Programme

P.6 -- Poster, VCTP-48

Date: Tuesday, 1 August 2023

Time: 08:30 - 10:00

Characterizing polar molecules via odd-even high-order harmonic generations

Kim-Ngan H. Nguyen (1,2), Cam-Tu Le (3,4), Ngoc-Loan Phan (5)

(1) Institute of Fundamental and Applied Sciences, Duy Tan University, Ho Chi Minh City 71008, Vietnam (2) Faculty of Natural Sciences, Duy Tan University, Da Nang City 50000, Vietnam (3) Atomic Molecular and Optical Physics Research Group, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City 72915, Vietnam (4) Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City 72915, Vietnam (5) Computational Physics Key Laboratory K002, Department of Physics, Ho Chi Minh City University of Education, Ho Chi Minh City 72711, Vietnam

High-order harmonic generation (HHG) is a powerful tool for probing the structural and dynamical information of matter. Particularly, HHG has been used to retrieve the HOMO (Highest Occupied Molecule Orbital) [1], probe the internuclear distance in a molecule [2], tracking nuclear and electron dynamics [3]. These studies have clearly confirmed that HHG reflects characteristics of the atomic or molecular targets through their interaction with strong laser field. One of the properties of HHG is its symmetry embodiment. Specifically, with the symmetric laser-target system, such as atoms/symmetric-molecules and linear laser fields, HHG spectra contains only odd harmonic orders. When breaking the symmetry of laser-target system either by using an asymmetry target or asymmetry laser pulse, even orders emerge. Thus, the appearance of the even order in the HHG spectra is the manifestation of the asymmetry of a system. The demonstration of the physical quantities characterizing the asymmetry of polar molecules via the odd-even HHG is essential. Recently, we have numerically and analytically proved that the even-to-odd ratio (i.e. the ratio between the intensity of the even order and two adjacent odd harmonic orders) is a universal quantity that only depends on the polar molecules, and is almost unchanged when varying laser parameters [4]. This statement leads us to the question that whether the harmonic phases can also be a characteristic of the polar-molecular target. Moreover, HHG is a quantity that is measured in the frequency domain. By time-frequency transforms such as Gabor transform, one can retrieve the emission of HHG in the time domain, which can give us the microscopic information of the system. In this way, HHG process can be explained by the interference of attosecond bursts emitted as a train with time. Regarding polar molecules, two adjacent attosecond bursts are distinctive both in amplitude and in phase due to the asymmetry of system [5]. The next question is whether the ratio of intensity and phase difference of two adjacent bursts can characterize the molecular asymmetry, i.e. whether they are stable with changing the laser parameters. If they are, it implies that these quantities are the intrinsic features of the system. Last but not least, is there any relation between these time-domain and frequency-domain quantities? In this work, we simulate the HHG spectra by numerically solving the time-dependent Schrödinger equation. We indicate that the even-to-odd ratio and the phase difference between two adjacent harmonic orders in HHG spectra are universal quantities that characterize the asymmetry of molecular targets. In the time domain, we also prove that the ratio and the phase difference between two adjacent attosecond bursts are also intrinsic properties and reflect the asymmetry of the system. Moreover, we also point out the relation between the time-domain (attosecond bursts) and the frequency-domain (HHG) quantities. [1] J. Itatani, J. Levesque, D. Zeidler, H. Niikura, H. Pepin, J. C. Kieffer, P. B. Corkum, and D. M. Villeneuve, Nature 432, 867 (2004). [2] M. Lein, N. Hay, R. Velotta, J. P. Marangos, and P. L. Knight, Phys. Rev. A 66, 023805 (2002). [3] B. Zhang, S. Yu, Y. Chen, X. Jiang, and X. Sun, Phys. Rev. A 92, 053833 (2015). [4] H. T. Nguyen, K.-N. H. Nguyen, N.-L. Phan, C.-T. Le, D. 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, D. Vu, and V.-H. Le, Phys. Rev. A 106, 063108 (2022).

Presenter: Nguyen Huynh Kim Ngan