Optimization

An effective method based on Bayesian compressive sensing (BCS) and convex optimization for near-field sparse array synthesis is presented in this paper. An algorithm to generate reference-shaped beams in the near-field region with controllable sidelobe levels is first proposed. Then, the multitask BC is modified and generalized to synthesize a near-field sparse array radiating a desired near-field pattern with the co-polarization component. After that, a postprocessing of the final array excitation is employed to put constraints on the minimum element spacing to make the sparse layout practicable. The degradation of the near-field pattern is mitigated through reestimating the array excitation by a convex optimization. Three numerical examples show the effectiveness of the proposed method with more than 50% of elements saved compared to the uniformly distributed layout. The comparison to the result obtained by a full-wave simulator FEKO is also presented to demonstrate the validity of this method considering strong antenna mutual couplings.

Auto-alignment of X-ray focusing mirrors with speckle-based at-wavelength metrology.

Significant improvements have been made in the fabrication of diffraction-limited X-ray optics used to pursue an aberration-free wavefront. Alignment of these optics plays a crucial role in the resultant beam quality. Here, we present a simple and fast alignment method based on imaging X-ray near-field speckle patterns, with experimental demonstration using a pair of Kirkpatrick-Baez mirrors. The proposed technique has the potential to be an alternative to conventional methods. It loosens the stringent demand for high-resolution scanning stages compared to conventional knife-edge scan and, hence, can be applied to nano-focusing optics. The flexibility and straightforward implementation of the method allow it to be applied to a wide range of experiments at synchrotron facilities and laboratory-based sources.

Mode Controlling of Surface Plasmon Polaritons by Geometric Phases

Metasurfaces are made of two-dimensional arrays of subwavelength nanostructures that form a spatially varying optical response, to control the wave fronts of optical waves. As the feature size of its constituent materials is nanoscale, investigation of the light-nanostructure interactions in the near field is critical for understanding the novel properties of metasurfaces. Here, we used a scanning near-field optical microscope (SNOM) to observe the near-field distribution of surface plasmon polaritons (SPPs) from a ring-shaped metasurface under illumination of circularly polarized light. It was found that with an additional degree of freedom of the geometric phase provided by the regularly arranged metamolecules, control over the near-field interference of the SPPs can be achieved, which is governed by the metasurface geometric symmetry that can be tuned by its topological charge. Meanwhile, the planar chiral character of the metamolecules exerts a deep influence on the near-field interference patterns. Our results can pave the way for active control of SPP propagation in near fields and have potential applications in highly integrated optical communication systems.