Pressure-mediated contact quality improvement between monolayer MoS_2 and graphite

Two-dimensional(2D) materials and their heterostructures have attracted a lot of attention due to their unique electronic and optical properties. MoS_2 as the most typical 2D semiconductors has great application potential in thin film transistors, photodetector, hydrogen evolution reaction, memory device, etc. However, the performance of MoS_2 devices is limited by the contact resistance and the improvement of its contact quality is important. In this work, we report the experimental investigation of pressure-enhanced contact quality between monolayer MoS_2 and graphite by conductive atom force microscope(C-AFM). It was found that at high pressure, the contact quality between graphite and MoS_2 is significantly improved. This pressure-mediated contact quality improvement between MoS_2 and graphite comes from the enhanced charge transfer between MoS_2 and graphite when MoS_2 is stretched. Our results provide a new way to enhance the contact quality between MoS_2 and graphite for further applications.

Thermally induced band hybridization in bilayer-bilayer MoS2/WS2 heterostructure

Transition metal dichalcogenides(TMDs),being valley selectively,are an ideal system hosting excitons.Stacking TMDs together to form heterostructure offers an exciting platform to engineer new optical and electronic properties in solid-state systems.However,due to the limited accuracy and repetitiveness of sample preparation,the effects of interlayer coupling on the electronic and excitonic properties have not been systematically investigated.In this report,we study the photoluminescence spectra of bilayer-bilayer MoS_(2)/WS_(2) heterostructure with a typeⅡband alignment.We demonstrate that thermal annealing can increase interlayer coupling in the van der Waals heterostructures,and after thermally induced band hybridization such heterostructure behaves more like an artificial new solid,rather than just the combination of two individual TMD components.We also carry out experimental and theoretical studies of the electric controllable direct and indirect infrared interlayer excitons in such system.Our study reveals the impact of interlayer coupling on interlayer excitons and will shed light on the understanding and engineering of layer-controlled spin-valley configuration in twisted van der Waals heterostructures.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light-matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications,which is intrinsically determined by the bandgap of the materials involved.To extend the applications beyond the bandgap limit,it is of great significance to study the light-matter interaction below the material bandgap.Here,we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from-0.86 pm to 1.4 pm.Even though this spectral range is below the bandgap,we observe a significant absorbance enhancement up to-4.2%in the monolayer molybdenum disulfide(comparable to its absorption within the bandgap region)due to pump-induced absorption by the excited carrier states.The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states(i.e.,real carrier states in the short-wavelength region of~<1μm,and exciton states in the long wavelength region of~>1μm).Our results elucidate the fundamental understanding regarding the optical properties,excited carrier states,and carrier dynamics in the technologically important near-infrared region,which potentially leads to various photonic and optoelectronic applications(e.g„excited-state-based photodetectors and modulators)of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

Nonlinear optical absorption properties of InP nanowires and applications as a saturable absorber

Indium phosphide(InP)nanowires(NWs)have attracted significant attention due to their exotic properties that are different from the bulk counterparts,and have been widely used for light generation,amplification,detection,modulation,and switching,etc.Here,high-quality InP NWs were directly grown on a quartz substrate by the Au-nanoparticle assisted vapor-liquid-solid method.We thoroughly studied their nonlinear optical absorption properties at 1.06μm by the open-aperture Z-scan method.Interestingly,a transition phenomenon from satu-rable absorption(SA)to reverse saturable absorption(RSA)was observed with the increase of the incident laser intensity.In the analysis,we found that the effective nonlinear absorption cofficient(βeff-10^2 cm/MW)under the SA process was 3 orders of magnitude larger than that during the RSA processes.Furthermore,the SA proper-ties of InP NWs were experimentally verified by using them as a saturable absorber for a passively Q-switched Nd:YVO4 solid-state laser at 1.06μm,where the shortest pulse width of 462 ns and largest single pulse energy of 1.32μJ were obtained.Moreover,the ultrafast carrier relaxation dynamics were basically studied,and the intraband and inter-band ultrafast carrier relaxation times of 8.1 and 63.8 ps,respectively,were measured by a degenerate pump-probe method with the probe laser of 800 nm.These results well demonstrate the nonlinear optical absorption properties,which show the excellent light manipulating capabilities of InP NWs and pave a way for their applications in ultrafast nanophotonic devices.

Quantitative in situ measurement of optical force along a strand of cleaved silica optical fiber induced by the light guided therewithin

We propose an optomechanical system to quantify the net force on a strand of cleaved silica optical fiber in situ as the laser light is being guided through it.Four strands of the fiber were bonded to both sides of a macroscopic oscillator,whose movements were accurately monitored by a Michelson interferometer.The laser light was propagating with variable optical powers and frequency modulations.Experimentally,we discovered that the driving force for the oscillator consisted of not only the optical force of the light exiting from the cleaved facets but also the tension along the fiber induced by the light guided therewithin.The net driving force was determined only by the optical power,refractive index of the fiber,and the speed of light,which pinpoints its fundamental origin.

Ultrafast carrier dynamics and nonlinear optical response of InAsP nanowires

Indium arsenide phosphide(In As P)nanowires(NWs),a member of theⅢ–Ⅴsemiconductor family,have been used in various photonic and optoelectronic applications thanks to their unique electrical and optical properties such as high carrier mobility and adjustable band gap.In this work,we synthesize In As P NWs and further explore their nonlinear optical properties.The ultrafast carrier dynamics and nonlinear optical response are thoroughly studied based on the nondegenerate pump probe and Z-scan experimental measurements.Two different characteristic carrier lifetimes(~2 and~15 ps)from In As P NWs are observed during the excited-carrier relaxation process.Based on the physical model analysis,the relaxation process can be ascribed to the carrier cooling process via carrier-phonon scattering and Auger recombination.In addition,based on the measured excited-carrier lifetime and Pauli-blocking principle,we discover that In As P NWs show strong saturable absorption properties at the wavelengths of 532 and 1064 nm.Last,we demonstrate for the first time a femtosecond(~426 fs)solid-state laser based on an In As P NWs saturable absorber at 1.04μm.We believe that our work provides a better understanding of the In As P NWs optical properties and will further advance their photonic applications in the near-infrared range.

Science Missions Using CubeSats

As the role of missions and experiments carried out in outer space becomes more and more essential in our understanding of many earthly problems,such as resource management,environmental problems,and disaster management,as well as space science questions,thanks to their lower cost and faster development process CubeSats can benefit humanity and therefore,young scientists and engineers have been motivated to research and develop new CubeSat missions.Not very long after their inception,CubeSats have evolved to become accepted platforms for scientific and commercial applications.The last couple of years showed that they are a feasible tool for conducting scientific experiments,not only in the Earth orbit but also in the interplanetary space.For many countries,a CubeSat mission could prompt the community and young teams around the world to build the national capacity to launch and operate national space missions.This paper presents an overview of the key scientific and engineering gateways opened up to the younger scientific community by the advent and adaptation of new technology into CubeSat missions.The role of cooperation and the opportunities for capacity-building and education are also explored.Thus,the present article also aims to provide useful recommendations to scientists,early-career researchers,engineers,students,and anyone who intends to explore the potential and opportunities offered by CubeSats and CubeSats-based missions.

High-quality hybrid dielectric materials for 2D electronics

Silicon-based field effect transistors(FETs)are the building blocks of modern electronics,serving as the cornerstone.However,current silicon technology is approaching its performance-downscaling limits[1].This challenge primarily stems from the substantial decline in electron mobility as silicon-based semiconductor thickness decreases,coupled with the pronounced emergence of short-channel effects upon FET miniaturization[1,2].

Real- and momentum-indirect neutral and charged excitons in a multi-valley semiconductor

Excitons dominate the photonic and optoelectronic properties of a material.Although significant advancements exist in understanding various types of excitons,progress on excitons that are indirect in both real-and momentum-spaces is still limited.Here,we demonstrate the real-and momentum-indirect neutral and charged excitons(including their phonon replicas)in a multi-valley semiconductor of bilayer MoS_(2),by performing electric-field/doping-density dependent photoluminescence.Together with first-principles calculations,we uncover that the observed real-and momentum-indirect exciton involves electron/hole from K/Γvalley,solving the longstanding controversy of its momentum origin.Remarkably,the binding energy of real-and momentum-indirect charged exciton is extremely large(i.e.,~59 meV),more than twice that of real-and momentum-direct charged exciton(i.e.,~24 meV).The giant binding energy,along with the electrical tunability and long lifetime,endows real-and momentum-indirect excitons an emerging platform to study many-body physics and to illuminate developments in photonics and optoelectronics.

Review of fabrication methods of large-area transparent graphene electrodes for industry

Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes;it has low sheet resistance,high optical transmission and is flexible.Whereas the most common transparent electrode material,tin-doped indium-oxide(ITO)is brittle,less transparent and expensive,which limit its compatibility in flexible electronics as well as in low-cost devices.Here we review two large-area fabrication methods for graphene based transparent electrodes for industry:liquid exfoliation and low-pressure chemical vapor deposition(CVD).We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results.State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43.5,slightly over the minimum required for industry of 35,while CVD reaches as high as 419.

Mode couplings of a semiconductor nanowire scanning across a photonic crystal nanocavity

The position-dependent mode couplings between a semiconductor nanowire(NW)and a planar photonic crystal(PPC)nanocavity are studied.By scanning an NW across a PPC nanocavity along the hexagonal lattice’sΓ– M and M– K directions,the variations of resonant wavelengths,quality factors,and mode volumes in both fundamental and second-order resonant modes are calculated,implying optimal configurations for strong mode-NW couplings and light-NW interactions.For the fundamental(second-order)resonant mode,scanning an NW along the M– K(Γ– M)direction is preferred,which supports stronger light-NW interactions with larger NW-position tolerances and higher quality factors simultaneously.The simulation results are confirmed experimentally with good agreements.

Microwave Tunneling and Robust Information Transfer Based on Parity-Time-Symmetric Absorber-Emitter Pairs

Robust signal transfer in the form of electromagnetic waves is of fundamental importance in modern technology,yet its operation is often challenged by unwanted modifications of the channel connecting transmitter and receiver.Parity-time-(PT-)symmetric systems,combining active and passive elements in a balanced form,provide an interesting route in this context.Here,we demonstrate a PT-symmetric microwave system operating in the extreme case in which the channel is shorted through a small reactance,which acts as a nearly impenetrable obstacle,and it is therefore expected to induce large reflections and poor transmission.After placing a gain element behind the obstacle,and a balanced lossy element in front of it,we observe full restoration of information and overall transparency to an external observer,despite the presence of the obstacle.Our theory,simulations,and experiments unambiguously demonstrate stable and robust wave tunneling and information transfer supported by PT symmetry,opening opportunities for efficient communication through channels with dynamic changes,active filtering,and active metamaterial technology.

Coherent modulation of chiral nonlinear optics with crystal symmetry

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS2) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).

Nanowire-assisted microcavity in a photonic crystal waveguide and the enabled high-efficiency optical frequency conversions

We report an indium phosphide nanowire(NW)-induced cavity in a silicon planar photonic crystal(PPC)waveguide to improve the light–NW coupling.The integration of NW shifts the transmission band of the PPC waveguide into the mode gap of the bare waveguide,which gives rise to a microcavity located on the NW section.Resonant modes with𝑄factors exceeding 103 are obtained.Leveraging on the high density of the electric field in the microcavity,the light–NW interaction is enhanced strongly for efficient nonlinear frequency conversion.Second-harmonic generation and sum-frequency generation in the NW are realized with a continuous-wave pump laser in a power level of tens of microwatts,showing a cavity-enhancement factor of 112.The hybrid integration structure of NW-PPC waveguide and the self-formed microcavity not only opens a simple strategy to effectively enhance light–NW interactions,but also provides a compact platform to construct NW-based on-chip active devices.

Erratum to “Microwave Tunneling and Robust Information Transfer Based on Parity-Time-Symmetric Absorber-Emitter Pairs”

In the article titled“Microwave Tunneling and Robust Information Transfer Based on Parity-Time-Symmetric Absorber-Emitter Pairs”[1],there were errors in Figure 2 which occurred during production.In panel(c),the red line should be attributed to“Re(ZNIC/Z0)”and the blue line should be attributed to“Im(ZNIC/Z0).”In panel(d),the blue line should read“∣S 21∣=∣S12∣”and the black line should read“|S22|.”The corrected figure is shown as Figure 1 below.

Unveiling bulk and surface radiation forces in a dielectric liquid

Precise control over light-matter interactions is critical for many optical manipulation and material characterization methodologies,further playing a paramount role in a host of nanotechnology applications.Nonetheless,the fundamental aspects of interactions between electromagnetic fields and matter have yet to be established unequivocally in terms of an electromagnetic momentum density.Here,we use tightly focused pulsed laser beams to detect bulk and boundary optical forces in a dielectric fluid.From the optical convoluted signal,we decouple thermal and nonlinear optical effects from the radiation forces using a theoretical interpretation based on the Microscopic Ampère force density.It is shown,for the first time,that the time-dependent pressure distribution within the fluid chiefly originates from the electrostriction effects.Our results shed light on the contribution of optical forces to the surface displacements observed at the dielectric air-water interfaces,thus shedding light on the long-standing controversy surrounding the basic definition of electromagnetic momentum density in matter.

Direct nanoscopic observation of plasma waves in the channel of a graphene field-effect transistor

Plasma waves play an important role in many solid-state phenomena and devices.They also become significant in electronic device structures as the operation frequencies of these devices increase.A prominent example is field-effect transistors(FETs),that witness increased attention for application as rectifying detectors and mixers of electromagnetic waves at gigahertz and terahertz frequencies,where they exhibit very good sensitivity even high above the cut-off frequency defined by the carrier transit time.Transport theory predicts that the coupling of radiation at THz frequencies into the channel of an antenna-coupled FET leads to the development of a gated plasma wave,collectively involving the charge carriers of both the two-dimensional electron gas and the gate electrode.In this paper,we present the first direct visualization of these waves.Employing graphene FETs containing a buried gate electrode,we utilize near-field THz nanoscopy at room temperature to directly probe the envelope function of the electric field amplitude on the exposed graphene sheet and the neighboring antenna regions.Mapping of the field distribution documents that wave injection is unidirectional from the source side since the oscillating electrical potentials on the gate and drain are equalized by capacitive shunting.The plasma waves,excited at 2 THz,are overdamped,and their decay time lies in the range of 25-70 fs.Despite this short decay time,the decay length is rather long,i.e.,0.3-0.5μm,because of the rather large propagation speed of the plasma waves,which is found to lie in the range of 3.5-7×10^(6)m/s,in good agreement with theory.The propagation speed depends only weakly on the gate voltage swing and is consistent with the theoretically predicted 1/4 power law.

Reconstructive spectrometers taper down in price

The development of a low-cost compact reconstructive spectrometer paves the way towards portable pm-resolution spectroscopy.

Two-dimensional MXenes for lithium-sulfur batteries

Rechargeable lithium-sulfur(Li-S)batteries have attracted significant research attention due to their high capacity and energy density.However,their commercial applications are still hindered by challenges such as the shuttle effect of soluble lithium sulfide species,the insulating nature of sulfur,and the fast capacity decay of the electrodes.Various efforts are devoted to address these problems through questing more conductive hosts with abundant polysulfide chemisorption sites,as well as modifying the separators to physically/chemically retard the polysulfides migration.Two dimensional transition metal carbides,carbonitrides and nitrides,so-called MXenes,are ideal for confining the polysulfides shuttling effects due to their high conductivity,layered structure as well as rich surface terminations.As such,MXenes have thus been widely studied in Li-S batteries,focusing on the conductive sulfur hosts,polysulfides interfaces,and separators.Therefore,in this review,we summarize the significant progresses regarding the design of multifunctional MXene-based Li-S batteries and discuss the solutions for improving electrochemical performances in detail.In addition,challenges and perspectives of MXenes for Li-S batteries are also outlined.

Giant Valley Coherence at Room Temperature in 3R WS_(2) with Broken Inversion Symmetry

Breaking the space-time symmetries in materials can markedly influence their electronic and optical properties.In 3R-stacked transition metal dichalcogenides,the explicitly broken inversion symmetry enables valley-contrasting Berry curvature and quantization of electronic angular momentum,providing an unprecedented platform for valleytronics.Here,we study the valley coherence of 3R WS_(2) large single-crystal with thicknesses ranging from monolayer to octalayer at room temperature.Our measurements demonstrate that both A and B excitons possess robust and thickness-independent valley coherence.The valley coherence of direct A(B)excitons can reach 0.742(0.653)with excitation conditions on resonance with it.Such giant and thickness-independent valley coherence of large single-crystal 3R WS_(2) at room temperature would provide a firm foundation for quantum manipulation of the valley degree of freedom and practical application of valleytronics.