Tunable topological edge and corner states in an all-dielectric photonic crystal.

Topological photonics has become a new and fascinating area in recent years, which enables electromagnetic waves to propagate with negligible backscattering and excellent robustness even when encountering sharp corners or defects. But the flexible tunability of edge and corner states is challenging once the topological photonic crystals (PhCs) have been fabricated. In this paper, we propose a new all-dielectric PhC with C3 symmetry constructed by hexagonal array of petal-like aperture embedded in silicon background. The proposed configuration has much wider energy gap than its triangular counterpart, and hence is suitable for wideband and high-capacity applications. When the apertures are filled with liquid crystals (LCs), the topologically-protected edge and corner states can be regulated through changing the refractive index of the LCs under different bias voltages. Moreover, the robustness of topological protection of edge and corner states is further demonstrated. This is the first demonstration of LC based tunable valley higher-order photonic topological insulator. The tunability of the proposed topological PhCs may be beneficial for development of tunable optical waveguides, reconfigurable topological microcavities, and other intelligent topological optical/terahertz devices.

Method to obtain the initial value for the inverse design in nanophotonics based on a time-reversal technique.

In the inverse design of nanophotonic devices, mathematical optimization methods are generally used to perform local optimization in the design region to obtain the physical structure that meets design expectations. These methods usually produce good structures. However, due to the lack of physical considerations, most of the inverse design methods for nanophotonic devices use random initial topology as the initialization for optimization, which will inevitably cause a waste of computing resources. In this Letter, we propose a method based on a time-reversal technique to quickly determine the induced source of the physical structure in the design region and, thus, obtain the initial topological structure of the nanophotonic devices. For a nanophotonic 90°-bend waveguide and 90°-bend power splitter waveguide, numerical examples show that the initial topology obtained by our method not only has good initial performance, but also can be used as a reasonable initialization for inverse design.

Axial localization using time reversal multiple signal classification in optical scanning holography.

This paper presents a method to identify the axial location of targets in an optical scanning holography (OSH) system. By combining time reversal (TR) technique with the multiple signal classification (MUSIC) method in OSH, axial location can be detected with high resolution. Both simulation and experimental work have been carried out to verify the feasibility of the proposed work.

New autofocus and reconstruction method based on a connected domain.

In this Letter, we propose a new method for auto-focusing and reconstruction without defocus noise in optical scanning holography. By using a connected domain (CD) to calculate the area of different domains, which are labeled by a connected component, the focus distance can be found via the smallest area of each CD. Meanwhile, the sectional images without defocus noise can also be reconstructed based on the labeled domains. The effectiveness of this method has been verified with a simulation and experiments.

Performance Comparison with Different Antenna Properties in Time Reversal Ultra-Wideband Communications for Sensor System Applications.

The complexity reduction of receivers in ultrawideband (UWB) communication when time reversal (TR) technique is applied makes it suitable for low-cost and low-power sensor systems. Larger antenna dispersion can generally lead to a less stable phase center and will increase the interference in UWB communications based on pulse radio, whereas a higher antenna gain will result in higher channel gain and further larger channel capacity. To find out the trade-off between antenna gain and dispersion, we performed the channel measurements using different antennas in a dense multipath environment and established the distribution of channel capacities based on the measured channel responses. The results show that the capacity loss caused by antenna dispersion cannot be compensated by antenna gain with line-of-sight transmission to some extent, the effect of phase center on the communication system is negligible, and antennas with smaller time dispersion will have a better energy focusing property and anti-interference performance in TR systems.

Compact Reconfigurable Antenna with an Omnidirectional Pattern and Four Directional Patterns for Wireless Sensor Systems.

A compact reconfigurable antenna with an omnidirectional mode and four directional modes is proposed. The antenna has a main radiator and four parasitic elements printed on a dielectric substrate. By changing the status of diodes soldered on the parasitic elements, the proposed antenna can generate four directional radiation patterns and one omnidirectional radiation pattern. The main beam directions of the four directional modes are almost orthogonal and the four directional beams can jointly cover a 360° range in the horizontal plane, i.e., the main radiation plane of omnidirectional mode. The whole volume of the antenna and the control network is approximately 0.70 λ × 0.53 λ × 0.02 λ, where λ is the wavelength corresponding to the center frequency. The proposed antenna has a simple structure and small dimensions under the requirement that the directional radiation patterns can jointly cover the main radiation plane of the omnidirectional mode, therefore, it can be used in smart wireless sensor systems for different application scenarios.

Defocus noise suppression with combined frame difference and connected component methods in optical scanning holography.

Random phase masks can transform the defocus noise into a speckle-like pattern in optical scanning holography (OSH). In this Letter, we presented a speckle reduction based on combined frame difference and connected component method in a random phase-coded OSH system. The image quality of the reconstructed sections is improved with better visibility.

Far-field detection of sub-wavelength Tetris without extra near-field metal parts based on phase prints of time-reversed fields with intensive background interference.

Time-reversal (TR) phase prints are first used in far-field (FF) detection of sub-wavelength (SW) deformable scatterers without any extra metal structure positioned in the vicinity of the target. The 2D prints derive from discrete short-time Fourier transform of 1D TR electromagnetic (EM) signals. Because the time-invariant intensive background interference is effectively centralized by TR technique, the time-variant weak indication from FF SW scatterers can be highlighted. This method shows a different use of TR technique in which the focus peak of TR EM waves is unusually removed and the most useful information is conveyed by the other part.

Principle and performance analysis of coherent tracking sensor based on local oscillator beam nutation.

By integrating coherent tracking sensor functionality into a coherent communication receiver, a dual-function receiver with coherent boresight error sensing is developed for tracking in coherent free-space optical communication systems. The sensor principles are analyzed according to optical interference theory, and the boresight error detection algorithm and small signal linear model are derived. Analysis of local-oscillator beam nutation on system performance shows that the best nutation half-angle is 0.5-1 µrad, the noise equivalent angle is less than 0.02 µrad, and the communication sensitivity degradation is 0.2-0.6 dB. This technology opens new avenues for free-space optical communication system design.

Polycentric spatial focus of time-reversal electromagnetic field in rectangular conductor cavity.

Polycentric focus effect of time-reversal (TR) electromagnetic field is found in a rectangular resonant cavity. Theoretical deduction shows that the effect is due to the mirror symmetry of the cavity and the maximum number of focus points is 27 including 1 main focus point and 26 secondary focus points. A case of 6 focus points is calculated, in which the numerical results are consistent with the theoretical predictions, and particularly the 5 secondary focus points have directly resulted in inaccurate imaging and pulse signal interception.

Four-domain dual-combination operation invariance and time reversal symmetry of electromagnetic fields.

Current experimental investigations about time reversal (TR) electromagnetic (EM) fields always depended on TR mirror (TRM). However, EM fields can perform reversal operation invariance in four domains connected by Fourier Transform. A multiplication table and an appropriate operating figure about EM fields' invariance are derived to describe a series of dual combined operations in the four domains, in which there are at least 10 dual-combination operations different from current TRM operations which can equivalently actualize TR symmetry. Theoretically, centrosymmetric operations of spatial position vector and spatial spectrum vector may have the potential to promote different reversal mirrors.

Publisher Correction: Planar array with bidirectional elements for tunnel environments.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Wide-angle scanning planar array with quasi-hemispherical-pattern elements.

A compact quasi-hemispherical-pattern antenna, two linear wide-angle scanning arrays, and a planar wide-angle scanning array are proposed. To increase the field-of-view scanning range of arrays, a compact low-profile antenna with a quasi-hemispherical pattern is introduced firstly. When the ground is infinite, the proposed antenna has a quasi-hemispherical pattern, i.e., an approximately uniform radiation in the upper half space. Afterward, two linear arrays arranged along the length and width of the proposed antenna's radiation patch are presented. The main beams of the two arrays with 16 active elements can scan from less than -75° to more than +75°. When the linear array element number is 128, the maximum scanning angle can reach 86°. At last, a planar array with 16 × 16 active elements is proposed. In two special planes, xz plane and yz plane, the main beams of the planar array can scan from less than -77° to more than +77° with a gain fluctuation less than 5 dB and a maximum side lobe level (SLL) less than -10 dB. An excellent wide-angle-scanning performance both in linear and planar arrays can be obtained using the proposed method.

Planar array with bidirectional elements for tunnel environments.

The growing demands for wireless communications in tunnel environments are driven by requests for maintaining uninterrupted internet access for users in tunnels as well as wireless connections for wireless sensors, security, and control networks. Nevertheless, wireless networks in tunnel environments are associated with technical challenges related to elongated wireless coverage in two opposite near-end-fire directions. Here, we introduce a low-profile bidirectional antenna that can be mounted on a large ground plane and that has a dual-magnetic-current mode exhibiting radiation patterns with 3-dB beamwidth coverage at near-end-fire angles. Furthermore, we realized a planar array with eight such bidirectional antennas that were configured as a sparse array in order to reduce the commonplace issues of mutual coupling and grating lobes. The radiation beams of the proposed antenna array can scan in the end-fire area (60° < φ < 120°, 45° < θ < 80°), with scanning gains of 11~15 dB in the near-end-fire directions. In addition, we demonstrate that the proposed array can adaptively generate a directional single beam or multiple beams according to the positions of users, which is suitable for intelligent communication systems with low power consumptions and high communication bandwidths in tunnel environments.

Far-field subwavelength imaging with near-field resonant metalens scanning at microwave frequencies.

A method for far-field subwavelength imaging at microwave frequencies using near-field resonant metalens scanning is proposed. The resonant metalens is composed of switchable split-ring resonators (SRRs). The on-SRR has a strong magnetic coupling ability and can convert evanescent waves into propagating waves using the localized resonant modes. In contrast, the off-SRR cannot achieve an effective conversion. By changing the switch status of each cell, we can obtain position information regarding the subwavelength source targets from the far field. Because the spatial response and Green's function do not need to be measured and evaluated and only a narrow frequency band is required for the entire imaging process, this method is convenient and adaptable to various environment. This method can be used for many applications, such as subwavelength imaging, detection, and electromagnetic monitoring, in both free space and complex environments.