Seminar về chủ đề: Hiệu ứng nhiều electron và điều khiển trường trong quá trình phát xạ sóng điều hoà bậc cao

Vào 14h30, ngày 29/10/2025 Viện IAST tổ chức buổi trao đổi học thuật tại Phòng họp C Đại học Tôn Đức Thắng với thông tin chi tiết như sau: 

1/ PGS.TS Phan Thị Ngọc Loan báo cáo về chủ đề: Terahertz-assisted high-order harmonic generation for waveform sampling and coherent XUV emission.
Abstract: 

Terahertz (THz) fields—particularly THz pulses with durations ranging from picoseconds to nanoseconds—play a vital role in fundamental science, material control, and practical applications such as high-speed wireless communication technologies [1, 2]. Beyond generating THz pulses, accurately characterizing them is a critical step toward their full utilization. Among existing techniques, THz time-domain spectroscopy (THz-TDS) is widely used as it provides both amplitude and phase information of the THz field. However, THz-TDS remains technically challenging due to its strong dependence on the properties of the detection material, which limits its accuracy and measurable bandwidth [1, 2]. This motivates the development of alternative, material-independent methods for THz waveform sampling.

In this report, we demonstrate that high-order harmonic generation (HHG)—a nonlinear optical process emitted when atoms or molecules interact with an intense ultrashort laser field—can serve as a powerful and general tool for THz field characterization. When the laser interaction is assisted by a THz field, the resulting HHG spectrum encodes detailed information about the THz waveform through symmetry-breaking signatures. By establishing analytical relationships between the THz electric field and the odd-even harmonic intensity ratio or frequency shift in the HHG spectrum, we propose two universal THz-TDS methods [3-5]. The first measures the even-to-odd harmonic intensity ratio in atoms or symmetric molecules irradiated by a multicycle mid-infrared laser pulse [3]. The second extracts the frequency shift of harmonic peaks in the cutoff region using few-cycle laser pulses [4]. More specifically, we show that the two proposed methods are general and robust, being independent of the atomic or molecular species and laser parameters. This universality paves the way for a compact, all-optical scheme for THz waveform sampling using standard tabletop HHG setups.

Additionally, we explore the potential of THz-assisted HHG for generating high-coherence extreme ultraviolet (XUV) and soft X-ray photons. We show that moderate THz fields can significantly extend the HHG cutoff, making it possible to achieve coherent high-energy radiation [6]. Such sources hold great promise for applications such as diagnosing microscopic defects in microelectronic components.
 

2/ ThS. NCS Nguyễn Huỳnh Kim Ngân báo cáo về chủ đề: Frequency shift in high-order harmonic generation induced by multielectron polarization
Abstract: 

High-order harmonic generation (HHG) has become a versatile tool for probing ultrafast electron dynamics. While most models rely on the single-active-electron (SAE) approximation [1], recent studies highlight the essential role of multielectron effects, particularly the dynamic polarization of inner-shell electrons—known as multielectron polarization (MEP). This effect strongly influences the emission from polar molecules [2-3], yet its impact on the frequency position of harmonics remains unexplored.

Here, we show that MEP can induce measurable frequency shifts in HHG spectra of polar molecules driven by few-cycle laser pulses. By combining analytical analysis with numerical simulations based on the time-dependent Schrödinger equation (TDSE), we demonstrate that MEP distorts the phase difference between adjacent attosecond bursts, leading to shifted harmonic peaks.

For CO, our results reveal frequency displacements up to 0.8 harmonic orders (≈1.24 eV) near the cutoff region. This behavior originates from the MEP-induced asymmetry between consecutive half-cycle emissions. Although direct experimental verification remains challenging, indirect evidence can be obtained from even–odd harmonic intensity ratios. Our findings identify the harmonic frequency shift as a spectral signature of multielectron polarization and a sensitive probe of molecular asymmetry in strong-field physics.

3/  ThS. NCS Triệu Đoan An báo cáo về chủ đề: Laser-target symmetry-breaking in high harmonic generation: from frequency shift to odd-even intensity modulation
Abstract:

 Although the frequency shift and odd-even intensity modulation in high-order harmonic generation (HHG) have both been observed for asymmetric laser-target systems, they are typically studied as two separate phenomena. In this report, we provide a comprehensive picture of these two nonlinear optical phenomena, unifying them through a common origin - asymmetry of the laser-target system. By tuning asymmetric laser-target systems, we discover a transition from the harmonic frequency shift to the odd-even intensity modulation upon increasing the duration of the driving laser pulse. Specifically, these phenomena are observed simultaneously for laser pulses with intermediate pulse duration. For numerical evidence, we solve the time-dependent Schrödinger equation, while insight into the underlying physics is obtained from a simplified analytically tractable model. Understanding the asymmetric characteristics reflected in the HHG as provided is crucial for retrieving laser-target information, sampling external fields, and probing molecular dynamics.

4/   ThS. NCS Hoàng Trọng Đại Dương báo cáo về chủ đề:  Multielectron effects in high harmonic generation of HCN: depending on laser pulse parameters
Abstract:

 In recent decades, the progress of laser technologies has resulted in the discovery of various nonlinear effects [1-4]. These nonlinear effects, such as above-threshold ionization (ATI), high-energy ATI (HATI), nonsequential double ionization (NSDI), and high-order harmonic generation (HHG) [1-4], have potential to imply in extracting time-resolved imaging and investigating dynamics within atoms and molecules [5, 6]. As a result, a deep physics understand and accurate theoretical explanations of these nonlinear effects that are consistent with experimental observations are required. Solving time-dependent Schrödinger equation (TDSE) using the SAE model which is a low-cost method is a common way for investigating these phenomena theoretically [7-9]. Previous works ansatz highest occupied molecule orbital (HOMO) dominantly contributes to highly nonlinear spectra, such as HHG, compared to lower-lying orbitals [11, 12]. However, subsequent studies have revealed the imprint of lower-lying orbitals. The coupling between HOMO and lower-lying orbitals leading to electron-electron interactions between MOs, called the multielectron effect. This effect offers to extract electron-electron dynamics on attosecond time scales, which is a significant topic in Strong-field Physics.

Our recent studies unveiled the imprint of the multielectron effect on the HHG spectra of CO [8, 9]. We discuss how the multielectron effect impacts the even-to-odd ratio and the cutoff [8, 9] of the spectra. However, in our researches and other publications [7], a simple correlation, called dynamic core electron polarization (DCeP), is implied in solving TDSE to consider the average effect of the core electrons. This correlation has limitations in determining the contributions of each MO.

To determine the contribution of each MO, an advanced approach such as time-dependent density functional theory (TDDFT) is applied [13-15]. However, due to the new target, HCN molecules, there is disagreement in these researches about the contribution of HOMO and HOMO-1 [13, 15]. Furthermore, these simulations did not take into account the permanent dipoles of each MO, which is required for dissymmetrical molecules like HCN.

In this work, we discerned the contribution of HOMO and HOMO-1 in HHG spectra of HCN by solving the TDSE with SAE approximation that mimic the energies and permanent dipoles of both HOMO and HOMO-1. We explored imprint of multielectron and the competation in contributing on HHG spectra of HOMO and HOMO-1 depending on the laser parameters. This explained the disagreement in previous studies [13, 15].