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We demonstrate that the extreme ultraviolet free induction decay emission can be significantly enhanced by employing isolated attosecond pulses. In this work, the near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate the populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of the dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecod pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX3 (X = S, Se) monolayers, through first-principles calculations. Attributed to their special braceletlike structure, the in-plane negative Poisson's ratio (NPR) of BiSbS3 and BiSbSe3 monolayers are as high as -0.25 and -0.26, respectively. The phonon dispersion calculations, ab initio molecular dynamics simulations, and elastic constants calculations demonstrate that these two monolayers possess excellent dynamic, thermal, and mechanical stabilities. The band gap values of BiSbS3 and BiSbSe3 calculated at the HSE level by considering the spin-orbit coupling (SOC) effect are 1.68 and 1.20 eV. The anisotropic carrier mobility and superior optical absorption indicate that they may shine in the next generation of electronic and optoelectronic devices. All of these discoveries not only enrich the types of auxetic materials but also provide a structural reference for designing new auxetic materials on the molecular level. Furthermore, they can provide theoretical guidance for future applications of BiSbX3 (X = S, Se) monolayers in various fields.

We propose a novel approach based on the subcycle injection of carriers to extend the high-energy cutoff in solid-state high harmonics. The mechanism is first examined by employing the standard single-cell semiconductor Bloch equation (SC SBE) method for one-dimensional (1D) Mathieu potential model for ZnO subjected to the intense linearly polarized midinfrared laser field and extreme-ultraviolet pulse. Then, we use coupled solution of Maxwell propagation equation and SC SBE to propagate the fundamental laser field through the sample, and find that the high-harmonics pulse train from the entrance section of the sample can inject carriers to the conduction bands with attosecond timing, subsequently leading to a dramatic extension of high-energy cutoff in harmonics from the backside. We predict that for a peak intensity at 2×10^{11} W/cm^{2}, as a result of the self-seeding, the high-energy cutoff shifts from 20th (7.75 eV) order to around 50th (19.38 eV) order harmonics.

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 (N2+) which are produced using a strong 800 nm femtosecond laser pulse. Such photon retention, facilitated by quantum coherence, keeps releasing directly-unmeasurable coherent photons for tens of picoseconds, but is able to be read-out by a time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant absorption, resulting in a strong radiation at 329.3 nm. We reveal a pivotal role of the excited-state population to transmit such extremely weak re-emitted photons in this system. This new finding unveils the nature of the coherent quantum control in N2+ for the potential platform for optical information storage in the remote atmosphere, and facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules.

We report an unexpected experimental observation in rotation-resolved N2+ lasing that the R-branch lasing intensity from a single rotational state in the vicinity of 391 nm can be greatly stronger than the P-branch lasing intensity summing over the total rotational states at suitable pressures. According to a combined measurement of the dependence of the rotation-resolved lasing intensity on the pump-probe delay and the rotation-resolved polarization, we speculate that the destructive interference can be induced for the spectrally-indistinguishable P-branch lasing due to the propagation effect while the R-branch lasing is little affected due to its discrete spectral property, after precluding the role of rotational coherence. These findings shed light on the air-lasing physics, and provide a feasible route to manipulate air lasing intensity.

The anisotropic transport properties of gallium telluride (GaTe) have been reported by several experiments, giving rise to many debates recently. The anisotropic electronic band structure of GaTe shows the extreme difference between the flat band and tilted band in two distinct directions, Γ ¯ - X ¯ and Γ ¯ - Y ¯ , and which we called as the mixed flat-tilted band (MFTB). Focusing on such two directions, the relaxation of photo-generated carriers has been studied using the non-adiabatic molecular dynamics (NAMD) method to investigate the anisotropic behavior of ultrafast dynamics. The results show that the relaxation lifetime is different in flat band direction and tilted band direction, which is evidence for the existence of anisotropic behavior of the ultrafast dynamic, and such anisotropic behavior comes from the different intensities of electron–phonon coupling of the flat band and tilted band. Furthermore, the ultrafast dynamic behavior is found to be affected strongly by spin–orbit coupling (SOC) and such anisotropic behavior of the ultrafast dynamic can be reversed by SOC. The tunable anisotropic ultrafast dynamic behavior of GaTe is expected to be detected in ultrafast spectroscopy experiments and it may provide a tunable application in nanodevice design. The results may also provide a reference for the investigation of MFTB semiconductors.

We develop a quantum optical description of radiation from a two-level system (TLS) in strong laser fields, which provides a clear insight into the final states of the TLS and the harmonics field. It is shown that there are two emission channels: the Rayleigh-like channel and the Raman-like channel, which correspond to the TLS ending up in the ground state and excited state after the emission, respectively. The numerical result shows that the harmonics are mainly produced by the Rayleigh-like channel. In addition, according to the coherence of emission among the emitters, the radiation is divided into coherent parts that result from the semi-classical dipole oscillation and incoherent parts that result from the quantum fluctuations of the dipole moment. In the weak field limits, the Rayleigh-like channel corresponds to the coherent parts, and the Raman-like channel corresponds to the incoherent parts. However, in strong laser fields, both channels contribute to coherent and incoherent radiation, and how much they contribute depends on the final excitation. By manipulating the laser field, we can make the Rayleigh-like channel produce either coherent or incoherent radiation.

We report the laser-chirp controlled terahertz (THz) wave generation from two-color-laser-induced air plasma. Our experimental results reveal that the THz wave is affected by both the laser energy and chirp, leading to radiation minima that can be quantitatively reconstructed using the linear-dipole-array model. The phase difference between the two colors, determined by the chirp and intensity of the laser, can account for the radiation minima. Furthermore, we observe an asynchronous variation in the generated THz spectrum, which suggests a THz frequency-dependent phase matching between the laser pulse and THz wave. These results highlight the importance of laser chirp during the THz wave generation and demonstrate the possibility of modulating the THz yields and spectrum through chirping the incident laser pulse. This work can provide valuable insights into the mechanism of plasma-based THz wave generation and offer a unique means to control THz emissions.

We investigate the crystal-momentum-resolved contributions to high-order harmonic generation in laser-driven graphene by semi-conductor Bloch equations in the velocity gauge. It is shown that each harmonic is generated by electrons with the specific initial crystal momentum. The higher harmonics are primarily contributed by the electrons of larger initial crystal momentum because they possess larger instantaneous energies during the intra-band motion. Particularly, we observe circular interference fringes in the crystal-momentum-resolved harmonics spectrum, which result from the inter-cycle interference of harmonic generation. These circular fringes will disappear if the inter-cycle interference is disrupted by the strong dephasing effect. Our findings can help to better analyze the mechanism of high harmonics in graphene.

We introduce a technique to store and manipulate single x-ray photons which relies on dynamically controlled absorption via nuclear hyperfine magnetic splitting. This scheme is inherently suitable for storage, on-demand generation and dynamical manipulation of single x-ray photons, for instance, the manipulation of the temporal shape, temporal splitting, the interference between x-ray photons and the control of the polarization. Our approach opens up new paths in x-ray quantum information.

The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX

We introduce and demonstrate a new approach to measure the electron-hole dynamics and coherence induced by strong-field ionization using hole-assisted high-harmonic spectroscopy. The coherent driving of the infrared and XUV pulses correlates the dynamics of the core-hole and the valence-hole by coupling multiple continua, which leads to the otherwise forbidden absorption and emission of high harmonics. An analytical model is developed based on the strong-field approximation by taking into account the essential multielectron configurations. The emission spectra from the core-valence transition and the core-hole recombination are found to modulate strongly as functions of the time delay between the two pulses, suggesting that the coherent electron wave packets in multiple continua can be utilized to temporally resolve the core-valence transition in attoseconds.

Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually, thereby greatly enhancing the scope of Stark decelerated molecules. For comparison, we detect decelerated and trapped ammonia molecules using two different schemes: (i) nonresonant multiphoton ionization using intense femtosecond (fs) pulses in the near infrared, and (ii) resonance-enhanced multiphoton ionization using nanosecond (ns) pulses from a tunable UV laser. The observed number of ions per shot for both schemes is similar. The fs laser detection scheme suffers from an increased background, which can be effectively eliminated by subsequent mass and velocity selection. To determine the detection volume of the ns laser detection scheme, we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below ∼100 μm. It is concluded that the detection volume for the ns laser detection scheme is 1.5–2 times larger than that of the fs laser detection scheme.

All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control the high-field processes at the nanoscale. Here we report enhanced non-perturbative high-harmonic and terahertz emission from a LiNbO3 metasurface. The harmonic and terahertz intensity are enhanced by more than one order of magnitude compared to unpatterned samples. The enhanced high harmonics and terahertz waves are highly anisotropic with respect to the excitation polarization and are selective by the excitation wavelength due to its resonant features.

New instrumentation and calibration procedures for terahertz time-domain spectroscopic ellipsometry (THz-TDSE) are demonstrated. The THz-TDSE is capable of simultaneous measurement of two orthogonal components of reflected THz electric fields with no need to rotate a polarizer. In the calibration, the TDSE response function was obtained via the simultaneous polarization measurements reflected by a flat metal mirror, adapted in conventional THz time-domain reflection spectroscopy, and used here for THz-TDSE without the problems of position accuracy. The calibration could be used to determine accurate ellipsometric parameters with high tolerance of imperfect polarizer extinction ratios and of nonideality in the THz reflection components. Results are presented for an opaque Si wafer with heavy doping. The simultaneous measurements rejected significant common-mode noise from the laser, and it extended reliable THz spectra into the frequency range with a low dynamic range of a photoconductive-antenna THz source, which is a fundamental breakthrough for reflection-based measurements and overcomes the hurdle of phase uncertainty.

Molecular spectroscopy is a powerful tool for accelerating advances in materials science, chemistry, biology, and astronomy. Typically, molecular spectroscopy uses nanotips or nanoparticles to locally detect the molecule properties in a sample resulting from absorption of infrared and terahertz radiations. Here, we present an alternative that transient localized plasma can be applied to enhance the molecular spectroscopy. Our experiments show a significant enhancement of rotational transitions in carbon monoxide plasma under the illumination of terahertz waves. We proposed a semiclassical model for this enhancement, dominated by the energy exchange from electrons to molecules or ions via a collision process in the localized plasma. Our findings gain more insight into the nonequilibrium dynamics in laser-generated localized plasma. We also propose that the localized plasma can be used to improve the sensitivity of molecular spectroscopy.

The novel strong field autoionization (SFAI) dynamics is identified and investigated by channel-resolved angular streaking measurements of two electrons and two ions for the double-ionized CO. Comparing with the laser-assisted autoionization calculations, we demonstrate the electrons from SFAI are generated from the field-induced decay of the autoionizing state with a following acceleration in the laser fields. The energy-dependent photoelectron angular distributions further reveal that the subcycle ac-Stark effect modulates the lifetime of the autoionizing state and controls the emission of SFAI electrons in molecular frame. Our results pave the way to control the emission of resonant high-harmonic generation and trace the electron-electron correlation and electron-nuclear coupling by strong laser fields. The lifetime modulation of quantum systems in the strong laser field has great potential for quantum manipulation of chemical reactions and beyond.

Understanding the evolution of molecular electronic structures is the key to explore and control photochemical reactions and photobiological processes. Subjected to strong laser fields, electronic holes are formed upon ionization and evolve in the attosecond timescale. It is crucial to probe the electronic dynamics in real time with attosecond-temporal and atomic-spatial precision. Here, we present molecular attosecond interferometry that enables the in situ manipulation of holes in carbon dioxide molecules via the interferometry of the phase-locked electrons (propagating in opposite directions) of a laser-triggered rotational wave packet. The joint measurement on high-harmonic and terahertz spectroscopy (HATS) provides a unique tool for understanding electron dynamics from picoseconds to attoseconds. The optimum phases of two-color pulses for controlling the electron wave packet are precisely determined owing to the robust reference provided with the terahertz pulse generation. It is noteworthy that the contribution of HOMO-1 and HOMO-2 increases reflecting the deformation of the hole as the harmonic order increases. Our method can be applied to study hole dynamics of complex molecules and electron correlations during the strong-field process. The threefold control through molecular alignment, laser polarization, and the two-color pulse phase delay allows the precise manipulation of the transient hole paving the way for new advances in attochemistry.

We theoretically investigate terahertz emission from solid materials pumped by intense two-color femtosecond laser field in the presence of decoherence effects. Quantum-mechanical simulations are based on the length gauge semiconductor Bloch equations describing the optical excitation and decoherence with phenomenological dephasing and depopulation times. Contributions of interband and intraband mechanisms are identified in time domain, and the latter has dominated THz generation in solid-state systems. It is found that dephasing is crucial for enhancing asymmetric intraband current and deduced that solid-state materials with short dephasing time and long depopulation time would be optimal selection for strong-field terahertz generation experiments.

We report on an experimental investigation of the five vibrational Raman lines at 358 nm, 388 nm, 391 nm, 428 nm, and 471 nm of

Physical processes near threshold in strong-field ionization are complex owing to the important role of many intermediate states. Two typical near-threshold processes in few-cycle laser pulses, the generations of Rydberg electrons and terahertz waves, are investigated using classical trajectory Monte Carlo simulations. Mapping final energies of electrons to the tunneling coordinates, the two processes are distinguished based on the contributions from different kinds of electron trajectories, which highlights the complex interplay between the Coulomb potential and the laser field on the electron dynamics. We clarify the dependence of momentum and energy spectra on the carrier-envelope phase. We find that the optimal phases for terahertz wave generation and creation of the Rydberg state in few-cycle laser pulses are opposite. This finding can be applied to further experiments on attosecond electron dynamics.

We theoretically investigate the coherent control of strong-field high-harmonic generation in the presence of an isolated attosecond pulse. It is found that the rapid modulation of the controlled signal exhibits interference fringe structures in the delay-dependent spectra. By comparing the classical trajectory model with quantum mechanical calculation, it is demonstrated that the fringes are resulted from the interference between the photon- and the tunnelling-initiated recombination pathways. The relative recombination times for the two paths are reconstructed from the interference fringes, which provides a novel scheme for optical observation of the interplay of the photionization and tunneling ionization electron dynamics in attosecond resolution.

We experimentally investigated the rotationally resolved polarization characteristics of N$_2^+$ lasing at 391 and 428 nm using a pump-seed scheme. By varying the relative angle between the linear polarizations of the pump and seed, it is found that the polarizations of the P and R branches of 391-nm lasing are counter-rotated. By contrast, both branches of 428-nm lasing remain polarized along the pump. The origin of the puzzled abnormal polarization characteristics is found based on a complete physical model that simultaneously includes the transient photoionization and the subsequent coupling among the electronic, vibrational and rotational quantum states of ions.It suggests that the cascaded resonant Raman processes following ionization create negative coherence between the rotational states of $J$ and $J$+2 in the ionic ground state X$^2\Sigma_g^+(\nu=0)$, which leads to mirror-symmetrical polarization for the P and R branches of 391-nm lasing. Both the experiment and theory indicate that the demonstrated rotational coherence plays an extremely pivotal role in clarifying the gain mechanism of N$_2^+$ lasing and opens up the route toward quantum optics under ultrafast strong fields., Comment: submitted to referenced Journal in Jan 12, 2020

Terahertz (THz) generation in a gas medium is simulated by quantum calculations with multicolor intense laser pulses of various frequency ratios. By correlating THz radiation with the above-threshold-ionization photoelectron spectrum, we confirm that our previously proposed free-free transition model for the generation of THz radiation is also applicable to multicolor pulses. THz photon emissions can proceed through transitions between continuous states with similar energies that have opposite parities. This mechanism predicts that THz waves can be efficiently generated at special two-color laser frequency ratios when the multiphoton mixing condition is satisfied. Applying this model to multicolor fields (sawtooth wave shape), we provide a quantum-mechanical interpretation for the reason for THz radiation enhancement. A scheme combining multiple lasers to raise THz radiation satisfying multiphoton mixing conditions is also proposed.

Molecular ions, produced via ultrafast ionization, can be quantum emitters with the aid of resonant electronic couplings, which makes them the ideal candidates to study strong-field quantum optics. In this work, we experimentally and numerically investigate the necessary condition for observing a collective emission arising from macroscopic quantum polarization in a population-inverted N 2 + gain system, uncovering how the individual ionic emitters proceed into a coherent collection within hundreds of femtoseconds. Our results show that for a relatively high-gain case, the collective emission behaviors can be readily initiated for all the employed triggering pulse area. However, for a low-gain case, the superradiant amplification is quenched since the building time of macroscopic interionic quantum coherence exceeds the dipole dephasing time, in which situation the seed amplification and free induction decay play an essential role. These findings not only clarify the contentious key issue regarding to the amplification mechanism of N 2 + lasing but also show the unique characteristics of ultrashort laser-induced amplification in a molecular ion system where both the microscopic and macroscopic quantum coherence might be present.

According to the asymmetric molecular orbital reconstruction algorithm, which divides orbital into gerade and ungerade components and which does not depend on the unidirectional recollisional condition, we obtain the two-dimensional highest occupied molecular orbital (HOMO) of CO based on the directly calculated transition dipole moment and the harmonic spectra calculated by the Lewenstein model, respectively, which is the three-dimensional (3D) HOMO projected onto the plane perpendicular to the laser propagation direction. In order to retrieve the full orbital function, a 3D molecular orbital tomography (MOT) method is developed and is successfully applied to the reconstructions of the HOMO of CO, which simplifies the 3D imaging process of orbitals of linear molecules, and is expected to be extended to reconstruct the 3D orbitals of nonlinear molecules. In addition, the time-dependent density functional theory is employed to acquire the harmonic spectra of CO in a 800 nm and 1500 nm wavelength laser, respectively. The comparison of these two reconstruction results helps identify the multi-electron effects for asymmetric MOT, which requires further study. This work advances the development of MOT and is expected to reveal multi-electron effects in orbital imaging of complex polyatomic molecules.

The vacuum ultraviolet (VUV) radiation is generated in the strong-field-ionized CO molecules through 2 + 1 resonance excitation with two-color femtosecond laser pulses. When scanning the relative delay between two pump pulses, the rotational-resolved VUV radiations show periodic oscillations lasting as long as 500 ps. Fourier analysis reveals that these oscillations correspond to rotational beat frequencies of the A 2 Π i state of C O + , which is the result of multi-channel interference during the resonant excitation process. High resolution of Fourier transform spectra up to 0.067 c m − 1 allows us to obtain the fine energy levels of the A 2 Π i state. The theoretical calculation is in good agreement with the experimental observation. This work reveals the rotational coherence of the ionic excited state and shows the prospect of rotational coherence spectroscopy in measuring fine structures of molecular ions.