Ab initio description of laser-induced ultrafast phenomena in the presence of surfaces and interfaces

在大多数情况下，飞秒激光脉冲诱导的超快现象被认为是非热的，或者至少有一个非热的起源。对于非热结构现象，有大量的实验证据。相干声子激发、声子压缩、超快熔化、键软化和硬化等效应可以直接测量，也可以由测量推断。我们团队在成功解释和预测超短激光和XUV脉冲诱导的非热结构现象方面拥有丰富的经验。我们开发了能够对激光激发材料进行从头计算分子动力学模拟的代码CHIES，并已被证明在相同精度的情况下比其他从头计算程序快约200倍。由于许多实验，特别是基于超快电子结晶学的实验都是在薄膜上进行的，而且大多数激光-固体相互作用的应用都是针对表面的加工(例如，纳米结构)，因此对表面和薄膜中激光诱导的超快现象的模拟是一个理论挑战。到目前为止，还没有关于表面和薄膜中非热效应的从头计算理论。在这个项目中，我们计划模拟超快非热现象(1)在硅薄膜中，(2)在硅/碳化硅超晶格中，和(3)在硅/碳化硅薄膜中。由于技术上的原因，对于这些系统的原子化描述收敛较差，这将使仿真速度慢得令人望而却步。因此，有必要通过提高底层自洽计算的收敛速度来进一步加快计算速度。通过所提出的模拟，我们有望预测新的非热效应，如抑制Si/C界面上C原子的表面熔化(抢占)或反常扩散，这可能开辟非平衡物理研究的新领域并导致新的应用。此外，对Si/SiC薄膜的处理是朝着真实描述氧化表面迈出的第一步。

Ultrafast phenomena induced by femtosecond-laser pulses are believed to be, in most cases, nonthermal or to have at least a nonthermal origin. There is plenty of experimental evidence for nonthermal structural phenomena. Effects like excitation of coherent phonons, phonon squeezing, ultrafast melting, bond-softening and hardening were either directly measured or inferred from measurements. Our group has a vast experience in successfully explaining and predicting nonthermal structural phenomena induced by ultrashort laser and XUV pulses. We developed the code CHIVES, able to perform ab initio molecular dynamics simulations of laser excited materials, and which has been shown to be about two hundred times faster than other ab initio codes with the same accuracy. Since many experiments, particularly those based on ultrafast electron crystallography, are performed on thin films and since most applications of laser-solid interactions are aimed at the processing (e.g., nanostructuring) of surfaces, it is a challenge for theory to simulate laser induced ultrafast phenomena on surfaces and in thin films. No ab initio theory of nonthermal effects on surfaces and in thin films could be performed so far. In this project we plan to simulate ultrafast non thermal phenomena (1) in silicon thin films, (2) in a Si/SiC superlattice, and (3) in a Si/SiC thin film. Due to technical reasons, for the atomistic description of these systems convergence is poor, which would make the simulations forbiddingly slow. Therefore, it is necessary to further speed up CHIVES by improving the convergence of the underlying self-consistent calculations. From the proposed simulations we expect to predict novel nonthermal effects, like, suppression of surface melting (preempting) or anomalous diffusion of C atoms at the Si/C interface, which might open a new field of research in nonequilibrium physics and lead to new applications.In addition, the treatment of a Si/SiC thin film is a first step towards a realistic description of oxidized surfaces.