Ya-Ting Sun†
a,
Xiao-Ning Yang†a,
Xin Shenb,
Feng Wang
*a and
Lan Jiang
*c
aSchool of Physics, Beijing Institute of Technology, Beijing 100081, China. E-mail: wangfeng01@tsinghua.org.cn
bChina Mobile (Hangzhou) Information Technology Co., Ltd., Hangzhou 310023, China
cLaser Micro/Nano-Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China. E-mail: jianglan@bit.edu.cn
First published on 18th September 2025
We conducted a study on the electron stopping power of α-particles in silicon crystals regulated by ultrashort pulse lasers, using time-dependent density functional theory non-adiabatically coupled with molecular dynamics. Our investigation focused on α-particles with initial velocities ranging from 0.1 to 1.0 a.u., providing detailed insights into the electronic states involved in the stopping process with exceptional spatial and temporal resolution. Our findings show how pulse lasers can influence electron stopping power and the degree to which this stopping power can be controlled by the laser pulses.
In general, a material's ability to slow down moving projectile particles is referred to as its stopping power.12–15 Conceptually, the stopping power of a material against a specific projectile can be categorized into two types based on the particles that make up the material. The first is nuclear stopping power (Sn), which occurs at low projectile velocities and involves primarily energy transfer to nuclei without electronic excitations.15,16 The second is electron stopping power (Se), which becomes significant at higher projectile velocities, where the predominant energy transfer occurs to electrons.16–19
While the stopping power of materials in their ground state under ion irradiation has been extensively studied through both experimental20–23 and theoretical24–27 methods, comparatively less attention has been given to the stopping power of materials when they are in excited states. Currently, there is limited research on how the excited states of target electrons influence the stopping power of target materials for projectile ions. For instance, Pang et al.28 performed a theoretical study analyzing the interaction of helium ions with aluminum targets at finite temperatures, employing time-dependent density functional theory (TDDFT) non-adiabatically coupled with molecular dynamics (TDDFT-MD).29,30 Their results provide important understanding of how temperature affects stopping power in aluminum targets for temperatures below 500 K, indicating that electron temperature exerts minimal impact on Se in this low-temperature regime. Furthermore, Sun et al.31 investigated the Se of protons in aluminum crystals at high electron temperatures using a quantum-blocking mechanism based on a physical picture of electronic transitions in energy levels for explaining the phenomenon of the Se decreasing with the increase of target electron temperature. Interestingly, by manipulating ultrashort pulse lasers in either the time or spatial domain, it becomes possible to control photon–electron interactions within materials.32,33 This capability, in turn, enables the regulation of the material's electronic states and ultimately results in changes to its properties. It appears that the capability to manipulate and control a material's properties with an ultrashort pulse laser offers a variety of fascinating applications.
Based on these achievements, our objective is to explore the effects of pulse lasers on Se and the fundamental physical mechanisms involved. Specifically, we focus on the Se of α-particles in silicon, modulated by ultrashort pulse lasers. We are motivated to carry out numerical simulations to understand how laser pulses control Se and to what extent Se can be controlled by these laser pulses.
The remainder of this paper is organized as follows. In Section 2, we describe the computational methodology and model employed to study the Se of α-particles in silicon regulated by ultrashort pulse lasers. Section 3 presents a detailed analysis of the simulation results, including explanations of the observed phenomena. Finally, Section 4 summarizes the key conclusions derived from this work.
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VKS(r,t) = Ven(r,R(t)) + VH[ρ](r,t) + VXC[ρ](r,t), | (3) |
![]() | (4) |
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Within the framework of density functional theory (DFT)34,35 and TDDFT, the standard approach is to fully occupy all KS orbitals with energies below the Fermi level, while orbitals above this level remain unoccupied. Before starting the time evolution calculation, the ground state electronic structure is computed by solving the static KS equation ĤKS(0)ψi(0) = εi(0)ψi(0), where ĤKS(0) is the KS Hamiltonian at the initial time, and εi(0) and ψi(0) are the corresponding eigenvalues and eigenstates. These ground state orbitals ψi(0) serve as the starting point for the subsequent time-dependent evolution. During evolution, the instantaneous single-particle orbital energy εi(t) at time t satisfy: εi(t) = 〈ψi(r,t)|ĤKS(t)|ψi(r,t)〉.
The laser field is described through the time-dependent vector potential A:
![]() | (6) |
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Meanwhile, the ionic core motion follows the classical Newton equation:
![]() | (8) |
![]() | (9) |
In this study, the pulse laser width was set to 2 fs, the wavelength to 800 nm (ħω = 1.55 eV), and the intensity ranged from I = 1 × 1011 W cm−2 to I = 1 × 1015 W cm−2, with propagation along the z-axis and polarization oriented along the x-axis. This configuration was chosen because ultrashort pulse lasers can induce profound, damage-free modifications in the electric and optical properties of dielectric materials, thereby enabling precise control of electron dynamics at the atomic scale.36,37
Based on the conventional cubic unit cell of silicon38 (a = b = c = 5.467 Å; Fig. 1(a)), we constructed a supercell with periodic boundary conditions. Cleaving this supercell along the (111) plane yielded a new supercell (10.8614 Å × 11.5203 Å × 15.3603 Å; Fig. 1(b)). Electronic structure calculations employ Troullier–Martins norm-conserving pseudopotentials for silicon (with core configuration 1s22s22p6) and helium, implemented within the local density approximation (LDA)39 to account for XC effects. A 2 × 2 × 2 Monkhorst–Pack k-point grid is used to sample the Brillouin zone. A real-space mesh spacing of 0.4 a.u., corresponding to a plane-wave energy cutoff of 52.06 Ry, ensures accurate unitary time propagation. The propagation step length, Δt × ν ∼ 3.9 × 10−3 Å,17,40 is adopted for various α-particle velocities to guarantee energy convergence of the time-dependent evolution with 800 empty states included. To numerically integrate the equations of motion, the velocity Verlet algorithm is employed for ionic cores, and an enforced time-reversal symmetry algorithm41 is used for the TDKS equations.
During real-time simulations, α-particles initially positioned at the left boundary of the simulation box move along the positive z-axis with velocity v (see Fig. 1(b)). To elucidate the variation trend of Se in lattice channeling and off-channeling regions, three impact parameter cases (i.e., the closest approach distance between incident particles and target atoms) were selected for comparative analysis. As illustrated in the inset of Fig. 3, these positions are determined based on the geometric features of the hexagonal lattice structure: the center of the silicon atom hexagon is defined as the channeling region (channel-1), the intermediate position at half the bond length perpendicular to the adjacent Si–Si bond is designated as channel-2, and the midpoint of the Si–Si bond is defined as the off-channeling region (channel-3).
All calculations were performed using Octopus code.42,43 Specifically, the calculation steps are divided into the following three steps: first, obtain a silicon crystal in the electronic ground state using DFT calculation. Then, TDDFT is used to simulate the dynamic evolution process of electrons in silicon crystals under the action of ultrashort laser pulses. Finally, TDDFT-MD is used to simulate α-particles passing through silicon crystals irradiated by ultrashort laser pulses.
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Fig. 2 The kinetic energy loss ΔEk = Ek(z0) − Ek(z) of the α-particle as a function of displacement along the z-axis for channel-1 (solid line) and channel-3 (dashed line) trajectories. |
Fig. 3 compares the Se calculated from TDDFT-MD simulations with experimental data44–46 and SRIM-2013 predictions (for silicon with density 2.3212 g cm−3).48 The TDDFT-MD results show higher Se values than SRIM-2013, but are in good agreement with the experimental data of Tran et al.44 and Pearce et al.,46 thereby validating the computational methods and silicon structural models used in this study. Experimental measurements typically involve averaging over trajectories in amorphous or polycrystalline targets, which diminishes channeling effects. SRIM-2013 accounts for this through averaging over impact parameters. TDDFT-MD simulations explicitly model the periodic crystalline environment, thereby enhancing channeling effects along specific trajectories (e.g., axial or planar channels). Several interesting findings are as follows: (i) velocity dependence: Se increases monotonically with α-particle velocity for all trajectories. (ii) Trajectory sensitivity: the influence of the trajectory becomes more pronounced at higher velocities.
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Fig. 3 The Se of silicon crystal against an α-particle moving is represented as a function of velocity. The symbols denote the calculated values along different channels: □ for channel-1, ○ for channel-2, and △ for channel-3 trajectories. Experimental data are represented by ★ (Tran et al.44), * (Konac et al.45), and + (Pearce et al.46). The green curve corresponds to SRIM-2013 predictions.47 The inset shows a top view of the channel geometry. |
As Fig. 3 shows, three different incident trajectories were calculated, and the trend of Se variation with α-particle velocity remains consistent across all channels. Based on this observation, channel-1 is selected as a representative trajectory for in-depth analysis to effectively illustrate the effect of laser irradiation on the Se of silicon crystals. Fig. 4 depicts the ΔEk of α-particle traveling along channel-1 at a velocity of 1.0 a.u. in silicon crystals after irradiation with different laser intensities is a function of displacement along the z-axis. Several interesting findings are as follows: (i) for silicon crystals irradiated with high laser intensity (I > 1 × 1014 W cm−2), the ΔEk of the α-particle is markedly lower than in unirradiated crystals. (ii) For silicon crystals irradiated with low to medium laser intensities (I ≤ 1 × 1014 W cm−2), the ΔEk is slightly increased relative to unirradiated crystals (see inset of Fig. 4). (iii) The ΔEk first increases and then decreases as the laser intensity rises.
To further clarify the effect of laser irradiation on the Se of crystals, Fig. 5 shows the variation of Se with laser intensity at three different α-particle velocities. Several interesting findings are as follows: (i) Se initially increases and then decreases with increasing laser intensity. (ii) The effect of laser intensity on Se is significantly stronger for faster α-particles (e.g., v = 1.0 a.u.) than for slower ones (e.g., v = 0.4 a.u.). (iii) Notably, the peak value of Se occurs at the same laser intensity of I = 1 × 1014 W cm−2 for all three velocities investigated.
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Fig. 5 The Se of silicon crystal against an α-particle at different velocities along channel-1 moving is represented as a function of laser intensity. |
To understand this non-monotonic dependence of Se on laser intensity, we consider two competing effects induced by the laser field. First, increasing the laser intensity can raise the concentration of free electrons in silicon, which enhances the collision probability between free electrons and incident particles. This mechanism can increase the Se. Second, increasing the laser intensity can enhance electronic excitation, leading to a reduction in Se through a quantum-blocking mechanism as follows: (i) the Se in silicon crystal originates from electron transitions between occupied and unoccupied states induced by the incident particles. (ii) The transition probability from low-energy occupied states to high-energy unoccupied states is higher than that from high-energy occupied states to even higher-energy empty states. (iii) At low electronic excitation, electrons mainly occupy low-energy states, leaving higher-energy states mostly empty, which facilitates electron transitions induced by incident particles. (iv) Conversely, at high electronic excitation, electrons occupying higher-energy states can block electron transitions induced by incident particles, leading to a reduction in Se.
Next, to validate the proposed electronic excitation mechanism, we introduce the occupation distribution function defined as
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![]() | (11) |
DOO(ε,t1,t2) = f(ε,t2) − f(ε,t1). | (12) |
Fig. 6 shows the DOO below the Fermi level of silicon crystals before and after laser irradiation with different intensities. The valley in the DOO signifies electron excitation, and its integrated area is the number of excited electrons. It can be seen that several valleys appeared, indicating significant electron excitation into the conduction band. Obviously, laser irradiation promotes electron excitation in the silicon crystal, with the number of excited electrons rising as the laser intensity increases. Moreover, electrons bound in deeper energy levels of a silicon crystal have been excited into the conduction band by more intense laser irradiation, contributing to an increase in the concentration of free electrons.
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Fig. 6 Differences of occupancy distribution functions below the Fermi level of silicon crystals before and after laser irradiation with different intensities. The Fermi energy is at 0.16 a.u. |
Fig. 7 shows the DOO below the Fermi level before and after α-particle collisions at different velocities along channel-1 within a silicon crystal, subsequent to irradiation by lasers of varying intensities. The solid line represents the case without laser irradiation for comparison. It can be observed that as the velocity of α-particles increases, fewer electrons are excited from shallow energy levels below the Fermi level to the conduction band, while more electrons are excited from deep energy levels below the Fermi level to the conduction band. From the perspective of photons, when projectile particles pass through silicon crystal, a changing electric field is created that includes virtual photons, leading to the generation of the DOO spectrum. Ref. 28 has demonstrated that higher-velocity projectiles carry more high-frequency photons, making them more likely to excite electrons bound in deeper energy levels.
To further verify the influence of laser irradiation on electron excitation in silicon crystals, Fig. 8 shows two-dimensional snapshots of the electron density distribution along the (010) and (111) planes under varying laser intensities. Without laser irradiation (I = 0 W cm−2), the electron density exhibits clear covalent-bond characteristics. As the laser intensity increases to 1 × 1012 W cm−2, minor changes in electron density distribution appear, while the overall bond structure remains largely intact. Upon further increasing the intensity to 1 × 1014 W cm−2, the electron density shows noticeable diffusion, accompanied by a reduction in the covalent-bond characteristics and increased metal-bond characteristics. At a higher laser intensity of 1 × 1015 W cm−2, the electron density becomes more dispersed and the covalent-bond characteristics are significantly diminished, suggesting severe disruption of the bond structure, potentially leading to metal-bonds or plasmonic behavior.
It is well established that during interactions between α-particles and target materials, the generation of electron–hole pairs due to electronic excitations introduces additional energy loss channels.49 Fig. 9 presents the electron density difference in the system before and after the incident of α-particles moving along silicon crystal channel-1 at a velocity of 1.0 a.u., subsequent to irradiation by lasers of varying intensities. Several interesting findings are as follows: (1) as the laser intensity increases, the spatial distribution of electron–hole pairs excited by α-particles becomes less localized, indicating that laser can regulate the spatial range of the interaction between α-particles and electrons in silicon crystals. (2) For the laser intensity of I = 1 × 1014 W cm−2, the number of electron–hole pairs generated by α-particle excitation appears to reach its maximum, implying the highest energy transfer between α-particles and silicon crystals, consistent with the position of the Se peak shown in Fig. 5.
The interaction between ultrashort laser pulses and matter is highly dependent on laser parameters and material intrinsic properties. As shown in Fig. 10(a), after laser irradiation of silicon crystals, the average number of excited electrons (ne) per silicon atom significantly increases with the increase of laser intensity, which is a mechanism causing Se to increase with the increase of laser intensity. To quantitatively characterize the degree of electron excitation, we define the sum of single-particle orbital energy (Eeig) as follows:
![]() | (13) |
A higher/lower Eeig value indicates that the electron is in a higher/lower excitation state. Fig. 10(b) shows that Eeig increases with increasing laser intensity, further confirming that electron are excited to higher-energy states with increasing laser intensity, which is another mechanism causing Se to decrease with the increase of laser intensity. The increase of laser intensity leads to competition between these two mechanisms, ultimately resulting in the peak phenomenon of Se as a function of laser intensity shown in Fig. 5.
While the Se of materials in their ground state under ion irradiation has been extensively explored through both experimental techniques and theoretical modeling, comparatively less research has focused on the Se of materials in excited electronic states. The ability to induce and precisely control localized transient electron states within materials using ultrashort pulse lasers offers a promising and innovative approach to modifying Se. This method allows for the intentional alteration of materials away from their conventional ground states, potentially enabling controlled changes in their Se properties. Exploring this phenomenon opens new avenues for scientific research and technological advancements, deserving to be investigated in future research both theoretically and experimentally.
Footnote |
† These authors contributed equally to this work. |
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