Second Harmonic Generation Of Femtosecond Laser Written Waveguides In Periodically Poled Lithium Tantalate Crystal

Second harmonic generation technology is an important aspect in the field of non-linear optics.With the application and development of laser technology,frequency doubling technology is widely used in solid-state lasers and other non-linear devices LiNbO3 crystal with superlattice structure has many advantages,such as high non-linear efficiency,good mechanical and physical properties,low cost and compact size,so its application in the field of green solid-state lasers is a promising hotspotLithium tantalate(LiTaO3)crystal is a ferroelectric oxide crystal.Its crystal structure is the same as that of lithium niobate,with excellent piezoelectric,ferroelectric,acousto-optic and electro-optic properties.Its nonlinearity coefficient is less than that of lithium niobate crystal,but it possess higher thermal conductivity and optical damage threshold,so phase mismatch can be avoided/reduced in case of high light intensity.In addition,lithium tantalate crystal has a wider transmittance range and lower coercive field compared with lithium niobate crystal,leading to larger periodic inversion thickness and aperture.Therefore,LiTaO3 has great research value and application potential in the field of mid-infrared integration.The sample used in this paper is near stoichiometric lithium tantalate crystal.The Li/Ta ratio of LiTaO3 ampoule grown in congruent melt composition will deviate from stoichiometric value and form congruent crystal with excessive tantalum and lithium deficient,which results in the loss of the non-linear properties in lithium tantalate crystals.The performance of optical damage and coercive electric field was also deteriorated severely.The Li/Ta ratio of near stoichiometric lithium tantalate is close to stoichiometric ratio,which eliminates the intrinsic defects of lithium vacancies and inverse tantalum in crystals,and improves the non-linear properties of crystals.In addition,by doping Mg into the sample,the coercive electric field of the crystal is greatly reduced,and the processing accuracy of periodic polarization domain inversion is improved.In recent years,as an efficient three-dimensional waveguide micromachining technology,femtosecond laser micromachining has been developed and investigated intensively.Compared with traditional waveguide fabrication processes such as metal diffusion,proton exchange and ion implantation,femtosecond laser lithography has the advantages of convenient machining technology,short fabrication cycle and low fabrication cost.The laser beam shows Gaussian distribution at the focal area,and the intensity of the light is strictly restricted inside the focal volume.In most cases,the multi-photon ionization process happens only in the central region of the focusing volume,this spatial confinement,combined with laser-beam scanning or sample translation,makes it possible to fabricate geometrically complex structure in three dimensions.Moreover,the absorption process is independent of the material,enabling optical devices to be fabricated in compound substrates of different materials.In this paper,the cladding waveguides was fabricated in periodically polarized lithium tantalate crystals by femtosecond laser micromachining.We have investigated its nonlinear properties and we found that the non-linear properties of cladding waveguides were well preserved.The fan-out domain inversion structure was fabricated by periodically polarized processing.The second harmonic generation was successfully achieved by end-facet coupling and temperature tuning.