Programme

O.13 -- Oral, VCTP-46

Date: Tuesday, 5 October 2021

Time: 14:30 - 14:50

Effect of static electric field on odd-even high-order harmonic generation

Doan-An Trieu, Ngoc-Loan Phan, Van-Hoang Le

Ho Chi Minh City University of Education

Interaction of atoms/molecules with ultrashort intense laser pulses leads to high-order harmonic generation (HHG), a fascinating non-linear effect. For atoms or symmetric molecules, the HHG spectra contain odd harmonics only. However, many studies have recently focused on asymmetric molecules that emit high-order harmonics with both odd and even orders due to symmetry breaking. This property is essential; thus, the ratio of adjacent even and odd harmonics, briefly called the even-to-odd ratio, is introduced to characterize such HHG spectra [1]. In this study, we utilize a simple model that mimics a polar molecule, a one-dimensional hydrogen atom in a weak electric field, to investigate the effect of asymmetricity on the even-to-odd ratio. Because the electric field also significantly broadens the plateau region in HHG spectra [2, 3], we also study the electric field effect on the plateau structure and extending the HHG cutoff. The HHG data are simulated by numerically solving the time-dependent Kohn-Sham equation embedded in the Octopus source codes [4]. We find out that the dependence of the even-to-odd ratio on the static electric field can be separated into two distinct regions with different behaviors. With increasing the static electric field, the even-to-odd ratio grows exponentially, and then, after reaching a critical point, it fluctuates irregularly. Remarkably, this dependence is stable for different laser pulses. The underlying mechanism that leads to this interesting dependence is also clarified. Besides, we also detect the multiple-plateau structure of HHG spectra and explain its formation by using the classical simulation. [1] N.L. Phan, C.T. Le, V.H. Hoang, and V.H. Le, “Odd-even harmonic generation from oriented CO molecules in linearly polarized laser fields and the influence of the dynamic core-electron polarization,” Phys. Chem. Chem. Phys. 21, 24177 (2019). [2] H. Liu, Z.Z. Zhang, Y.J. Wu, S.C. Jiang, and C. Yu, “Isolated attosecond pulses generation from coherent superposition state of helium ion in static electric fields and spatial nonhomogeneous fields,” Int. J. Mod. Phys. B 30, 1650229 (2016). [3] C.L. Xia, G.T. Zhang, J. Wu, and X.S. Liu, “Single attosecond pulse generation in an orthogonally polarized two-color laser field combined with a static electric field,” Phys. Rev. A 81, 043420 (2010). [4] M.A.L. Marques, A. Castro, G.F. Bertsch, and A. Rubio, “Octopus: A first-principles tool for excited electron-ion dynamics,” Comput. Phys. Commun. 151, 60 (2003).

Presenter: Trieu Doan An