Mechanical Performance of Bio-inspired Bidirectional Corrugated Sandwich Pressure Shell Under External Hydrostatic Pressure

This paper aims to enhance the compression capacity of underwater cylindrical shells by adopting the corrugated sandwich structure of cuttlebone.The cuttlebone suffers uniaxial external compression,while underwater cylindrical shells are in a biaxial compressive stress state.To suit the biaxial compressive stress state,a novel bidirectional corrugated sandwich structure is proposed to improve the bearing capacity of cylindrical shells.The static and buckling analysis for the sandwich shell and the unstiffened cylindrical shell with the same volume-weight ratio are studied by numerical simulation.It is indicated that the proposed sandwich shell can effectively reduce the ratio between circumferential and axial stress from 2 to 1.25 and improve the critical buckling load by about 1.63 times.Numerical simulation shows that optimizing and adjusting the structural parameters could significantly improve the advantage of the sandwich shell.Then,the hydrostatic pressure tests for shell models fabricated by 3D printing are carried out.According to the experimental results,the overall failure position of the sandwich shell is at the center part of the sandwich shell.It has been found the average critical load of the proposed sandwich shell models exceeds two times that of the unstiffened shell models.Hence,the proposed bio-inspired bidirectional corrugated sandwich structure can significantly enhance the pressure resistance capability of cylindrical shells.

Inverse design of nonlinear phononic crystal configurations based on multi-label classification learning neural networks

Phononic crystals,as artificial composite materials,have sparked significant interest due to their novel characteristics that emerge upon the introduction of nonlinearity.Among these properties,second-harmonic features exhibit potential applications in acoustic frequency conversion,non-reciprocal wave propagation,and non-destructive testing.Precisely manipulating the harmonic band structure presents a major challenge in the design of nonlinear phononic crystals.Traditional design approaches based on parameter adjustments to meet specific application requirements are inefficient and often yield suboptimal performance.Therefore,this paper develops a design methodology using Softmax logistic regression and multi-label classification learning to inversely design the material distribution of nonlinear phononic crystals by exploiting information from harmonic transmission spectra.The results demonstrate that the neural network-based inverse design method can effectively tailor nonlinear phononic crystals with desired functionalities.This work establishes a mapping relationship between the band structure and the material distribution within phononic crystals,providing valuable insights into the inverse design of metamaterials.

Nonlinear wave propagation in acoustic metamaterials with bilinear nonlinearity

Nonlinear phononic crystals have attracted great interest because of their unique properties absent in linear phononic crystals.However,few researches have considered the bilinear nonlinearity as well as its consequences in acoustic metamaterials.Hence,we introduce bilinear nonlinearity into acoustic metamaterials,and investigate the propagation behaviors of the fundamental and the second harmonic waves in the nonlinear acoustic metamaterials by discretization method,revealing the influence of the system parameters.Furthermore,we investigate the influence of partially periodic nonlinear acoustic metamaterials on the second harmonic wave propagation,and the results suggest that pass-band and band-gap can be transformed into each other under certain conditions.Our findings could be beneficial to the band gap control in nonlinear acoustic metamaterials.

Finite barrier bound state

A boundary mode localized on one side of a finite-size lattice can tunnel to the opposite side which results in unwanted couplings.Conventional wisdom tells that the tunneling probability decays exponentially with the size of the system which thus requires many lattice sites before eventually becoming negligibly small.Here we show that the tunneling probability for some boundary modes can apparently vanish at specific wavevectors.Thus,similar to bound states in the continuum,a boundary mode can be completely trapped within very few lattice sites where the bulk bandgap is not even well-defined.More intriguingly,the number of trapped states equals the number of lattice sites along the normal direction of the boundary.We provide two configurations and validate the existence of this peculiar finite barrier-bound state experimentally in a dielectric photonic crystal at microwave frequencies.Our work offers extreme flexibility in tuning the coupling between localized states and channels as well as a new mechanism that facilitates unprecedented manipulation of light.

A hybrid invisibility cloak based on integration of transparent metasurfaces and zero-index materials

The invisibility cloak,a long-standing fantastic dream for humans,has become more tangible with the development of metamaterials.Recently,metasurface-based invisibility cloaks have been proposed and realized with significantly reduced thickness and complexity of the cloaking shell.However,the previous scheme is based on reflection-type metasurfaces and is thus limited to reflection geometry.In this work,by integrating the wavefront tailoring functionality of transparent metasurfaces and the wave tunneling functionality of zero-index materials,we have realized a unique type of hybrid invisibility cloak that functions in transmission geometry.The principle is general and applicable to arbitrary shapes.For experimental demonstration,we constructed a rhombic double-layer cloaking shell composed of a highly transparent metasurface and a double-zero medium consisting of dielectric photonic crystals with Dirac cone dispersions.The cloaking effect is verified by both full-wave simulations and microwave experimental results.The principle also reveals exciting possibilities for realizing skin-thick ultrathin cloaking shells in transmission geometry,which can eliminate the need for spatially varying extreme parameters.Our work paves a path for novel optical and electromagnetic devices based on the integration of metasurfaces and metamaterials.

Highly efficient achromatic subdiffraction focusing lens in the near field with large numerical aperture

The achromatic subdiffraction lens with large numerical aperture(NA)is of significant importance in optical imaging,photolithography,spectroscopy,and nanophotonics.However,most of the previous research on sub-diffraction lenses has been restricted by limited bandwidth and efficiency as well as severe chromatic aberrations.In this paper,a semicircular gradient index lens(sGRIN)with a modified refractive index profile originated from a Maxwell fish-eye lens is put forward to achieve highly efficient(above 81%)achromatic(4–20 GHz)subdiffraction focusing at the focusing line(around 0.28λ)with large NA of 1.3 and broadband diffraction-limited far-field radiation(4–16 GHz)theoretically,which overcomes the drawbacks of previous works.The presented lens is designed by gradient dielectric metamaterials.Evanescent waves ignited at the lens/air interface and transformation of electromagnetic(EM)waves with high spatial frequency in sGRIN to EM waves with low spatial frequency in air are responsible for subdiffraction focusing and diffraction-limited far-field radiation,respectively.Experimental results demonstrate the excellent performance of achromatic subdiffraction focusing and diffraction-limited far-field radiation.The presented lens has great potential to be applied in subdiffraction imaging systems.

Ultra-broadband reflectionless Brewster absorber protected by reciprocity

The Brewster’s law predicts zero reflection of p-polarization on a dielectric surface at a particular angle.However,when loss is introduced into the permittivity of the dielectric,the Brewster condition breaks down and reflection unavoidably appears.In this work,we found an exception to this long-standing dilemma by creating a class of nonmagnetic anisotropic metamaterials,where anomalous Brewster effects with independently tunable absorption and refraction emerge.This loss-independent Brewster effect is bestowed by the extra degrees of freedoms introduced by anisotropy and strictly protected by the reciprocity principle.The bandwidth can cover an extremely wide spectrum from dc to optical frequencies.Two examples of reflectionless Brewster absorbers with different Brewster angles are both demonstrated to achieve large absorbance in a wide spectrum via microwave experiments.Our work extends the scope of Brewster effect to the horizon of nonmagnetic absorptive materials,which promises an unprecedented wide bandwidth for reflectionless absorption with high efficiency.

Experimental observation of multiple edge and corner states in photonic slabs heterostructures

The photonic topological insulator has become an important research topic with a wide range of applications.Especially the higher-order topological insulator,which possesses gapped edge states and corner or hinge states in the gap,provides a new scheme for the control of light in a hierarchy of dimensions.In this paper,we propose a heterostructure composed of ordinary-topological-ordinary(OTO)photonic crystal slabs.Two coupled edge states(CESs)are generated due to the coupling between the topological edge states of the ordinary-topological interfaces,which opens up an effective way for high-capacity photonic transport.In addition,we obtain a new band gap between the CESs,and the two kinds of coupled corner states(CCSs)appear in the OTO bend structure.In addition,the topological corner state is also found,which arises from the filling anomaly of a lattice.Compared with the previous topological photonic crystal based on C-4 lattice,CESs,CCSs,and the topological corner state are all directly observed in experiment by using the near-field scanning technique,which makes the manipulation of the electromagnetic wave more flexible.We also verify that the three corner states are all robust to defects.Our work opens up a new way for guiding and trapping the light flow and provides a useful case for the coupling of topological photonic states.

Breakdown of Maxwell Garnett theory due to evanescent fields at deep-subwavelength scale

Deep-subwavelength all-dielectric composite materials are believed to tightly obey the Maxwell Garnett effective medium theory. Here, we demonstrate that the Maxwell Garnett theory could break down due to evanescent fields in deep-subwavelength dielectric structures. By using two-and three-dimensional dielectric composite materials with inhomogeneities at a scale of λ∕100, we show that local evanescent fields generally occur near the dielectric inhomogeneities. When tiny absorptive constituents are placed there, the absorption and transmission of the whole composite will show strong dependence on the positions of the absorptive constituents. The Maxwell Garnett theory fails to predict such position-dependent characteristics because it averages out the evanescent fields. By taking the distribution of the evanescent fields into consideration, we have made a correction to the Maxwell Garnett theory so that the position-dependent characteristics become predictable. We reveal not only the breakdown of the Maxwell Garnett theory, but also a unique phenomenon of "invisible" loss induced by the prohibition of electric fields at deep-subwavelength scales. We believe our work promises a route to control the macroscopic properties of composite materials without changing their composition, which is beyond the traditional Maxwell Garnett theory.

Toxic Shock Syndrome Due to Streptococcus pyogenes Presenting with Purpura Fulminans inan Older Adult

Introduction Purpura fulminans(PF)was first described in 18861 and is characterized as a rare syndrome with a rapidly progressive course of hemorrhagic infarction of the skin and soft tissue necrosis.It mainly occurs in children and rarely in adults.1 PF is a life-threatening disease that can be a clinical presentation of toxic shock syndrome(TSS).2 The most common infectious etiology is cocci,and routine use of antibiotics for this disease is well recognized.However,another etiology,influenza virus,is often neglected.We herein report a case involving an older patient with TSS presenting with PF,in which Streptococcus pyogenes(S.pyogenes)accompanied by influenza virus might have played an important role,in order to arise the attention of clinician to the infection of the influenza virus in the pathogenesis of TSS.