Dynamic Polarization Control of Nonlinear Terahertz Photoresponse via Topological Phase Transitions.

Precise modulation of topologically protected states via external stimuli, such as electric, optical, and magnetic fields, is a cornerstone for advancing robust topological photonics and quantum technologies. However, the realization of dynamic and noninvasive control remains constrained by the high-energy thresholds of conventional stimuli, which can disrupt delicate topological states. Here, we employ low-energy terahertz excitation to directly probe the photoresponse across a temperature-induced topological phase transition in ultrathin ZrTe₅, a material at the intersection of topological physics and low-dimensional systems, leveraging its unique ability to interact with low-energy quasiparticle states without compromising coherence in the system. We observe a giant and robust nonlinear terahertz photoresponse characterized by in situ tunable geometric properties of Bloch quasiparticles. The response exhibits colossal behavior and a sign reversal across a temperature-driven topological phase transition, linked to a nonvanishing Berry curvature dipole that serves as a direct marker of symmetry-breaking evolution between weak (m < 0) and strong (m > 0) topological insulator phases. The observed device exhibits a response time of ~1 μs with a noise equivalent power of 5.6 pW/Hz^0.5 across the 0.5-THz range, demonstrating the potential of topological phase transitions for terahertz detection. These findings underscore the potential of low-energy terahertz excitation for dynamically polarizing and controlling topological states in ultrathin materials, offering a versatile framework for exploring symmetry-breaking phenomena and advancing next-generation optoelectronic devices.