High-Order Harmonic Generation in Photoexcited Three-Dimensional Dirac Semimetals.

High-order harmonic generation (HHG) in condensed matter is highly important for potential applications in various fields, such as materials characterization, all-optical switches, and coherent light source generation. Linking HHG to the properties or dynamic processes of materials is essential for realizing these applications. Here, a bridge has been built between HHG and the transient properties of materials through the engineering of interband polarization in a photoexcited three-dimensional Dirac semimetal (3D-DSM).

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.

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.

Layer-dependent ultrafast carrier dynamics of PdSe investigated by photoemission electron microscopy.

For atomically thin two-dimensional materials, variations in layer thickness can result in significant changes in the electronic energy band structure and physicochemical properties, thereby influencing the carrier dynamics and device performance. In this work, we employ time- and energy-resolved photoemission electron microscopy to reveal the ultrafast carrier dynamics of PdSe with different layer thicknesses. We find that for few-layer PdSe with a semiconductor phase, an ultrafast hot carrier cooling on a timescale of approximately 0.

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 PO·2HO + HO ⇌ PO·3HO in the liquid phase, which helps to clarify the hydration pathway of the metaphosphate.

Controlling the Polarization of Nitrogen Ion Lasing.

Air lasing provides a promising technique to remotely produce coherent radiation in the atmosphere and has attracted continuous attention. However, the polarization properties of N lasing with seeding have not been understood since it was discovered 10 years ago, in which the polarization behaviors appear disordered and confusing. Here, we performed an experimental and theoretical investigation of the polarization properties of N lasing and successfully revealed its underlying physical mechanism.

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.

Spatiotemporal imaging and shaping of electron wave functions using novel attoclock interferometry.

Electrons detached from atoms by photoionization carry valuable information about light-atom interactions. Characterizing and shaping the electron wave function on its natural timescale is of paramount importance for understanding and controlling ultrafast electron dynamics in atoms, molecules and condensed matter. Here we propose a novel attoclock interferometry to shape and image the electron wave function in atomic photoionization.

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.

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.

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.

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.

Holistic quality evaluation of Callicarpae Formosanae Folium by multi-chromatography-based qualitative and quantitative analysis of polysaccharides and small molecules.

Callicarpae Formosanae Folium (CFF), derived from the leaves of Callicarpa formosana Rolfe, is a common Chinese medicinal herb used for the treatment of hematemesis. Phytochemical studies found that phenylpropanoids, flavonoids, terpenoids and polysaccharides were the main ingredients of CFF. However, there is limited scientific information concerning holistic quality method and quality consistency evaluation of CFF.

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.

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.

Photon retention in coherently excited nitrogen ions.

Quantum coherence in quantum optics is an essential part of optical information processing and light manipulation. Alkali metal vapors, despite the numerous shortcomings, are traditionally used in quantum optics as a working medium due to convenient near-infrared excitation, strong dipole transitions and long-lived coherence. Here, we proposed and experimentally demonstrated photon retention and subsequent re-emittance with the quantum coherence in a system of coherently excited molecular nitrogen ions (N) which are produced using a strong 800 nm femtosecond laser pulse.

Ultrafast extreme ultraviolet photoemission electron microscope.

Here, we report our newly built table-top ultrafast extreme ultraviolet (EUV) photoemission electron microscope. The coherent ultrafast EUV light is served by a single order harmonic, which is generated by the interaction between the intense 800-nm femtosecond laser and noble gases in the hollow core fiber. The required order of the harmonic is selected out by a single grating in the off-plane mount and focused on the sample in the ultrahigh vacuum chamber of the photoemission electron microscope.

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.

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.

Ramsey interferometry through coherent -+-+ coupling and population transfer in 2+ air laser.

Motivated by the hot debate on the mechanism of laser-like emission at 391 nm from 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 2+. 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 (=2)-+ states and the +(=0)-+ states. Our results show that the 391 nm optical gain is susceptible to the population inversion within 2+ states manipulated by the Ramsey technique and thus clearly reveal their symbiosis.

Formation Mechanism of Excited Neutral Nitrogen Molecules Pumped by Intense Femtosecond Laser Pulses.

Backward amplified spontaneous emission of neutral nitrogen molecules has been reported from laser-induced plasma filaments. The cavity-free UV emission has great potential applications in remote atmospheric sensing. However, the formation mechanism for the excited nitrogen molecules inside filaments remains controversial.

Enhanced Coherent Emission from Ionized Nitrogen Molecules by Femtosecond Laser Pulses.

The forward emission spectra were experimentally measured for ionized nitrogen molecules by an 800 nm pump laser and a delayed seed laser. It was found that emission lines around both 428 and 391 nm are greatly enhanced upon use of a 391 or 428 nm seed laser. The emission lines around 391 and 428 nm can be assigned to the rotational transitions of N [BΣ(' = 0) → XΣ( = 0)] and N [BΣ(' = 0) → XΣ( = 1)], respectively.

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+(BΣu+,ν=0) and N2+(XΣg+,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.

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.