It becomes more and more challenging to satisfy the long-term demand of transmission capacity in wireless networks if we limit our research within the frame of traditional electromagnetic wave characteristics (e.g., frequency, amplitude, phase and polarization). The potential of orbital angular momentum (OAM) for unleashing new capacity in the severely congested spectrum of commercial communication systems is generating great interest in wireless communication field. The OAM vortex wave/beam has different topological charges, which are orthogonal to each other. It provides a new way for multiplexing in wireless communications. Electromagnetic wave or synthetic beam carrying OAM has a spiral wavefront phase structure, which may provide a new degree of freedom or better orthogonality in spatial domain. In this paper, we introduce the fundamental theory of OAM. Then, OAM generation and reception methods are equally demonstrated. Furthermore, we present the latest development of OAM in wireless communication. We further discuss the controversial topic ``whether OAM provides a new degree of freedom'' and illustrate our views on the relationship between OAM and MIMO. Finally, we suggest some open research directions of OAM.
In this paper, we develop numerical methods for using vector spherical and spheroidal waves in the hybrid method to calculate the multiple scattering of objects of complex shapes, based on the rigorous solutions of Maxwell equations in the form of Foldy-Lax multiple scattering equations (FL). The steps in the hybrid method are: (1) calculating the T-matrix of each single object using vector spherical/spheroidal waves and (2) vector spherical/spheroidal waves addition theorem. We utilize the commercial software HFSS to calculate the scattered fields of a complex object on the circumscribing sphere or spheroid for multiple incidences and polarizations. The T-matrix of spherical waves or spheroidal waves are then obtained from these scattered fields. To perform wave transformations (i.e. addition theorem) for vector spherical/spheroidal waves, we develop robust numerical methods. Numerical results are illustrated for T-matrices and numerical vector addition theorems.
Potential-based integral equations are being explored to develop numerical methods that avoid low frequency breakdown issues and are better suited to couple to quantum physics computations. Important classes of quantum electrodynamics problems are typically formulated in the radiation gauge, leading to interest in efficient numerical solutions able to be performed directly in this gauge. This work presents time domain integral equations for penetrable regions that are developed in the radiation gauge. An appropriate marching-on-in-time discretization scheme is developed that fully conforms to the spatial and temporal Sobolev space properties of the integral equations. It is shown that following this approach leads to a discrete system with improved stability properties that produces accurate results down to very low frequencies. The accuracy and stability of this formulation at low frequencies are shown through numerical results.
A new design for a cylindrical dielectric resonator antenna (DRA) with a capability of switching between circular, linear horizontal and linear vertical polarizations is introduced. The DRA, operating at the center frequency of 3.25 GHz, is fed by a microstrip line through two dog-bone slots. In this design, only two PIN diodes are employed as switching elements which significantly decreases the complexity of DC biasing circuits compared to existing designs. The PIN diodes are embedded in transformers connected to the feeding microstrip lines. This technique conveniently allows to make compensations for parasitic effects of the PIN diodes junction capacitors on the antenna matching bandwidth. The circular, linear horizontal and linear vertical polarizations have a bandwidth of 22%, 17% and 18%, respectively. The 3-dB axial ratio bandwidth for the circular polarization is 12%. The measured results obtained from prototyped antenna agree well with simulated results of the designed antenna system, which confirms the validity of the design process.
This paper presents an overview and review of the fundamental implicit finite-difference time-domain (FDTD) schemes for computational electromagnetics (CEM) and educational mobile apps. The fundamental implicit FDTD schemes are unconditionally stable and feature the most concise update procedures with matrix-operator-free right-hand sides (RHS). We review the developments of fundamental implicit schemes, which are simpler and more efficient than all previous implicit schemes having RHS matrix operators. They constitute the basis of unification for many implicit schemes including classical ones, providing insights into their inter-relations along with simplifications, concise updates and efficient implementations. Based on the fundamental implicit schemes, further developments can be carried out more conveniently. Being the core CEM on mobile apps, the multiple one-dimensional (M1-D) FDTD methods are also reviewed. To simulate multiple transmission lines, stubs and coupled transmission lines efficiently, the M1-D explicit FDTD method as well as the unconditionally stable M1-D fundamental alternating direction implicit (FADI) FDTD and coupled line (CL) FDTD methods are discussed. With the unconditional stability of FADI methods, the simulations are fast-forwardable with enhanced efficiency. This is very useful for quick concept illustrations or phenomena demonstrations during interactive teaching and learning. Besides time domain, many frequency-domain methods are well-suited for further developments of useful mobile apps as well.
A periodic millimeter wave leaky-wave antenna (LWA), which has two different types of radiator elements that enable backward to forward radiation, is proposed. The unit-cell of the LWA consists of two quarter-wavelength microstrip lines and two corrugated substrate integrated waveguide (CSIW) cells with S-shaped quarter-wavelength open-circuit stubs. In addition to two parallel edge radiators, a single etched transverse slot with a tilt angle acts as an ancillary radiator, which ensures impedance matching in a large frequency range and achieves the backward to forward scanning. We analyze the proposed design through simulations, characterize a fabricated prototype and find it to have good radiation properties including broad impedance bandwidth. The measurement results show a high peak gain from 11 to 15.8 dBi with a large scanning angle range from -34° to +22° in the K-band operating frequency range.
We report the second-harmonic generation (SHG) from single GaN nanowire. The diameter of the GaN nanowire varies from 150 to 400 nm. We present a model for the SHG process in the GaN nanowire; the analysis shows quantitatively that the SHG is dominated by its surface area. The effective second order nonlinear optical susceptibility (χ(2)eff) increases as the diameter of the GaN nanowire decreases. For 150-nm diameter GaN nanowire, χ(2)eff reaches 136 pm/V.
Enhancing the scattering of light from subwavelength structures is of both fundamental and practical significance. While the scattering cross section from each channel cannot exceed the single-channel limit, it is recently reported that the total cross section can far exceed this limit if one overlaps the contribution from many channels. Such a phenomenon about enhancing the scattering from subwavelength structures in free space is denoted as the superscattering in some literature. However, the scatterer in practical scenarios is not always in free space but may be embedded in environments with non-unity refractive index n. The influence of environments on the superscattering remains elusive. Here the superscattering from subwavelength structures in the isotropic environment with near-zero index are theoretically investigated. Importantly, a smaller n can lead to a larger total cross section for superscattering. The underlying mechanism is that a smaller n can give rise to a larger single-channel limit. Our work thus indicates that the scattering from subwavelength structures can be further enhanced if one simultaneously maximizes the single-channel limit and the contribution from many channels.
The zeroing of second order correlation functions between output fields after interferences in a 50/50 beam splitter has been accepted decades-long in the quantum optics community as an indicator of the quantum nature of lights. But, a recent work  presented some notable discussions and experiments that classical electromagnetic fields can still exhibit the zero correlation under specific conditions. Here, we examine analytically classical and quantum electromagnetic field interferences in a 50/50 beam splitter in the context of the second order correlation function for various input conditions. Adopting the Heisenberg picture in quantum electromagnetics, we examine components of four-term interference terms in the numerator of second order correlation functions and elucidate their physical significance. As such, we reveal the fundamental difference between the classical and quantum interference as illustrated by the Hong-Ou-Mandel (HOM) effect. The quantum HOM effect is strongly associated with: (1) the commutator relation that does not have a classical analogue; (2) the property of Fock states needed to stipulate the one-photon quantum state of the system; and (3) a destructive wave interference effect. Here, (1) and (2) imply the indivisibility of a photon. On the contrary, the classical HOM effect requires the presence of two destructive wave interferences without the need to stipulate a quantum state.
A dual-mode hyperspectral imager using field of view scanning needs no moving macro parts. It could work in dual-mode (macro imaging and micro imaging) and is equipped with a conjugated camera for quick object-selection and focusing. By adjusting the imaging lens and achieving the image clarity on the conjugated camera, we could find the correct location and focusing of the ROIs simultaneously instead of inefficiently checking the hyperspectral image after the whole scanning process. The whole system was applied to the study of spectral characteristics of blood oxygen in human hands and the microscopic identification of algae, showing a great potential of clinical and marine applications of our system.