Frontiers of sub-cycle terahertz science: The fast, the strong and the small

Phase-locked few-cycle pulses in the ultrabroadband terahertz (THz) spectral window <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-4</sup> have become a powerful tool to access low-energy elementary dynamics in solids, on time scales shorter than a single oscillation cycle of light <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5-14</sup> . Here, we tackle the role of excitons and their dynamics in modern dichalcogenide monolayers and exciton-polariton condensates <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-9</sup> . Furthermore, intense THz pulses are used as atomically strong bias fields to explore high-harmonic generation by dynamical Bloch oscillations in bulk solids <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11-13</sup> . Finally, we develop a novel microscope that introduces sub-cycle resolution on the few-nm length scale <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> . The experiments reviewed here challenge modern quantum theories and spark hope for electronics and magnetic storage at optical clock rates.