Switching between the Functions of Half-Wave Plate and Quarter-Wave Plate Simply by Using a Vanadium Dioxide Film in a Terahertz Metamaterial

We propose a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate.The two switchable functions can be simply achieved by inserting a VO2 film in the metamaterial design.Finite-difference time-domain(FDTD)simulation results show that the proposed metamaterial can convert x-polarized incident wave to y-polarized reflected wave when VO2 is at metal phase,and convert x-polarized wave to circularly polarized wave when VO2 is at insulator phase.The metamaterial performs well in the two functions,i.e.,the same broad working frequency band and near perfect polarization conversion.The switching effect originates from the switchable Fabry-Perot cavity length induced by the phase change of VO2.We believe that our findings provide a reference in designing switchable metamaterials.

Numerical study of optical trapping properties of nanoparticle on metallic film with periodic structure

Based on the three-dimensional dispersive finite difference time domain method and Maxwell stress tensor equation,the optical trapping properties of nanoparticle placed on the gold film with periodic circular holes are investigated numerically. Surface plasmon polaritons are excited on the metal-dielectric interface, with particular emphasis on the crucial role in tailoring the optical force acting on a nearby nanoparticle. Utilizing a first order corrected electromagnetic field components for a fundamental Gaussian beam, the incident beam is added into the calculation model of the proposed method. To obtain the detailed trapping properties of nanoparticle, the selected calculations on the effects of beam waist radius, sizes of nanoparticle and circular holes, distance between incident Gaussian beam and gold film, material of nanoparticle and polarization angles of incident wave are analyzed in detail to demonstrate that the optical-trapping force can be explained as a virtual spring which has a restoring force to perform positive and negative forces as a nanoparticle moves closer to or away from the centers of circular holes. The results of optical trapping properties of nanoparticle in the vicinity of the gold film could provide guidelines for further research on the optical system design and manipulation of arbitrary composite nanoparticles.

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method.Based on the plane-wave angular spectrum expansion and Fresnel approximation,the analytical expressions of the reflected field,internal field as well as transmitted field in each region are obtained.Through numerical simulations,the intensity distributions of the incident beam,reflected beam,internal beam as well as transmitted beam are presented at oblique incidence.Besides,we also compare the intensity distributions of the geometrical-optics beam field,the first order beam mode field and the actual beam field,which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab.Meanwhile,the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated.Finally,the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail.The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.

Ratchet transport of self-propelled chimeras in an asymmetric periodic structure

We studied the rectified transport of underdamped particles subject to phase lag in an asymmetric periodic structure.When the inertia effect is considered,it is possible to observe reversals of the average velocity with small self-propelled force,whereas particles always move in the positive direction with large self-propelled force.The introduction of phase lag leads particles to follow circular orbits and suppress the polar motion.In addition,this can adjust the direction of particle motion.There exists an optimal value of polar interaction strength at which the rectification is maximal.These results open the way for many application processes,such as spatial sorting of particles mixture and separation based on their physical properties.

Characteristics and mechanism analysis of Fano resonances in Π-shaped gold nano-trimer

Fano interference of metallic nanostructure is an effective way to reduce the irradiation loss and improve the spectral resolution. A Π-shaped gold nano-trimer, which is composed of a gold nanorod and two gold nanorices, is presented to investigate the properties of Fano resonances in the visible spectrum by using the finite element method （FEM）. The theoretical analysis demonstrates that the Fano resonance of the Π-shaped gold nano-trimer is attributed to the near-field interaction between the bright mode of the nanorice pair and the dark quadrupole mode of the nanorod. Furthermore, by breaking the geometric symmetry of the nanostructure the line-shape spectrum with double Fano resonances of Π-shaped gold nano-trimer is obtained and exhibits structure-dependent and medium-dependent characteristics. It is a helpful strategy to design a plasmonic nanostructure for implementing multiple Fano resonances in practical applications.

Quantum plasmon enhanced nonlinear wave mixing in graphene nanoflakes

A distant-neighbor quantum-mechanical method is used to study the nonlinear optical wave mixing in graphene nanoflakes(GNFs),including sum-and difference-frequency generation,as well as four-wave mixing.Our analysis shows that molecular-scale GNFs support quantum plasmons in the visible spectrum region,and significant enhancement of nonlinear optical wave mixing is achieved.Specifically,the second-and third-order wave-mixing polarizabilities of GNFs are dramatically enhanced,provided that one(or more) of the input or output frequencies coincide with a quantum plasmon resonance.Moreover,by embedding a cavity into hexagonal GNFs,we show that one can break the structural inversion symmetry and enable otherwise forbidden second-order wave mixing,which is found to be enhanced by the quantum plasmon resonance too.This study reveals that the molecular-sized graphene could be used in the quantum regime for nanoscale nonlinear optical devices and ultrasensitive molecular sensors.

Multi-functional vanadium dioxide integrated metamaterial for terahertz wave manipulation

We proposed a vanadium dioxide(VO2)-integrated multi-functional metamaterial structure that consists of three metallic grating layers and two VO2 films separated by SiO2 dielectric spacers.The proposed structure can be flexibly switched among three states by adjusting temperature,incident direction,and polarization.In state 1,the incident wave is strongly transmitted and perfectly converted to its orthogonal polarization state.In state 2,the incident wave is perfectly absorbed.In state 3,incident wave is totally reflected back.The working frequency of the multi-functional metamaterial can be arbitrarily tuned within a broad pass band.We believe that our findings are beneficial in designing temperature-controlled metadevices.

Difference scattering field properties between periodic defect particles and three-dimensional slightly rough optical surface

Based on the practical situation of nondestructive examination, the calculation model of the composite scattering is established by using a three-dimensional half-space finite difference time domain, and the Monte Carlo method is used to solve the problem of the optical surface with roughness in the proposed scheme. Moreover, the defect particles are observed as periodic particles for a more complex situation. In order to obtain the scattering contribution of defects inside the optical surface, a difference radar cross section is added into the model to analyze the selected calculations on the effects of numbers, separation distances, different depths and different materials of defects. The effects of different incident angles are also discussed. The numerical results are analyzed in detail to demonstrate the best position to find the defects in the optical surface by detecting in steps of a fixed degree for the incident angle.

Two-tier control structure design methodology applied to heat exchanger networks

Because of its paramount importance in the successful industrial control strategy of a given heat exchanger network(HEN),the control structure designs for providing appropriate manipulated variable(MV)and controlled variable pairings have received considerable attention.However,quite frequently HENs with such control structures face the problem of hard constraints,typically holding the HENs at less controlled operating space.So both the MV pairings and the above control pairings should be considered to design a control structure.This paper investigates the systematic incorporation of the two pairings,and presents a methodology for designing such two-tier control structure.This is developed based on the sequential strategy,coupling an indirect-tier with direct-tier control structure design,wherein the intention is realized in the former stage and the latter is implemented for further optimization.The MV identification and pairing are achieved through variations in heat load of heat exchangers to design the indirect-tier control structure.Then the direct-tier control structure is followed the relative gain array pairing rules.With the proposed methodology,on the one hand,it generates an explicit connection between the MV pairings and the HEN configuration,and the quantitative interaction measure is improved to avoid the multiple solutions to break the relationship among all the control pairings into individuals;on the other hand,a two-tier control structure reveals control potentials and control system design requirements,this may avoid complex and economically unfavourable control and HEN structures.The application of proposed framework is illustrated with two cases involving the dynamic simulation analysis,the quantitative assessment and the random test.

Optical solitons supported by finite waveguide lattices with diffusive nonlocal nonlinearity

We investigate the properties of fundamental,multi-peak,and multi-peaked twisted solitons in three types of finite waveguide lattices imprinted in photorefractive media with asymmetrical diffusion nonlinearity.Two opposite soliton selfbending signals are considered for different families of solitons.Power thresholdless fundamental and multi-peaked solitons are stable in the low power region.The existence domain of two-peaked twisted solitons can be changed by the soliton self-bending signals.When solitons tend to self-bend toward the waveguide lattice,stable two-peaked twisted solitons can be found in a larger region in the middle of their existence region.Three-peaked twisted solitons are stable in the lower(upper)cutoff region for a shallow(deep)lattice depth.Our results provide an effective guidance for revealing the soliton characteristics supported by a finite waveguide lattice with diffusive nonlocal nonlinearity.

Hydroxylated metal–organic-layer nanocages anchoring single atomic cobalt sites for robust photocatalytic CO_(2) reduction

Assembly of two-dimensional(2D)metal–organic layers(MOLs)based on the hard and soft acid–base theorem represents an exquisite strategy for the construction of photocatalytic platforms in virtue of the highly exposed active sites,much improved mass transport,and greatly elevated stability.Herein,nanocages composed of MOLs are produced for the first time through a cosolvent approach utilizing zirconium-based UiO-66-(OH)2 as the structural precursor.To endow the catalytic activity for CO_(2) conversion,single atomic Co^(2+)sites are appended to the Zr-oxo nodes of the MOL cages,demonstrating a remarkable CO yield of 7.74 mmol·g^(-1)·h^(-1) and operational stability of 97.1%product retention after five repeated cycles.Such an outstanding photocatalytic performance is mainly attributed to the unique nanocage morphology comprising enormous 2D nanosheets for augmented Co^(2+)exposure and the abundant surface hydroxyl groups for local CO_(2) enrichment.This work underlines the tailoring of both metal–organic framework(MOF)morphology and functionality to boost the turnover rate of photocatalytic CO_(2) reduction reaction(CO_(2)RR).

Fast-chargingand dendrite-free lithiummetal anodeeenabledby partial lithiation ofgraphene aerogel

The development of deeply cyclable lithium metal batteries with fast-charging capability offers a promising solution to relieve the“range anxiety”in driving electric vehicles.Conventional lithium metal anodes suffered from low operating current densities and shallow charge/discharge depths,owing to the intrinsic dendrite growth governed by Sand’s law.Herein,we come up with a novel design of heavy-duty lithium metal anode fabricated by partially infusing the three-dimensional(3D)porous graphene aerogel with molten Li.Both electroanalytical measurements and simulations show that the unique electrode architecture brings notable advantages in mediating smooth Li plating/stripping,including reduced local current density,inhibited dendrite growth,buffered volume fluctuation,as well as more efficient Li utilization.Consequently,a remarkable cycling performance in symmetric cells for over 400 cycles(800 h)with an ultrahigh cycling capacity of 15 mAh·cm^(−2) at 15 mA·cm^(−2) is achieved,which,to our best knowledge,has been never seen in literature.LiFePO4 full cells demonstrate a superb rate capability up to 10 C and a prolonged cycling of 1,600 cycles at 2 C with the per-cycle capacity decay of only 0.023%.This study paves the way for the ultimate deployment of lithium metal batteries in real-world applications that require fast charging and deep cycling.

Chemical vapor deposition growth of phase-selective inorganic lead halide perovskite films for sensitive photodetectors

Inorganic lead halide perovskites are attractive optoelectronic materials owing to their relative stability compared to organic cation alternatives.The chemical vapor deposition(CVD) method offers potential for high quality perovskite film growth.The deposition temperature is a critical parameter determining the film quality owing to the melting difference between the precursors.Here,perovskite films were deposited by the CVD method at various temperatures between 500-800℃.The perovskite phase converts from CsPb_(2)Br_(5) to CsPbBr_(3) gradually as the deposition temperature is increased.The grain size of the perovskite films also increases with temperature.The phase transition mechanism was clarified.The photoexcited state dynamics were investigated by spatially and temporally resolved fluorescence measurements.The perovskite film deposited under 750℃ condition is of the CsPbBr_(3) phase,showing low trap-state density and large crystalline grain size.A photodetector based on perovskite films shows high photocurrent and an on/off ratio of ~2.5×10^(4).

Electrochemical CO_(2) reduction catalyzed by organic/inorganic hybrids

Electroreduction of CO_(2) into value-added chemicals and fuels utilizing renewable electricity offers a sustainable way to meet the carbon-neutral goal and a viable solution for the storage of intermittent green energy sources.At the core of this technology is the development of electrocatalysts to accelerate the redox kinetics of CO_(2) reduction reactions(CO_(2)RR)toward high targeted-product yield at minimal energy input.This perspective focuses on a unique category of CO_(2)RR electrocatalysts embodying both inorganic and organic components to synergistically promote the reaction activity,selectivity and stability.First,we summarize recent progress on the design and fabrication of organic/inorganic hybrids CO_(2)RR electrocatalysts,with special attention to the assembly protocols and structural configurations.We then carry out a comprehensive discussion on the mechanistic understanding of CO_(2)RR processes tackled jointly by the inorganic and organic phases,with respect to the regulation of mass and charge transport,modification of double-layer configuration,tailoring of intermediates adsorption,and establishment of tandem pathways.At the end,we outline future challenges in the rational design of organic/inorganic hybrids for CO_(2)RR and further extend the scope to the device level.We hope this work could incentivize more research interests to construct organic/inorganic hybrids for mobilizing electrocatalytic CO_(2)RR towards industrialization.