2D-THz spectroscopy: exploring the nonlinear dynamics in quantum materials.

Unraveling the nonlinear regime of light-matter interaction in quantum materials at ultrafast timescales has remained elusive over the past few decades. The primary obstacle entailed finding a resonant pump as well as a suitable, resonant probe that could effectively excite and capture the interaction pathways of the collective modes within their inherent timescales. Intriguingly, the characteristic energyscales of the said interactions and the timescales of ensuing dynamics lie in the THz range, making THz radiation not only an apt probe but also an ideal resonant tool for driving the collective modes out of equilibrium. In the said direction, 2D-THz spectroscopy serves as a state-of-the-art technique for unveiling the correlation dynamics of quantum materials through table-top experiments. On a microscopic level, this offers valuable insights into the competing interactions among the charge, spin, lattice, and orbital degrees of freedom. Though the field of 2D-THz spectroscopy is relatively new and yet to be explored in its full potential, this review highlights the progress made in investigating various coupling channels of collective modes, namely magnons, phonons, polaritons, etc in different insulating and semiconducting systems. We also provide pedagogical introduction to the 2D-THz spectroscopy and foresee its emergence alongside cutting-edge experimental tools, reshaping our understanding of quantum materials with new perspectives.