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

本研究的主要目的是将飞秒光脉冲产生超快声学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产生机制。这里使用的是直径为几微米的紧密聚焦的激发光束和探测光束，允许以-10 μm的空间分辨率检测声子。为了做到这一点，我们已经利用了超快反射和干涉测量法在正常入射。我们进行了第一次产生和检测的相干声子在非晶和液态金属。汞的结果表明，电子扩散很可能是影响声子产生。观察到一个有趣的不对称脉冲形状。首次在非晶态金属中成功地实现了皮秒声子脉冲的产生和探测，相干声子脉冲的产生和探测是一个新的课题。 ...更多信息 在过渡金属（特别是Cr和Ni）中进行的-100fs光脉冲实验表明，考虑到实验中测得的超声衰减，我们可以成功地用双温度模型来模拟声子脉冲的形状，并首次尝试了在过渡金属中进行的-10fs超短光脉冲实验。对于泵浦光束和探测光束，在红外线中使用~ 20 fs量级的光脉冲宽度，在一系列不同厚度的膜的数据中，超快电子扩散对超短时间尺度的影响是明显的。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在多层结构中反射或透射时受应变调制的方式。这不仅可以对纵向相干声子进行，而且可以对剪切相干声子进行。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