Efficient Second- and Third-Harmonic Generations in Er3+/Fe2+-Doped Lithium Niobate Single Crystal with Engineered Surficial Cylindrical Hole Arrays

Herein, significant enhancement of second- and third-harmonic generation efficiencies in a 1 mol% Er3+ and 0.07 mol% Fe2+-doped lithium niobate single-crystal plate were achieved after ablating periodic cylindrical pit arrays on the surface. Enhanced absorption and reduced transmittance of light were measured when the incident light signal passed through the patterned sample. Enhanced photoluminescence and two-photon-pumped upconversion emission spectra were also explored to obtain more details on the efficiency gains. The excitation-energy-dependent second-harmonic generation efficiency was measured, and an enhancement as high as 20-fold was calculated. The conversion efficiency of second-harmonic generation is 1 to 3 orders higher than that from other lithium niobite metasurfaces and nanoantennas. This work provides a convenient and effective method to improve the nonlinear conversion efficiency in a thin lithium niobite plate, which is desirable for applying to integrated optical devices.