Programme

P.52 -- Posters, VCTP-50

Date: Tuesday, 5 August 2025

Time: 08:30 - 10:00

Orientation asymmetry of planar molecules reflected in high-harmonic generation spectra

T. Tran (1), D.-A. Trieu (2,3), K.-N. H. Nguyen (2,3), D. D. Hoang-Trong (4,5), and N.-L. Phan (1)

(1) Computational Physics Key Laboratory K002, Department of Physics, Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam (2) Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City, Vietnam (3) Faculty of Natural Sciences, Duy Tan University, Da Nang City, Vietnam (4) Simulation in Materials Science Research Group, Science and Technology Advanced Institute, Van Lang University, Ho Chi Minh City, Vietnam (5) Faculty of Applied Technology, School of Technology, Van Lang University, Ho Chi Minh City, Vietnam

High-order harmonic generation (HHG) has emerged as a powerful tool for probing molecular electronic structure [1,2] and dynamics [2,3]. Numerous studies have shown that the HHG spectra emitted from linear polar molecules can exhibit both odd and even harmonics when the symmetry of the laser – target system is broken, typically due to the intrinsic permanent dipole moment of the molecule. Recent research further reveals that the intensity ratio between even and odd harmonic orders – referred to as even-to-odd ratio [4], and the phase difference between two consecutive HHG bursts contributing to the same harmonic order [5], remain stable under different laser conditions and can be used to characterize the molecule’s intrinsic asymmetry [4,5]. However, these research have so far focused on linear polar molecules. For nonlinear molecules with symmetric geometric structures, there still exist certain orientations for which the two-fold rotational symmetry is broken. The use of HHG spectra to characterize the directional asymmetry of such molecules has not yet been systematically investigated. In this study, we employ time-dependent density functional theory (TDDFT) to simulate HHG from the planar, C₃-symmetric molecule BH₃. We analyze two key observables extracted from the HHG signal: the even-to-odd ratio and the phase difference between two consecutive HHG bursts, as functions of the laser polarization direction. Our results show that the HHG spectra vary significantly with the polarization direction, transitioning from purely odd harmonics to mixed odd-even harmonics and then back to purely odd. This behavior directly reflects the directional asymmetry in the molecular structure. Notably, this phenomenon is observed not only for the highest occupied molecular orbital (HOMO) but also persists for lower-lying orbitals such as HOMO−1. Furthermore, when using few-cycle laser pulses with suitable wavelengths, the extracted even-to-odd ratio and phase difference can even reconstruct the geometric and electronic asymmetry of the molecule along different orientations. [1] M. Lein, N. Hay, R. Velotta, J. P. Marangos, and P. L. Knight, Phys. Rev A. 66, 1 (2002). [2] P. Peng, C. Marceau, and D. M. Villeneuve, Nat. Rev. Phys. 1, 144 (2019). [3] N. T. Nguyen, B. V. Tang, and V. H. Le, J. Mol. Struct. 949, 52 (2010). [4] H. T. Nguyen, K.-N. H. Nguyen, N.-L. Phan, C.-T. Le, DD. Vu, L.-P. Tran, and V.-H. Le, Phys. Rev. A 105, 023106 (2022). [5] K.-N. H. Nguyen, N.-L. Phan, C.-T. Le, DD. Vu, and V.-H. Le, Phys. Rev. A 106, 063108 (2022).

Presenter: Trần Thành