Chengyin Wu

• Peking University

A single intense near-infrared femtosecond laser beam can generate ultraviolet N$_2^{+}$ lasing at 391 nm in the air, offering a promising tool for remote sensing. One of the key debates regarding its mechanism is whether it is seeded by a self-generated spectral component, such as the second harmonic, which is inevitably produced by the charge gra...

Solid-state high-order harmonic generation (HHG) presents a promising approach for achieving controllable broadband coherent light sources and dynamically detecting materials. In this study, we demonstrate the all-optical control of HHG in a strongly correlated system, vanadium dioxide ( VO 2 ), through photo-carrier doping. It has been discovered...

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 it...

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 t...

ZnO nanowire (NW) lasing driven by mid-infrared (MIR) laser pulses has attracted significant attention owing to its remarkable wavelength-independent lasing threshold and potential applications in diverse situations. However, the properties of MIR laser-driven ZnO microwire (MW) lasing are rarely studied when the wire diameter is increased from nan...

The Berry curvature of molecular electronic wave function serves as an electron-spin-dependent effective magnetic field, which results in an effective magnetic force that can lead to electron-spin-dependent molecular dynamics. We propose an experimental scheme to observe this Berry curvature effect in molecular dynamics. We show that in CH2OH photo...

This study utilized time- and energy-resolved photoemission electron microscopy with ultrahigh spatial resolution and wide-field real-time detection advantages to reveal the layer-dependent ultrafast dynamics of excited-state electrons in PdSe 2 .

Air lasing provides a promising technique to remotely produce coherent radiation in the atmosphere and has attracted continuous attention. However, the polarization properties of N2+ 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 perfor...

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...

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regar...

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...

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 observe...

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...

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...

The spin-dependent Berry force is a genuine effect of Berry curvature in molecular dynamics, which can dramatically result in spatial spin separation and change of reaction pathways. However, the way to probe the effect of Berry force remains challenging, because the time-reversal (TR) symmetry required for opposite Berry forces conflicts with TR s...

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 observe...

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 observe...

Air lasing is a simple and promising technique to efficiently produce coherent light in the atmosphere. Yet, its capability to obtain structured light like optical vortices or vector beams is still unexplored. Here, we demonstrate the generation of vortex superfluorescent radiation with the same orbital angular momentum as the pump beam, obtained v...

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 att...

We investigate the ultrafast carrier dynamics in a ZnO crystal irradiated by a strong, linearly polarized midinfrared laser pulse. Depending on the laser intensity, there exist two dominant light wave-driven processes, i.e., the electron-hole collision recombination and the electron-electron impact excitation. The corresponding optical signatures a...

Structured light has attracted great interest in scientific and technical fields. Here, we demonstrate the first generation of structured air lasing in N2+ driven by 800 nm femtosecond laser pulses. By focusing a vortex pump beam at 800 nm in N2 gas, we generate a vortex superfluorescent radiation of N2+ at 391 nm, which carries the same photon orb...

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 th...

We investigate the subcycle temporal property of multiphoton ionization of Ar atoms in a linearly polarized laser pulse at 400 nm by employing the two-color phase-of-the-phase photoelectron spectroscopy. A perturbative weak 800-nm circularly polarized field is used to clock the intrinsic multiphoton ionization dynamics. The resulting phase-of-the-p...

Einstein, Podolsky and Rosen's prediction on incompleteness of quantum mechanics was overturned by experimental tests on Bell's inequality that confirmed the existence of quantum entanglement. In X-ray optics, entangled photon pairs can be generated by X-ray parametric down conversion (XPDC), which is limited by relatively low efficiency. Meanwhile...

Einstein, Podolsky and Rosen's prediction on incompleteness of quantum mechanics was overturned by experimental tests on Bell's inequality that confirmed the existence of quantum entanglement. In X-ray optics, entangled photon pairs can be generated by X-ray parametric down conversion (XPDC), which is limited by relatively low efficiency. Meanwhile...

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regar...

In strong field ionization, the pump pulse not only photoionizes the molecule, but also drives efficient population exchanges between its ionic ground and excited states.In this study, we investigated the population dynamics accompanying strong field molecular photoionization, using angular distribution of dissociative fragments after ionization.Ou...

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 P21/2 ionic state are observed as compared with those from P23/2 ionic...

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...

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 sel...

The interaction between a quantum particle’s spin angular momentum¹ and its orbital angular momentum² is ubiquitous in nature. In optics, the spin–orbit optical phenomenon is closely related with the light–matter interaction³ and has been of great interest4,5. With the development of laser technology⁶, the high-power and ultrafast light sources now...

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 s...

Motivated by the hot debate on the mechanism of laser-like emission at 391 nm from ${{\rm N}_2}$ N 2 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 ${\rm N}_2^ +$ N 2 + . Our calculation sheds light on the long standing co...

Quantum memory is an essential part of quantum computer and quantum information storage. Alkali metal vapors are traditionally used as a medium due to convenient near-infrared excitation, strong dipole transitions and long-lived coherence. Here, we proposed and experimentally demonstrated the optical storage in coherently excited molecular nitrogen...

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

The laser-like coherent emission at 391nm from N$_2$ gas irradiated by strong 800nm pump laser and weak 400nm seed laser is theoretically investigated. Recent experimental observations are well simulated, including temporal profile, optical gain and periodic modulation of the 391nm signal from N$_2^+$. Our calculation sheds light on the long standi...

We observe the parity-unfavored photoelectron emission in multiphoton single ionization of krypton atoms in an intense ultraviolet laser field. With systematic experiments of successively varying the light ellipticity and intensity, we identify that this parity-unfavored emission is associated with the first excited ionic state via a resonant pathw...

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 nm and 391 nm are greatly enhanced with injecting either a 391-nm or 428-nm seed laser. The emission lines around 391 nm and 428 nm can be assigned to the rotatio...

We report a combined theoretical and experimental investigation on the unambiguous role of electronic coherence underlying the generation of laserlike coherent emission from N2+ ions launched in a strong laser field. The coherence manifests itself by giving rise to a dramatic enhancement of N2+ laserlike radiation with two temporally separated pump...

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 N 2 + ( B 2 Σ u + , ν ″ = 0 ) and N 2 + ( X 2 Σ g + , ν = 0 ) . When another weak 800-nm probe laser is injected and scanned after the pump laser, the...

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 obse...

We experimentally study multiphoton ionization of xenon atoms by 266 nm, 400 nm and 800 nm femtosecond laser pulses. The measurements of photoelectron kinetic energy and photoelectron angular distribution indicate that nonresonant multiphoton ionization occurs in the present experimental condition. It obeys the dipolar selection rule and the angula...

Rescattering mediated by intense laser fields is one of the dominant processes in strong-field physics. It was usually described classically or semiclassically. Recently, laser-induced recollisions with the tunneling barriers have attracted great theoretical interest. However, fully distinguishing the “quantum” effect of rescattering with the tunne...

We report on the generation of electronic quantum coherence in the ionized nitrogen molecules. The coherence is revealed by observing N2+ air lasing with a near-infrared laser and a delayed mid-infrared laser.

We have observed collective spontaneous emission (superfluorescence) from ionized nitrogen molecules generated by 800-nm laser pulses. The superfluorescence intensity around 391 nm was found to be greatly enhanced when injecting an external seed with various wavelengths. Our finding provides insights into controlling the coherent emission from air...

The existence of electronic coherence can fundamentally change the scenario of nonlinear interaction of light with quantum systems such as atoms and molecules, which, however, has escaped from observation in the investigations of strong field nonlinear optics in the past several decades. Here, we report on the generation of electronic quantum coher...

Singly ionized nitrogen molecules in ambient air pumped by near-infrared femtosecond laser give rise to superradiant emission. Here we demonstrate coherent control of this superradiance by injecting a pair of resonant seeding pulses inside the nitrogen gas plasma. Strong modulation of the 391.4 nm superradiance with a period of 1.3 fs is observed w...

We investigate the 391-nm lasing dynamics from ionized nitrogen molecules in 800-nm femtosecond laser fields. By comparing the radiation intensity, spectrum shape, and temporal profile of the 391-nm lasing at various experimental conditions, we conclude that the lasing dynamics contains not only the generation and the decay of ionized nitrogen mole...

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, res...

Air pumped by mid-infrared femtosecond pulses gives rise to coherent emission at 391 nm. Based on ellipticity dependence measurement and time-resolved characterization, we attribute this emission to resonance-enhanced harmonics in perturbative regime.

Air pumped by mid-infrared femtosecond pulses gives rise to coherent emission at 391/428 nm. Based on ellipticity dependence measurement and time-resolved characterization, we attribute this emission to resonance-enhanced low order harmonics.

We measure the wavelength-dependent correlated-electron momentum (CEM) spectra of strong-field double ionization of Xe atoms, and observe a significant change from a roughly nonstructured (uncorrelated) pattern at 795 nm to an elongated distribution with V-shaped structure (correlated) at higher wavelengths of 1320 and 1810 nm, pointing to the tran...

Nitrogen molecules are ionized when an intense 800-nm femtosecond laser propagates through a nitrogen gas cell. We experimentally probe the relative population in different vibrational states and electronic states of ionized nitrogen molecules by using an external seed with various wavelengths. It is found that the seed is amplified or attenuated d...

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-...

A column of bright filament is generated when a strong 800 nm femtosecond laser propagates in pure nitrogen gas, from which some fluorescence emission lines are observed. These emission lines can be assigned to the transitions of and N2 (C³Πu → B³Πg). By using a picosecond streak camera, we investigated the fluorescence decay dynamics of It was fou...

DOI:https://doi.org/10.1103/PhysRevA.95.069902

We perform high-resolution measurement of ellipticity-resolved momentum distributions from tunneling ionization of atoms along the major and minor axes in strong elliptically polarized fields, respectively. With developing a subcycle nonadiabatic strong-field tunneling theory for arbitrary laser polarization, we show that the electron initial condi...

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) (Ar2(2+)→Ar(+)+Ar(+)) and (2,1) (Ar2(3+)→Ar(2+)+Ar(+)) using a cold t...

Deuteron momentum distributions from the dissociative ionization of D2 in intense elliptically polarized laser fields have been explored in a joint experimental and numerical study. The asymmetrical charge localization in the dissociative D2+ offers a large torque, and thus an elliptically polarized laser field efficiently rotates the molecular ion...

When molecules are irradiated by intense femtosecond laser pulses, multiple ionization occurs and multiply charged molecular ions are generated. Due to the Coulomb repulsive force, multiply charged molecular ions will quickly dissociate into fragmental ions with high kinetic energy through the process of so-called Coulomb explosion. In this review...

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...

We study the photon energy sharing between the photoelectron and the nuclei in the process of above-threshold multiphoton dissociative ionization of CO molecules by measuring the joint energy spectra. The experimental observation shows that the electron-nuclear energy sharing strongly depends on the vibrational state. The experimental observation s...

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoe...

We experimentally studied three-body fragmentation dynamics of (CO2)23+ generated by intense femtosecond laser fields. Three-dimensional momentum vectors as well as kinetic energies were measured for correlated fragmental ions using the technology of coincidence measurement. The results demonstrate that sequential fragmentation channel dominates fo...

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...

We investigate nuclear dynamics and population transfer among multiple electronic states in tunnel ionized nitrogen molecules, and clarify the underlying mechanism behind the population inversion responsible for free-space N2 + lasers.

Tunneling ionization is one of the fundamental processes for molecules in intense laser fields, and the tunnel-ionized molecules are in various electronic states. Here, we report an experimental study of the population dynamics of N2+(B2Σu+) and N2+(X2Σg+) generated in intense femtosecond laser fields by simultaneously measuring the fluorescence em...

Rare gas clusters are model systems to investigate structural properties at finite size. However, their structures are difficult to be determined with available experimental techniques because of the strong coupling between the vibration and the rotation. Here we experimentally investigated multiple ionization and fragmentation dynamics of argon tr...

We have experimentally and theoretically studied the fragmentation dynamics of argon trimer (Ar3) in intense laser fields. By coincidently measuring the momentum vectors, we obtained the emission geometry of the three fragmental ions produced in the three-body fragmentation process. In addition to the direct Coulomb explosion channels, we observed...

We measure photoelectron energy distributions from single ionization of xenon atoms by a linearly polarized laser pulse (800nm,25fs) with successively varying the laser intensity within the region of 1.4−7.0×1013W/cm2. By measuring the photoelectron energy shifted with the ponderomotive potential, we have calibrated the laser peak intensity precise...

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states...

The structure distribution of argon trimer was experimentally reconstructed with laser-driven Coulomb explosion imaging technique and compared with our finite-temperature ab initio calculations.

Dissociative ionization dynamics were studied experimentally for CO2 driven by intense laser pulses. Three-dimensional momentum vectors of correlated atomic ions were obtained for each three-body fragmentation event using triple ion coincidence measurement. Newton diagram demonstrated that three-body fragmentation of CO2 n+ (n = 3-6) can occur thro...

Photoelectrons ionized from atoms and molecules in a strong laser field are either emitted directly or rescattered by the nucleus, both of which can serve as efficiently useful tools for molecular orbital imaging. We measure the photoelectron angular distributions of molecules (N2, O2 and CO2) ionized by infrared laser pulses (1320 nm, 0.2 ~ 1 × 10...

We perform a fully differential measurement on strong-field double ionization of Xe by 25 fs, 790 nm laser pulses in intensity region (0.4-3)×10^{14} W/cm^{2}. We observe that the two-dimensional correlation momentum spectra along the laser polarization direction show a nonstructured distribution for double ionization of Xe when decreasing the lase...

Tunneling ionization is one of the fundamental processes for molecules in intense laser fields. Depending on the ionizing molecular orbitals, molecular ions are in ground or excited electronic states. Here, we report an experimental study of tunneling ionization of nitrogen molecules using spectroscopic methods. The molecular ions in the excited el...

DOI:https://doi.org/10.1103/PhysRevLett.113.139902

Clusters usually exhibit unique structures as an intermediate form of matter. However, their structures are still difficult to be determined with available experimental techniques. There are many structure distinct isomers for carbon-dioxide clusters. Here we report an experimental and theoretical joint study to determine the structures of the lowe...

DOI:https://doi.org/10.1103/PhysRevLett.112.189901

We experimentally reconstructed the structure of the N2Ar van der Waals complex with the technique of laser-based channel-selected Coulomb explosion imaging. The internuclear distance between the N2 center of mass and the Ar atom, i.e., the length of the van der Waals bond, was determined to be 3.88 Å from the two-body explosion channels. The angle...

DOI:https://doi.org/10.1103/PhysRevA.89.049909

We measure high resolution photoelectron angular distributions (PADs) for above-threshold ionization of xenon atoms in infrared laser fields. Based on the Ammosov-Delone-Krainov theory, we develop an intuitive quantum-trajectory Monte Carlo model encoded with Feynman's path-integral approach, in which the Coulomb effect on electron trajectories and...

We perform a comparison study on strong-field ionization of the rare-gas dimers (Kr2 and Ar2) with their rare-gas monomers (Kr and Ar) in infrared laser fields at wavelengths of 795 and 1320 nm in the intensity range of (0.4-2)×1014 W/cm2. The photoelectron longitudinal momentum distributions of the rare-gas dimers reveal prominent intensity and wa...

We perform a triple coincidence study on differential momentum distributions of strong-field double ionization of Ar atoms in linearly polarized fields (795 nm, 45 fs, 7×10^{13} W/cm^{2}). Using a three-dimensional two-electron atomic-ensemble semiclassical model including the tunneling effect for both electrons, we retrieve differential momentum d...

Geometric structures of some van der Waals Complexes were determined by precisely measuring three-dimensional momentum vectors of correlated atomic ions produced in the laser-driven Coulomb explosion of these van der Waals complexes.

We investigate strong-field ionization dynamics of atoms in circularly polarized laser fields by a three-dimensional electron ensemble method which is validated by comparison with ab initio results of solving the time-dependent Schrödinger equation. We show that the Coulomb potential and the electron recollision play very crucial roles for single i...

Using photoelectron angular streaking by strong circularly polarized laser pulses, we measure angular-dependent tunneling rates from the inner orbitals of O2 in the molecular frame, which correlate with the strong-field molecular dissociative ionization process. We show that the electronic density of molecular inner orbitals can be visualized by an...

DOI:https://doi.org/10.1103/PhysRevA.88.069905

Dissociative ionization of N2Ar was experimentally studied in an intense femtosecond laser field. The break of the weak van der Waals bond generates an argon atomic ion and a nitrogen molecular ion. Both of them can recapture the tunneling electron. The probability measurement indicates that the tunneling electron prefers to be recaptured by the at...

In recent years, reaction microscopes have become a powerful technique to image ultrafast dynamics in atoms and molecules. Here, we present the specific details of our reaction microscope that was designed to study molecular dynamics driven by intense femtosecond laser pulses. A supersonic molecular beam and laser focusing system was specially desi...