Lixin Ran

Experimental observation of Weyl points

Weyl physics emerges in the laboratory Weyl fermions—massless particles with half-integer spin—were once mistakenly thought to describe neutrinos. Although not yet observed among elementary particles, Weyl fermions may exist as collective excitations in so-called Weyl semimetals. These materials have an unusual band structure in which the linearly dispersing valence and conduction bands meet at discrete “Weyl points.” Xu et al. used photoemission spectroscopy to identify TaAs as a Weyl semimetal capable of hosting Weyl fermions. In a complementary study, Lu et al. detected the characteristic Weyl points in a photonic crystal. The observation of Weyl physics may enable the discovery of exotic fundamental phenomena. Science, this issue p. 613 and 622 Microwave measurements are used to identify Weyl points in a double-gyroid photonic crystal. [Also see Research Article by Xu et al.] The massless solutions to the Dirac equation are described by the so-called Weyl Hamiltonian. The Weyl equation requires a particle to have linear dispersion in all three dimensions while being doubly degenerate at a single momentum point. These Weyl points are topological monopoles of quantized Berry flux exhibiting numerous unusual properties. We performed angle-resolved microwave transmission measurements through a double-gyroid photonic crystal with inversion-breaking where Weyl points have been theoretically predicted to occur. The excited bulk states show two linear dispersion bands touching at four isolated points in the three-dimensional Brillouin zone, indicating the observation of Weyl points. This work paves the way to a variety of photonic topological phenomena in three dimensions.

Experimental confirmation of negative refractive index of a metamaterial composed of Ω-like metallic patterns

A one-dimensional metamaterial is realized using three connected Ω rings printed back-to-back and reversed on two sides of a dielectric substrate. Both transmission and prism experiments are reported, yielding concordant results of the presence of a left-handed frequency band. Experiments show reduced losses and an enlarged left-handed frequency band.

Design and analytical full-wave validation of the invisibility cloaks, concentrators, and field rotators created with a general class of transformations

We investigate a general class of electromagnetic devices created with any continuous transformation functions by rigorously calculating the analytical expressions of the electromagnetic field in the whole space. Some interesting phenomena associated with these transformation devices, including the invisibility cloaks, concentrators, and field rotators, are discussed. By carefully choosing the transformation function, we can realize cloaks, which are insensitive to perturbations at both the inner and outer boundaries. Furthermore, we find that when the coating layer of the concentrator is realized with left-handed materials, energy will circulate between the coating and the core, and the energy transmitted through the core of the concentrator can be much bigger than that transmitted through the concentrator. Therefore, such concentrator is also a power flux enhancer. Finally, we propose a spherical field rotator, which functions as not only a wave vector rotator but also a polarization rotator, depending on the orientations of the spherical rotator with respect to the incident wave direction. The functionality of these transformation devices are all successfully confirmed by our analytical full-wave method, which also provides an alternate computational efficient validation method in contrast to numerical validation methods.

Instrument-Based Noncontact Doppler Radar Vital Sign Detection System Using Heterodyne Digital Quadrature Demodulation Architecture

In this paper, we present a fast solution to build a Doppler radar system for noncontact vital sign detection (VSD) using instruments that are generally equipped in radio-frequency and communication laboratories. This paper demonstrates the feasibility of conducting research on VSD in ordinary radio-frequency laboratories. The system is designed with a heterodyne digital quadrature demodulation architecture that helps mitigate quadrature channel imbalance and eliminate the complicated dc offset calibration required for arctangent demodulation. Moreover, its tunable carrier frequency helps select different optimal frequencies for different human objects. Two sets of extensive experiments have been carried out in the laboratory environment with a self-designed 2.4-GHz patch antenna array and a 1-18-GHz broadband antenna. The test results are satisfactory: for a 0-dBm transmit power, the detection range can be extended to 2.5 m with accuracy higher than 80%. The system is also capable of detecting vital signs in the presence of different obstructions between the subject and the antenna.

Experimental Study on Several Left-Handed Matamaterials

Left-handed materials (LHM) are engineered structures that exhibit electromagnetic properties not found in nature. Real applications of LHM need substrates with low loss, wide bandwidth as well as stable mechanical characteristics. In this paper, we summarize some experimental as well as numerical results of lefthanded materials with different configurations of rods and splitring resonators (SRRs). Hot-press technics utilized in PC board manufacture are used to produce solid-state multi-layer left-handed materials. Either mechanical or electromagnetic characteristics of LH samples are notably improved.

Metamaterial exhibiting left-handed properties over multiple frequency bands

We present experimental measurements at microwave frequencies of a double band left-handed metamaterial. The sample of metamaterial is composed of an extended version of the S-shaped resonators that exhibit simultaneously a negative permittivity and a negative permeability response at comparable frequencies. The experimental results clearly show that there exist two frequency bands where the refraction index is negative. The double left-handed passbands are due to the multiple capacitances and inductances induced in the structure, which can be further tuned to realize a metamaterial with multiple (more than two) left-handed frequency bands.

Negative refraction of a combined double S-shaped metamaterial

We present a combined double S-shaped metamaterial structure that exhibits a left-handed property over a wide frequency band of 6GHz. The transmission experiment, phase measurement, and prism refraction experiment from the structure confirm the presence of the negative index of refraction.

Experimental retrieval of the effective parameters of metamaterials based on a waveguide method

A waveguide-based retrieval method for measuring complex permittivity and permeability tensors of metamaterials is presented. In the proposed scheme, multiple independent sets of scattering data for the material under test with different orientations are measured in the frequency range corresponding to the dominant TE(10) mode. The method is applied to various metamaterials and shows its effectiveness in the effective parameters extraction.

Controllable left-handed metamaterial and its application to a steerable antenna

A controllable left-handed metamaterial based on a varactor-loaded S-shaped resonator structure is presented to realize an active radome. By controlling the phase properties of the radome, the antenna exhibits continuous scanning capability in a range of ±40°, which is confirmed by theoretical analysis.

Magnetic Properties of S - Shaped Split - Ring Resonators

We present a theoretical analysis of the radiation of an S-shaped split ring resonator (S-SRR) for the realization of a metamaterial exhibiting left-handed properties. It is shown that the structure is resonant due to its internal capacitances and inductances, which can be adjusted such that the electric plasma frequency and magnetic plasma frequency, both due to the S-SRR only, appear within the same frequency band. Using the same idea, we also present some extended S-shaped split-ring resonator structures with improved performance.