Shiyi Xiao

Spin-dependent optics with metasurfaces

Abstract Optical spin-Hall effect (OSHE) is a spin-dependent transportation phenomenon of light as an analogy to its counterpart in condensed matter physics. Although being predicted and observed for decades, this effect has recently attracted enormous interests due to the development of metamaterials and metasurfaces, which can provide us tailor-made control of the light-matter interaction and spin-orbit interaction. In parallel to the developments of OSHE, metasurface gives us opportunities to manipulate OSHE in achieving a stronger response, a higher efficiency, a higher resolution, or more degrees of freedom in controlling the wave front. Here, we give an overview of the OSHE based on metasurface-enabled geometric phases in different kinds of configurational spaces and their applications on spin-dependent beam steering, focusing, holograms, structured light generation, and detection. These developments mark the beginning of a new era of spin-enabled optics for future optical components.

Flexible coherent control of plasmonic spin-Hall effect

The surface plasmon polariton is an emerging candidate for miniaturizing optoelectronic circuits. Recent demonstrations of polarization-dependent splitting using metasurfaces, including focal-spot shifting and unidirectional propagation, allow us to exploit the spin degree of freedom in plasmonics. However, further progress has been hampered by the inability to generate more complicated and independent surface plasmon profiles for two incident spins, which work coherently together for more flexible and tunable functionalities. Here by matching the geometric phases of the nano-slots on silver to specific superimpositions of the inward and outward surface plasmon profiles for the two spins, arbitrary spin-dependent orbitals can be generated in a slot-free region. Furthermore, motion pictures with a series of picture frames can be assembled and played by varying the linear polarization angle of incident light. This spin-enabled control of orbitals is potentially useful for tip-free near-field scanning microscopy, holographic data storage, tunable plasmonic tweezers, and integrated optical components.

Effective PT-symmetric metasurfaces for subwavelength amplified sensing

We propose a novel design principle for ultrathin metasurfaces to realize optically amplified sensing with a performance that exceeds those of passive coherent perfect absorbers by several orders of magnitude. Our strategy is based on a generalized condition of lasing, coherent perfect absorption and their coexistence in metamaterials that feature an effective PT-symmetry. The devices we introduce here can be operated in configurations that involve both a one-sided or a two-sided wave incidence, where the latter case allows us to tune the degree of amplified absorption through the coherent phase between the two input beams. We also discuss how the conditions on the material parameters can be relaxed, away from the ideal case, such that a substantial amplification of the sensing performance can easily be reached in practical applications.

Polarization-dependent optics using gauge-field metamaterials

We show that effective gauge field for photons with polarization-split dispersion surfaces, being realized using uniaxial metamaterials, can be used for polarization control with unique opportunities. The metamaterials with the proposed gauge field correspond to a special choice of eigenpolarizations on the Poincare sphere as pseudo-spins, in contrary to those from either conventional birefringent crystals or optical active media. It gives rise to all-angle polarization control and a generic route to manipulate photon trajectories or polarizations in the pseudo-spin domain. As demonstrations, we show beam splitting (birefringent polarizer), all-angle polarization control, unidirectional polarization filter, and interferometer as various polarization control devices in the pseudo-spin domain. We expect that more polarization-dependent devices can be designed under the same framework.

Perfect Co-Circular Polarization Reflector: A Class of Reciprocal Perfect Conductors With Total Co-Circular Polarization Reflection

We establish a general class of reciprocal perfect conductors, namely perfect co-circular polarization (CP) reflector, from which the reflected waves are completely co-polarized when a circularly polarized electromagnetic (EM) wave impinges on it, in contrast to a conventional perfect conductor like a perfect electric conductor (PEC), a perfect magnetic conductor (PMC) or their interpolated version as a perfect electromagnetic conductor (PEMC). We discuss the impedance boundary conditions and the reflection characteristics of the perfect conductors, including perfect co-CP reflectors and perfect cross-CP reflectors, in terms of a circularly polarized EM wave. We also discuss the realization of one of the perfect co-CP reflectors approximated by coating a layer of anisotropic, yet lossless and reciprocal, material on a PEC. In addition, we discuss its realization using metamaterial structures.

Unidirectional zero reflection as gauged parity-time symmetry

We introduce here the concept of establishing parity-time (PT)-symmetry through a gauge transformation involving a shift of the mirror plane for the parity operation. The corresponding unitary transformation on the systems constitutive matrix allows us to generate and explore a family of equivalent PT-symmetric systems. We further derive that unidirectional zero reflection for a reciprocal two-port system can always be associated with a passively gauged PT-symmetry. We demonstrate this experimentally using a microstrip transmission-line with magnetoelectric coupling. This study allows us to use bianisotropy as a practical route to realise and explore exceptional point behaviour of PT-symmetric or generally non-Hermitian systems.

Metasurface-loaded waveguide for transformation optics applications

We theoretically investigate a two-dimensional metasurface-loaded waveguide as a generic platform for transformation optics (TO) applications. The mode indices can achieve values much less or greater than one by tuning the reflection phase from the metasurface. Due to the subwavelength feature size of the metasurface, we develop an effective description of the wave propagation using an artificial electromagnetic boundary approach, which replaces the effective medium description of TO for bulk media. We numerically demonstrate a constant zero-index medium for wave collimation, gradient index profiles as Luneburg and Maxwell fisheye lenses and a wave bender based on the finite embedded coordinate transformation. These investigations provide a feasible route to perform TO with metasurfaces as waveguide boundaries, yet the designs can still be obtained using an effective boundary approach with only a few constitutive parameters.

Observation of PT-symmetric exceptional point from magnetoelectric bianisotropy

Recently, we have witnessed a wave of investigations on PT-symmetric optical systems due to the promising features of tunable optical properties given by exceptional points and PT-phase transitions. Although many exotic optical properties have been demonstrated with PT-symmetry, some of these demonstrated properties are commonly believed not special to PT-symmetry. For example, unidirectional zero reflection [1, 2], which can occur at the exceptional point of PT-symmetry, is such a property. A typical bianisotropic medium (medium with cross coupling between magnetic and electric resonance) yields asymmetric reflection and therefore can also have unidirectional zero reflection when the impedance is only matched in one direction. Thus, the question arises: Can these systems, which naturally give rise to asymmetric light propagation, also have an underlying PT-symmetry? If this is the case, it should also be possible to establish the associated PT-symmetric Hamiltonian for these systems. Here, we propose a microwave transmission-line to simulate the bianisotropic medium [3], which exhibits unidirectional zero reflection at a particular condition. Such a system, seemingly lacking even a mirror symmetry in the lossless situation, is experimentally shown to have a PT-symmetric Hamiltonian by defining a new parity operator with magnetoelectric coupling [4]. The results reveal the necessary “hidden” PT-symmetry of any systems with unidirectional zero reflection and also open up a simple route to realize and explore exceptional point behavior of PT-symmetric system.

Experimental demonstration of flexible plasmonic spin-Hall effect and its application on coherent control

Optical spin-Hall effect (OSHE) is a spin-dependent transport phenomenon of light. Here, the spin of light is associated with the circular polarization. Although being observed one decade ago [1], OSHEs are usually extremely weak and in uncontrollable manner owning to the inability to fully control the spin-orbit coupling. Recently, with the development of metasurfaces, the spin-dependent splitting of light beam can be significantly enhanced by using metamaterials [2]. Many attentions have been paid to the spin-orbit interaction (SOI) of light using geometric-phase enabled optical and plasmonic systems. However, these previous works on OSHE only demonstrated simple and symmetric splitting of the two spins [1-4], far from the capability to generate independent OSHEs, which can then work coherently together for more flexible and tunable functionalities. Here, we present a plasmonic metasurface consisting of an array of nano-slots with tailor-made orientation profiles to generate independent surface plasmon polariton (SPP) patterns on the metasurface with the two incident spins [5]. For example a “cross” pattern for one spin and a “triangle” pattern for another as incidence. With such flexible control of OSHEs, we also demonstrate that it is possible to make the two spins work coherently together by controlling the relative phase between the two polarizations. It allows us to play motion pictures, for example, to write a letter “b” with a series of picture frames by varying the phase difference between the two spins. Our work may stimulate more applications in near-field optics, such as optical integrated circuits, tip-free near-field scanning optical microscopy, and plasmonic tweezers to trap and move micron size particles.

Metasurface-based illusion optics

Metasurface, an optically thin layer of metarmaterials (MTMs), has recently attracted great interest for its capability to manipulate the phase of a propagating wave [1-3]. Such a surface approach of metamaterials has paved a way to escape from the difficulties in fabricating three-dimension (3D) metamaterials. Recently, optically thin metasurface has been demonstrated to achieve invisibility carpet cloaking devices from microwave [4] to optical frequencies [5]. The present design paradigm of metasurface cloaks is based on using a tailor-made reflection phase profile to compensate the distortion introduced by the curved surface. In the view of light, it is equivalent to reshape a curved to a flat wavefront and vice versa. On the other hand, it is not straight-forward to achieve more complicated transformation optics devices, for example, illusion optics: cloaking an external object and creating a perception of another object with different shape and material properties [6]. Here, we propose a generic metasurface design scheme to achieve illusion optics. In particular, we are interested on the realizations using completely passive metasurfaces. Our simulation results show that the passive metasurfaces can already conceal an external object as well as restore another object with a tolerable range of incident angles. Our scheme for metasurface-enabled illusion optics provides an alternative way for performing transformation optics applications beyond a carpet cloak.