Coherent ultraviolet light generation in widegap wurtzite ZnMgO thin films through frequency conversion

Laser frequency conversion in wide bandgap nonlinear optical thin films enables compact coherent ultraviolet (UV) light sources for UV photonics integrated circuits. This work unveils the generation of coherent 350 nm wavelength UVA light in a-axis oriented wurtzite Zn0.67Mg0.33O and Zn0.51Mg0.49O thin films through frequency doubling of a femtosecond laser. The conversion efficiency of the 0.66 μm-thick Zn0.67Mg0.33O and 0.40 μm-thick Zn0.51Mg0.49O thin films reached 3.25 × 10−4 and 1.07 × 10−4, respectively. The single-phase wurtzite Zn0.51Mg0.49O thin film shows an optical bandgap of 4.13 eV, a light absorption coefficient of less than 1 cm−1 at 350 nm wavelength, and a second-order nonlinear susceptibility element χ33(2) of 10.83 pm/V at 700 nm wavelength. Increasing Mg incorporation into wurtzite ZnO causes lattice distortion and deteriorates the non-centrosymmetry of the wurtzite structure, resulting in the reduction in χ33(2). The introduction of a Zn0.79Mg0.21O buffer layer enhances the χ33(2) of the Zn0.51Mg0.49O thin film by 1.3 times through improving film growth quality and relieving the lattice distortion. These results suggest the great prospects of high-efficiency coherent UV light generation in widegap Zn1−xMgxO thin films integrated with reversible polarization.