LAUSR

Extremely strong‐field terahertz (THz) radiation in free space has compelling applications in nonequilibrium condensed matter state regulation, all‐optical THz electron acceleration and manipulation, THz biological effects, etc. However, these practical applications are constrained by the absence of high‐intensity, high‐efficiency, high‐beam‐quality, and stable solid‐state THz light sources. Here, the generation of single‐cycle 13.9‐mJ extreme THz pulses from cryogenically cooled lithium niobate crystals and a 1.2% energy conversion efficiency from 800 nm to THz are demonstrated experimentally using the tilted pulse‐front technique driven by a home‐built 30‐fs, 1.2‐Joule Ti:sapphire laser amplifier. The focused peak electric field strength is estimated to be 7.5 MV cm−1. A record of 1.1‐mJ THz single‐pulse energy at a 450 mJ pump at room temperature is produced and observed that the self‐phase modulation of the optical pump can induce THz saturation behavior from the crystals in the substantially nonlinear pump regime. This study lays the foundation for the generation of sub‐Joule THz radiation from lithium niobate crystals and will inspire more innovations in extreme THz science and applications.