Physical Biology Center for Ultrafast Science and Technology, Arthur Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91125, USA.
Department of Applied Physics and Materials Science, Steele Laboratory, California Institute of Technology, Pasadena, California 91125, USA.
Nat Commun. 2017 May 11;8:15177. doi: 10.1038/ncomms15177.
The ultrafast spatial and temporal dynamics of excited carriers are important to understanding the response of materials to laser pulses. Here we use scanning ultrafast electron microscopy to image the dynamics of electrons and holes in silicon after excitation with a short laser pulse. We find that the carriers exhibit a diffusive dynamics at times shorter than 200 ps, with a transient diffusivity up to 1,000 times higher than the room temperature value, D≈30 cms. The diffusivity then decreases rapidly, reaching a value of D roughly 500 ps after the excitation pulse. We attribute the transient super-diffusive behaviour to the rapid expansion of the excited carrier gas, which equilibrates with the environment in 100-150 ps. Numerical solution of the diffusion equation, as well as ab initio calculations, support our interpretation. Our findings provide new insight into the ultrafast spatial dynamics of excited carriers in materials.
激发载流子的超快时空动力学对于理解材料对激光脉冲的响应非常重要。在这里，我们使用扫描超快电子显微镜来成像硅中电子和空穴在短激光脉冲激发后的动力学。我们发现，载流子在短于 200 ps 的时间内表现出扩散动力学，瞬态扩散率高达室温值的 1000 倍，D≈30cms。然后，扩散率迅速下降，在激发脉冲后约 500 ps 达到 D 值。我们将瞬态超扩散行为归因于激载流子气体的快速膨胀，该气体在 100-150 ps 内与环境达到平衡。扩散方程的数值解以及从头算计算都支持我们的解释。我们的发现为材料中激发载流子的超快空间动力学提供了新的见解。
