High efficient Raman sideband cooling and strong three-body recombination of atoms

We report a highly efficient three-dimensional degenerated Raman sideband cooling(3D dRSC)that enhances the loading of a magnetically levitated optical dipole trap,and observe the strong atom loss due to the three-body recombination.The 3D dRSC is implemented to obtain 5×10^(7)Cs atoms with the temperature of~480 nK.The cold temperature enables 1.8×10^(7)atoms loaded into a crossed dipole trap with an optimized excessive levitation magnetic gradient.Compared to the loading of atoms from a bare magneto-optical trap or the gray-molasses cooling,there is a significant increase in the number of atoms loaded into the optical dipole trap.We derive for the three-body recombination coefficient of L_(3)=7.73×10^(-25)cm^(6)/s by analyzing the strong atom loss at a large scattering length of 1418 Bohr radius,and discover the transition from the strong three-body loss to the dominant one-body loss.Our result indicates that the lifetime of atoms in the optical dipole trap is finally decided by the one-body loss after the initial strong three-body loss.

Mechanisms and efficient elimination approaches of self-absorption in LIBS

Laser-induced breakdown spectroscopy(LIBS) is a promising analytical spectroscopy technology based on spectroscopic analysis of the radiation emitted by laser-produced plasma.However, for quantitative analysis by LIBS, the so-called self-absorption effects on the spectral lines, which affect plasma characteristics, emission line shapes, calibration curves, etc, can no longer be neglected. Hence, understanding and determining the self-absorption effects are of utmost importance to LIBS research. The purpose of this review is to provide a global overview of self-absorption in LIBS on the issues of experimental observations and adverse effects,physical mechanisms, correction or elimination approaches, and utilizations in the past century.We believe that better understanding and effective solving the self-absorption effect will further enhance the development and maturity of LIBS.

Accurate quantitative CF-LIBS analysis of both major and minor elements in alloys via iterative correction of plasma temperature and spectral intensity

The chemical composition of alloys directly determines their mechanical behaviors and application fields.Accurate and rapid analysis of both major and minor elements in alloys plays a key role in metallurgy quality control and material classification processes.A quantitative calibration-free laser-induced breakdown spectroscopy(CF-LIBS)analysis method,which carries out combined correction of plasma temperature and spectral intensity by using a secondorder iterative algorithm and two boundary standard samples,is proposed to realize accurate composition measurements.Experimental results show that,compared to conventional CF-LIBS analysis,the relative errors for major elements Cu and Zn and minor element Pb in the copperlead alloys has been reduced from 12%,26%and 32%to 1.8%,2.7%and 13.4%,respectively.The measurement accuracy for all elements has been improved substantially.

Effect of external magnetic field on the shift of resonant frequency in photoassociation of ultracold Cs atoms

We study the influence of external magnetic field on the shift of the resonant frequency in the photoassociation of ultracold Cs atoms, which are captured in a magnetically levitated optical crossed dipole trap. With the increase of the photoassociation laser intensity, the linear variation of the frequency shift is measured by recording the photoassociation spectra of the long-range 0_u^+ state of Cs molecule below the 6S_(1/2)+ 6P_(1/2) dissociation limit at different magnetic fields.The slope of the frequency shift to the intensity of the photoassociation laser exhibits a strong dependence on the external magnetic field. The experimental data is simulated with an analytic theory model, in which a single channel rectangular potential with the tunable well depth is introduced to acquire the influence of the magnetic field on the atomic behavior in the effective range where photoassociation occurs.

High-precision three-dimensional Rydberg atom localization in a four-level atomic system

Rydberg atoms have been widely investigated due to their large size,long radiative lifetime,huge polarizability and strong dipole-dipole interactions.The position information of Rydberg atoms provides more possibilities for quantum optics research,which can be obtained under the localization method.We study the behavior of three-dimensional(3D)Rydberg atom localization in a four-level configuration with the measurement of the spatial optical absorption.The atomic localization precision depends strongly on the detuning and Rabi frequency of the involved laser fields.A 100%probability of finding the Rydberg atom at a specific 3D position is achieved with precision of~0.031λ.This work demonstrates the possibility for achieving the 3D atom localization of the Rydberg atom in the experiment.

Highly sensitive detection of Rydberg atoms with fluorescence loss spectrum in cold atoms

Fluorescence loss spectrum for detecting cold Rydberg atoms with high sensitivity has been obtained based on lock-in detection of fluorescence of 6 P3/2 state when cooling lasers of the magneto-optical trap are modulated.The experiment results show that the signal to noise ratio has been improved by 32.64 dB when the modulation depth(converted to laser frequency)and frequency are optimized to 4 MHz and 6 kHz,respectively.This technique enables us to perform a highly sensitive non-destructive detection of Rydberg atoms.

Numerical simulation of laser-induced plasma in background gas considering multiple interaction processes

Laser-induced plasma is often produced in the presence of background gas,which causes some new physical processes.In this work,a two-dimensional axisymmetric radiation fluid dynamics model is used to numerically simulate the expansion process of plasma under different pressures and gases,in which the multiple interaction processes of diffusion,viscosity and heat conduction between the laser ablated target vapor and the background gas are further considered,and the spatio-temporal evolutions of plasma parameters(species number density,expansion velocity,size and electron temperature)as well as the emission spectra are obtained.The consistency between the actual and simulated spectra of aluminum plasma in 1 atm argon verifies the correctness of the model and the numerical simulation,thus providing a refinement analysis method for the basic research of plasma expansion in gases and the application of laser-induced breakdown spectroscopy.

Superfluid to Mott-insulator transition in a one-dimensional optical lattice

Bose–Einstein condensates(BEC)of sodium atoms are transferred into one-dimensional(1D)optical lattice potentials,formed by two laser beams with a wavelength of 1064 nm,in a shallow optical trap.The phase coherence of the condensate in the lattice potential is studied by changing the lattice depth.A qualitative change in behavior of the BEC is observed at a lattice depth of~13.7Er,where the quantum gas undergoes a transition from a superfluid state to a state that lacks well-to-well phase coherence.

Microwave Induced Ultralong-Range Charge Migration in a Rydberg Atom

A microwave induced superposition of the 40S_(1/2) and 40P_(1/2) states of a Cs atom has been investigated in detail.Ultralong-range charge migration which spans a region more than 200 nm has been discovered. As far as we know, this is the first time to discover charge migration in such a long range. This leads to a large dipole moment which oscillates periodically. The present discovery may stimulate new applications such as quantum simulation of many body physics dominated by periodic interactions. In addition, we find an interesting phenomenon that Cs atoms in the superposition of 40S_(1/2) and 40P_(1/2) have a much larger blockade radius than those of Cs(40S_(1/2))or Cs(40P_(1/2)) atoms.

Generalized Aubry–André–Harper Models in Optical Superlattices

Ultracold atoms trapped in optical superlattices provide a simple platform for realizing the seminal Aubry–André–Harper(AAH)model.However,this model ignores the periodic modulations on the nearest-neighbor hoppings.We establish a generalized AAH model by which an optical superlattice system can be approximately described when V_(1)≫V_(2),with periodic modulations on both on-site energies and nearest-neighbor hoppings.This model supports much richer topological properties absent in the standard AAH model.Specifically,by calculating the Chern numbers and topological edge states,we show that the generalized AAH model possesses multifarious topological phases and topological phase transitions,unlike the standard AAH model supporting only a single topological phase.Our findings can uncover more opportunities for using optical superlattices to study topological and localization physics.

Spin current in a spinor Bose–Einstein condensate induced by a gradient magnetic field

We develop a research of spin currents in a^(23)Na spinor Bose–Einstein condensate(BEC)by applying a magnetic field gradient.The spin current is successfully induced by the spin-dependent force arising from the magnetic field gradient.The dynamics of the spin components under the magnetic force is investigated.The study is promising to be extended to produce a longer spin-coherence and to enhance the sensitivity of the spin-mixing interferometry in a spinor BEC.

Deep learning in two-dimensional materials:Characterization,prediction,and design

Since the isolation of graphene,two-dimensional(2D)materials have attracted increasing interest because of their excellent chemical and physical properties,as well as promising applications.Nonetheless,particular challenges persist in their further development,particularly in the effective identification of diverse 2D materials,the domains of large-scale and highprecision characterization,also intelligent function prediction and design.These issues are mainly solved by computational techniques,such as density function theory and molecular dynamic simulation,which require powerful computational resources and high time consumption.The booming deep learning methods in recent years offer innovative insights and tools to address these challenges.This review comprehensively outlines the current progress of deep learning within the realm of 2D materials.Firstly,we will briefly introduce the basic concepts of deep learning and commonly used architectures,including convolutional neural and generative adversarial networks,as well as U-net models.Then,the characterization of 2D materials by deep learning methods will be discussed,including defects and materials identification,as well as automatic thickness characterization.Thirdly,the research progress for predicting the unique properties of 2D materials,involving electronic,mechanical,and thermodynamic features,will be evaluated succinctly.Lately,the current works on the inverse design of functional 2D materials will be presented.At last,we will look forward to the application prospects and opportunities of deep learning in other aspects of 2D materials.This review may offer some guidance to boost the understanding and employing novel 2D materials.

Visible-infrared-terahertz optical modulation of few-layer graphene through lithium intercalation

Optical modulation is significant and ubiquitous to telecommunication technologies,smart windows,and military devices.However,due to the limited tunability of traditional doping,achieving broadband optical property change is a tough problem.Here,we demonstrate a remarkable transformation of optical transmittance in few-layer graphene(FLG)covering the electromagnetic spectra from the visible to the terahertz wave after lithium(Li)intercalation.It results in the transmittance being higher than 90%from the wavelengths of 480 to 1040 nm,and it increases most from 86.4%to 94.1%at 600 nm,reduces from∼80%to∼68%in the wavelength range from 2.5 to 11μm,has∼20%reduction over a wavelength range from 0.4 to 1.2 THz,and reduces from 97.2%to 68.2%at the wavelength of 1.2 THz.The optical modification of lithiated FLG is attributed to the increase of Fermi energy(Ef)due to the charge transfer from Li to graphene layers.Our results may provide a new strategy for the design of broadband optical modulation devices.

Single-photon frequency-modulated continuous-wave Lidar based on quantum compressed sensing

Frequency-modulated continuous-wave(FMCW)Lidar has the characteristics of high-ranging accuracy,noise immunity,and synchronous speed measurement,which makes it a candidate for the next generation of vehicle Lidar.In this work,an FMCW Lidar working at the single-photon level is demonstrated based on quantum compressed sensing,and the target distance is recovered from the sparse photon detection,in which the detection sensitivity,bandwidth,and compression ratio are improved significantly.Our Lidar system can achieve 3 GHz bandwidth detection at photon count rates of a few thousand,making ultra-long-distance FMCW Lidar possible.

Testing universality of Feynman-Tan relation in interacting Bose gases using high-order Bragg spectra

The Feynman-Tan relation,obtained by combining the Feynman energy relation with the Tan’s two-body contact,can explain the excitation spectra of strongly interacting 39K Bose-Einstein condensate(BEC).Since the shift of excitation resonance in the Feynman-Tan relation is inversely proportional to atomic mass,the test of whether this relation is universal for other atomic systems is significant for describing the effect of interaction in strongly correlated Bose gases.Here we measure the high-momentum excitation spectra of 133Cs BEC with widely tunable interactions by using the second-and third-order Bragg spectra.We observe the backbending of frequency shift of excitation resonance with increasing interaction,and even the shift changes its sign under the strong interactions in the high-order Bragg spectra.Our finding shows good agreement with the prediction based on the Feynman-Tan relation.Our results provide significant insights for understanding the profound properties of strongly interacting Bose gases.

Photonic graphene with reconfigurable geometric structures in coherent atomic ensembles

Photonic graphene,possesses a honeycomb-like geometric structure,provides a superior platform for simulating photonic bandgap,Dirac physics,and topological photonics.Here,the photonic graphene with reconfigurable geometric structures is demonstrated in a 5S_(1/2)–5P_(3/2)–5D_(5/2) cascade-type 85Rb atomic ensembles.A strong hexagonal-coupling field,formed by the interference of three identical coupling beams,is responsible for optically inducing photonic graphene in atomic vapor.The incident weak probe beam experiences discrete diffraction,and the observed pattern at the output plane of vapor cell exhibits a clear hexagonal intensity distribution.The complete photonic graphene geometries from transversely stretched to longitudinally stretched are conveniently constructed by varying the spatial arrangement of three coupling beams,and the corresponding diffraction patterns are implemented theoretically and experimentally to map these distorted geometric structures.Moreover,the distribution of lattice sites intensity in photonic graphene is further dynamically adjusted by two-photon detuning and the coupling beams power.This work paves the way for further investigation of light transport and graphene dynamics.

Breaking the symmetry of colloidal 2D nanoplatelets:Twist induced quantum coupling

Twist provides a new degree of freedom for nanomaterial modifications,which can provide novel physical properties.Here,colloidal two-dimensional(2D)twisted CdSe nanoplatelets(NPLs)are successfully fabricated and their morphology can change from totally flat to edge-twisted,and then to middle-twisted with prolonged reaction time.By combining experiments and corresponding theoretical analyses,we have established the length-dependent relationships between the surface energy and twist,with a critical lateral dimension of 30 nm.We found that the defects formed during the synthesis process play a vital role in generating intense stress that develops a strong torsion tensor around the edges,resulting in edge-twisted and final middletwisted NPLs.Furthermore,due to the geometric asymmetry of twisted NPLs,the dissymmetry factor of single particle NPLs can reach up to 0.334.Specifically,quantum coupling occurs in middle-twisted NPLs by twisting one parent NPL into two daughter NPLs,which are structurally and electronically coupled.This work not only further deepens our understanding of the twist mechanism of 2D NPLs during colloidal synthesis,but also opens a pathway for applications using twistronics and quantum technology.

Generation of coherent blue light via bichromatic pumping in cesium vapor

Diode-pumped alkali lasers,possessing high efficiency and narrow linewidth,can provide feasible solutions for wavelength ranges difficult to reach by commercial lasers.In this study,we investigate a generation of coherent blue light(CBL)via four-wave mixing(FWM)-based up-conversion processes in cesium(Cs)vapor.A bichromatic pumping scheme with 852-and 917-nm lasers drives the Cs atoms to the 6D5/2 excited level,followed by cascaded decay of 6D5/2→7 P3/2→6 S1/2,producing 456-nm CBL under phase matching conditions.The fluorescence in multiple bands from blue to near-and far-infrared in the FWM process is demonstrated under different experimental conditions.To optimize the experimental parameters,we investigate the dependence of 456-nm CBL on the vapor temperature,frequency,and intensity of the two pump lasers.A maximum power of 2.94 mW is achieved with pump powers of 430 mW(for 852 nm)and 470 mW(for 917 nm).The corresponding conversion efficiency is 1.5%/W,three-fold higher than those in previous studies.Our results can contribute to fundamental research on atom–photon interactions and quantum metrology.

A monolithically sculpted van der Waals nano-opto-electro-mechanical coupler

The nano-opto-electro-mechanical systems(NOEMS)are a class of hybrid solid devices that hold promises in both classical and quantum manipulations of the interplay between one or more degrees of freedom in optical,electrical and mechanical modes.To date,studies of NOEMS using van der Waals(vdW)heterostructures are very limited,although vdW materials are known for emerging phenomena such as spin,valley,and topological physics.Here,we devise a universal method to easily and robustly fabricate vdW heterostructures into an architecture that hosts opto-electro-mechanical couplings in one single device.We demonstrated several functionalities,including nano-mechanical resonator,vacuum channel diodes,and ultrafast thermo-radiator,using monolithically sculpted graphene NOEMS as a platform.Optical readout of electric and magnetic field tuning of mechanical resonance in a CrOCl/graphene vdW NOEMS is further demonstrated.Our results suggest that the introduction of the vdW heterostructure into the NOEMS family will be of particular potential for the development of novel lab-on-a-chip systems.

Laser intensity induced transparency in atom-molecular transition process

We propose a new transparency mechanism,which is based on photoassociation(PA)laser intensity induced transitional frequency shift for ultracold cesium molecules formed in PA scheme.The PA laser intensity is supposed to change before the atom-molecule resonance.Thus,a remarkable transparent effect for PA laser is expected to appear in the vicinity of original resonant transitional line,where the variation of PA laser intensity induces the shift of the excited molecular levels.The mechanism is different from electromagnetically induced transparency effect and interesting for further research on the scattering length for cesium atomic condensate.