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1T-TaS$_2$ is a layered quantum material hosting an insulating commensurate charge density wave (CDW) phase at low temperatures. The insulating nature of this phase arises from a complex interplay between electron-phonon coupling, electron-electron correlations and interlayer orbital hybridization, making it highly susceptible to external perturbations and atomic-scale disorder. In particular, it is known that ultrafast laser excitation, electrical pulses, or surface defects can drive 1T-TaS$_2$ into a metastable metallic mosaic phase at low temperatures. However, achieving a detailed understanding of how the nanoscopic environment influences the formation and ultrafast dynamics of such phases remains challenging. Terahertz-scanning tunneling microscopy (THz-STM) has emerged as a powerful technique for investigating nonequilibrium states with femtosecond-time and sub-nanometer spatial resolution [1,2]. Moreover, it has been shown recently that the tip-enhanced THz-fields can drive metastable phases via altering interlayer degrees of freedom in layered materials [3]. Here, we use THz-STM to investigate the local formation as well as the photoinduced dynamics of a THz-induced metastable state in 1T-TaS$_2$ [4]. We show that THz excitation drives the TaS$_2$ surface into a long-lived metastable state with a locally altered interlayer stacking order. In addition, by using optical-pump THz-probe measurements to probe its ultrafast response to optical excitation, we locally probe coherent oscillations of the local charge density, revealing the presence of multiple phonon modes including the intralayer CDW amplitude mode and two previously unreported interlayer shear and breathing modes. Our work establishes THz-STM as a powerful platform for imaging and controlling nonequilibrium quantum phases at their intrinsic spatiotemporal scales.
[1] M. Müller. Prog. Surf. Sci. 99, 100727 (2024)
[2] T. Cocker et al., Nature Phot. 15, 558–569 (2021)
[3] Jelic et al., Nature Phot. 19, 1048–1055 (2025)
[4] L. Parra López et al., arXiv:2505.20541 (2025)