Probing Ionic Vibrational Dynamics via Terahertz Fluctuations in Nanopore Ionic Current.

The analysis of current noise in nanopores has emerged as a powerful tool for probing the size, dynamics, and interactions of ions and molecules. However, while the low-frequency regime (<1 MHz) is well-characterized, the fundamental question of what physical information is encoded in the high-frequency spectrum remains open, largely due to experimental limitations. Here, we employ all-atom molecular dynamics simulations to map the ionic current power spectrum in the frequency range from MHz to THz. We reveal a transition in the spectral density: at low frequencies, the power spectrum reflects ionic conductance and is modulated by ion correlations, whereas distinct peaks emerge in the THz range. These peaks are nonstochastic but represent vibrational fingerprints of cations and their hydration shells, with characteristic resonances at 1-3 THz for Na+ and 4 THz for La3+. These resonant modes are intrinsic to the ion's local hydration environment, independent of applied voltage or ion correlation. This work establishes nanopore-based high-frequency current noise spectroscopy as a direct probe of ultrafast hydration dynamics, offering new insights for studying local ionic environments in confined liquids.