We describe a three-dimensional (3D) magneto-optical trap (MOT) capable of simultaneously capturing ⁸⁵Rb and ¹³³Cs atoms. Unlike conventional setups, our system utilizes two separate laser systems that are combined before entering the vacuum chamber, enabling the simultaneous trapping of two different atomic species. We trapped ⁸⁵Rb and ¹³³Cs atoms using relatively low total power: 8 mW cooling and 4 mW repump for ⁸⁵Rb, and 7.5 mW cooling and 1.5 mW repump for ¹³³Cs. The number of trapped atoms was \(1.6 \times 10^8\) for ⁸⁵Rb and \(1.4 \times 10^8\) for ¹³³Cs. The optical depths were 3.71 for ⁸⁵Rb and 3.45 for ¹³³Cs. The temperature of trapped atoms was $\sim200$ μK for ⁸⁵Rb and $\sim 200$ μK for ¹³³Cs. Our 3D MOT setup allows full horizontal optical access to the trapped atomic ensembles without spatial interference from the trapping or repump laser beams. Our vacuum system is also quite simple, avoiding much of the complexity typically encountered in similar dual-species systems. However, the red detuning of the cooling laser used for atomic trapping in our system is relatively small, leaving room for further optimization. This system offers a versatile platform for exploring complex phenomena in ultracold atom physics, such as Rydberg molecule formation and interspecies interactions.

Terahertz (THz) radiation, spanning the frequency range 100 GHz to 10 THz, offers diverse applications in spectroscopy, materials characterization, medical diagnostics and environmental monitoring. Despite its potential, the generation of high-intensity, tunable THz radiation remains a significant challenge. In this work, we explore a novel approach to the efficient generation of THz radiation based on laser-plasma interactions, utilizing the principles of photon deceleration. When a relativistic CO₂ laser passes through a pre-ionized plasma, the laser induces a nonlinear wakefield, creating a strong refractive index gradient. This gradient, combined with the lower-density region of the wakefield, slows down the laser, facilitating the accumulation of THz radiation. The resulting THz pulse exhibits extreme collimation, high energy efficiency and tunability. Our work shows that this method can achieve up to 10% conversion efficiency with optimal plasma density near the critical density. This technique presents a promising solution for overcoming current limitations in THz source development and offers potential for diverse applications.

We performed real-time and real-space numerical simulations of high-order harmonic generation in the three-dimensional structured molecule methane (CH$_{4}$) using time-dependent density functional theory. By irradiating the methane molecule with an elliptically polarized laser pulse polarized in the \( x \)-\( y \) plane, we observed significant even-order harmonic emission in the \( z \)-direction. By analyzing the electron dynamics in the electric field and the multi-orbital effects of the molecule, we revealed that electron recombination near specific atoms in methane is the primary source of high-order harmonic generation in the \( z \)-direction. Furthermore, we identified the dominant molecular orbitals responsible for the enhancement of harmonics in this direction and demonstrated the critical role played by multi-orbital effects in this process.

The attosecond extreme ultraviolet (XUV) pulse pump and femtosecond infrared (IR) pulse probe scheme is commonly used to study the dynamics and attosecond transient absorption (ATA) spectra of microscopic systems. In a recent report [Proc. Natl. Acad. Sci. USA 121 e2307836121 (2024 )], we showed that shaped XUV pulses with spectral minima can significantly alter the absorption line shape of helium's 2s2p doubly excited state within a few tens of attoseconds. However, it remains unclear if similar effects could be observed in a singly excited state. In this study, we use shaped XUV pulses to excite helium's 2p singly excited state and couple the 2p and 3d states with a delayed IR pulse. Comparing these results with those from Gaussian XUV pulses, we find that the ATA spectra for the shaped XUV pulses exhibit more pronounced changes with delay, while the changes for the Gaussian pulses are gradual. We also explain these differences through population changes and analytical models. Our findings show that shaped XUV pulses can regulate the dynamics and absorption spectra of a singly excited state

Molecular high-order harmonic spectroscopy is a significant advancement in ultrafast science, enabling the measurement of multielectron dynamics with attosecond temporal resolution. The fine structures observed in the molecular harmonic spectrum provide crucial insights into the structural or multielectron dynamical effects induced by intense laser fields. In this study, we measure the high-order harmonic spectrum of aligned CO$_{2}$ molecules contributed from short trajectories. Two distinct groups of minima are identified in the plateau region. Our findings indicate that the deeper-lying molecular orbitals and two-center interference play significant roles in molecular harmonic generation. The results pave the way for advancing the understanding of multielectron dynamics in polyatomic molecules under intense laser fields.

The two-body fragmentation dynamics of water isotopologues dications ($\rm{{H_2O}^{2+}}$, $\rm{{HOD}^{2+}}$, and $\rm{{D_2O}^{2+}}$) induced by 200 eV electron impact is investigated. Two fragment ions and an emitted electron are detected in coincidence, and their momentum vectors are determined by employing a reaction microscope. The complete kinematical information of four two-body fragmentation channels of $\rm {H}^{+} + \rm {OH}^{+}$, $\rm {H}^{+} + \rm {OD}^{+}$, $\rm {D}^{+} + \rm {OH}^{+}$, and $\rm {D}^{+} + \rm {OD}^{+}$ is obtained. By analyzing the projectile energy-loss spectrum, the initial electronic state of the two-body dissociation channel is determined. Upon examining the kinetic energy release (KER) distributions of the four fragmentation channels, a clear difference is found between the two-body fragmentation channel $\rm {H}^{+} + \rm {OD}^{+}$ and the other three channels. The isotopic effect in the two-body fragmentation is demonstrated by the analysis of the relative yields of the two-body fragmentation channels originating from different isotopologues, which shows preferential cleavage of the O-H bond over the O-D bond. These results provide deeper insight into the microscopic dynamic mechanisms in water radiolysis.

We propose a method to characterize the features of a cold strontium cloud in a magneto-optical trap (MOT) through the photoionization of cold Sr atoms in a custom-designed reaction microscope. Sr atoms in the dark state of $\mathrm{5s5p \, ^3P_2}$ populated via the cascade transition $\mathrm{5s5p \, ^1P_1 \rightarrow 5s4d \, ^1D_2 \rightarrow 5s5p \, ^3P_2}$ accumulate a significant fraction, giving a long lifetime of 520 s. These atoms in the dark state are subsequently trapped by the gradient magnetic field of the MOT. By scanning the Sr$^+$ momentum distributions ionized with an 800 nm infrared femtosecond laser, we are able to outline the size of $\sim0.55$ mm in radius and the temperature of $\sim0.40$ mK for the dark-state atoms, which is significantly cooler than the MOT temperature of 3.3 mK trapped in the 461 nm. The size of MOT exhibits an oblate spheroidal distribution with a radius of approximately 0.35 mm and 0.55 mm, extracted with momenta of photoion and absorption imaging, respectively. The results using the photoion momenta are consistent with the expected results from absorption imaging, which confirms the method's reliability. The advantage of this method is the ability to simultaneously characterize the distribution information of atoms in different initial states within the cold atomic cloud.

This review comprehensively explores the theory and applications of attosecond transient absorption spectroscopy (ATAS) in studying ultrafast electronic dynamics across various systems, from atoms to solids. Driven by significant advancements in ultrafast laser technology, such as generating isolated attosecond pulses, ATAS enables detailed investigations of ultrafast electronic processes with unprecedented time resolution. The article introduces the fundamental principles and historical development of ATAS. Applications of ATAS are discussed in three main domains: in atoms, where it has been used to study build-up dynamics of Autler-Townes splitting, Fano resonance, light-induced states, etc.; in molecules, where it has revealed coherent molecular wavepacket dynamics and non-adiabatic dynamics near conical intersections; and in solids, where it has been extended to investigate ultrafast charge carrier dynamics in metals, semiconductors, and insulators. The review highlights the potential of ATAS in developing ultrafast optical switches and petahertz electronics. The ability of ATAS to probe and manipulate electronic dynamics at the attosecond timescale provides a powerful tool for exploring the fundamental limits of electronic and optical processes in materials.

Cavity-free lasing of nitrogen molecules pumped by intense femtosecond laser pulses holds the potential for remote sensing of electric fields. Here we compared the influence of an external direct current (DC) electric field on both the directional lasing radiation and omnidirectional fluorescence emission of neutral nitrogen molecules. It was found that the forward lasing radiation in both pure nitrogen gas and ambient air shows a sensitive dependence on the direction and strength of the DC field, while the fluorescence is not influenced. The effect of pump laser polarization was also investigated. The distinct behavior of the lasing and fluorescence in response to the DC field was attributed to their different dependences on the population distribution of excited nitrogen molecules. This study consolidates the method for standoff detection of electric field with an air lasing effect in the atmosphere.

This study analytically examines the ionization of atoms in strong near-circular laser fields. The classic Keldysh-Rutherford (KR) Coulomb-scattering (CS) model [Phys. Rev. Lett. 121 123201 (2018)] successfully explained the attoclock experimental curve for the H atom at lower laser intensities. Here, we develop a semiclassical model that includes the initial conditions related to the quantum properties of tunneling in the KR model at the beginning of the scattering process. This model is able to explain recent attoclock experimental curves over a wider range of laser and atomic parameters. Our results show the importance of system symmetry and quantum effects in attoclock measurements, suggesting the complex role of the Coulomb potential in strong-field ionization.

The dephasing time $ T_2 $ is a fundamental parameter that characterizes the coherence of electronic states and electron-phonon interactions in condensed matter physics. Accurate measurement of $ T_2 $ is essential for elucidating ultrafast electronic and phononic processes, which are crucial for the development of advanced electronic, optoelectronic, and quantum devices. However, due to the complexity of solid-state systems with their intricate band structures and strong many-body interactions, reconstructing $ T_2 $ remains a long-term challenge for both condensed matter physics and optical science. In this work, we introduce a machine learning (ML) approach to retrieve $ T_2 $ from the high-order harmonic generation (HHG) spectrum resulting from the interaction between a strong infrared (IR) laser pulse and solid-state material. The consistency between the experimental and reconstructed HHG spectra validates the efficiency of our scheme. Our ML method offers two key advantages: first, it does not require stringent experimental conditions, and second, the optimization process is fully automated and more reliable than empirical selection of dephasing time values. The ability of our method to reconstruct dephasing time from solid HHG spectra provides a powerful tool for probing the intrinsic properties of materials under extreme conditions. Besides, our method provides another significant advantage, which offers a direct approach to calculating the quantum tunneling time of carriers between different energy bands under light-induced excitation.

Light-induced conical intersections (LICIs) present a distinctive mechanism for nonadiabatic coupling, thereby facilitating ultrafast chemical reactions, including the indirect photodissociation of diatomic molecules. In contrast to static conical intersections, LICIs are dynamically tunable, providing a pathway for precise control of molecular dissociation. In this study, we employ the time-dependent quantum wave packet method to investigate the dissociation dynamics of the OH molecule, focusing on its ground state X$^{2}\Pi$ and repulsive state 1$^{2}\Sigma^{-}$. By varying laser field parameters (intensity, full width at half maximum (FWHM), and wavelength), we elucidate how nonadiabatic coupling governs selective dissociation channel control. Our findings reveal that the choice of initial vibrational states and the tailoring of laser conditions significantly influence dissociation pathways, providing theoretical insights into manipulating molecular dynamics via LICIs. These results provide a foundation for future experimental studies and the development of advanced molecular control techniques.

We present a fully time-dependent quantum wave packet evolution method for investigating molecular dynamics in intense laser fields. This approach enables the simultaneous treatment of interactions among multiple electronic states while simultaneously tracking their time-dependent electronic, vibrational, and rotational dynamics. As an illustrative example, we consider neutral H$_2$ molecules and simulate the laser-induced excitation dynamics of electronic and rotational states in strong laser fields, quantitatively distinguishing the respective contributions of electronic dipole transitions (within the classical-field approximation) and non-resonant Raman processes to the overall molecular dynamics. Furthermore, we precisely evaluate the relative contributions of direct tunneling ionization from the ground state and ionization following electronic excitation in the strong-field ionization of H$_2$. The developed methodology shows strong potential for performing high-precision theoretical simulations of electronic-vibrational-rotational state excitations, ionization, and dissociation dynamics in molecules and their ions under intense laser fields.

We investigate the zeptosecond-timescale delayed ionization process induced by ultrafast laser propagation in different directions across the molecule. The experimental measurements by Grundmann et al.[Science 370 339 (2020)] serve as a basis for our study, where they extract the birth time delay of photoelectron emission from two nuclei, amounting to a few hundred zeptoseconds. By comparing and analyzing the results, we observe that asymmetric systems, such as the 2p$\sigma $ state of HeH$^{2+}$, exhibit nonequivalent responses to forward and backward laser propagation, resulting in an asymmetric dependence of the interference structure in the photoelectron momentum spectra. This process is considered as an ultrafast nonreciprocal phase shift with zeptosecond resolution. Through computational simulations, we explore the relationship between this kind of ultrafast nonreciprocity effect and molecular orbital symmetry. This study broadens our understanding of nonreciprocal physical mechanisms in the field of strong-field ultrafast dynamics, and provides a theoretical basis for the experimental investigation of the nonreciprocal phase shift within the zeptosecond timescale in the response processes of matter under ultrafast laser irradiation.

We present a comprehensive study on the role of various excited states in high-order harmonic generation of hydrogen atoms driven by a long-wavelength (1500 nm) laser field. By numerically solving the time-dependent Schr?dinger equation (TDSE) and performing a time-frequency analysis, we investigate the influence of individual excited states on the harmonic spectrum. Our results reveal that the 2s excited state primarily contributes to the enhancement of high-energy harmonic yields by facilitating long electron trajectories, while the 2p excited state predominantly suppresses harmonic yields in the lower-energy region (20th-50th orders) by altering the contributions of electron trajectories. Our results highlight the critical role of the excited states in the HHG process, even at longer laser wavelengths.