The Study On Nonlinear Optical Properties Of Monochalcogenides And Applications For Ultrafast Fiber Lasers

Compared with their bulk counterparts,two-dimensional layered materials(2DMLs)possess many unique physical properties which have been widely applied in optoelectronic and flexible electronic devices.When the thickness of the material is reduced to several nanometers the surface-to-volume ratio was increased.The modulation ability for the incident light of 2DMLs was tuned due to the quantum restriction effect.In addition,the optical bandgap engineering can be changed by applying external voltage,stress or changing the thickness and defect state of 2DMLs,so that the 2DMLs can produce a strong optical modulation effect in a wide band.Generally speaking,with the different input intensity of excitation light,the absorption of light can be divided into linear absorption and nonlinear absorption.The linear absorption mainly includes the absorption,scattering and transmission of light at different wavelengths.When the strength of filed is strong enough,the high-order susceptibility of the material is excited,resulting in higher conductivity.The nonlinear properties of materials mainly include harmonic generation,multiphoton absorption and Kerr effect.Recently,extensively researches have been focused on two-dimensional transition metal dichalcogenides with which the nonlinear properties have been widely used in ultrafast lasers,ultrafast photodetectors and new energy devices,showing a strong potential application in the nextgeneration optoelectronics.Some novel two-dimensional layered materials,such as monochalcogenides(Ga Te,Ga Se,In Te,etc.),tri-chalcogenides(Mo SSe),and novel transition metal chalcogenides(Zr Te5,Hf Te5),are of great interest to researchers because of their novel and excellent physical properties.In this paper,the nonlinear optical properties of novel 2DMLs,IIIA/IVA monochalcogendides(Ga Te,Ge Te,Ga Se and Ge Se),and their applications as saturable absorbers(SAs)in ultrafast optics are studied.In the second chapter,several typical preparation methods for 2DMLs are introduced,including top-to-down mechanical exfoliation,ultrasonic method,bottom-to-up chemical vapor deposition and magnetron sputtering method.Ultrasound-assisted liquid phase exfiliation is a widely used method due to its low-cost and batch preparation.The lattice structure and electronic bandgap of 2DMLs are analyzed by a series of characterization methods including Raman spectroscopy,photoluminescence,X-ray diffraction and optical absorption,which is helpful to evaluate the crystal quality and optical properties of 2DMLs.Finally,the paper introduces the operation mechanism of SAs,different fabraction types of SAs and the measurement of saturable absorption properties of SAs.The third chapter mainly discusses the basic nonlinear optical theory and nonlinear measurement technology.Nonlinear optical properties include second-order nonlinearity,thirdorder nonlinearity and high-order nonlinear effects.The third-order nonlinear effect includes third-order harmonic generation(THG),four-wave mixing(FWM),intensity dependent refractive index change(kerr effect,SA),which can be used in optical modulators and other nonlinear optical devices.Additionally,Z-Scan/I-scan and pump-probe are typical technologies for the measurement of third-order nonlinearity and carrier relaxation time.The nonlinear absorption coefficient and nonlinear refractive index of the material can be obtained by open Z-Scan and closed Z-Scan measurements,respectively.Ultrafast time resolved transient absorption spectroscopy(TAS)is a common pump-detection technique,which provides a powerful analysis for the relaxation process of excitation energy of materials.In the fourth section,we demonstrate the applications of two-dimensional layered materials which can be used in ultrafast optical fiber lasers.We prepared Ga Te and Ge Te based microfiber SAs and applied them into an erbium-doped fiber laser and a thulium-doped fiber laser,respectively.By adjusting the nonlinear effect and dispersions in the cavity,mode-locking operations are achieved with the ultrashort pulse duration of 450 fs and 600 fs in 1.5 ?m laser cavity and for 2 ?m,the shortest pulse durations are 980 fs and 1.5 ps,respectively.Secondly,we prepared organic films based on Ga Se and Ge Se nanosheets and embedded them into a 1.5 ?m fiber laser to obtain stable Q-switching operation.In the last chapter,we summarize the work done and put forward the further optimization scheme for the shortcomings in the experiment and make a prospect for the following works.We hope to achieve the anticipated results through continuous optimization.