Coherent Discriminatory Modal Manipulation of Acoustic Phonons at the Nanoscale.

Understanding and controlling the phononic characteristics in solids is crucial to elucidate many physical phenomena and develop new phononic devices with optimal performance. Although substantial progress on the spatial control of phonons by material design has been achieved, the manipulation of phonons in the time domain has been less studied but can elucidate in-depth insight into various phonon-coupling processes. In this work, we explore different time-domain pump-control(s)-probe phonon manipulation schemes in both simulations and experiments with good consistency. In particular, we use an Au-Ag core-shell nanoparticle with a manifestation of multiple phonon vibrational modes as a model system for multimodal-phonon manipulation, and we demonstrate that the simple addition of a femtosecond optical control pulse to an all-optical pump-probe phonon measurement can enhance or suppress the fundamental breathing phonon mode of nanoparticles depending on the time separation between the pump and the control pulses. A more advanced control of the higher-order phonon modes and their interplay has also been achieved using two sequential and independently tunable optical control pulses, which enables the discriminatory modal manipulation of phonons for the first time. This work represents a significant step toward a deep understanding of the phonon-mediated physical and chemical processes and a development of new nanoscale materials with desirable functionalities and properties.