Transmissive 2-bit anisotropic coding metasurface

Coding metasurfaces have attracted tremendous interests due to unique capabilities of manipulating electromagnetic wave. However, archiving transmissive coding metasurface is still challenging. Here we propose a transmissive anisotropic coding metasurface that enables the independent control of two orthogonal polarizations. The polarization beam splitter and the orbital angular momentum (OAM) generator have been studied as typical applications of the anisotropic 2-bit coding metasurface. The simulated far field patterns illustrate that the x and y polarized electromagnetic waves are deflected into two different directions, respectively. The anisotropic coding metasurface has been experimentally verified to realize an OAM beam with l = 2 of right-handed polarized wave, resulting from both contributions from linear-to-circular polarization conversion and the phase profile modulation. This work is beneficial to enrich the polarization manipulation field and develop transmissive coding metasurfaces.     

Controlling acoustic orbital angular momentum with artificial structures: From physics to application

Acoustic orbital angular momentum (OAM) associated with helicoidal wavefront recently attracts rapidly-growing attentions, offering a new degree of freedom for acoustic manipulation. Due to the unique dynamical behavior and inherent mode orthogonality of acoustic OAM, its harnessing is of fundamental interests for wave physics, with great potential in a plethora of applications. The recent advance in materials physics further boosts efforts into controlling OAM-carrying acoustic vortices, especially acoustic metasurfaces with planar profile and subwavelength thickness. Thanks to their unconventional acoustic properties beyond attainable in the nature, acoustic artificial structures provide a powerful platform for new research paradigm for efficient generation and diverse manipulation of OAM in ways not possible before, enabling novel applications in diverse scenarios ranging from underwater communication to object manipulation. In this article, we present a comprehensive view of this emerging field by delineating the fundamental physics of OAM-metasurface interaction and recent advances in the generation, manipulation, and application of acoustic OAM based on artificial structures, followed by an outlook for promising future directions and potential practical applications.     

非线性超表面新模式：时空编码数字超表面

超表面是由亚波长结构组成的超薄人工表面,对电磁波具有突出的调控能力。近年来,因其在非线性效应增强方面的巨大潜力,超表面引起了人们越来越多的重视。具有现场可编程能力和时空信息调制能力的时空编码数字超表面可精准、高效地操控非线性电磁效应和电磁信息,因此成为非线性超表面的一种新模式。文章首先介绍时空编码数字超表面的基本概念,重点讨论近几年在非线性电磁调控方面的研究进展,包括谐波生成、电磁参数控制和非线性波前调控等相关工作。最后,对时空编码数字超表面面临的挑战及未来的发展趋势进行了总结和展望。     

Exotic spectra, wavefunctions and transport in incommensurate lattices

We present numerical results on a range of related issues for a number of incommensurate TMB’s, each of which shows a metal-insulator type transition as a binding-to-hopping ratio is made to increase through some limiting value. These supplement a series of similar results on a couple of 1D lattices in a number of recent works (see below). A brief review pertaining to spectral properties and wavefunctions in incommensurate lattices is followed by results on the above TBM’s relating to an interesting correlation between the gross features of wavefunctions and the energies arranged in a particular sequence termed thelattice-ordered sequence, and also between the lattice-ordered energies and the on-site potentials. We present a qualitative explanation of these correlations on the basis of perturbation theory. Basic results on dynamics of wavepackets in relation to spectral characteristics of incommensurate TBM’s are also reviewed. Features of lattice-ordered energies and wavefunctions for the TBM’s under study are used in the framework of the so-called Maryland construction, leading to a qualitative prediction of criteria for recurrent and non-recurrent wavepacket dynamics in these lattices, and these predictions are checked against numerical iterations of the relevant ‘quantum maps’. Closely related to the dynamics of wavepackets are the transport properties of these lattices. Results are available to indicate that the unusual spectral characteristics of pseudorandom lattices lead to novel features in transport properties of these systems. In this context, low temperature a.c conductivity in these lattices is a good probe for the spectral characteristics and wavefunctions. However, not much is known about the a.c conductivity, excepting a set of early results pertaining to the low frequency regime, principally because of the fact that the a.c conductivity depends on global characteristics of the spectrum and the entire set of wavefunctions. We present a simple model whereby the gross structure of variation of the a.c conductivity with frequency can be obtained from a knowledge of the spectrum alone for the set of TMB’s under consideration. Numerical computations show that despite its simplicity, the model leads to results in good agreement with those from the Kubo-Greenwood formula for a.c conductivity.     

Schrödinger's What is Life?—The 75th Anniversary of a Book that Inspired Biology

In his book What is Life?—The Physical Aspect of the Living Cell, Erwin Schrödinger gives a “naïve physicist's” answer to the question “how can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?” Although his book was met with criticism from some of his colleagues, it has had a large impact and has served as profound inspiration for pioneers of molecular biology as well as for later generations of both scientists and laymen.     

Experimental Investigation of Mixing in Aperiodic Heterogeneous Porous Media: Comparison with Stochastic Transport Theory

An electrochemical technique was used to measure concentration distributions in an aperiodic heterogeneous model for comparison with a stochastic transport theory. Four identical columns, each filled with a homogeneous distribution of glass beads, were threaded together to create a single model with aperiodic heterogeneity. The layers in the model were arranged in different ways providing 24 realizations of the permeability distribution. Comparisons between experimental moment data and moments of simulated mean concentration distributions showed that the model was not able to accurately predict experimentally observed mixing behavior.     

逆周期的交换半群生成的簇的子簇

In this paper, all subvarieties of the varieties Ak （k ∈N） generated by aperiodic commutative semigroups are characterized. Based on this characterization, the structure of lattice of subvarieties of Ak is investigated.     

Tunable On‐Chip Sources with Aperiodic Metasurface

Surface plasmons show tremendous capability in integrated communication, quantum computing and sensing. Excitations and manipulations of surface plasmons are essential in developing integrated photonic devices. Here, a systematic study of tunable emission of surface plasmons with an eightfold quasicrystal metasurface, which acts as an on‐chip source, is presented. It is shown that the quasicrystal structure can switch on or off the surface plasmons propagation channels in the desired direction. Meanwhile, such a quasicrystal structure can be polarization‐dependent or polarization‐independent based on different constituent slit pairs. The proposed quasicrystal design provides more freedom for steering surface plasmons in the launching process. Thus, it may significantly simplify the design and fabrication of integrated plasmonic devices.     

A comparison theorem on convergence rates of random walks on groups

We give a comparison theorem which helps us to estimate the change in the convergence rate when we enlarge the support of a probability that generates a random walk, and/or change the original mass assignment. Some examples of applications of this theorem are discussed.Research supported in part by the Office of Research and Sponsored Programs, University of Oregon.