Copper Nanoparticles Embedded in Lithium Tantalate Crystals for Multi-GHz Lasers

Embedded copper nanoparticles (Cu NPs) with spherical geometry are synthesized by Cu+ ion implantation in lithium tantalate (LiTaO3) crystal. Because of the localized surface plasmon resonance (LSPR) effect, strong optical absorption in the visible light band has been observed in LiTaO3 with embedded Cu NPs. By changing the Cu+ ion fluence, the diameter of Cu NPs and the correlated linear optical responses could be tailored. The open-aperture ultrafast Z-scan measurement reveals that system also possesses saturable absorption in the near-infrared band. Further processing of spherical NPs by swift heavy ion (SHI) irradiation results in elongation of NPs along the incident direction, which significantly enhances the optical nonlinearity. It is found that the saturation intensity of the LiTaO3 with elongated Cu NPs is decreased by ∼62%, and the modulation depth is increased by 125% in comparison to that with spherical NPs. A Q-switched mode-locked waveguide laser operating at 1 μm is realized with repetition rate of 8.5 GHz and pulse duration of 23.5 ps by utilizing LiTaO3 with elongated Cu NPs as saturable absorber. This work indicates that ion beam techniques provide an effective way to tailor the optical nonlinearies of dielectric materials by engineering the shapes of embedded NPs.