Strong nonlinear-polarization in ZnMgO epitaxial thin-films with Li incorporation

Abstract

The second-order nonlinear-polarization originated from the interaction between thin-film materials with second-order nonlinear susceptibility (χ (2)) and high-power laser is essential for integrated optics and photonics. In this work, strong second-order nonlinear-polarization was found in a-axis oriented Zn1-x Mg x O (ZnMgO) epitaxial thin-films with Li incorporation, which were deposited by radio-frequency magnetron sputtering. Mg incorporation (x > 0.3) causes a sharp fall in the matrix element χ 33 of χ (2) tensor, although it widens optical bandgap (E opt). In contrast, moderate Li incorporation significantly improves χ 33 and resistance to high-power laser pulses with a little influence on E opt. In particular, a Zn0.67Mg0.33O:Li [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ 33| of 36.1 pm V−1 under a peak power density (E p) of 81.2 GW cm−2, a resistance to laser pulses with E p of up to 124.9 GW cm−2, and an E opt of 3.95 eV. Compared to that of ZnO, these parameters increase by 37.8%, 53.4%, and 18.6%, respectively. Specially, the Zn0.67Mg0.33O:Li shows higher radiation resistance than a Mg-doped LiNbO3 crystal with a comparable E opt. First-principle calculations reveal the Li occupation at octahedral interstitial sites of wurtzite ZnO enhances radiation resistance by improving structural stability. X-ray photoelectron spectroscopy characterizations suggest moderate Li incorporation increases χ 33 via enhancing electronic polarization. These findings uncover the close relationship between the octahedra interstitial defects in wurtzite ZnMgO and its nonlinear-polarization behavior under the optical frequency electric field of high-power laser.