Fabrication And Optical Nonlinear Studies Of Metallic Nanoparticles In Lithium Niobate Crystals By Ion Implantation

Lithium niobate（LiNbO₃）has huge nonlinear optical coefficients and wide range of light transmission,which makes it ideal optical components in applications like electro-optic modulators,optical limiters,integrated optical devices and etc.However,LiNbO₃ has low laser damage threshold,thereby limiting its applications.Due to the large third-order nonlinear optical coefficients and fast optical responses of metallic nanoparticles（MNPs）,the combination of MNPs and LiNbO₃ crystal is an effective way to enhance its nonlinear optical effect.In this paper,MNPs with different forms were synthesized in LiNbO₃ crystals by metallic ion implantation.Theoretical and experimental researches were performed with focusing on fabrication,microstructures,linear and nonlinear optical properties of MNPs in LiNbO₃ crystals.The main research contents and results are as follows:（1）Z cut LiNbO₃ crystals were singly and sequentially implanted with 100 keV Cu at a fluence of 5×10¹⁶/cm² and 70 keV Zn at fluences of 3×10¹⁶/cm² and 5×10¹⁶/cm².Combining with subsequent thermal annealing and various analytical techniques,the microstructures,optical properties and thermal stabilities of MNPs were mainly investigated.UV-Vis absorption results indicated that the Cu implanted samples exhibited a significant absorption peak around 614 nm due to Cu NPs formation according to TEM results.The Cu and Zn implanted samples showed a wider absorption peak with a blue shift of about 40 nm,and the corresponding TEM results indicated Cu Zn alloy NPs formation.After subsequent annealing,the Cu samples showed weak thermal stability,and the surface plasmon resonance（SPR）peak of Cu NPs disappeared after 300℃ annealing.For the Cu and Zn samples,the absorption peak red-shifted to SPR peak position of Cu NPs after 300℃ annealing.TEM results indicated that Cu Zn alloy NPs were de-alloyed,Cu NPs were formed and then decomposed after 400℃ annealing.Z-Scan results showed that the nanocomposites had third-order nonlinear polarizabilities four orders of magnitude higher than the un-implanted LiNbO₃ crystals.Especially,the nonlinear optical enhancement effect of Cu Zn alloy NPs was stronger than that of single Cu NPs.In addition,nanocomposites had strong saturation absorption properties,which helped to broaden the application of LiNbO₃ materials.（2）Z cut LiNbO₃ crystals were processed by single and sequential implantation of70 keV Zn and 60 keV Cu ions at the same fluence of 5×10¹⁶/cm² together with subsequent annealing in N₂ ambient.The effects of Zn pre-implantation on the structures,thermal stabilities and optical properties of subsequent Cu NPs were studied in detail.The results showed that the Cu implanted samples exhibited an obvious SPR absorption peak at 626 nm,whereas the Zn pre-implantation resulted in a blue shift of the SPR peak and an enhanced SPR absorption peak at 592 nm.TEM results confirmed that Cu Zn/Cu core/shell NPs were synthesized,in which Cu Zn alloys mainly occupied around the core and Cu mainly distributed in the shell.Two samples exhibited completely different thermal evolution patterns:The SPR peak disappeared in the Cu implanted samples after 250℃ annealing,whereas the SPR peak of the Zn pre-implanted samples gradually red-shifted as the annealing temperature increased,and the much stronger and narrower SPR peak was maintained after 300℃ annealing.The internal mechanism was that Zn pre-implantation could largly suppress the sputtering loss of Cu implants during post Cu ion implantation,therefore the volume fraction of Cu NPs increased.Besides,Zn provided a protective effect and greatly improved the thermal stability of the Cu NPs.Z scan results showed that the Zn pre-implanted samples exhibited enhanced third-order nonlinear optical effects,which could improve the nonlinear optical properties of LiNbO₃ bulk materials.