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.

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.

Rydberg electromagnetically induced transparency and Autler–Townes splitting in a weak radio-frequency electric field

We utilize an electromagnetically induced transparency(EIT) of a three-level cascade system involving Rydberg state in a room-temperature cell, formed with a cesium 6 S_(1/2)–6 P_(3/2)–66 S_(1/2) scheme, to investigate the Autler–Townes(AT)splitting resulting from a 15.21-GHz radio-frequency(RF) field that couples the |66 S_(1/2) → |65 P_(1/2) Rydberg transition.The radio-frequency electric field induced AT splitting, γAT, is defined as the peak-to-peak distance of an EIT-AT spectrum.The dependence of AT splitting γAT on the probe and coupling Rabi frequency, ?_p and ?_c, is investigated. It is found that the EIT-AT splitting strongly depends on the EIT linewidth that is related to the probe and coupling Rabi frequency in a weak RF-field regime. Using a narrow linewidth EIT spectrum would decrease the uncertainty of the RF field measurements.This work provides new experimental evidence for the theoretical framework in [J. Appl. Phys. 121, 233106(2017)].

Radiation-Pressure Effects in Cold-Atom Absorption Spectroscopy and Electromagnetically Induced Transparency

Radiation pressure due to the interaction between a probe light and cold atoms is investigated in a standard cesium magneto-optical trap. The radiation pressure alters the absorption spectroscopy of cold atoms, leading to line shapes and linewidths after resonant interaction that are different for positive and negative probe chirps. The difference is attributed to the radiation pressure of the probe laser, due to which atoms become accelerated at the resonance. The effect of the radiation pressure is also seen in electromagnetically induced transparency (EIT) involving an excited Rydberg level. The density matrix equation accounting for the radiation pressure is used to simulate the experiments. The simulations agree well with the measurements both for absorption and EIT spectra. We find that the effect of the radiation pressure is reduced at low probe intensities, and can be neglected when the probe intensity is smaller than Isat/2 .

Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy

The extension of dual-comb spectroscopy(DCS)to all wavelengths of light along with its ability to provide ultralarge dynamic range and ultra-high spectral resolution,renders it extremely useful for a diverse array of applications in physics,chemistry,atmospheric science,space science,as well as medical applications.In this work,we report on an innovative technique of quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy(QEMR-PAS),in which the beat frequency response from a dual comb is frequency down-converted into the audio frequency domain.In this way,gas molecules act as an optical-acoustic converter through the photoacoustic effect,generating heterodyne sound waves.Unlike conventional DCS,where the light wave is detected by a wavelengthdependent photoreceiver,QEMR-PAS employs a quartz tuning fork(QTF)as a high-Q sound transducer and works in conjunction with a phase-sensitive detector to extract the resonant sound component from the multiple heterodyne acoustic tones,resulting in a straightforward and low-cost hardware configuration.This novel QEMRPAS technique enables wavelength-independent DCS detection for gas sensing,providing an unprecedented dynamic range of 63 dB,a remarkable spectral resolution of 43 MHz(or~0.3 pm),and a prominent noise equivalent absorption of 5.99×10^(-6)cm^(-1)·Hz-1/2.

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.

Step-edge controlled fast growth of wafer-scale MoSe_(2)films by MOCVD

Two-dimensional(2D)transition metal dichalcogenides(TMDCs),due to their unique physical properties,have a wide range of applications in the next generation of electronics,optoelectronics,and valleytronics.Large-scale preparation of high-quality TMDCs films is critical to realize these potential applications.Here we report a study on metal-organic chemical vapor deposition(MOCVD)growth of wafer-scale MoSe_(2)films guided by the crystalline step edges of miscut sapphire wafers.We established that the nucleation density and growth rate of MoSe_(2)films were positively correlated with the step-edge density and negatively with the growth temperature.At a certain temperature,the MoSe_(2)domains on the substrate with high step-edge density grow faster than that with low density.As a result,wafer-scale and continuous MoSe_(2)films can be formed in a short duration(30 min).The MoSe_(2)films are of high crystalline quality,as confirmed by systematic Raman and photoluminescence(PL)measurements.The results provide an important methodology for the rapid growth of wafer-scale TMDCs,which may promote the application of 2D semiconductors.

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.

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.

Synthesizing Metal Oxide Semiconductors on Doped Si/SiO_(2) Flexible Fiber Substrates for Wearable Gas Sensing

Traditional metal oxide semiconductor(MOS)gas sensors have limited applications in wearable devices owing to their inflexibility and high-power consumption by substantial heat loss.To overcome these limitations,we prepared doped Si/SiO_(2)flexible fibers by a thermal drawing method as substrates to fabricate MOS gas sensors.A methane(CH_(4))gas sensor was demonstrated by subsequently in situ synthesizing Co-doped ZnO nanorods on the fiber surface.The doped Si core acted as the heating source through Joule heating,which conducted heat to the sensing material with reduced heat loss;the SiO_(2)cladding was an insulating substrate.The gas sensor was integrated into a miner cloth as a wearable device,and the concentration change of CH_(4)was monitored in real time through different colored lightemitting diodes.Our study demonstrated the feasibility of using doped Si/SiO_(2)fibers as the substrates to fabricate wearable MOS gas sensors,where the sensors have substantial advantages over tradition sensors in flexibility,heat utilization,etc.

Electrophoretic deposition of graphene-based materials:A review of materials and their applications

Recently,graphene-based materials have been successfully fabricated by the electrophoretic deposition(EPD)technique and exhibited various extraordinary properties.Here,research progress of the field of graphene-based materials prepared by the EPD process in recent 5 years is reviewed,including graphene films,graphene/non-metal composites,graphene/metal-based nanoparticles composites,graphene/polymer composites.We also summarize the experimental deposition conditions and the applications of the deposited graphene-based materials that have been reported.It can be concluded that EPD is a simple and reliable manipulation technique and promises a bright future for the production of graphenebased materials in the field of advanced nanocomposite materials.Finally the current issues and outlook of the development direction of EPD in future are also proposed.

Quantum superposition demonstrated higher-order topological bound states in the continuum

Higher-order topological insulators,as newly found non-trivial materials and structures,possess topological phases beyond the conventional bulk-boundary correspondence.In previous studies,in-gap boundary states such as the corner states were regarded as conclusive evidence for the emergence of higher-order topological insulators.Here,we present an experimental observation of a photonic higher-order topological insulator with corner states embedded into the bulk spectrum,denoted as the higher-order topological bound states in the continuum.Especially,we propose and experimentally demonstrate a new way to identify topological corner states by exciting them separately from the bulk states with photonic quantum superposition states.Our results extend the topological bound states in the continuum into higher-order cases,providing an unprecedented mechanism to achieve robust and localized states in a bulk spectrum.More importantly,our experiments exhibit the advantage of using the time evolution of quantum superposition states to identify topological corner modes,which may shed light on future exploration between quantum dynamics and higher-order topological photonics.

Digital quantum simulation of Floquet topological phases with a solid-state quantum simulator

Harnessing the dynamics of complex quantum systems is an area of much interest and a quantum simulator has emerged as a promising platform to probe exotic topological phases.Since the flexibility offered by various controllable quantum systems has helped gain insight into the quantum simulation of such complicated problems,an analog quantum simulator has recently shown its feasibility to tackle the problems of exploring topological phases.However,digital quantum simulation and the detection of topological phases still remain elusive.Here,we develop and experimentally realize the digital quantum simulation of topological phases with a solid-state quantum simulator at room temperature.Distinct from previous works dealing with static topological phases,the topological phases emulated here are Floquet topological phases.Furthermore,we also illustrate the procedure of digitally simulating a quantum quench and observing the nonequilibrium dynamics of Floquet topological phases.Using a quantum quench,the 0-andπ-energy topological invariants are unambiguously detected through measuring time-averaged spin polarizations.We believe our experiment opens up a new avenue to digitally simulate and detect Floquet topological phases with fast-developed programmable quantum simulators.

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.

Atom-optically synthetic gauge fields for a noninteracting Bose gas

Synthetic gauge fields in synthetic dimensions are now of great interest.This concept provides a convenient manner for exploring topological phases of matter.Here,we report on the first experimental realization of an atom-optically synthetic gauge field based on the synthetic momentum-state lattice of a Bose gas of ^(133)Cs atoms,where magnetically controlled Feshbach resonance is used to tune the interacting lattice into noninteracting regime.Specifically,we engineer a noninteracting one-dimensional lattice into a two-leg ladder with tunable synthetic gauge fields.We observe the flux-dependent populations of atoms and measure the gauge field-induced chiral currents in the two legs.We also show that an inhomogeneous gauge field could control the atomic transport in the ladder.Our results lay the groundwork for using a clean noninteracting synthetic momentum-state lattice to study the gauge field-induced topological physics.

Quantum description and measurement for single photon modulation

Single photon modulation has been proposed to overcome the defects of the low signal-to-noise ratio(SNR)and slow process rate of photon counting. In this paper, we present the quantum theory of single photon modulation, and then experimentally investigate the modulation spectroscopy both in the time domain and frequency domain. It is found that the SNR reached 150 in approximately the MHz modulation bandwidth.

Superfluid-superradiant mixed phase of the interacting degenerate Fermi gas in an optical cavity

We investigate the ground-state properties of an attractively interacting degenerate Fermi gas coupling with a high-finesse optical cavity. We predict a new mixed phase with both the superfluid and superradiant properties for the intermediate fermion-fermion interaction and fermion-photon coupling strengths. Moreover, in this mixed phase a relatively large ratio of the scaled polarization to the dimensionless mean-field gap, which is in contrast to that in the conventional superfluid regime can be obtained. We also figure out rich phase diagrams depending crucially on the atomic resonant frequency(effective Zeeman field) and address briefly the experimental detection of our predicted quantum phases.

Method for studying diatomic rovibrational spectra at a given vibrational state

An algebraic method for rotational energies at a given vibrational state(AMr(v)) is proposed in this study in order to obtain unknown high-lying rovibrational energies. Applications of this method to the ground electronic state X^1Σ^+of CO and the excited state C^1Σ^+of^(39)K^7Li molecules show the following:(1) the AMr(v) can give the rational upper limit J of a rotational quantum number of a diatomic electronic state;(2) the full AMr(v) rovibrational energies {E_(υJ)}_υ of given vibrational states not only reproduce all known experimental data excellently but also predict precisely the values of all high-lying rovibrational energies,which may not be available experimentally.

The role of surface charges in the blinking mechanisms and quantum-confined Stark effect of single colloidal quantum dots

The two frequently observed phenomena,photoluminescence(PL)blinking and quantum-confined Stark effect(QCSE)-induced spectral diffusion,are not conducive to the applications of colloidal quantum dots(QDs).It remains elusive how these two phenomena are linked to each other.Unraveling the potential link between blinking and QCSE could facilitate the adoption of appropriate strategies that can simultaneously suppress both PL blinking and spectral diffusion.In this work,we investigated the blinking mechanism and QCSE of single CdSe/CdS/ZnS QDs in the presence of positive and negative surface charges using single-dot PL spectroscopy.We found that the negative surface charges can simultaneously suppress PL blinking and spectral diffusion of single QDs.On the other hand,the positive surface charges could change the blinking mechanisms of QDs from Auger-blinking to band-edge carrier(BC)-blinking.Two types of QCSE were observed,and a significant QCSE-induced spectral broadening of 5.25 nm was measured,which could be attributed to the hopping of surface charges between different surface-trap sites.Based on these findings,several theoretical models are proposed to explain various phenomena observed.

Wide and fast-frequency tuning for a stabilized diode laser

External-cavity diode laser(ECDL)has important applications in many fundamental and applied researches.Here we report a method to fast and widely tune the frequency of a stabilized ECDL.The beat frequency between the ECDL and a frequency-locked reference laser is identified by the voltagecontrolled oscillator contained in a phase detector,whose output voltage is subtracted from the flexibly controlled PC signal to generate an error signal for stabilizing the ECDL.The output frequency of the stabilized ECDL can be shifted at a short characteristic time of∼150µs within a range of∼620 MHz.The wide and fast-frequency tuning achieved by our method is compared with other previous works.We demonstrated the performance of our method by the efficient sub-Doppler cooling of Cs atoms with the temperature as low as 6µK.