Xiaoxiao Wu

Direct observation of valley-polarized topological edge states in designer surface plasmon crystals

The extensive research of two-dimensional layered materials has revealed that valleys, as energy extrema in momentum space, could offer a new degree of freedom for carrying information. Based on this concept, researchers have predicted valley-Hall topological insulators that could support valley-polarized edge states at non-trivial domain walls. Recently, several kinds of photonic and sonic crystals have been proposed as classical counterparts of valley-Hall topological insulators. However, direct experimental observation of valley-polarized edge states in photonic crystals has remained difficult until now. Here, we demonstrate a designer surface plasmon crystal comprising metallic patterns deposited on a dielectric substrate, which can become a valley-Hall photonic topological insulator by exploiting the mirror-symmetry-breaking mechanism. Topological edge states with valley-dependent transport are directly visualized in the microwave regime. The observed edge states are confirmed to be fully valley-polarized through spatial Fourier transforms. Topological protection of the edge states at sharp corners is also experimentally demonstrated.The photonic valley-Hall effect can enable the unidirectional propagation of edge states, but often require covers which shield the states from direct measurement. Here, Wu et al. realize photonic valley-Hall effect using designer surface plasmons, enabling the direct observation of topological states.

Low-frequency tunable acoustic absorber based on split tube resonators

We demonstrate a high-efficiency tunable acoustic absorber for low frequencies (<500 Hz) with subwavelength thickness. The acoustic absorber is based on split tube resonators and could reach high-efficiency absorption at tunable resonance frequency with wavelength in air at least 30 times larger than its total thickness in simulations and experiments. The resonance frequency and high-efficiency absorption of the absorber are robust under oblique incidence even at large angles. The absorber could have potential applications for acoustic engineering due to its high structural stability, ease of fabrication, subwavelength thickness, and robust high-efficiency.

Broadband reflective metasurface for focusing underwater ultrasonic waves with linearly tunable focal length

We report a metasurface for focusing reflected ultrasonic waves over a wide frequency band of 0.45–0.55 MHz. The broadband focusing effect of the reflective metasurface is studied numerically and then confirmed experimentally using near-field scanning techniques. The focusing mechanism can be attributed to the hyperboloidal reflection phase profile imposed by different depths of concentric grooves on the metasurface. In particular, the focal lengths of the reflective metasurface are extracted from simulations and experiments, and both exhibit good linear dependence on frequency over the considered frequency band. The proposed broadband reflective metasurface with tunable focal length has potential applications in the broad field of ultrasonics, such as ultrasonic tomographic imaging, high intensity focused ultrasound treatment, etc.

Functionalized PDMS with Versatile and Scalable Surface Roughness Gradients for Cell Culture.

This manuscript describes a simple and versatile approach to engineering surface roughness gradients via combination of microfluidics and photopolymerization. Through UV-mediated polymerization, N-isopropylacrylamide with concentration gradients are successfully grafted onto PDMS surface, leading to diverse roughness degrees on the obtained PDMS substrate. Furthermore, the extent of surface roughness can be controllably regulated via tuning the flow rate ratio between the monomer solution and deionized water. Average roughness ranging from 2.6±0.7 nm to 163.6±11.7 nm has been well-achieved in this work. Such PDMS samples are also demonstrated to be capable of working as supporting substrates for controlling cell adhesion or detachment. Because of the different degrees of surface roughness on a single substrate, our method provides an effective approach for designing advanced surfaces for cell culture. Finally, the thermosensitive property of N-isopropylacrylamide makes our sample furnish as another means for controlling the cell detachment from the substrates with correspondence to the surrounding temperature.

Topological interface states in multiscale spoof-insulator-spoof waveguides.

The spoof-insulator-spoof (SIS) structure can serve as a waveguide for spoof surface plasmon polaritons (spoof SPPs). If a periodic geometry modulation in the wavelength scale is introduced to the SIS waveguide, this multiscale SIS (MSIS) waveguide possesses band gaps for spoof SPPs analogous to the band gaps in a photonic crystal. Inspired by the topological interface states found in photonic crystals, we construct an interface by connecting two MSIS waveguides with different topological properties (inverted Zak phases of bulk bands). The topological interface states in the MSIS waveguides are observed experimentally. The measured decay lengths of the interface states agree excellently with the numerical results. These localized interface states may find potential applications in miniaturized microwave devices.

High-efficiency ventilated metamaterial absorber at low frequency

We demonstrate a ventilated metamaterial absorber operating at low frequency ( 90%) has been achieved in both simulations and experiments. This high-efficiency absorption under the ventilation condition originates from the weak coupling of two identical split tube resonators constituting the absorber, which leads to the hybridization of the degenerate eigenmodes and breaks the absorption upper limit of 50% for conventional transmissive symmetric acoustic absorbers. The absorber can also be extended to an array and work in free space. The absorber should have potential applications in acoustic engineering where both noise reduction and ventilation are required.

Control the Drying Configuration of Suspensions via Regulating the Surface Topologies for Surface-Enhanced Raman Scattering Optimization

In this work, the authors present an innovative method to efficiently control the drying configuration of gold nanoparticles (AuNPs) for optimization of surface-enhanced Raman scattering (SERS) performance with improved sensitivity and re-producibility. Via repeated grafting-casting processes, we have simply regulated the surface topologies of polydimethylsiloxane (PDMS) with average surface roughness ranging from 1.4nm to 651.3nm. The assembling configurations of AuNPs after completed evaporation of solvent have been systematically studied on substrates with different roughness degrees. Furthermore, we found that based on the suspended droplet drying method, the inter-gaps among AuNPs can be well optimized on rougher substrate than on flat PDMS. The SERS spectra based on diverse substrates are investigated and compared, with the best available results arising from the roughest PDMS substrate accompanied by suspended drying means. By introducing rhodamine 6G as the probe molecule, such convenient method enables the detection limit down to 10-14mol/L with linear relationship between the Raman intensity and analyte concentrations within 10-8-10-14mol/L. The experimental results confirm the superiorities of our proposed method for preparation of SERS substrate, and thus providing a straightforward avenue for future construction of efficient and sensitive platform in fields from environmental monitoring to biological sensors.

A valve-free 2D concentration gradient generator

We report a concentration gradient generator featuring valve-free and simultaneous two-dimensional (2D) concentration generation. Each analyte is first diluted by a 1D gradient generator to five different concentrations and then directed into different chambers to generate combinations of different concentrations. The chip consists of three layers, which realizes a 3D fluid path, forming a skywalk structure that allows two perpendicular channels to cross each other in different layers. In this way, this chip could get rid of the pneumatic actuated valves (PAVs) or complicated channel designs that are conventionally adopted in controlling the infusion of multi-analytes. We believe that this design, which makes the chip independent of cumbersome external apparatus in controlling multi-analytes infusion, may potentially benefit the process of making microfluidic chips a portable and cost-effective product.

A metasurface with bidirectional hyperbolic surface modes and position-sensing applications

We have theoretically and experimentally studied resonance-induced hyperbolic metasurfaces and proved that they offer an efficient way to introduce Fano-resonance and decrease the Q-factor in our system in order to create hyperbolic isofrequency contours (IFCs) along two orthogonal directions. A metasurface with a continuous topological transition for such IFCs has been designed and experimentally implemented. In particular, two independent collimation frequencies can be found to correspond to the transition frequencies in orthogonal directions. As a consequence, we experimentally demonstrated that the metasurface can function as a position sensor by utilizing bidirectional hyperbolic surface waves, introducing a new avenue for coordinate sensing.Metasurfaces: Engineered light expands the power of touchAntenna-like planar structures known as metasurfaces may make touch-screen technology even more widespread. Devised to receive optical signals and re-emit different forms of light, metasurfaces have been targeted for applications including all-optical computers. Chinese researchers led by Weijia Wen from the Hong Kong University of Science and Technology and Bo Hou from Soochow University, Suzhou, have developed a technique that enhances control over metasurface-guided light. The team used simulations to design an array of asymmetric H-shaped metal antennas that introduce a new resonance feature into the metasurface device. The resonance effect causes guided light to align into straight beams similar to lasers and running on the metasurface. Based on the remarkable self-collimated beams, the team produced a working, high-resolution touch sensor with technological potentials compatible with a broad range of substrates.Textual caption summarizing the main findings of this workA metasurface with bidirectional self-collimation modes at independent frequencies has been numerically designed and experimentally demonstrated. The unique feature provides a new working principle of utilizing surface electromagnetic waves for position-sensors and thus opens a new avenue towards coordinate sensing. A position-sensing prototype device based on the metasurface has been constructed to manifest such applications.

Designing topological interface states in phononic crystals based on the full phase diagrams

The topological invariants of a periodic system can be used to define the topological phase of each band and determine the existence of topological interface states within a certain bandgap. Here, we propose a scheme based on the full phase diagrams, and design the topological interface states within any specified bandgaps. As an example, here we propose a kind of one-dimensional phononic crystals. By connecting two semi-infinite structures with different topological phases, the interface states within any specific bandgap or their combinations can be achieved in a rational manner. The existence of interface states in a single bandgap, in all odd bandgaps, in all even bandgaps, or in all bandgaps, are verified in simulations and experiments. The scheme of full phase diagrams we introduce here can be extended to other kinds of periodic systems, such as photonic crystals and designer plasmonic crystals.