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

超快时间分辨x射线散射将用于研究短脉冲激光辐照后硫族化合物基相变材料的结构动力学。特别是，我们想确定其非晶相和结晶相之间的热转变和可能的非热转变途径，研究结构动力学如何依赖于激光参数，如激发强度和脉冲持续时间，并澄清哪些过程决定和限制了最终的开关速度。用短（fs ~ ps）激光脉冲照射典型PCM薄膜Ge2Sb2Te5 （GST225）和其他硫系合金。在激光诱导下从晶态到非晶态以及从晶态到非晶态的转变过程中，结构的变化将通过一个时间延迟硬x射线探针脉冲在正入射传输几何结构中的散射来监测。充分利用LCLS的高通量和短脉冲持续时间，计划测量的散射模式作为泵浦-探针时间延迟的函数提供了瞬态结构因子S(q)，从而提供了开关过程中PCMs结构特性的信息。使用单个lcls脉冲获得高信噪比散射数据的可能性允许在“真实生活”开关条件下进行动态结构测量，这在以前或任何其他现有x射线源上都是不可能的。

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.