7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

First-principles calculations for ultrafast andnonlinear dynamics of light pulses and electrons
Shunsuke Yamada1 and Tomohito Otobe1
1Kansai Institute for Photon Science, National Institutes for Quantum Science and Technology
e-mail (speaker): yamada.shunsuke@qst.go.jp(a)vacuum region

Si thin ﬁlm

vacuum regionz

Si unit cell

Electron density change(b) t = 0 fs

Electric ﬁeld (V/Å)

0.4

Incident ﬁeld

0.2
0−0.2
−0.4

−60−40−2002040z (µm)(c) t = 200 fs

0.5StrongWeak

Energy (eV/atom)

0.4

0.4

Energydeposition

0.3
0.2
0.100

Electric ﬁeld (V/Å)

Ultrashort and high-intensity laser pulses yield novel
ultrafast and nonlinear phenomena of electrons incondensed matter. These phenomena are crucial forunderstanding the generation of warm dense matter(WDM). To describe such phenomena, we aredeveloping combined computational methods of theMaxwell equations for light propagation and thetime-dependent density functional theory (TDDFT) for
electron dynamics [1,2]. In this talk, we introduce
application examples of the combined methods andrecent studies such as high-order harmonic generation insemiconductors [3,4]. Also, we introduce a calculation
example of the laser intensity dependence of thereflection and transmission coefficients in laser irradiatedSi films. This calculation considers the transition fromthe multi-photon absorption regime to the classicalplasma regime. These computational methods areimplemented in our free-to-use open-source software
SALMON [5].
Figure 1(a) shows schematic illustration of ourmethod. We consider an irradiation of a free-standing Si
thin film (gray area) in a vacuum by an ultrashort lightpulse of a linearly polarized plane wave at the normal
incidence. The light propagation is described by theone-dimensional wave equation on a real-space grid. For
each grid point in the material, a TDDFT system forelectrons in a periodic unit cell of bulk Si is attached. InFig. 1(a), the electron density changes driven by the lightpulse are illustrated for the first three grid points. The
time evolution of the electronic wavefunctions and lightfield is solved simultaneously by exchanging the electron
current and vector potential in each time step. We callthis method “multiscale Maxwell-TDDFT.”
Figure 1(b) and (c) show snapshots of the electricfield for the incident and scattered light pulses,
respectively, as a typical calculation example. The Si
film has the thickness of 3 𝜇m. In Fig. 1(b), the initial
pulse (t = 0 fs) that locates in front of the film is shown
where the film is exhibited as a thin gray area. In Fig.
1(c), we display results corresponding to initial pulses of
two different maximum intensities: a strong pulse (I = 5×1012 W/cm2, red solid line) and a weak pulse (I = 109
W/cm2, blue dotted line). For the weak pulse, linear
propagation is expected. In the figure, the weak pulse ismultiplied by a factor of √5000 so that the differencesof two lines manifest nonlinear effects in the stronger
pulse. From these calculations, we can elucidate themechanisms of nonlinear phenomena in laser-irradiatedmaterials.
As shown by the above example, our computational
method is a powerful tool for elucidating extreme
phenomena in the latest optical science.

123z (µm)

0.20−0.2−0.4−60

Reﬂected ﬁeld

−40−20Transmitted ﬁeld

02040z (µm)

Fig. 1: (a) Overview of the multiscale Maxwell-TDDFT
method for a light propagation through a Si thin film. (b)Electric field at t = 0. The incident pulse is prepared in front of
the Si thin film, which is exhibited as a gray area. (c) Electricfield at t = 150 fs. In the inset, the energy deposition is shown.

References[1] K. Yabana et al., Phys. Rev. B 85, 045134 (2012).
[2] S. Yamada et al., Phys. Rev. B 98, 245147 (2018).
[3] S. Yamada et al., Phys. Rev. B 103, 155426 (2021).
[4] S. Yamada et al., Phys. Rev. B 107, 035132 (2023).
[5] https://salmon-tddft.jp