Nanoscale Coherent Phonons with Broadband Frequency Tunability

Achieving resonant detection and coherent control of phonons with simultaneous nanometer spatial and femtosecond temporal precision is of great importance for developing phonon-based technologies, yet remains a long-standing challenging task. In this Letter, we succeed to fabricate nanoscale coherent phonons with broadband frequency tunability using a femtosecond laser-combined scanning tunneling microscope. An ultrafast photocurrent, enhanced by localized surface plasmons, was used to track the high-frequency mechanical motion of nanoclusters at their intrinsic length and time scales. We demonstrate that the coherent acoustic phonon modes of the nanoclusters can be launched by the impulsive pressure of plasmon-induced hot electrons and that their frequency can be tuned by the size of the nanoparticles. Through tip manipulation, we generated a series of on-tip coherent phonons with an ultrabroad frequency range spanning from 15 GHz to 1 THz in a highly controlled manner. Such a tip with broadband-tunable coherent phonons holds great promise for detecting high-frequency mechanical and electromagnetic fluctuations in resonant modes.