Thickness Evolution and Polarization Dependence of Coherent Acoustic Phonons in Layered NbOCl2

NbOCl2 is an in-plane ferroelectric van der Waals semiconductor that combines weak interlayer electronic hybridization with a pronounced optical anisotropy. Whether this weak interlayer hybridization also implies a weak cross-plane mechanical coupling remains unclear. We used broadband transient reflectivity to study NbOCl2 flakes 6–880 nm thick while independently controlling the pump and probe polarizations. An 869 nm flake exhibits a long-lived oscillation at 0.017 THz (17 GHz), whereas the dominant Fourier component reaches approximately 0.106 THz at 6 nm. The full-thickness series does not follow a single 1/d dependence. Near 30 nm, the dominant spectral weight crosses from a weakly thickness-dependent propagating strain/Brillouin response to higher-frequency features with stronger finite-thickness character. Rotating the pump polarization mainly changes the oscillation amplitude, whereas rotating the probe polarization shifts the dominant Fourier maximum from approximately 0.017 to 0.021 THz. Weak interlayer electronic hybridization does not prevent a coherent cross-plane acoustic response. The response depends on the thickness and boundary conditions, while in-plane optical anisotropy affects excitation and detection.