Extreme-ultraviolet laser generation at 118 nm via adaptive random additional periodic-phase engineering in a LiF crystal

Extreme ultraviolet (EUV) coherent sources below 120nm are of paramount significance for promoting next-generation nano-scale lithography,precision spectroscopy, and exploring the emerging physical phenomena in quantum materials. Nonlinear optical conversion serves as the only feasible approach to obtain solid state EUV lasers, yet the intrinsic strong absorption at EUV and giant phase mismatch among light waves have hindered the realization of highly-efficient EUV light sources. Herein, we propose a random additional periodic phase (RAPP) strategy in third-order nonlinear crystals to overcome these problems, that an artificially designed random phase grating at micrometer-scales is embedded in the homogeneous bulk crystal, thus adaptively compensating the phase mismatch between fundamental-wave and third-harmonic waves. For the first time, the EUV laser at 118nm is demonstrated in the RAPP lithium fluoride (LiF) crystals with wide period distributions, where the highest output power is over 90uW. To the best our knowledge, this is the shortest wavelength among all solid-state laser systems, which represents a significant advance in nonlinear optical materials and opens new roadmap toward high-brightness EUV sources.