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Ultrafast physics in atomic, molecular and optical systems

超快激光技术的发展经历了从飞秒激光到阿秒脉冲的跨越, 相关突破性进展为探索物质微观动力学提供了全新视角. 20世纪80年代, 啁啾脉冲放大技术的发明使得产生超短超强激光成为可能, 极大地推动了强场物理、非线性光学和精密光谱学的发展. 2018年, Gérard Mourou和Donna Strickland因该技术获得诺贝尔物理学奖, 奠定了现代超快激光科学的基础. 进入 21世纪后, 高次谐波产生技术的成熟使得阿秒脉冲的生成和测量成为现实. 2023年, Anne L’Huillier、Pierre Agostini和Ferenc Krausz因阿秒脉冲技术的突破性贡献获得诺贝尔物理学奖, 标志着人类正式进入阿秒科学时代. 阿秒脉冲为实时观测电子运动提供了“超快相机”, 使得化学键断裂、电荷迁移、量子隧穿等超快过程的研究成为可能, 为原子分子物理、量子化学和材料科学带来了革命性的研究手段. 原子分子尺度的微观动力学过程不仅是理解物质宏观性质与功能的基石, 更与超快能量传递、光化学调控、量子信息处理等前沿科学问题密切相关, 为新一代光电器件、精密测量和量子计算等技术的发展提供了关键支撑.

为展示我国在超快原子分子光物理领域的最新成果, 《物理学报》特邀该领域一线科学家组织本期专题, 聚焦强场物理、阿秒科学及超快光学等方向的创新突破. 专题内容涵盖理论方法与实验技术的双重创新, 包括量子动力学新方法实现分子动力学几何相位的直接提取; 半经典响应时间理论揭示分子隧穿电离的超快动力学机制; 非微扰量子电动力学的频域理论构建强 X射线场中单光子康普顿散射的理论框架; 另有研究阐明椭圆偏振强场中分子电离的缀饰态与非缀饰态演化规律;基于光场调控实现太赫兹波的可控产生等研究成果和综述将被报道.

本专题旨在为相关研究提供方法学启示与交叉合作契机. 我们期待这些成果能激发读者对超快科学的探索热情, 推动该领域向更小时间尺度、更高调控精度迈进, 为揭示物质微观动力学规律及新型调控技术开辟新路径.

客座编辑：李辉　华东师范大学; 王春成　吉林大学; 吴健　华东师范大学; 丁大军　吉林大学

Acta Physica Sinica. 2025, 74(15).

[Abstract](287) HTML PDF

Dressed-state and undressed-state during molecular ionization induced by elliptically polarized laser field

LIU Jie, HAO Xiaolei

2025, 74 (9): 093202. doi: 10.7498/aps.74.20250064

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Despite the molecular strong-field approximation (SFA) theory has made remarkable achievements in describing the ultrafast dynamics of molecules in intense laser fields, there are basic inconsistencies in the theory itself. On the one hand, the basic principle of SFA requires that the initial state be an eigenstate of the system in the absence of the field, and on the other hand, the spatial translation invariance of the physical process requires that the initial state of the system be a laser-field-dressed state. These two conflicting requirements correspond to the two forms of molecular SFA theories, namely, the undressed state and the dressed state. The two theoretical validity and applicability conditions are widely disputed. In this paper, we investigate the ionization processes of N₂ and Ne₂ molecules in an elliptically polarized laser field and a circularly polarized laser field, aiming to solve the above-mentioned controversies. Elliptically polarized laser can efficiently suppress the re-scattering process and the influence of various interference effects, which makes the ionization process cleaner, and thus can effectively screen the applicable conditions for the dressed and undressed states. We calculate the photoelectron momentum distributions corresponding to different molecular orbitals in the dressed and undressed states by using the SFA and the Coulomb-corrected strong-field approximation and compare them with previous experimental results. For molecules with large nuclear spacing such as Ne₂, we find that the dressed state is necessary to accurately characterise their ionization, however, for molecules with small nuclear spacing such as N₂, the dressed state description is inapplicable. The conclusions of this work provide a reference for accurately describing laser-induced molecular ultrafast processes and further developing corresponding theories and molecular ultrafast imaging schemes.

[Abstract](287) HTML PDF

Preface to the special topic: Ultrafast physics in atomic, molecular and optical systems

2025, 74 (15): 150101. doi: 10.7498/aps.74.150101

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[Abstract](287) HTML PDF

Geometric phase in molecular dynamics

YANG Huan, ZHENG Yujun

2025, 74 (15): 150201. doi: 10.7498/aps.74.20250388

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The geometric phase effect of molecules, also known as the molecular Aharonov-Bohm effect, arises from the study of the conical intersections of potential energy surfaces. When encircling a conical intersection in the nuclear configuration space, the adiabatic electronic wave function acquires a π phase, leading to a change in sign. Consequently, the nuclear wave function must also change its sign to maintain the single-valued nature of the total wave function. This phase is topologically related to the conical intersection structure. Only by appropriately introducing the molecular geometric phase can the quantum dynamical behavior in the adiabatic representation be accurately described. In the diabatic representation, both the geometric phase effects and the non-adiabatic couplings between nuclei and electrons can be implicitly handled.In this paper, according to the quantum kinematic approach to the geometric phase, we propose a method for directly extracting the geometric phase in molecular dynamics. To demonstrate the unique features of this method, we adopt the $E \otimes e $ Jahn-Teller model, which is a standard model that includes a cone intersection point. This model comprises two diabatic electronic states coupled with two vibrational modes. The initial wave function is designed in such a way that it can circumnavigate the conical intersection in an almost adiabatic manner within approximately 2.4 ms. Subsequently, the quantum kinematic approach is utilized to extract the geometric phase during the evolution. In contrast to the typical topological effect of a quantized geometric phase of π, this extracted geometric phase in this case varies in a continuous manner. When a quantum system performs a path in its projected Hilbert space, it is a representation-independent and gauge-invariant formula of the geometric phase. This research provides a new perspective for exploring molecular geometric phases and the geometric phase effects. It may also provide a possible observable for experimentally studying geometric phases in molecular dynamics.

[Abstract](287) HTML PDF

Study of single-photon Compton scattering process of bound electrons in intense laser fields by using frequency-domain theory

QIU Yuanyuan, YANG Yujun, GUO Yingchun, WEI Zhiyi, WANG Bingbing

2025, 74 (15): 150301. doi: 10.7498/aps.74.20250483

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Compton scattering is defined as an inelastic scattering process in which the interaction between strong laser fields and electrons in matter leads to photon emission. In recent years, with the rapid development of X-ray free-electron lasers, the intensity of X-ray lasers has steadily increased, and the photon energy in Compton scattering process has risen correspondingly. Previous studies focus on single-photon Compton scattering of free electrons. However, the mechanism of non-relativistic X-ray photon scattering by bound electrons remains to be elucidated. Therefore, we develop a frequency-domain theory based on non-perturbative quantum electrodynamics to investigate single-photon Compton scattering of bound electrons in strong X-ray laser fields. Our results show that the double-differential probability of Compton backscattering decreases with the increase of incident photon energy. This work establishes a relationship between Compton scattering and atomic ionization in high-frequency intense laser fields, thereby providing a platform for studying atomic structure dynamics under high-intensity laser conditions.

[Abstract](287) HTML PDF

Applications of semiclassical response time theory in strong-field molecular ionization

YE Sheng, WANG Shang, CHEN Ziyu, LI Weiyan, SHEN Shiqi, CHEN Yanjun

2025, 74 (15): 153303. doi: 10.7498/aps.74.20250459

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The attosecond technology provides a powerful tool for studying the ultrafast dynamics of electrons during the strong-field ionization of atoms and molecules. This technology relies on quantitative theoretical models to invert the ultrafast time-domain information of the system in the ionization process from the photoelectron spectra obtained through experimental measurements. One of the key issues in constructing quantitative strong-field theoretical models is the theoretical description of the Coulomb effect. The Coulomb potential of molecules, compared with the single-center Coulomb potential of atoms, exhibits a multi-center distribution. This fundamental geometric structure feature results in many new effects of molecules in the external field, such as orientation effect, charge resonance effect, intrinsic dipole effect, and vibration effect. Therefore, it can be expected that the tunneling ionization process of molecules contains more phenomena than that of atoms, which is worthy of in-depth study in experiment and theory. Especially for stretched molecular ions, such as $ {\mathrm{H}}_{2}^{+} $, those exhibiting charge resonance effects in external fields, the difference between near-nucleus and far-nucleus Coulomb effects, which is of great significance for constructing quantitative theoretical models, becomes more complex, providing a platform for testing the applicability of quantitative theoretical models.This work systematically compares the predictions of different theoretical models for the attoclock characteristic observables in molecular systems with large internuclear distances. Through comparative analysis, it is found that the recently proposed semiclassical response time theory, which incorporates near-nucleus Coulomb corrections, shows better agreement with numerical experimental results than the developed strong-field approximation models that consider far-nucleus Coulomb corrections. The semiclassical response time theory establishes a theoretical framework for describing strong-field ultrafast ionization dynamics of stretched molecular systems by considering dual-center Coulomb potential corrections and excited-state contributions. Specifically, it approximates the complex four-body interactions (electron-laser-dual nuclei) in stretched molecular systems to a three-body interaction (electron-laser-dressed-up barrier-proximal nucleus), while using the influence of the other nucleus on the potential barrier as a correction term for the tunnel-exit position. This framework highlights the significant influence of quantum-property-dominated near-nucleus Coulomb effects on molecular tunneling ionization. Furthermore, the theory provides an explicit formula for the response time determined by fundamental laser and molecular parameters. By calculating this response time, the values of attoclock observables are deduced from the theory, thus enabling a clear discussion of ionization time delays in stretched molecular tunneling ionization and revealing that such delays reflect the timescale of strong four-body interactions between the laser, electron, and molecular nucleus. In contrast, the developed strong-field approximation model that simultaneously considers excited-state effects and numerically solves Newton’s equations to describe far-nucleus Coulomb effects cannot fully describe the above-mentioned four-body interaction, making it difficult to quantitatively describe the complex tunneling ionization dynamics under the combined action of coulomb and excited states. Additionally, since this model cannot clearly define the ionization time, the related ionization time delay issues cannot be well discussed. Computational results show that the semi-classical response time theoretical model has improved in terms of calculation accuracy and efficiency, thereby verifying the applicability of this theoretical model in the study of molecular ultrafast ionization dynamics.Moreover, for $ {\mathrm{H}}_{2}^{+} $ with intermediate internuclear distances, the charge resonance effect induces a significant ionization enhancement effect. We present relevant numerical experimental attoclock results and explore the potential applications of the response time theory in such systems. We also envision the extension of this theory to strong-field tunneling ionization in polar molecules, multi-center linear molecules, planar and three-dimensional molecules, and oriented molecules, where interference and Coulomb-acceleration effects compete with each other.

[Abstract](287) HTML PDF

Modulation of terahertz wave generation on lithium niobate chip by temporal dispersion of femtosecond laser

DUAN Haoyu, XU Xitan, ZHENG Ziyang, HUANG Yibo, LU Yao, WU Qiang, XU Jingjun

2025, 74 (15): 158702. doi: 10.7498/aps.74.20250573

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Femtosecond laser excitation of nonlinear materials is one of the key technologies for generating terahertz waves at present. Due to its advantages such as ultrashort time resolution and ultrabroad frequency spectrum, the technology has been widely used to characterize, measure, sense and image terahertz waves. However, the methods of controlling terahertz waves through microstructures can only regulate their transmission process, and they will face obstacles such as design difficulty and complex processes, making it hard to be widely used in industry. In this work, by introducing a pulse-shaping system to change the time dispersion of femtosecond laser pulses, the interaction process between femtosecond laser and lithium niobate crystals can be directly regulated, therefore the terahertz generation process can be directly controlled. Taking the second-order time dispersion for example, the terahertz signals generated by pump light with different second-order time dispersion in lithium niobate is detected by using the pump-probe phase-contrast imaging system. Meanwhile, the generation process of terahertz waves is simulated using the impact stimulated Raman scattering model and Huang-Kun equation, demonstrating the feasibility of using femtosecond laser pulses to adjust the time dispersion of terahertz waves. The experimental and simulation results show that when the time dispersion of femtosecond laser causes the pulse width to increase, the time in which the lithium niobate lattice is subjected to the impact stimulated Raman scattering force is prolonged, and the macroscopic polarization of the lithium niobate lattice is correspondingly extended. On the one hand, the longer duration of polarization results in a wider terahertz signal in the time domain and a narrower one in the frequency domain. On the other hand, since the impact stimulated Raman scattering force is proportional to the pump light intensity and is in the same direction during the interaction time, when the Raman scattering force ends, the lattice reaches a maximum displacement. The longer Raman scattering force causes the lattice to move to one side for a longer time, and correspondingly, the subsequent vibration of one period takes a longer time, ultimately resulting in a lower center frequency. In addition, this work also points out that the modulation of terahertz signals by pump light pulse width may be affected by the thickness of the wafer, and the modulation effect on thinner media may be more obvious. This result is of great reference significance for the active regulation of on-chip terahertz sources based on lithium niobate crystals in the future.

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Ultrafast physics in atomic, molecular and optical systems

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