Probing Phonon-Matter Interactions at the Nanoscale

固体中太赫兹声子的波长只有几纳米。与光学访问相比，这种长度尺度是前所未有的，并激发了在纳米尺度上探索新型量子现象的机会。用超短光脉冲辐照平面金属多层结构，可以在简单实验中产生相干太赫兹声子。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振幅，并且在纳米尺度上表现出声学非线性，而不会破坏材料[V]。Temnov，自然摄影。(2012);自然通讯，（2013）。这些巨大的应变脉冲以几GPa的压力为特征，在纳米尺度上集中在空间（一维）上，能够驱动磁致伸缩材料中的非热磁声开关[j]。Kovalenko et al., Phys。启。(2013)]。该项目解决了产生强烈的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。这将扩展当前最先进的强声脉冲形成技术，并允许对纳米尺度凝聚态系统进行新的研究。首先，我们将实验探索相干皮秒声脉冲在二维和三维纳米尺度上的聚焦。在第一种方法中，二维声聚焦[T。Pezeril et al.，物理学。启。（2011）]将扩展到纳米尺度，通过光学激发薄钴层中单个孤立的亚波长空穴。在第二种更具挑战性的方法中，纳米制造声学金钴菲涅耳透镜的概念将被用于将声波集中到100纳米以下的焦点。我们开发的新型压力集中器将为干扰和控制纳米尺度凝聚态系统中多体相互作用的量子特性提供理想的实验平台。应变波和位于焦点点的纳米磁体的铁磁进动之间的磁弹性相互作用的超快测量既可以作为成功的3d聚焦的新指纹，也将用于研究纳米尺度上的磁弹性相互作用的物理学。同时，将在超快时间尺度上研究单半导体量子点的应变诱导激子能级位移[F]。Sotier et al, Nature physics。（2009）， J. Huneke et al.，物理学。Rev. B(2011)]。这种能力将使我们首次能够在高度稳定的胶体量子点中进行基于压力的少费米子动力学研究。de Roo et al., ad . Funct。板牙。（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器件提供基础背景。

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.