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.
