THz-Driven Coherent Phonon Fingerprints of Hidden Symmetry Breaking in 2D Layered Hybrid Perovskites

Metal-halide perovskites (MHPs) emerged as a family of novel semiconductors with outstanding optoelectronic properties for applications in photovoltaics and light emission. Recently, they also attract interest as promising candidates for spintronics. In materials lacking inversion symmetry, spin-orbit coupling (SOC) leads to the Rashba-Dresselhaus effect, offering a pathway for spin current control. Therefore, inversion symmetry breaking in MHPs, which are characterized by strong SOC, has crucial implications. Yet, in complex low-dimensional hybrid organic-inorganic perovskites (HOIPs), the presence of and structural contributions to inversion symmetry breaking remain elusive. Here, employing intense THz fields, lattice dynamics carrying spectroscopic fingerprints of inversion symmetry breaking are coherently driven and observed in Ruddlesden-Popper (PEA)₂(MA)_n-1Pb_nI_3n+1 perovskites, which are globally assigned to a centrosymmetric space group. We demonstrante coherent control by THz pulses over specific phonons, which are assigned to either purely inorganic or highly anharmonic hybridized cage-ligand vibrations. By developing a general polarization analysis for THz-driven phonons, linear and nonlinear driving mechanisms are pinpointed. From this, simultaneous IR- and Raman-activity of inorganic cage modes below 1.5 THz is identified, indicating mode-selective inversion symmetry breaking. By exploring the driving pathways of these coherent phonons, the groundwork is laid for simultaneous ultrafast control of optoelectronic and spintronic properties in 2D HOIPs.