Programme

O.22 -- Oral, VCTP-50

Date: Thursday, 7 August 2025

Time: 11:45 - 12:10

MULTIELECTRON POLARIZATION-INDUCED FREQUENCY SHIFTS IN HIGH-ORDER HARMONIC GENERATION: A MANIFESTATION OF MOLECULAR ASYMMETRY

Kim-Ngan H. Nguyen (1), Doan-An Trieu (1), Cam-Tu Le (2), Ngoc-Loan Phan (3)

(1) Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City, Viet Nam (2) Ton Duc Thang University, Ho Chi Minh City, Vietnam (3) Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam

High-order harmonic generation (HHG) from atoms and molecules under intense laser fields has been a useful tool in ultrafast science, often modeled using the single-active-electron (SAE) approximation [1]. This approximation assumes that only the outermost electron responds to the laser field, while inner-shell electrons remain frozen. However, advance experimental evidence has reveals that strong-field behaviors cannot be explained without considering multielectron interactions. One prominent manifestation of multielectron effects is the dynamic polarization of the core electron, commonly referred to as multielectron polarization (MEP) [2]. This effect becomes especially important in targets with large polarizabilities, such as polar molecules. Considering HHG emission, MEP has been shown to influence the emission intensity, and the spectral structure [2-4]. While previous work has focused on MEP-induced modulation of HHG intensity, such as the even-to-odd ratio for polar molecule, i.e. the intensity ratio between the even-order harmonic and the neighboring odd-order harmonic [4], the influence of MEP on the frequency positions of HHG has not been figured out. Given that HHG results from the coherent interference of attosecond bursts generated every half optical cycle, any phase difference between two adjacent bursts—arising from molecular asymmetry or the laser waveform—can shift the resulting harmonic frequencies [5]. In this study, we investigate how MEP induces frequency shifts in HHG spectra from polar molecule driven by few-cycle laser pulses by distorting the phase difference between attosecond bursts. We first present an analytical demonstration to this effect and subsequently validate it through numerical simulations by solving the time-dependent Schr\”odinger equation (TDSE). Furthermore, we demonstrate the universality of the MEP-induced frequency shift when varying laser parameters. Finally, we indirectly confirm this effect by analyzing available HHG data when adopting multicycle laser pulses. By investigating the interaction between CO molecules and few-cycle laser pulses, we reveal that MEP causes a significant shift in harmonic peak frequencies, reaching up to 0.8 harmonic orders (equivalent to ~1.24 eV) near the cutoff region. This phenomenon is attributed to the MEP-induced asymmetry in the phase difference between two adjacent attosecond bursts. Although direct experimental confirmation is currently limited, we provide indirect validation by connecting measured odd-even intensity ratios with predicted peak shifts. Our results highlight the role of HHG frequency shifts as a spectral feature for characterizing multielectron effects and probing the asymmetry of polar molecules. [1] P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993). [2] B. Zhang, J. Yuan, and Z. Zhao, Phys. Rev. A 90, 035402 (2014). [3] C.-T. Le, V.-H. Hoang, L.-P. Tran, and V.-H. Le, Phys. Rev. A 97, 043405 (2018) [4] H. T. Nguyen, K.-N. H. Nguyen, N.-L. Phan, C.-T. Le, D. D. Vu, L.-P. Tran, and V.-H. Le, Phys. Rev. A 105, 023106 (2022). [5] D.-A. Trieu, V.-H. Le, and N.-L. Phan, Phys. Rev. A 110, L041101 (2024).

Presenter: Nguyen Huynh Kim Ngan