2021-12-24 PIER Vol. 172, 33-40. doi:10.2528/PIER21111602

Non-Hermitian Skin Effect and Delocalized Edge States in Photonic Crystals with Anomalous Parity-Time Symmetry

Qinghui Yan, Hongsheng Chen, and Yihao Yang

Non-Hermitian skin effect denotes the exponential localization of a large number of eigen-states at boundaries in a non-Hermitian lattice under open boundary conditions. Such a non-Hermiticity-induced skin effect can offset the penetration depth of in-gap edge states, leading to counterintuitive delocalized edge modes, which have not been studied in a realistic photonic system such as photonic crystals. Here, we analytically reveal the non-Hermitian skin effect and the delocalized edge states in Maxwell's equations for non-Hermitian chiral photonic crystals with anomalous parity-time symmetry. Remarkably, we rigorously prove that the penetration depth of the edge states is inversely proportional to the frequency and the real part of the chirality. Our findings pave a way towards exploring novel non-Hermitian phenomena and applications in continuous Maxwell's equations.

2021-12-20 PIER Vol. 172, 13-22. doi:10.2528/PIER21102002

Tunable Topological Refractions in Valley Sonic Crystals with Triple Valley Hall Phase Transitions (Invited Paper)

Ding Jia, Yin Wang, Yong Ge, Shou-Qi Yuan, and Hong-Xiang Sun

Topological refractions created by valley sonic crystals (VSCs) have attracted great attentions in the communities of physics and engineering owing to the advantage of zero reflection of sound and the potential for designing advanced acoustic devices. In previous works, topological refractions of valley edge states are demonstrated to be determined by the projections of the valleys K and K′, and two types of topological refractions generally exist at opposite terminals or different frequency bands. However, the realization of tunable topological refractions at the fixed frequency band and terminal still poses great challenge. To overcome this, we report the realization of tunable topological refractions by VSCs with triple valley Hall phase transitions. By simply rotating rods, we realize 3 types of topological waveguides (T1, T2 and T3) composed of two VSCs, in which the projections of the observed valley edge states can be modulated between K and K′. Additionally, based on the measured transmittance spectra, we experimentally demonstrate that these valleyedge states are almost immune to backscattering against sharp bends. More importantly, we realize tunable topological refractions at the fixed frequency band and terminal, and experimentally observe the coexistence of positive and negative refractions for T1 and T3, and negative refractions for T2. The proposed tunable topological refractions have potential applications in designing multi-functional sound antennas and advanced communication devices.

2020-12-31 PIER Vol. 169, 117-127. doi:10.2528/PIER20120701

A Parameter-Free Calibration Process for a Scheimpflug LIDAR for Volumetric Profiling

Longqiang Luo, Xiang Chen, Zhanpeng Xu, Shuo Li, Yaoran Sun, and Sailing He

Scheimpflug LIDAR has attracted considerable attention in the recent years, and has been widely applied in many fields due to its infinite depth of field. In this study, we reconstruct a series of formulas to demonstrate the Scheimpflug principles, with reference at the hinge point. These formulas based on directly measurable parameters are simple in form. Base on this, we report a new calibration for the Scheimpflug system, without measuring the instrument parameters. We also confirm that the result of calibration is accordance with the actual setting of the system. To take full advantage of the infinite depth of field of the Scheimpflug system, we have designed and carried out the system, combining with a rotary stage, to obtain the entire volumetric profile for a target of interest in a cycle rotation. To the best of our knowledge, this is the first time Scheimpflug system is utilized to perform a three-dimensional volumetric profile measurement.

2020-12-31 PIER Vol. 169, 87-101. doi:10.2528/PIER20112801

Electromagnetic-Circuital-Thermal Multiphysics Simulation Method: a Review (Invited)

Huan Huan Zhang, Pan Pan Wang, Shuai Zhang, Long Li, Ping Li, Wei E. I. Sha, and Li Jun Jiang

Electromagnetic-circuital-thermal multiphysics simulation is a very important topic in the field of integrated circuit (IC), microwave circuits, antennas, etc. This paper gives a comprehensive review of the state of the art of electromagnetic-circuital-thermal multiphysics simulation method. Most efforts were focused on electromagnetic-circuital co-simulation and electromagnetic-thermal co-simulation. A brief introduction of related theory like governing equations, numerical methods, and coupling mechanisms is also included.

2020-12-31 PIER Vol. 169, 73-86. doi:10.2528/PIER20102202

Distinguishing Bipolar Depression from Major Depressive Disorder Using fNIRS and Deep Neural Network

Tengfei Ma, Hailong Lyu, Jingjing Liu, Yuting Xia, Chao Qian, Julian Evans, Weijuan Xu, Jianbo Hu, Shaohua Hu, and Sailing He

A variety of psychological scales are utilized at present as the most important basis for clinical diagnosis of mood disorders. An experienced psychiatrist assesses and diagnoses mood disorders based on clinical symptoms and relevant assessment scores. This symptom based clinical criterion is limited by the psychiatrist's experience. In practice, it is difficult to distinguish the patients with bipolar disorder with depression episode (bipolar depression, BD) from those with major depressive disorder (MDD). The functional near-infrared spectroscopy (fNIRS) technology is commonly used to perceive the emotions of a human. It measures the hemodynamic parameters of the brain, which correlate with cerebral activation. Here, we propose a machine learning classification method based on deep neural network for the brain activations of mood disorders. Large time scale connectivity is determined using an attention long short term memory neural network and short-time feature information are considered using the InceptionTime neural network in this method. Our combined method is referred to as AttentionLSTM-InceptionTime (ALSTMIT). We collected fNIRS data of 36 MDD patients and 48 BD patients who were in the depressed state. All the patients were monitored by fNIRS during conducting the verbal fluency task (VFT). We trained the model with the ALSTMIT network. The algorithm can distinguish the two types of patients effectively: the average accuracy of classification on the test set can reach 96.2% stably. The classification can provide an objective diagnosis tool for clinicians, and this algorithm may be critical for the early detection and precise treatment for the patients with mood disorders.

2020-12-31 PIER Vol. 169, 103-115. doi:10.2528/PIER20062703

Wideband RCS Reduction of High Gain Fabry-Perot Antenna Employing a Receiver-Transmitter Metasurface

Peng Xie, Guang-Ming Wang, Hai-Peng Li, Ya-Wei Wang, and Binfeng Zong

This paper presents a high gain Fabry-Perot antenna with radar cross section (RCS) reduction property. A receiver-transmitter metasurface is designed and used as the partially reflective surface (PRS) of the antenna to realize high gain and wideband RCS reduction. Firstly, the working principle of the unit cell is similar to the reception and radiation of two patch antennas. The unit cell is designed to present high reflectivity through tuning the impedance matching between two patches. This can ensure that the antenna obtains high gain. Then, the ground plane in the middle makes the reflection phase from different sides of the unit cell be tuned independently. Two unit cells with same reflection phase from the bottom side and 180° reflection phase difference from the top side are obtained through tuning the size of the transmitter patch. With the improved chessboard arrangement of these two unit cells, the incident wave can be scattered into many directions. So the metasurface presents a good RCS reduction property. More importantly, thanks to the high reflectivity of the metasurface, almost all the electromagnetic waves from the outside are reflected and rarely enter the cavity. Therefore, the antenna achieves good in band RCS reduction. The measured results of the fabricated antenna agree well with the simulated ones, which verify the correctness of the design. The antennas reaches the maximum gain of 18.2 dBi at 10 GHz. Wideband RCS reduction and good in band RCS reduction are also obtained by the antenna.

2020-12-30 PIER Vol. 169, 59-71. doi:10.2528/PIER20120401

A Review of Algorithms and Hardware Implementations in Electrical Impedance Tomography (Invited)

Zheng Zong, Yusong Wang, and Zhun Wei

In recent years, electrical impedance tomography (EIT) has attracted intensive interests due to its noninvasive, ionizing radiation-free, and low-cost advantages, which is promising for both biomedical imaging and industry nondestructive tests. The purpose of this paper is to review state-of-the-art methods including both algorithms and hardwares in EIT. More specifically, for the advanced reconstruction algorithms in mainstream, we offer some insights on classification and comparison. As for the measurement equipment, the structure, configuration modes, and typical systems are reviewed. Furthermore, we discuss the limitations and challenges in EIT technique, such as low-spatial resolution and nonlinear-inversion problems, where future directions, such as solving EIT problems with deep learning, have also been addressed.

2020-12-12 PIER Vol. 169, 45-57. doi:10.2528/PIER20050801

Rotman Lens Design with Wideband DRA Array

Mohammad Ranjbar Nikkhah, Manish Hiranandani, and Ahmed A. Kishk

For rapid Rotman lens design, the symmetry plane is utilized to reduce the structure size by employing the odd and even mode characteristics. Solutions of half the structure for odd and even modes (short and open walls or electrical and magnetic walls, respectively) are much more efficient than the one-time solution for the whole structure. Then, s-parameters from both solutions are processed to obtain the s-parameters of the full lens. To support the wideband and wide scanning range, DRA array is used because of its ability to support these characteristics. Two examples are considered. The first example that employs four cylindrical DRA elements is built and measured to test the concept of terminating the dummy ports by absorbing materials instead of matching loads. This termination tremendously simplifies the structure and reduces the cost by saving the terminating connectors and the matching loads. Here, thin planar absorbing material is used on top of the microstrip lines of the dummy ports. The simulated and measured results are in good agreement. The second example utilizes 8 rectangular DRA array elements and is studied numerically.

2020-11-30 PIER Vol. 169, 33-43. doi:10.2528/PIER20083101

One-Way Topological States Along Vague Boundaries in Synthetic Frequency Dimensions Including Group Velocity Dispersion (Invited)

Qingrou Shan, Danying Yu, Guangzhen Li, Luqi Yuan, and Xianfeng Chen

We recently proposed a two-dimensional synthetic space including one spatial axis and one synthetic frequency dimension in a one-dimensional ring resonator array [Opt. Lett. 41, 741 (2016)]. Nevertheless, the group velocity dispersion (GVD) of the waveguides that compose rings was ignored for simplicity. In this paper, we extend the previous work and study the topological one-way edge states in such a synthetic space involving GVD. We show that the GVD brings a natural vague boundary in the frequency dimension, so the topological edge state still propagates at several frequency modes unidirectionally along the spatial axis. Positions of such vague boundary can be controlled by changing the magnitude of the GVD. In particular, a relatively strong GVD can degrade this two-dimensional synthetic space to one-dimensional spatial lattice, but yet the one-way state is still preserved in simulations. Our work therefore exhibits the impact of the GVD on topological photonics in the synthetic space, which will be important for future practical experimental implementations.

2020-11-25 PIER Vol. 169, 25-32. doi:10.2528/PIER20102708

Designer Surface Plasmons Enable Terahertz Cherenkov Radiation (Invited)

Jie Zhang, Xiaofeng Hu, Hongsheng Chen, and Fei Gao

Cherenkov radiation (CR) is a promising method to generate high-power terahertz (THz) electromagnetic (EM) waves, which are highly desired in numerous practical applications. For the purpose of economy energy, naturally occurred materials with flat surface (e.g. graphene), which can support highly-confined surface-plasmon-polariton (SPP) modes, have been proposed to construct high-efficiency terahertz CR source; however, these emerging materials cannot be easily fabricated nor flexibly designed. Here, we propose a designer-SPP metamaterial scheme to pursue terahertz CR. The metamaterial is a structure-decorated metal surface, which is compatible with planar fabrication, and can support SPP-like EM modes in terahertz frequencies, also named as designer SPP. Due to the structure dependence of designer SPP, its dispersions can be flexibly designed by changing the structure geometries as well as choosing proper dielectric medias. Numerical results clearly demonstrated this scheme. Our proposal may promise future high-efficiency and intense THz source with design flexibilities.