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First-Principles Calculations of the Electron Dynamics During Femtosecond Laser Pulse Train Material Interactions ∗ C

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First-Principles Calculations of the Electron Dynamics During Femtosecond Laser Pulse Train Material Interactions ∗ C Physics Letters A 375 (2011) 3200–3204 Contents lists available at ScienceDirect Physics Letters A www.elsevier.com/locate/pla First-principles calculations of the electron dynamics during femtosecond laser pulse train material interactions ∗ C. Wang a,L.Jianga, ,F.Wangb,X.Lia,Y.P.Yuana,H.L.Tsaic a Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China b Department of Applied Physics, Beijing Institute of Technology, Beijing 100081, China c Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA article info abstract Article history: This Letter presents first-principles calculations of nonlinear electron–photon interactions in crystalline Received 23 May 2011 SiO2 ablated by a femtosecond pulse train that consists of one or multiple pulses. A real-time and real- Received in revised form 29 June 2011 space time-dependent density functional method (TDDFT) is applied for the descriptions of electrons Accepted 8 July 2011 dynamics and energy absorption. The effects of power intensity, laser wavelength (frequency) and number Availableonline18July2011 of pulses per train on the excited energy and excited electrons are investigated. Communicated by R. Wu © 2011 Elsevier B.V. All rights reserved. Keywords: TDDFT Electron dynamics Laser pulse train 1. Introduction of dielectrics [8]. Once the critical free electron density is created, the transparent material becomes opaque, and the absorbed en- Compared with a long pulse, a femtosecond pulse in some as- ergy is mainly deposited in a very thin layer within a short period pects fundamentally changes the laser–material interaction mech- of time, which leads to the ablation of the thin layer. A number of anisms. A femtosecond pulse laser can easily achieve very high different mechanisms were proposed to describe the generation of peak power, which is powerful enough for full ionization of almost free electrons. In femtosecond ablation, avalanche ionization [9], any solid material with greatly reduced recast, microcracks, and electron impact ionization, and photoionization including multi- heat-affected zone. Hence, femtosecond lasers are promising for photon ionization and/or tunnel ionization [10], are major compet- the micro-/nanoscale fabrication of all types of materials [1–4].The ing mechanisms for free electron generation. The key parameter interaction of femtosecond pulse laser with wide band gap mate- which controls√ the ionization mechanism is the Keldysh parame- = rials especially dielectrics such as transparent materials [3,4] is an ter γ ω 2I P /F [11], where ω is the laser frequency, I P is the active research area which has very significant scientific and engi- ionization potential and F is the field strength. The multiphoton neering merits [5–7]. Laser ablation of a wide band gap material is ionization is expected to dominate when γ 1, while tunnel ion- sometimes called laser-induced breakdown in which the material ization dominates when γ 1. is first transformed into absorbing plasma with metallic properties Recent developments of optical devices make it possible to ob- and subsequent laser–plasma interaction causes the phase changes tain almost any arbitrary pulse shapes. A large number of stud- of bulk materials. Energy transport within the bulk material during ies have been reported regarding pulse shaping and its effects on the ablation process can be divided into two stages: (1) the pho- laser–material interactions [12–23]. For example, using a shaped ton energy absorption, mainly through free electrons generation, pulse train, ionization process can be controlled [12];atomscanbe heating, and electron excitation in a time scale from a few fem- selectively ionized [13]; molecular ground state rotational dynam- toseconds to a few picoseconds and (2) the redistribution of the ics can be manipulated [14]; chemical reactions can be controlled absorbed energy to lattice leading to material removal in a time [15]; and X-ray line emission from plasmas under the femtosecond scale from a few picoseconds to a few nanoseconds. Building up pulse can be significantly enhanced [16]. Also, it has been ex- of free electrons is necessary in order to initialize laser ablation perimentally demonstrated that by using temporally shaped pulse trains, the quality of femtosecond laser microstructuring of di- electrics can be improved [18,24]. In theory, the plasma model * Corresponding author. Tel.: +86 010 68914517; fax: +86 010 68914517. is used to investigate the pulse train ablation of fused silica, in E-mail address: [email protected] (L. Jiang). which quantum theories is employed to calculate the time and 0375-9601/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2011.07.009 C. Wang et al. / Physics Letters A 375 (2011) 3200–3204 3201 space dependent optical and thermal properties, including the The number of excited electrons per unit cell nex(t) is defined as electron heat capacity, electron relaxation time, electron conductiv- [30], ity, reflectivity, and absorption coefficient. The numerical calcula- 2 tion shows that nanoscale constant ablation depth can be achieved nex(t) = δnn − φnk ψnk(t) (7) by the pulse train technology [17]. nnk Although many studies have been conducted, it remains a sig- where k is the Bloch wave number and n is the band index. nificant challenge to fundamentally understand the phenomena The electron distribution in the conduction band is defined by associated with complex non-equilibrium and nonlinear processes, the projection of the time-dependent wave function to the initial especially the dissipation of the absorbed energy and the corre- particle states in the conduction band [31], sponding phase change mechanisms. For further understanding of laser–material interactions, description of quantum electronic dy- 2 Nex(m,k) = φmk ψnk(T ) (8) namics of many-electron systems is essential, which attracts many n active electron theoretical treatments for the laser–material inter- actions system. Up to now, the time-dependent density-functional where m represents the particle states. theory (TDDFT) appears to be the most feasible theoretical frame- work to treat the electrons quantum mechanically [25]. TDDFT is 3. Results and discussion used to investigate the electron dynamics not only for isolated atoms and molecules [26–28],butalsoforclusters[29] and bulk In our calculations of silica, an orthorhombic unit cell of α-SiO2 systems [30–33] irradiated by intense ultrashort pulses. is considered. Lattice constants in three directions are 9.29 a.u. This work employs TDDFT to calculate the generation of elec- (x axis), 16.09 a.u. (y axis), 10.21 a.u. (z axis), respectively. The ir- trons and the photon energy absorption of α-SiO2 under single reducible wedge of the Brillouin zone is presented by 154 k points, pulse and a pulse train. Fundamental understanding of electron ex- which corresponds to sampling the full Brillouin zone by 63 k citations and dynamics irradiated by shaped pulse trains is critical points to represent the Bloch wave functions for the momentum- to control and improve micro-/nanoscale processing of materials. space integration. Our calculations are based on the density func- tional theory (DFT) within the adiabatic local density approxima- 2. Theory tion. The Ceperley–Alder exchange-correlation functional is used for the uniform electron gas as parameterized by Perdew and TDDFT is a reformulation of time dependent quantum mechan- Zunger [35]. Interactions between valence electrons and ions are ics, where the fundamental variable is no longer the many-body treated by the norm-conserving pseudopotentials [36] with separa- wave function but the density. This time-dependent density is de- ble nonlocal operators. The electron wave functions are expanded termined by solving an auxiliary set of non-interacting Schrödinger in a plane-wave basis with an energy cutoff of 37 Ry. equations, the Kohn–Sham equations [25]. To describe the motion We represent the laser irradiation by subjecting our system to of electrons, the time-dependent Kohn–Sham (TDKS) equation for an external alternating electric field parallel to the z axis. The time single particle orbitals is applied as follows, step of t = 0.02 a.u. is used. In addition, the ion position is frozen in all the calculations. ∂ = ih¯ ψ(r,t) HKS(r,t)ψ(r,t) (1) Fig. 1 showselectricfieldoftheappliedlaserpulse,excited ∂t electron, excitation energy and multipoles as a function of time, = 2 n(r,t) ψi(r,t) (2) in which single laser train burst consisting of two pulses is ap- i plied. Each of the pulse is a Gaussian wave packet, 800 nm, 16 fs, where n(r,t) is the electron density, HKS(r,t) is the Kohn–Sham as shown in Fig. 1(a). The delay (separation) time between the Hamiltonian which is conventional separated in the following way two pulses within a train is 16 fs. The laser peak power density 14 2 [31], is 5 × 10 W/cm . Figs. 1(a) and 1(e) show that the signal of the multipoles along z axis follows the laser profile with a 1–2 fs 2 1 e delay. Compared with the multipoles along z axis, there is only HKS(r,t) = p + A(t) + V ion(r,t) 2m c slight disturbance along x axis and y axis from Fig. 1(d). It achieves the amplitude at around 8 fs and 24 fs, but in smaller magnitude + V Hartree(r,t) + V xc(r,t) (3) during the second pulse. This is due to the impact of a poten- where e is an elementary charge, A(t) is a time-dependent tial induced by free electron generations, which weakens the laser spatially-uniform vector potential which is related to the exter- field. nal electric field, V ion(r,t) is the electron–ion potential, V xc (r,t) Fig.
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