Probing Phonon-Matter Interactions at the Nanoscale

固体中的太赫兹声学声子的特征是波长只有几个纳米。与光学访问相比，这种长度尺度是史无前例的，并激发了在纳米尺度上探索新量子现象的机会。在超短光脉冲辐照平面金属多层结构的简单实验中，可以产生相干的THz声学声子。最近已经证明，在金-钴双层结构中激发的相干声子脉冲可以具有异常大的幅度，并在不破坏材料的情况下在纳米尺度上显示出声学非线性[V.Temnov，Natural Phot.(2012)；《自然通讯》。(2013)]。这些巨大的应变脉冲，以几Gpa的压力为特征，集中在空间(一维)纳米尺度上，能够驱动磁致伸缩材料中的非热磁-声开关[O.Kovalenko等，Phys。莱特牧师。(2013)]这个项目提出了产生强烈时空局域声扰动的想法，目的是利用它们来控制复杂量子系统中的动力学。这将扩展目前最先进的形成强烈声脉冲的技术，并允许在纳米级凝聚态系统中进行新的研究。首先，我们将在实验上探索相干皮秒声脉冲在2D和3D中聚焦到纳米尺度。在第一种方法中，2D声聚焦[T.Pezeril等人，Phys.莱特牧师。(2011)]将通过光学激发薄钴层中单个孤立的亚波长孔而扩展到纳米尺度。在第二个也是更具挑战性的方法中，将使用纳米硬化金-钴菲涅耳透镜的概念来将声波集中到100 nm以下的焦点。我们开发的新型压力集中器将提供一个理想的实验平台来扰动和控制纳米凝聚态系统中多体相互作用的量子性质。超快测量应变波之间的磁弹性相互作用和位于焦点中的纳米磁铁的进动既是成功的3D聚焦的新指纹，也将被用于在纳米尺度上研究磁弹性相互作用的物理。同时，应变诱导的单个半导体量子点的激子能级移动将在超快时间尺度上被研究[F.Sotier等人，自然物理。(2009)，J.Huneke等人，Phys.修订本B(2011)]。这种能力将使我们第一次能够对高度稳定的胶体量子点中的少费米子动力学进行压力依赖的研究[T.de Roo等人，Adv.Funct.马特。(2014)]在低温下，有可能驱动系统进入结构相变。概述的太赫兹声子脉冲实验有望为超快纳米级磁弹性和光弹性设备提供基本背景。

THz acoustic phonons in solids are characterized by a wavelength of only a few nanometers. Such length scales are unprecedented in comparison to optical access and motivate an opportunity to probe novel quantum phenomena at the nanoscale. Coherent THz acoustic phonons can be generated in simple experiments where planar metallic multilayer structures are irradiated by ultrashort optical pulses. Very recently it has been demonstrated that coherent phonon pulses excited in gold-cobalt bi-layer structures can possess unusually large amplitudes and show acoustic nonlinearities at the nanoscale without destroying the materials [V. Temnov, Nature Phot. (2012); Nature Comm. (2013)]. These giant strain pulses, characterized by a pressure of a few GPa and concentrated in space (in one dimension) on the nanometer scale are capable of driving the non-thermal magneto-acoustic switching in magneto-strictive materials [O. Kovalenko et al., Phys. Rev. Lett. (2013)].This project addresses ideas for generation of intense spatio-temporally localized acoustic perturbations with the aim of using them to control dynamics in complex quantum systems. This would both extend current state-of-the-art techniques for intense acoustic pulse formation and allow for novel investigations in nanoscale condensed-matter systems. First, we will experimentally explore focusing of coherent picosecond acoustic pulses to the nano-scale in 2D and 3D. In the first approach, 2D acoustic focusing [T. Pezeril et al., Phys. Rev. Lett. (2011)] will be extended to the nanoscale by optically exciting a single isolated sub-wavelength hole in a thin cobalt layer. In the second and more challenging approach, the concept of the nanofabricated acoustic gold-cobalt Fresnel lens will be used to concentrate acoustic waves to the sub-100 nm focus. The novel pressure concentrators we develop will provide an ideal experimental playground to perturb and control quantum nature of many-body interactions in nanoscale condensed-matter systems. Ultrafast measurements of magneto-elastic interactions between strain waves and the ferromagnetic precession of a nanomagnet located in the focal spot serve both as a novel fingerprint of successful 3D-focusing and will also be used to study the physics of magneto-elastic interactions at the nanoscale. In parallel, strain-induced exciton level shifts of single semiconductor quantum dots will be investigated on ultrafast timescales [F. Sotier et al, Nature Phys. (2009), J. Huneke et al., Phys. Rev. B (2011)]. This capability will allow us for the first time to perform pressure-dependent studies of few-fermion dynamics in a highly-stable colloidal quantum dot [T. de Roo et al., Adv. Funct. Mater. (2014)] at cryogenic temperatures, with the possibility of driving the system into a structural phase transition.The outlined experiments with THz phonon pulses are expected to provide the fundamental background for ultrafast nanoscale magneto-elastic and opto-elastic devices.