Direct Hot-Electron Transfer at the Au Nanoparticle/Monolayer Transition-Metal Dichalcogenide Interface Observed with Ultrahigh Spatiotemporal Resolution.

Plasmon-induced hot-electron transfer at the metallic nanoparticle/semiconductor interface is the basis of plasmon-enhanced photocatalysis and energy harvesting. However, limited by the nanoscale size of hot spots and femtosecond time scale of hot-electron transfer, direct observation is still challenging. Herein, by using spatiotemporal-resolved photoemission electron microscopy with a two-color pump-probe beamline, we directly observed such a process with a concise system, the Au nanoparticle/monolayer transition-metal dichalcogenide (TMD) interface. The ultrafast hot-electron transfer from Au nanoparticles to monolayer TMDs and the plasmon-enhanced transfer process were directly measured and verified through an in situ comparison with the Au film/TMD interface and free TMDs. The lifetime at the Au nanoparticle/MoSe2 interface decreased from 410 to 42 fs, while the photoemission intensities exhibited a 27-fold increase compared to free MoSe2. We also measured the evolution of hot electrons in the energy distributions, indicating the hot-electron injection and decay happened in an ultrafast time scale of ∼50 fs without observable electron cooling.

Fluorescence and lasing of neutral nitrogen molecules inside femtosecond laser filaments in air: mechanism and applications.

High power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studied. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered "air lasing" effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented.

Structure and Ultrafast X-ray Diffraction of the Hydrated Metaphosphate.

We study the pathway of metaphosphate hydration when a metaphosphate anion is dissolved in liquid water with an explicit water model. For this purpose, we propose a sequential Monte Carlo algorithm incorporated with the ab initio quantum mechanics/molecular mechanics (QM/MM) method, which can reduce the amount of ab initio QM/MM sampling while retaining the accuracy of the simulation. We demonstrate the numerical calculation of the standard enthalpy change for the successive addition reaction PO3-·2H2O + H2O ⇌ PO3-·3H2O in the liquid phase, which helps to clarify the hydration pathway of the metaphosphate. With the obtained hydrated structure of the metaphosphate anion, we perform ab initio calculations for its relaxation dynamics upon vibrational excitation and characterize the energy transfer process in solution with simulated ultrafast X-ray diffraction signals, which can be experimentally implemented with X-ray free-electron lasers.

Acoustic phonon excitation in gold probed by time-resolved photoemission electron microscopy.

Electron-phonon coupling is an important energy transfer mechanism in solids after ultrafast laser excitation. In this study, we present an extreme ultraviolet (EUV) and infrared (IR) pump-probe photoemission experiment to investigate the electron-phonon coupling in nonequilibrium gold. The energy of IR-laser-emitted photoelectrons is shifted due to the EUV photoemission and oscillates with a ∼4THz frequency. Such oscillation is considered as the effective excitation of the longitudinal acoustic phonon mode in gold through the spectral-dependent electron-phonon coupling. Our study showcases the capability of time-resolved photoemission electron microscopy to monitor the non-equilibrium lattice vibrations with ultrahigh spatial and temporal resolution.

Imaging and Controlling Ultrafast Electron Pulses Emitted from Plasmonic Nanostructures.

We experimentally study photoemission from gold nanodisk arrays using space-, time-, and energy-resolved photoemission electron microscopy. When excited by a plasmonic resonant infrared (IR) laser pulse, plasmonic hotspots are generated owing to local surface plasmon resonance. Photoelectrons emitted from each plasmonic hotspot form a nanoscale and ultrashort electron pulse. When the system is excited by an extreme ultraviolet (EUV) laser pulse, a uniformly distributed photoelectron cloud is formed across the sample surface. When excited by the IR and EUV laser pulses together, both the photoemission image and kinetic energy vary significantly for the IR laser-generated electrons depending on the time delay between the two laser pulses. These observations are well explained by the Coulomb interaction with the EUV laser-generated electron cloud. Our study offers a feasible approach to manipulate the energy of electron pulse emitted from a plasmonic nanostructure on an ultrafast time scale.

Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing.

Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical amplification in the ultraviolet range. Here, we demonstrated that a seed light pulse carrying orbital angular momentum (OAM) can be significantly amplified in nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the topological charge of ℓ = ±1, we observed an energy amplification of the seed light pulse by two orders of magnitude, while the amplified pulse carries the same OAM as the incident seed pulse. Moreover, we show that a spatial misalignment of the plasma amplifier with the OAM seed beam leads to an amplified emission of Gaussian mode without OAM, due to the special spatial profile of the OAM seed pulse that presents a donut-shaped intensity distribution. Utilizing this misalignment, we can implement an optical switch that toggles the output signal between Gaussian mode and OAM mode. This work not only certifies the phase transfer from the seed light to the amplified signal, but also highlights the important role of spatial overlap of the donut-shaped seed beam with the gain region of the nitrogen plasma for the achievement of OAM beam amplification.

Trajectory-controlled high-order harmonic generation in ZnO crystals.

We experimentally and theoretically study high-order harmonic generation in zinc oxide crystals irradiated by mid-infrared lasers. The trajectories are mapped to the far field spatial distribution of harmonics. The divergence angles of on-axis and off-axis parts exhibit different dependences on the order of the harmonics. This observation can be theoretically reproduced by the coherent interference between the short and long trajectories with dephasing time longer than 0.5 optical cycle. Further, the relative contribution of the short and long trajectories is demonstrated to be accurately controlled by a one-color or two-color laser on the attosecond time scale. This work provides a reliable method to determine the electron dephasing time and demonstrates a versatile control of trajectory interference in the solid high-order harmonic generation.

Entangled X-Ray Photon Pair Generation by Free-Electron Lasers.

We investigate entangled x-ray photon pair emissions in a free-electron laser (FEL) and establish a quantum electrodynamical theory for coherently amplified entangled photon pair emission from microbunched electron pulses in the undulator. We provide a scheme to generate highly entangled x-ray photon pairs and numerically demonstrate the properties of entangled emission, which is of great importance in x-ray quantum optics. Our work shows a unique advantage of FELs in entangled x-ray photon pair generation.

Geometric Phase Effect in Attosecond Stimulated X-ray Raman Spectroscopy.

Conical intersections (CIs) are diabolical points in the potential energy surfaces generally caused by point-wise degeneracy of different electronic states, and give rise to the geometric phases (GPs) of molecular wave functions. Here we theoretically propose and demonstrate that the transient redistribution of ultrafast electronic coherence in attosecond Raman signal (TRUECARS) spectroscopy is capable of detecting the GP effect in excited state molecules by applying two probe pulses including an attosecond and a femtosecond X-ray pulse. The mechanism is based on a set of symmetry selection rules in the presence of nontrivial GPs. The model of this work can be realized for probing the geometric phase effect in the excited state dynamics of complex molecules with appropriate symmetries, using attosecond light sources such as free-electron X-ray lasers.

Effect of harvest time span on physicochemical properties, antioxidant, antimicrobial, and anti-inflammatory activities of Meliponinae honey.

BACKGROUND: The maturity of honey has a great impact on its quality and contents. Additionally, stingless bee honey contains high moisture, which allows microorganisms to survive and ferment, contributing to honey's variable flavor and physicochemical properties. Therefore, there is a need for better quality control of the honey process, especially the harvest time of honey. RESULTS: We gathered honey from the nest of stingless bees Heterotrigona itama and Tetrigona binghami over different time periods, i.e. 15, 30, and 45 days. The results show harvest time considerably affects the physicochemical properties, antioxidant activity, and antimicrobial activity of honey. Good antioxidant activity and antimicrobial activity can be found in honey produced from a longer harvest time. Compared with 15-day harvest time, at 30- or 45-day harvest time water, trehalulose, and protein content and total acidity increased, and the content of reducing sugars, fructose and glucose, and pH values, decreased in both types of honey. Moreover, compared with 15-day harvest time, the sum of six organic acids in the 45-day honey of H. itama fluctuated between 2.78 to 4.12 g 100 g-1 and in the 45-day honey of T. binghami increased from 1.66 to 3.61 g 100 g-1 , respectively. CONCLUSION: Honey harvest time had a significant effect on the physicochemical properties, antioxidant activity, and antimicrobial activity of stingless bee honey (H. itama or T. binghami). This study provides a reference for beekeepers to adjust harvest time to obtain honey with suitable physicochemical parameters. © 2022 Society of Chemical Industry.

Optical amplification from high vibrational states of ionized nitrogen molecules generated by 800-nm femtosecond laser pulses.

We experimentally investigated the interaction between nitrogen molecules and intense femtosecond laser pulses. When irradiated by an 800-nm pump laser and a delayed 355-nm seed laser, the spectral lines around 353.3 nm and 353.8 nm are observed to be greatly amplified, no matter whether the pump laser is circularly or linearly polarized. The two spectral lines correspond to the transition of N2+ (B, ν' = 5 → X, ν = 4) and N2+ (B, ν' = 4 → X, ν = 3), respectively. In comparison with the spectral lines related with ground vibrational states of nitrogen molecular ion, the observed amplification exhibits different polarization dependence of the pump laser. This distinctive change can be explained by the population variation of high vibrational states caused by the pump laser with different polarizations.

Probing the Spin-Orbit Time Delay of Multiphoton Ionization of Kr by Bicircular Fields.

We study multiphoton ionization of Kr atoms by circular 400-nm laser fields and probe its photoelectron circular dichroism with the weak corotating and counterrotating circular fields at 800 nm. The unusual momentum- and energy-resolved photoelectron circular dichroisms from the ^{2}P_{1/2} ionic state are observed as compared with those from ^{2}P_{3/2} ionic state. We identify an anomalous ionization enhancement at sidebands related to the ^{2}P_{1/2} ionic state on photoelectron momentum distribution when switching the relative helicity of the two fields from corotating to counterrotating. By performing the two-color intensity-continuously-varying experiments and the pump-probe experiment, we find a specific mixed-photon populated resonant transition channel in counterrotating fields that contributes to the ionization enhancement. We then probe the time delay between the two spin-orbit coupled ionic states (^{2}P_{1/2} and ^{2}P_{3/2}) using bicircular fields and reveal that the resonant transition has an insignificant effect on the relative spin-orbit time delay.

Ramsey interferometry through coherent A2Πu-X2Σg+-B2Σu+ coupling and population transfer in N2+ air laser.

Motivated by the hot debate on the mechanism of laser-like emission at 391 nm from N2 gas irradiated by a strong 800 nm pump laser and a weak 400 nm seed laser, we theoretically study the temporal profile, optical gain, and modulation of the 391 nm signal from N2+. Our calculation sheds light on the long standing controversy on whether population inversion is indispensable for optical gain and show the Ramsey fringes of the emission intensity at 391 nm formed by additionally injecting another 800 nm pump or 400 nm seed, which provides strong evidence for the coherence driven modulation of transition dipole moment and population transfer between the A2Πu(ν=2)-X2Σg+ states and the B2Σu+(ν=0)-X2Σg+ states. Our results show that the 391 nm optical gain is susceptible to the population inversion within N2+ states manipulated by the Ramsey technique and thus clearly reveal their symbiosis. This study reveals not only the physical picture of producing N2+ population inversion but also versatile control of the N2+ air laser.

Subfemtosecond-resolved modulation of superfluorescence from ionized nitrogen molecules by 800-nm femtosecond laser pulses.

Superfluorescence emission around 391 nm is generated when nitrogen molecules are irradiated by a strong 800-nm pump laser and a delayed seed laser. The emission corresponds to the transition between N2+(B2Σu+,ν″=0) and N2+(X2Σg+,ν=0). When another weak 800-nm probe laser is injected and scanned after the pump laser, the superfluorescence intensity is observed to exhibit periodical modulation. The period is determined to be ~2.63 fs, corresponding to the transition frequency between N2+(A2Πu,ν'=2) and N2+(X2Σg+,ν=0). Based on theoretical derivation, these observations can be attributed to the laser-induced population transfer and polarization variation between the relevant electronic states of ionized nitrogen molecules.

Coherent modulation of superradiance from nitrogen ions pumped with femtosecond pulses.

Singly ionized nitrogen molecules in ambient air pumped by 800 nm femtosecond laser give rise to superradiant emission. Here, we study this superradiance by injecting a pair of resonant seeding pulses at different intensity ratios inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiant emission with a period of 1.3 fs is observed when the delay between the two seeding pulses is finely tuned. The modulation contrast is increased and then decreased with the delay time when the second seed pulse is stronger than the first one, and the maximum modulation contrast occurs at longer delay time when the second seeding pulse is stronger. This reveals the increase of the macroscopic polarization with time after the seeding pulse. Moreover, these observations provide a new level of control on the "air lasing" based on nitrogen ions, which can find potential applications in optical remote sensing.

Energy- and Momentum-Resolved Photoelectron Spin Polarization in Multiphoton Ionization of Xe by Circularly Polarized Fields.

We perform a joint experimental and theoretical study on momentum- and energy-resolved photoelectron spin polarization in multiphoton ionization of Xe atoms by circularly polarized fields. We experimentally measure the photoelectron momentum distributions of Xe atoms in circularly polarized near-infrared (800 nm) and ultraviolet (400 nm) light, respectively. We analyze the momentum- and energy-resolved photoelectron spin polarization by comparing the experimental photoelectron momentum distributions with the simulations, although we cannot derive the spin polarization solely from the experiment. We show that the use of circularly polarized ultraviolet light at 400 nm can create better than 90% spin polarization with focal volume effect considered, which enables the separation of the spin states by momentum gating. This paves the way to produce high-degree spin-polarized electron sources from strong-field multiphoton ionization.

Population inversion in the rotational levels of the superradiant N2 + pumped by femtosecond laser pulses.

Nitrogen molecular ions (N2 +) in air plasma pumped by femtosecond laser pulses give rise to superradiant emission at 391.4 nm in the presence of an external seed pulse at proper wavelength. Due to the transient alignment of the nitrogen molecular ions, the superradiance signal presents a strong modulation as a function of the temporal delay between the pump and the seed pulses. Through Fourier transformation with high frequency resolution, we distinguished the contribution of the finely separated rotation levels of the upper and lower states. It was found that the population density of certain rotational levels in the upper state is higher than that in the lower one, indicating that population inversion of the rotation levels of the two involved states is a key enabling factor for this superradiant emission.

Revealing the Sub-Barrier Phase using a Spatiotemporal Interferometer with Orthogonal Two-Color Laser Fields of Comparable Intensity.

We measure photoelectron momentum distributions of Ar atoms in orthogonally polarized two-color laser fields with comparable intensities. The synthesized laser field is used to manipulate the oscillating tunneling barrier and the subsequent motion of electrons onto two spatial dimensions. The subcycle structures associated with the temporal double-slit interference are spatially separated and enhanced. We use such a spatiotemporal interferometer to reveal sub-barrier phase of strong-field tunneling ionization. This study shows that the tunneling process transfers the initial phase onto momentum distribution. Our work has the implication that the sub-barrier phase plays an indispensable role in photoelectron interference processes.

Dissociative Ionization of Argon Dimer by Intense Femtosecond Laser Pulses.

We experimentally and theoretically studied dissociative ionization of argon dimer driven by intense femtosecond laser pulses. In the experiment, we measured the ion yield and the angular distribution of fragmental ions generated from the dissociative ionization channels of (1,1) (Ar22+ → Ar+ + Ar+) and (2,1) (Ar23+ → Ar2+ + Ar+) using a cold target recoil ion momentum spectroscopy. The channel ratio of (2,1)/(1,1) is 4.5-7.5 times of the yield ratio of double ionization to single ionization of argon monomer depending on the laser intensity. The measurement verified that the ionization of Ar+ is greatly enhanced if there exists a neighboring Ar+ separated by a critical distance. In addition, the fragmental ions exhibit an anisotropic angular distribution with the peak along the laser polarization direction and the full width at half maximum becomes broader with increasing laser intensity. Using a full three-dimensional classical ensemble model, we calculated the angle-dependent multiple ionization probability of argon dimer in intense laser fields. The results show that the experimentally observed anisotropic angular distribution of fragmental ions can be attributed to the angle-dependent enhanced ionization of the argon dimer in intense laser fields.

Phase Structure of Strong-Field Tunneling Wave Packets from Molecules.

We study the phase structure of the tunneling wave packets from strong-field ionization of molecules and present a molecular quantum-trajectory Monte Carlo model to describe the laser-driven dynamics of photoelectron momentum distributions of molecules. Using our model, we reproduce and explain the alignment-dependent molecular frame photoelectron spectra of strong-field tunneling ionization of N_{2} reported by M. Meckel et al. [Nat. Phys. 10, 594 (2014)]. In addition to modeling the low-energy photoelectron angular distributions quantitatively, we extract the phase structure of strong-field molecular tunneling wave packets, shedding light on its physical origin. The initial phase of the tunneling wave packets at the tunnel exit depends on both the initial transverse momentum distribution and the molecular internuclear distance. We further show that the ionizing molecular orbital has a critical effect on the initial phase of the tunneling wave packets. The phase structure of the photoelectron wave packet is a key ingredient for modeling strong-field molecular photoelectron holography, high-harmonic generation, and molecular orbital imaging.