Ultrafast switching dynamics in short pulse laser-driven phase change materials

利用超快时间分辨X射线散射研究了硫系相变材料在短脉冲激光辐照下的结构动力学。特别是，我们想确定其非晶相和晶相之间的热和可能的非热转变途径，研究结构动力学如何取决于激光参数，如激发强度和脉冲持续时间，并澄清哪些过程确定和限制最终的开关速度。薄膜的原型PCM Ge 2Sb 2 Te 5（GST 225）以及其他硫属化物合金将照射短（fs至ps）的光脉冲。在激光诱导的过渡过程中的结构变化，从结晶到非晶态，反之亦然，将监测的时间延迟的硬X射线探测脉冲在正常入射透射几何散射。充分利用LCLS的高通量和短脉冲持续时间，作为泵浦-探测时间延迟的函数的散射图案的计划测量提供瞬态结构因子S（q），从而提供关于切换期间PCM的结构性质的信息。用单个LCLS脉冲获得高S/N散射数据的可能性允许在“真实的寿命”切换条件下进行动态结构测量，这在任何其他现有的X射线源之前或使用任何其他现有的X射线源都是不可能的。

Ultrafast time-resolved X-ray scattering will be used to study the structural dynamics in chalcogenide-based phase change materials (PCMs) after short pulse laser irradiation. In particular, we would like to identify thermal and possibly non-thermal transition pathways between their amorphous and crystalline phases, investigate how the structural dynamics depend on laser parameters such as excitation strength and pulse duration, and clarify which processes determine and limit the ultimate switching speed. Thin films of the prototypical PCM Ge2Sb2Te5 (GST225) as well as other chalcogenide alloys will be irradiated by short (fs to ps) optical laser pulses. The structural changes during the laser-induced transitions from the crystalline to the amorphous state and vice versa will be monitored by scattering of a time-delayed hard X-ray probe pulse in normal-incidence transmission geometry. Taking full advantage of the high flux and the short pulse duration of the LCLS the planned measurements of the scattering pattern as a function of the pump-probe time delay provide the transient structure factor S(q) and thus information on the structural properties of PCMs during switching. The possibility to obtain high S/N scattering data with single LCLS-pulses allow dynamic structure measurements under "real life" switching conditions which have not been possible before or with any other existing X-ray source.