The mechanism and effects of subgrade fluidisation under ballasted railway tracks

The rapid growth in railway infrastructure and the construction of high-speed heavy-haul rail network,especially on ground that is basically unsuitable,poses challenges for geotechnical engineers because a large part of the money invested in the development of railway lines is often spent on track maintenance.In fact around the world,the mud pumping of subgrade fines is one of the common reasons why track performance deteriorates and track stability is hindered.This article presents a series of laboratory tests to examine following aspects of mud pumping:(1)the mechanisms of subgrade fluidisation under undrained condition,(2)the effects of mud pumping on the engineering characteristics of ballast,and(3)the use of vertical drains to stabilize subgrade under cyclic loads.The undrained cyclic triaxial testing on vulnerable soft subgrade was performed by varying the cyclic stress ratio(CSR)from 0.2 to 1.0 and the loading frequency f from 1.0 to 5.0 Hz.It is seen from the test results that for a specimen compacted at an initial dry density of 1790 kg/m3,the top portion of the specimen fluidises at CSR=0.5,irrespective of the applied loading frequency.Under cyclic railway loading,the internal redistribution of water at the top of the subgrade layer softens the soil and also reduces its stiffness.In response to these problems,this paper explains how the inclusion of vertical drains in soft subgrade will help to prevent mud pumping by alleviating the build-up of excess pore pressures under moving train loads.

Diffraction-Limited Imaging with a Graphene Metalens

Planar graphene metalens has demonstrated advantages of ultrathin thickness(200 nm),high focusing resolution(343 nm)and efficiency(>32%)and robust mechanical strength and flexibility.However,diffraction-limited imaging with such a graphene metalens has not been realized,which holds the key to designing practical integrated imaging systems.In this work,the imaging rule for graphene metalenses is first derived and theoretically verified by using the Rayleigh-Sommerfeld diffraction theory to simulate the imaging performance of the 200 nm ultrathin graphene metalens.The imaging rule is applicable to graphene metalenses in different immersion media,including water or oil.Based on the theoretical prediction,high-resolution imaging using the graphene metalens with diffraction-limited resolution(500 nm)is demonstrated for the first time.This work opens the possibility for graphene metalenses to be applied in particle tracking,microfluidic chips and biomedical devices.

Ultra-high NA graphene oxide flat lens on a fiber facet with near diffraction-limited focusing

The realization of a high numerical aperture(NA) fiber lens is critical for achieving high imaging resolution in endoscopes, enabling subwavelength operation in optical tweezers and high efficiency coupling between optical fibers and photonic chips. However, it remains challenging with conventional design and fabrication. Here we propose an ultrathin(400 nm) graphene oxide(GO) film lens fabricated in situ on a standard single-mode fiber facet using the femtosecond laser direct writing technique. An extremely high NA of 0.89 is achieved with a near diffraction-limited focal spot(FWHM = 0.68λ), which is verified theoretically and experimentally. The diameter of the fabricated fiber GO lens is as small as 12 μm with no beam expansion structure. The proposed fiber GO lens is promising for applications such as super-resolution imaging, compact optical tweezers, medical endoscopes,and on-chip integration.

Integrated optical parametric amplifiers in silicon nitride waveguides incorporated with 2D graphene oxide films

Optical parametric amplification(OPA)represents a powerful solution to achieve broadband amplification in wavelength ranges beyond the scope of conventional gain media,for generating high-power optical pulses,optical microcombs,entangled photon pairs and a wide range of other applications.Here,we demonstrate optical parametric amplifiers based on silicon nitride(Si3N4)waveguides integrated with two-dimensional(2D)layered graphene oxide(GO)films.We achieve precise control over the thickness,length,and position of the GO films using a transfer-free,layer-by-layer coating method combined with accurate window opening in the chip cladding using photolithography.Detailed OPA measurements with a pulsed pump for the fabricated devices with different GO film thicknesses and lengths show a maximum parametric gain of~24.0 dB,representing a~12.2 dB improvement relative to the device without GO.We perform a theoretical analysis of the device performance,achieving good agreement with experiment and showing that there is substantial room for further improvement.This work represents the first demonstration of integrating 2D materials on chips to enhance the OPA performance,providing a new way of achieving high performance photonic integrated OPA by incorporating 2D materials.

Broadband Diffractive Graphene Orbital Angular Momentum Metalens by Laser Nanoprinting

Optical beams carrying orbital angular momentum(OAM)play an important role in micro-/nanoparticle manipulation and information multiplexing in optical communications.Conventional OAM generation setups require bulky optical elements and are unsuitable for on-chip integration.OAM generators based on metasurfaces can achieve ultracompact designs.However,they generally have limited working bandwidth and require complex designs and multistep time-consuming fabrication processes.In comparison,graphene metalenses based on the diffraction principle have simple designs and can be fabricated by laser nanoprinting in a single step.Here,we demonstrate that a single ultrathin(200 nm)graphene OAM metalens can integrate OAM generation and high-resolution focusing functions in a broad bandwidth,covering the entire visible wavelength region.Broadband graphene OAM metalenses with flexibly controlled topological charges are analytically designed using the detour phase method considering the dispersionless feature of the graphene material and fabricated using ultrafast laser nanoprinting.The experimental results agree well with the theoretical predictions,which demonstrate the accuracy of the design method.The broadband graphene OAM metalenses can find broad applications in miniaturized and integrated photonic devices enabled by OAM beams.

Diffraction-limited imaging with monolayer 2D material-based ultrathin flat lenses

Ultrathin flat optics allow control of light at the subwavelength scale that is unmatched by traditional refractive optics.To approach the atomically thin limit,the use of 2D materials is an attractive possibility due to their high refractive indices.However,achievement of diffraction-limited focusing and imaging is challenged by their thickness-limited spatial resolution and focusing efficiency.Here we report a universal method to transform 2D monolayers into ultrathin flat lenses.Femtosecond laser direct writing was applied to generate local scattering media inside a monolayer,which overcomes the longstanding challenge of obtaining sufficient phase or amplitude modulation in atomically thin 2D materials.We achieved highly efficient 3D focusing with subwavelength resolution and diffractionlimited imaging.The high focusing performance even allows diffraction-limited imaging at different focal positions with varying magnifications.Our work paves the way for downscaling of optical devices using 2D materials and reports an unprecedented approach for fabricating ultrathin imaging devices.

Graphene metalens for particle nanotracking

Particle nanotracking(PNT)is highly desirable in lab-on-a-chip systems for flexible and convenient multiparameter measurement.An ultrathin flat lens is the preferred imaging device in such a system,with the advantage of high focusing performance and compactness.However,PNT using ultrathin flat lenses has not been demonstrated so far because PNT requires the clear knowledge of the relationship between the object and image in the imaging system.Such a relationship still remains elusive in ultrathin flat lens-based imaging systems because they operate based on diffraction rather than refraction.In this paper,we experimentally reveal the imaging relationship of a graphene metalens using nanohole arrays with micrometer spacing.The distance relationship between the object and image as well as the magnification ratio is acquired with nanometer accuracy.The measured imaging relationship agrees well with the theoretical prediction and is expected to be applicable to other ultrathin flat lenses based on the diffraction principle.By analyzing the high-resolution images from the graphene metalens using the imaging relationship,3D trajectories of particles with high position accuracy in PNT have been achieved.The revealed imaging relationship for metalenses is essential in designing different types of integrated optical systems,including digital cameras,microfluidic devices,virtual reality devices,telescopes,and eyeglasses,and thus will find broad applications.

Surface and interface chemistry in metal‐free electrocatalysts for electrochemical CO_(2) reduction

The electrochemical reduction of carbon dioxide(CO_(2))into value‐added fuels and chemicals presents a sustainable route to alleviate CO_(2) emissions,promote carbon‐neutral cycles and reduce the dependence on fossil fuels.Considering the thermodynamic stability of the CO_(2) molecule and sluggish reaction kinetics,it is still a challenge to design highly efficient electrocatalysts for the CO_(2) reduction reaction(CO_(2)RR).It has been found that the surface and interface chemistry of electrocatalysts can modulate the electronic structure and increase the active sites,which is favorable for CO_(2) adsorption,electron transfer,mass transport,and optimizing adsorption strength of reaction intermediates.However,the effect of surface and interface chemistry on metal‐free electrocatalysts(MFEs)for CO_(2)RR has not been comprehensively reviewed.Herein,we discuss the importance of the surface and interface chemistry on MFEs for improving the electrochemical CO_(2)RR performance based on thermodynamic and kinetic views.The fundamentals and challenges of CO_(2)RR are firstly presented.Then,the recent advances of the surface and interface chemistry in improving reaction rate and overcoming reaction constraints are reviewed from regulating electronic structure,active sites,electron transfer,mass transport,and intermediate binding energy.Finally,the research challenges and prospects are proposed to suggest the future designs of advanced MFEs in CO_(2)RR.

Theoretical investigation of CoTa_(2)0_(6)/graphene heterojunctions for oxygen evolution reaction

Water electrolysis is to split water into hydrogen and oxygen using electricity as the driving force.To obtain low-cost hydrogen in a large scale,it is critical to develop electrocatalysts based on earth abundant elements with a high efficiency.This computational work started with Cobalt on CoTh_(2)C)_(6)surface as the active site,CoTa_(2)O_(6)/Graphene heterojunctions have been explored as potential oxygen evolution reaction(OER)catalysts through density functional theory(DFT).We demonstrated that the electron transfer(_(6))from CoTa_(2)C)_(6)to graphene substrate can be utilized to boost the reactivity of Co-site,leading to an OER overpotential as low as 0.30 V when N-doped graphene is employed.Our findings offer novel design of heterojunctions as high performance OER catalysts.

High tolerance detour-phase graphene-oxide flat lens

Flat lenses thinner than a wavelength promise to replace conventional refractive lenses in miniaturized optical systems.However,Fresnel zone plate flat lens designs require dense annuli,which significantly challenges nanofabrication resolution.Herein,we propose a new implementation of detour phase graphene flat lens with flexible annular number and width.Several graphene metalenses demonstrated that with a flexible selection of the line density and width,the metalenses can achieve the same focal length without significant distortions.This will significantly weaken the requirement of the nanofabrication system which is important for the development of large-scale flat lenses in industry applications.

Broadband angular momentum cascade via a multifocal graphene vortex generator

Light beams carrying orbital angular momentum(OAM)have inspired various advanced applications,and such abundant practical applications in turn demand complex generation and manipulation of optical vortices.Here,we propose a multifocal graphene vortex generator,which can produce broadband angular momentum cascade containing continuous integer non-diffracting vortex modes.Our device naturally embodies a continuous spiral slit vortex generator and a zone plate,which enables the generation of high-quality continuous vortex modes with deep depths of foci.Meanwhile,the generated vortex modes can be simultaneously tuned through incident wavelength and position of the focal plane.The elegant structure of the device largely improves the design efficiency and can be fabricated by laser nanofabrication in a single step.Moreover,the outstanding property of graphene may enable new possibilities in enormous practical applications,even in some harsh environments,such as aerospace.