Attosecond spectroscopy for filming the ultrafast movies of atoms,molecules and solids

Three decades ago,a highly nonlinear nonpertubative phenomenon,now well-known as the high harmonic generation(HHG),was discovered when intense laser irradiates gaseous atoms.As the HHG produces broadband coherent radiation,it becomes the most promising source to obtain attosecond pulses.The door to the attosecond science was opened ever since.In this review,we will revisit the incredible adventure to the attoworld.Firstly,the progress of attosecond pulse generation is outlined.Then,we introduce the efforts on imaging the structures or filming the ultrafast dynamics of nuclei and electrons with unprecedented attosecond temporal and Angstrom spatial resolutions,utilizing the obtained attosecond pulses as well as the high harmonic spectrum itself.

Controllable Absorption and Dispersion Properties of an RF-driven Five-Level Atom in a Double-Band Photonic-Band-Gap Material

驾驶五水平的原子在 photonic 晶体嵌入的无线电频率(RF ) 的探查吸收分散系列被考虑各向同性的双乐队 photonic-band-gap (PBG ) 水库调查。在使用的模型，分别地，二转变被上面、更低的乐队在如此的 PBG 材料联合，因此导致一些好奇的现象。数字模拟为光系列被执行。当一转变频率在乐队差距内，其它在差距外面时，它被发现那，在那里出现在吸收系列的三座山峰。然而，为案例，二转变频率躺着在内或在乐队差距外面，系列显示四吸收侧面。特别，在那里出现在系列的二座锋利的山峰当两转变频率在乐队内存在时，豁开。吸收性、散的反应上的驾驶 RF 的地的紧张和频率的影响在不同乐队边位置下面被分析。一个透明性窗口出现在吸收系列，这被发现并且被调整系统参数被分散侧面的一个很陡峭的变化伴随。这些结果证明系统的吸收分散性质强烈取决于导致 RF 的量干扰和 PBG 水库的状态(DOS ) 的密度。

Yield enhancement of elliptical high harmonics driven by bicircular laser pulses

We theoretically investigate the yield enhancement of elliptical high harmonics in the interaction of molecules with bicircular laser pulses by solving the time-dependent Schrodinger equation.It is shown that by adjusting the relative intensity ratio of the two bicircular field components in specific ranges the yield of the molecular high harmonics for the plateau and cutoff regions can be respectively enhanced.To analyze this enhancement phenomenon,we calculate the weights of the electron classical trajectories.Additionally,we also study the ellipticity distribution of harmonics for different intensity ratios.We find that these enhanced harmonics are elliptically polarized,which we mainly attribute to the recombination dipole moment of the major weighted trajectories.These enhanced elliptical extreme ultraviolet and soft x-ray radiations may serve as essential tools for exploring the ultrafast dynamics in magnetic materials and chiral media.

Multifunctional light-field modulation based on hybrid nonlinear metasurfaces

The generation characteristics of nonlinear optical signals and their multi-dimensional modulation at micro-nano scale have become a prominent research area in nanophotonics,and also the key to developing various novel nonlinear photonics devices.In recent years,the demand for higher nonlinear conversion efficiency and device integration has led to the rapid progress of hybrid nonlinear metasurfaces composed of nanostructures and nonlinear materials.As a joint platform of stable wavefront modulation,nonlinear metasurface and efficient frequency conversion,hybrid nonlinear metasurfaces offer a splendid opportunity for developing the next-generation of multipurpose flat-optics devices.This article provides a comprehensive review of recent advances in hybrid nonlinear metasurfaces for light-field modulation.The advantages of hybrid systems are discussed from the perspectives of multifunctional light-field modulation,valleytronic modulation,and quantum technologies.Finally,the remaining challenges of hybrid metasurfaces are summarized and future developments are also prospected.

Attosecond probing and control of charge migration in carbon-chain molecule

In quantum mechanics,when an electron is quickly ripped off from a molecule,a superposition of new eigenstates of the cation creates an electron wave packet that governs the charge flow inside,which has been called charge migration(CM).Experimentally,extracting such dynamics at its natural(attosecond)timescale is quite difficult.We report the first such experiment in a linear carbon-chain molecule,butadiyne(C_(4)H_(2)),via high-harmonic spectroscopy(HHS).By employing advanced theoretical and computational tools,we showed that the wave packet and the CM of a single molecule are reconstructed from the harmonic spectra for each fixed-in-space angle of the molecule.For this onedimensional molecule,we calculate the center of charge <x>(t) to obtain v_(cm),to quantify the migration speed and how it depends on the orientation angle.The findings also uncover how the electron dynamics at the first few tens to hundreds of attoseconds depends on molecular structure.The method can be extended to other molecules where the HHS technique can be employed.

Microwave-Controlled Light-Pulse Propagation in a Driven A-Type Atomic System with Two-Folded Levels

The temporal and spatial dynamics of one weak probe laser pulse,propagating through a Λ-type atomicmedium with two-folded levels under the resonant excitation of one microwave driving field and one strong control field,is investigated in this paper.By numerically solving coupled Bloch-Maxwell equations,it is found that,in the absenceof the microwave driving field,the atomic medium is transparent to the probe pulse at line center,which propagatesover sufficiently long distances.By contrast,when the microwave driving field is applied,the probe pulse at line centercan be rapidly absorbed on propagation.This substantial reduction of probe transmittance caused by the microwavedriving field may lead to potential applications in designing a new kind of optical switching.

Measurement of molecular alignment with deep learningbased M-XFROG technique [Invited]

We demonstrate a deep-learning neural network(DNN) method for the measurement of molecular alignment by using the molecular-alignment-based cross-correlation polarization-gating frequency resolved optical gating(M-XFROG) technique.Our network has the capacity for direct measurement of molecular alignment from the FROG traces. In a proof-of-principle experiment, we have demonstrated our method in O^(2) molecules. With our method, the molecular alignment factor<cos^(2)θ>(t) of O_(2), impulsively excited by a pump pulse, was directly reconstructed. The accuracy and validity of the reconstruction have been verified by comparison with the simulations based on experimental parameters.

260 fs,403 W coherently combined fiber laser with precise high‑order dispersion management

An ultrafast fiber laser system comprising two coherently combined amplifier channels is reported.Within this system,each channel incorporates a rod-type fiber power amplifier,with individual operations reaching approximately 233 W.The active-locking of these coherently combined channels,followed by compression using gratings,yields an output with a pulse energy of 504μJ and an average power of 403 W.Exceptional stability is maintained,with a 0.3%root mean square(RMS)deviation and a beam quality factor M^(2)<1.2.Notably,precise dispersion management of the front-end seed light effectively compensates for the accumulated high-order dispersion in subsequent amplification stages.This strategic approach results in a significant reduction in the final output pulse duration for the coherently combined laser beam,reducing it from 488 to 260 fs after the gratings compressor,while concurrently enhancing the energy of the primary peak from 65%to 92%.

Review of cavity optomechanics in the weak-coupling regime: from linearization to intrinsic nonlinear interactions

Recently, cavity optomechanics has become a rapidly developing research field exploring the coupling between the optical field and mechanical oscillation. Cavity optomechanical systems were predicted to exhibit rich and nontrivial effects due to the nonlinear optomechanical interaction. However, most progress during the past years have focused on the linearization of the optomechanical interaction, which ignored the intrinsic nonlinear nature of the optomechanical coupling. Exploring nonlinear optomechanical interaction is of growing interest in both classical and quantum mechanisms, and nonlinear optomechanical interaction has emerged as an important new frontier in cavity optomechanics. It enables many applications ranging from single-photon sources to generation of nonclassical states. Here, we give a brief review of these developments and discuss some of the current challenges in this field.

Background-free detection of molecular chirality using a single-color beam [Invited]

We propose and numerically demonstrate a simple and background-free all-optical chiral spectroscopy technique for gas molecules.Our approach is based on high harmonic generation driven by a new type of laser beam that is produced by one linearly polarized single-color beam passing through a lens and a prism.It is shown that chiral and achiral signals are completely separated in frequency,indicating strong background-free and highly sensitive chirality detection.We believe this all-optical method can open new opportunities for ultrafast detection for chiral dynamics in the femtoseond to attosecond time scale.

Controllable alignment of elongated microorganisms in 3D microspace using electrofluidic devices manufactured by hybrid femtosecond laser microfabrication

This paper presents a simple technique to fabricate new electrofluidic devices for the three-dimensional(3D)manipulation of microorganisms by hybrid subtractive and additive femtosecond(fs)laser microfabrication(fs laser-assisted wet etching of glass followed by water-assisted fs laser modification combined with electroless metal plating).The technique enables the formation of patterned metal electrodes in arbitrary regions in closed glass microfluidic channels,which can spatially and temporally control the direction of electric fields in 3D microfluidic environments.The fabricated electrofluidic devices were applied to nanoaquariums to demonstrate the 3D electro-orientation of Euglena gracilis(an elongated unicellular microorganism)in microfluidics with high controllability and reliability.In particular,swimming Euglena cells can be oriented along the z-direction(perpendicular to the device surface)using electrodes with square outlines formed at the top and bottom of the channel,which is quite useful for observing the motions of cells parallel to their swimming directions.Specifically,z-directional electric field control ensured efficient observation of manipulated cells on the front side(45 cells were captured in a minute in an imaging area of~160×120μm),resulting in a reduction of the average time required to capture the images of five Euglena cells swimming continuously along the z-direction by a factor of~43 compared with the case of no electric field.In addition,the combination of the electrofluidic devices and dynamic imaging enabled observation of the flagella of Euglena cells,revealing that the swimming direction of each Euglena cell under the electric field application was determined by the initial body angle.

Manipulation of Spontaneous Emission via Quantum Interference in an Elliptically Polarized Laser Field

Manipulation of spontaneous emission from an atom confined in three kinds of modified reservoirs has been investigated by means of an elliptically polarized laser field. Some interesting phenomena such as the multi-peak structure, extreme spectral narrowing, and cancellation of spontaneous emission can be observed by adjusting controllable system parameters. Moreover, these phenomena depend on the constructive or destructive quantum interference between multiple decay channels and which can be changed appreciably by varying the phase difference between the two circularly polarized components of the probe field. These results demonstrate the importance of an elliptically polarized laser field in controlling the spontaneous emission and its potential applications in high-precision spectroscopy.

Extracting the phase distribution of the electron wave packet ionized by an elliptically polarized laser pulse

We use an interferometic scheme to extract the phase distribution of the electron wave packet from above-threshold ionization in elliptically polarized laser fields.In this scheme,an electron wave packet released from a circularly polarized laser pulse acts as a reference wave and interferes with the electron wave packet ionized by a time-delayed counter-rotating elliptically polarized laser field.The generated vortex-shaped interference pattern in the photoelectron momentum distribution enables us to extract the phase distribution of the electron wave packet in the elliptically polarized laser pulse with high precision.By artificially screening the ionic potential at different ranges when solving the time-dependent Schördinger equation,we find that the angle-dependent phase distribution of the electron wave packet in the elliptically polarized laser field shows an obvious angular shift as compared to the strong-field approximation,whose value is the same as the attoclock shift.We also show that the amplitude of the angle-dependent phase distribution is sensitive to the ellipticity of the laser pulse,providing an alternative way to precisely calibrate the laser ellipticity in the attoclock measurement.

Full experimental determination of tunneling time with attosecond-scale streaking method

Tunneling is one of the most fundamental and ubiquitous processes in the quantum world.The question of how long a particle takes to tunnel through a potential barrier has sparked a long-standing debate since the early days of quantum mechanics.Here,we propose and demonstrate a novel scheme to accurately determine the tunneling time of an electron.In this scheme,a weak laser field is used to streak the tunneling current produced by a strong elliptically polarized laser field in an attoclock configuration,allowing us to retrieve the tunneling ionization time relative to the field maximum with a precision of a few attoseconds.This overcomes the difficulties in previous attoclock measurements wherein the Coulomb effect on the photoelectron momentum distribution has to be removed with theoretical models and it requires accurate information of the driving laser fields.We demonstrate that the tunneling time of an electron from an atom is close to zero within our experimental accuracy.Our study represents a straightforward approach toward attosecond time-resolved imaging of electron motion in atoms and molecules.

Structuring Nonlinear Wavefront Emitted from Monolayer Transition-Metal Dichalcogenides

The growing demand for tailored nonlinearity calls for a structure with unusual phase discontinuity that allows the realization of nonlinear optical chirality,holographic imaging,and nonlinear wavefront control.Transition-metal dichalcogenide(TMDC)monolayers offer giant optical nonlinearity within a few-angstrom thickness,but limitations in optical absorption and domain size impose restriction on wavefront control of nonlinear emissions using classical light sources.In contrast,noble metal-based plasmonic nanosieves support giant field enhancements and precise nonlinear phase control,with hundred-nanometer pixellevel resolution;however,they suffer from intrinsically weak nonlinear susceptibility.Here,we report a multifunctional nonlinear interface by integrating TMDC monolayers with plasmonic nanosieves,yielding drastically different nonlinear functionalities that cannot be accessed by either constituent.Such a hybrid nonlinear interface allows second-harmonic(SH)orbital angular momentum(OAM)generation,beam steering,versatile polarization control,and holograms,with an effective SH nonlinearityχ^((2))of~25 nm/V.This designer platform synergizes the TMDC monolayer and plasmonic nanosieves to empower tunable geometric phases and large field enhancement,paving the way toward multifunctional and ultracompact nonlinear optical devices.

利用准非线性自旋轨道耦合产生高阶的二次谐波涡旋光束

谐波涡旋光束是一种典型的矢量光束,即产生的谐波信号具有螺旋型波前,在高容量光通信和非线性微纳光器件中有潜在应用前景.然而,目前的谐波涡旋光束产生器具有设计复杂,谐波转化效率低的局限性.本文提出并实验验证了基于准非线性自旋轨道耦合的高阶二次谐波涡旋光束,其拓扑荷数高达28阶,这是现有报道中的最高数值.结果表明金螺旋相位板的准轨道角动量(拓扑荷数q)可以传递到单层二硫化钨所产生的谐波信号上,其拓扑荷数为l_(n)=2nq(n为谐波阶次).该工作为新型高质量谐波涡旋光束的产生及应用开辟了新的途径.

Nonlinear Talbot self-healing in periodically poled LiNbO_3 crystal [Invited]

The nonlinear Talbot effect is a near-field nonlinear diffraction phenomenon in which the self-imaging of periodic objects is formed by the second harmonics of the incident laser beam. We demonstrate the first, to the best of our knowledge, example of nonlinear Talbot self-healing, i.e., the capability of creating defect-free images from faulty nonlinear optical structures. In particular, we employ the tightly focused femtosecond infrared optical pulses to fabricate LiNbO_(3) nonlinear photonic crystals and show that the defects in the form of the missing points of two-dimensional square and hexagonal periodic structures are restored in the second harmonic images at the first nonlinear Talbot plane. The observed nonlinear Talbot self-healing opens up new possibilities for defect-tolerant optical lithography and printing.

Temporal Airy–Talbot effect in linear optical potentials

We investigate the Airy–Talbot effect of an Airy pulse train in time-dependent linear potentials.The parabolic trajectory of self-imaging depends on both the dispersion sign and the linear potential gradient.By imposing linear phase modulations on the pulse train,the Airy–Talbot effects accompanied with positive and negative refractions are realized.For an input composed of stationary Airy pulses,the self-imaging follows straight lines,and the Airy–Talbot distance can be engineered by varying the linear potential gradient.The effect is also achieved in symmetric linear potentials.The study provides opportunities to control the self-imaging of aperiodic optical fields in time dimension.