Air nonlinearity triggered by an ultra-intense sub-5 THz light bullet

Summary form only given. Air turns into a nonlinear medium for electromagnetic waves under exceptionally strong fields [1]. However up to present, its minuscule nonlinear response has limited the exploration to the optical frequency regime owing to the availability of intense near-infrared lasers. Here, we report on the observation of large-amplitude nonlinearity in air induced by an extremely intense light bullet [2] at THz frequencies provoking strong air birefringence [3]. The observed nonlinearity manifests itself as third order susceptibility. The presented nonlinear observations break the barrier for the entire exciting THz-induced nonlinear phenomena in air ranging from THz-induced self-focusing and self-phase modulation to THz solitons and filamentation. Our pulse was 3.9 THz-centered and its peak field and intensity were 3.3 GV/m and 14.4 PW/m², respectively.