Nanoscale Coherent Phonon Dynamics Probed by Ultrafast Scanning Probe Microscopy

Tracking the dynamical process of coherent phonons in solids can help us understand and control the interactions between coherent phonons and other quasiparticles, providing fresh insights into the areas such as electron–phonon coupling, photo-induced phase transitions, and thermal dissipation. However, due to the diffraction limit, conventional ultrafast spectroscopy measurement cannot probe coherent phonons at the nanoscale. Here, based on our home-built femtosecond laser-combined scanning probe microscopy (SPM) [1], we succeed to capture the nanoscale dynamical evolution of coherent phonons of NaCl nanoclusters [2]. Through tip manipulation, we obtain a series of on-tip coherent phonons with an ultrabroad frequency range spanning from 40 GHz to 1 THz. Such a tip with broadband-tunable coherent phonons holds great promise for detecting high-frequency mechanical and electromagnetic fluctuations in resonant modes. Furthermore, we extended this approach to resolve the coherent phonons in the strongly correlated system bulk 1T-TaSe₂ [3], which play a critical role in the formation of charge density waves and photo-induced insulator-metal transition.