Nanoscale Probing of Ultrafast Coherent Phonon Generation and Electron Diffusion in Metals

本研究的主要目的是将利用飞秒光脉冲产生超快声子的知识扩展到基本金属材料和纳米结构。可以研究声子脉冲在这种材料中的传播动力学和声子的产生机制。利用紧密聚焦的激发和直径几微米的探测光束，可以探测到-10 μm空间分辨率的声子。为了做到这一点，我们使用了超快反射仪和干涉测量法进行正常入射的测量。我们在非晶金属和液态金属中进行了相干声子的第一代和探测。汞的结果表明，电子扩散很可能影响声子的产生。观察到一个有趣的不对称脉冲形状。皮秒声子脉冲可以在非晶金属中产生和检测的首次演示也取得了成功。在过渡金属中，特别是Cr和Ni的-100 fs光脉冲实验表明，考虑到实验中测量到的超声衰减，我们可以成功地用双温模型模拟声子脉冲形状。首次在过渡金属中进行了- 10fs光脉冲的超短光脉冲实验。采用-20 fs阶的红外光脉冲宽度对泵浦和探测光束进行测量，在一系列不同厚度薄膜的数据中，超快电子扩散对超短时间尺度的影响非常明显。在较厚的薄膜中，观察到信号极性的显著反转。在多层金属结构中产生相干声子脉冲的初步结果表明，为了解释所观察到的声学回波，必须考虑电子扩散。相关的理论工作成功地阐明了光通过多层结构反射或透射时的应变调制的方式。这不仅适用于纵向相干声子，也适用于剪切相干声子。少

In main aim of this study is to extend our knowledge of ultrafast acoustic phonon generation using femtosecond light pulses to basic metallic materials and nanostructures. The phonon pulse propagation dynamics in such materials and the phonon generation mechanisms can be studied. The use here of a tightly focused excitation and probe light beams a few microns in diameter allow the detection of phonons with -10 μm spatial resolution. In order to do this we have made use of an ultrafast reflectometry and interferometry for measurements at normal incidence.We have carried out the first generation and detection of coherent phonons in amorphous and liquid metals. The results for Hg demonstrate that electron diffusion is very likely to be influencing the phonon generation. An interesting asymmetric pulse shape was observed. The first demonstration that picosecond phonon pulses could be generated and detected in an amorphous metal was also successful.Coherent phonon generation and detection e … More xperiments in transition metals with -100 fs optical pulses, in particular for Cr and for Ni, shows that we can successfully model the phonon pulse shapes with the two-temperature model, taking due account of ultrasonic attenuation measured in experiment.Ultrashort optical pulse experiments in transition metals with -10 fs optical pulses have been attempted for the first time. Using a -20 fs order optical pulse width in the infrared for both the pump and probe beams, the effect of ultrafast electron diffusion on ultrashort timescales is clearly evident in the data for a series of films of different thicknesses. A striking reversal in polarity of the signals for thicker films was observed.Preliminary results for the generation of coherent phonon pulses in multilayer metal structures show that electron diffusion must be taken into account in order to explain the observed acoustic echoes.Related theoretical work was successful in elucidating the way light is modulated by strain on reflection or transmission through a multilayer structure. This can be done not only for longitudinal coherent phonons but also for shear coherent phonons. Less