arXiv subjects

Shuang Zhang

Publications and source records attributed to Shuang Zhang.

At least 127 records · Page 7Linked to original sources

arXiv2018

Chiral-reversing vortex radiation from a single emitter by eigenstates phase locking

The radiation of an emitter does not depend only on its intrinsic properties but also on the surrounding photonic environment, the notion of which is essential in the developments of lasers, quantum optics and other light-matter interaction related fields. However, in conventional wisdom, an emitter radiates into photonic eigenstates in the weak coupling regime and does not alter the property of the latter. Here, we report a counterintuitive phenomenon where the radiation field of a dipole in a parity-time symmetric ring resonator displays the opposite handedness to the eigenstates of the system. This chiral-reversing radiation takes place at an exception point of the underlying non-Hermitian system, where the singularity at the exceptional point forces a phase locking of the coalesced eigenstates when interacting with the dipole emitter. Such an intriguing phenomenon has been employed to construct vortex radiation with controllable topological charge from a single quantum dot embedded plasminic nanocavity with Purcell enhancement factor up to 1000. Our scheme enriches the interesting physics of an exception point in the quantum region and may open a new paradigm for chiral quantum optics and vortex lasers at nanoscale.

physics.optics↗

arXiv2018

Generating Spatial Spectrum with Metasurfaces

Fourier optics, the principle of using Fourier Transformation to understand the functionalities of optical elements, lies at the heart of modern optics, and has been widely applied to optical information processing, imaging, holography etc. While a simple thin lens is capable of resolving Fourier components of an arbitrary optical wavefront, its operation is limited to near normal light incidence, i.e. the paraxial approximation, which put a severe constraint on the resolvable Fourier domain. As a result, high-order Fourier components are lost, resulting in extinction of high-resolution information of an image. Here, we experimentally demonstrate a dielectric metasurface consisting of high-aspect-ratio silicon waveguide array, which is capable of performing Fourier transform for a large incident angle range and a broad operating bandwidth. Thus our device significantly expands the operational Fourier space, benefitting from the large numerical aperture (NA), and negligible angular dispersion at large incident angles. Our Fourier metasurface will not only facilitate efficient manipulation of spatial spectrum of free-space optical wavefront, but also be readily integrated into micro-optical platforms due to its compact size.

physics.optics↗

arXiv2017

Three dimensional photonic Dirac points in metamaterials

Topological semimetals, representing a new topological phase that lacks a full bandgap in bulk states and exhibiting nontrivial topological orders, recently have been extended to photonic systems, predominantly in photonic crystals and to a lesser extent, metamaterials. Photonic crystal realizations of Dirac degeneracies are protected by various space symmetries, where Bloch modes span the spin and orbital subspaces. Here, we theoretically show that Dirac points can also be realized in effective media through the intrinsic degrees of freedom in electromagnetism under electromagnetic duality. A pair of spin polarized Fermi arc like surface states is observed at the interface between air and the Dirac metamaterials. These surface states show linear k-space dispersion relation, resulting in nearly diffraction-less propagation. Furthermore, eigen reflection fields show the decomposition from a Dirac point to two Weyl points. We also find the topological correlation between a Dirac point and vortex/vector beams in classic photonics. The theoretical proposal of photonic Dirac point lays foundation for unveiling the connection between intrinsic physics and global topology in electromagnetism.

physics.optics↗

arXiv2017

Discovery of ideal Weyl points with helicoid surface states

Weyl points, serving as monopoles in the momentum space and laying the foundation of topological gapless phases, have recently been experimentally demonstrated in various physical systems. However, none of the observed Weyl degeneracies are ideal: they either offset in energy or coexist with trivial dispersions at other momenta. The lack of an ideal Weyl system sets a serious limit to the further development of Weyl physics and potential applications. Here, by constructing a photonic metamaterial, we experimentally observe an ideal Weyl system, whose nodal frequencies are pinned by symmetries to exactly the same value. Benefitting from the ideal Weyl nodes, we are able to map out the complete evolution of the helicoid surface states spinning around the projections of each Weyl nodes. Our discovery provides an ideal photonic platform for Weyl systems and novel topological devices.

cond-mat.mtrl-sci↗

arXiv2017

Observation of photonic Fermi arcs in topological metamaterials

The discovery of topological phases introduces new perspectives and platforms for various interesting physics originally investigated in quantum context and then, on an equal footing, in classic wave systems, such as photonics, acoustics and mechanics. As a characteristic feature, nontrivial Fermi arcs, connecting between topologically distinct Fermi surfaces, play vital roles in the clarification of Dirac and Weyl semimetals and have been observed in quantum materials very recently. However, in classical systems, no direct experimental observation of Fermi arcs in the momentum space has been reported so far. Here, using near-field scanning measurements, we show the observation of a photonic Fermi arcs connecting topologically distinct bulk states in chiral hyperbolic metamaterials. To verify the topological nature of this system, we further observe backscattering-immune propagation of nontrivial surface wave across a three-dimension physical step. Our results demonstrate metamaterial approach towards topological photonics and offer a deeper understanding of topological phases in three-dimension classical systems.

physics.optics↗

arXiv2017

The minimal ideal in multiplier algebras

Let $\mathcal A$ be a simple, $σ$-unital, non-unital, non-elementary C*-algebra and let $I_{min}$ be the intersection of all the ideals of $\mathcal M(\mathcal A)$ that properly contain $\mathcal A$. $I_{min}$ coincides with the ideal defined by Lin (Simple C*-algebras with continuous scales and simple corona algebras. 112, (1991) Proc. Amer.Math. Soc) in terms of approximate units of $\mathcal A$ and $I_{min}/\mathcal A$ is purely infinite and simple. If $\mathcal A$ is separable, or if $\mathcal A$ has the (SP) property and its dimension semigroup $D(\mathcal A)$ of Murray-von Neumann equivalence classes of projections of $\mathcal A$ is order separable, or if $\mathcal A$ has strict comparison of positive elements by traces, then $\mathcal A\ne I_{min}$. If the tracial simplex $ \mathcal T(\mathcal A)$ is nonempty, let $ I_{con}$ be the closure of the linear span of the elements $A\in\mathcal M(\mathcal A)_+$ such that the evaluation map $\hat A(τ)=τ(A)$ is continuous. If $\mathcal A$ has strict comparison of positive element by traces then $I_{min}= I_{con}$. Furthermore, $I_{min}$ too has strict comparison of positive elements in the sense that if $A, B\in (I_{min})_+$, $B\not \in\mathcal A$ and $d_τ(A)< d_τ(B)$ for all $τ\in \mathcal T(\mathcal A)$ for which $d_τ(B)< \infty$, then $A\preceq B$. However if $\mathcal A$ does not have strict comparison of positive elements by traces then $I_{min}\ne I_{con}$ can occur: a counterexample is provided by Villadsen's AH algebras without slow dimension growth. If the dimension growth is flat, $ I_{con}$ is the largest proper ideal of $\mathcal M(\mathcal A)$.

math.OA↗

arXiv2017

A Reconfigurable Active Huygens' Metalens

Metasurfaces enable a new paradigm of controlling electromagnetic waves by manipulating subwavelength artificial structures within just a fraction of wavelength. Despite the rapid growth, simultaneously achieving low-dimensionality, high transmission efficiency, real-time continuous reconfigurability, and a wide variety of re-programmable functions are still very challenging, forcing researchers to realize just one or few of the aforementioned features in one design. In this study, we report a subwavelength reconfigurable Huygens' metasurface realized by loading it with controllable active elements. Our proposed design provides a unified solution to the aforementioned challenges of real-time local reconfigurability of efficient Huygens' metasurfaces. As one exemplary demonstration, we experimentally realized a reconfigurable metalens at the microwave frequencies which, to our best knowledge, demonstrates for the first time that multiple and complex focal spots can be controlled simultaneously at distinct spatial positions and re-programmable in any desired fashion, with fast response time and high efficiency. The presented active Huygens' metalens may offer unprecedented potentials for real-time, fast, and sophisticated electromagnetic wave manipulation such as dynamic holography, focusing, beam shaping/steering, imaging and active emission control.

physics.optics↗

arXiv2017

Translocation time of a polymer chain through an energy gradient nanopore

The translocation time of a polymer chain through an interaction energy gradient nanopore was studied by Monte Carlo simulations and the Fokker-Planck equation with double-absorbing boundary conditions. Both the simulation and calculation revealed three different behaviors for polymer translocation. These behaviors can be explained qualitatively from free-energy landscapes obtained for polymer translocation at different parameters. Results show that the translocation time of a polymer chain through a nanopore can be tuned by suitably designing the interaction energy gradient.

cond-mat.soft↗

arXiv2016

Molecule-Induced Conformational Change in Boron Nitride Nanosheets with Enhanced Surface Adsorption

Surface interaction is extremely important to both fundamental research and practical application. Physisorption can induce shape and structural distortion (i.e. conformational changes) in macromolecular and biomolecular adsorbates, but such phenomenon has rarely been observed on adsorbents. Here, we demonstrate theoretically and experimentally that atomically thin boron nitride (BN) nanosheets as an adsorbent experience conformational changes upon surface adsorption of molecules, increasing adsorption energy and efficiency. The study not only provides new perspectives on the strong adsorption capability of BN nanosheets and many other two-dimensional nanomaterials but also opens up possibilities for many novel applications. For example, we demonstrate that BN nanosheets with the same surface area as bulk hBN particles are more effective in purification and sensing.

cond-mat.mes-hall↗

arXiv2016

Controlling plasmonic orbital angular momentum by combining geometric and dynamic phases

Tunable orbit angular momentum (OAM) of surface plasmon polaritons (SPPs) is theoretically studied with appropriately designed metasurfaces. By controlling both the orientation angle and spatial position of nano aperture array on an ultrathin gold film, the field distributions of the surface waves can be engineered to contain both spin dependent and independent OAM components. Simultaneous control over the geometric phase and optical path difference induced phase (dynamic phase) provides extra degrees of freedom for manipulating OAM of SPPs. We show that arbitrary combination of OAM numbers can be realized for the SPPs excited by incident light of different circular polarizations. The results provides powerful control over the OAM of SPPs, which will have potential applications on optical trapping, imaging, communications and quantum information processing.

physics.optics↗

arXiv2016

A Dual Field-of-View Zoom Metalens

Conventional optical zoom system is bulky, expensive and complicated for real time adjustment. Recent progress in the metasurface research has provided a new solution to achieve innovative compact optical systems. In this paper, we propose a highly integrated zoom lens with dual field-of-view (FOV) based on double sided metasurfaces. With silicon nanobrick arrays of spatially varying orientations sitting on both side of a transparent substrate, this ultrathin zoom metalens can be designed to focus an incident circular polarized beam with spin-dependent FOVs without varying the focal plane, which is important for practical applications. The proposed dual FOV zoom metalens, with the advantages such as ultracompactness, flexibility and replicability, can find applications in fields which require ultracompact zoom imaging and beam focusing.

physics.optics↗

arXiv2016

Pancharatnam-Berry phase induced spin-selective transmission in herringbone dielectric metamaterials

Manipulating the polarisation of light is crucial for sensing and imaging applications. One such aspect in particular is selective transmission of one circular polarisation (spin) when light is transmitted through a medium or a device. However, most present methods of achieving this have relatively low efficiency and selectivity, whilst high selectivity examples rely on lossy and complex three-dimensional helical or multilayer structures. Here, we propose a dielectric metamaterial approach for achieving spin-selective transmission of electromagnetic waves, utilizing spin-controlled constructive or destructive interference between two Pancharatnam-Berry (PB) phases in conjunction with propagative dynamic phase. The dielectric metamaterial, consisting of monolithic silicon herringbone structures, exhibits a broadband operation in the terahertz regime whilst obtaining a spin-selective efficiency upwards of 60%. Such a device is robust and is not easily degraded by errors in fabrication.

physics.optics↗

arXiv2016

Elastic Spin Hall effect in Mechanical Graphene

We show that spin-orbit interaction and elastic spin-Hall effect can exist in a classical mechanical system consisting of a two-dimensional honeycomb lattice of masses and springs. The band structure shows the presence of splitting at K point induced by the difference of longitudinal and transverse elastic constant, and this splitting can be regarded as an effective Dresselhaus-type spin-orbit coupling. Interestingly, as an initial displacement away from the equilibrium is applied, the time evolution simulation shows that waves of different spin polarization propagates along different directions at the Gamma and K point, which is characteristic of spin-Hall effect. Several cases for spin-Hall effect are also discussed.

cond-mat.mes-hall↗

arXiv2016

Geometric Metasurface Fork Gratings for Vortex Beam Generation and Manipulation

In recent years, optical vortex beams possessing orbital angular momentum have caught much attention due to their potential for high capacity optical communications. This capability arises from the unbounded topological charges of orbital angular momentum (OAM) that provides infinite freedoms for encoding information. The two most common approaches for generating vortex beams are through fork diffraction gratings and spiral phase plates. While realization of conventional spiral phase plate requires complicated 3D fabrication, the emerging field of metasurfaces has provided a planar and facile solution for generating vortex beams of arbitrary orbit angular momentum. Here we realize a novel type of geometric metasurface fork grating that seamlessly combine the functionality of a metasurface phase plate for vortex beam generation, and that of a linear phase gradient metasurface for controlling the wave propagation direction. The metasurface fork grating is therefore capable of simultaneously controlling both the spin and the orbital angular momentum of light.

physics.optics↗

arXiv2016

Wide-angle and high-efficiency achromatic metasurfaces for visible light

Recently, an achromatic metasurface was successfully demonstrated to deflect light of multiple wavelengths in the same direction and it was further applied to the design of planar lenses without chromatic aberrations [Science, 347, 1342(2015)]. However, such metasurface can only work for normal incidence and exhibit low conversion efficiency. Here, we present an ultrawide-angle and high-efficiency metasurface without chromatic aberration for wavefront shaping in visible range. The metasurface is constructed by multiple metallic nano-groove gratings, which support enhanced diffractions for an ultrawide incident angle range from 10o to 80o due to the excitations of localized gap plasmon modes at different resonance wavelengths. Incident light at these resonance wavelengths can be efficiently diffracted into the same direction with complete suppression of the specular reflection. This approach is applied to the design of an achromatic flat lens for focusing light of different wavelengths into the same position. Our findings provide a facile way to design various achromatic flat optical elements for imaging and display applications.

physics.optics↗

arXiv2016

Pseudospin induced chirality with Staggered Optical Graphene

Pseudospin plays a very important role in understanding various interesting physical phenomena associated with 2D materials such as graphene. It has been proposed that pseudospin is directly related to angular momentum, and it was recently experimentally demonstrated that orbit angular momentum is an intrinsic property of pseudospin in a photonic honeycomb lattice. However, in photonics, the interaction between spin and pseudospin for light has never been investigated. In this Letter, we propose that, in an optical analogue of staggered graphene, i.e. a photonic honeycomb lattice waveguide with in-plane inversion symmetry breaking, the pseudospin mode can strongly couple to the spin of an optical beam incident along certain directions. The spin-pseudospin coupling, caused by the spin-orbit conversion in the scattering process, induces a strong optical chiral effect for the transmitted optical beam. Spin-pseudospin coupling of light opens door to the design of pseudospin-mediated spin or valley selective photonic devices.

physics.optics↗

arXiv2015

Phenomenological modeling of Geometric Metasurfaces

Metasurfaces, with their superior capability in manipulating the optical wavefront at the subwavelength scale and low manufacturing complexity, have shown great potential for planar photonics and novel optical devices. However, vector field simulation of metasurfaces is so far limited to periodic-structured metasurfaces containing a small number of meta-atoms in the unit cell by using full-wave numerical methods. Here, we propose a general phenomenological method to analytically model metasurfaces made up of arbitrarily distributed meta-atoms based on the assumption that the meta-atoms possess localized resonances with Lorentz-Drude forms, whose exact form can be retrieved from the full wave simulation of a single element. Applied to phase modulated geometric metasurfaces, our analytical results show good agreement with full-wave numerical simulations. The proposed theory provides an efficient method to model and design optical devices based on metasurfaces.

physics.optics↗

arXiv2015

Plasmon Weyl Degeneracies in Magnetized Plasma

In this letter, we report the presence of novel type of plasmon Weyl points in a naturally existing material - magnetized plasma. In such a medium, conventional, purely longitudinal bulk plasma oscillations exists only along the direction of applied magnetic field (z direction). With strong enough magnetic field, there exist helical propagating modes along z direction with circular polarizations. The orthogonality between the longitudinal bulk plasmon mode and the transverse helical propagating modes guarantees their crossing at the bulk plasmon frequency. These crossing points, embedded in the bulk plasmon dispersion line, serve as monopoles in the k space - the so called Weyl points. These Weyl points lead to salient observable features. These include the highly intriguing observation that, at a magnetized plasma surface which is parallel to the applied magnetic field, reflection of an electromagnetic wave with in-plane wave-vector close to the Weyl points exhibits chiral behavior only in half of the k plane, which is bounded by the projection of the bulk plasmon dispersion line. We also verify the presence of 'Fermi arcs' connecting the two Weyl points with opposite chiralities when magnetized plasma interfaces with trivial photonic materials. Our study introduces the concept of Weyl photonics into homogeneous strongly dispersive photonic materials, which could pave way for realizing new topological photonic devices.

physics.optics↗