Loss of optical nonlinearity in proton-exchanged LiNbO3 waveguides

Abstract
The second‐order optical nonlinearity in a planar waveguide of proton‐exchanged LiNbO3 is directly probed by second‐harmonic generation from 532 to 266 nm in a reflection geometry. Compared to untreated LiNbO3, a 30‐fold reduction in the optical nonlinearity is observed. Contrary to previous reports, we find that the optical nonlinearity cannot be effectively restored by thermal annealing.