Investigation Of Lasing And Nonlinear Optical Properties In Crystal-based Micro/Nano Optical Structures

Integrated circuits enable optical signals transmission and information processing with high speed,high stability and low loss in an integrated space.By on-chip integration of varieties of functional optical components(such as lasers,splitters,couplers,optical switches,polarizers,gratings,etc.),photonics systems with small volume,multi-function,high speed and large capacity can be fabricated on one substrate and could be widely applied in the fields of fiber-optic communications,chemical sensing,biomedicine,environmental monitoring,spectroscopy,materials science,photonic computers and so on.In order to realize the integration of optical system with high performance,the optical structure size develops to micro or nano scale.Micro/nano optical structures could be obtained by directly reducing the volume of functional components or modifying the bulk materials to achieve new functions and improve optical properties of materials.In our work,the micro/nano optical structures including optical waveguide microstructure and metal nanoparticles mainly fabricated by bulk materials modification.Optical waveguide microstructure consists of a guiding core with relatively high refractive index surrounding by a cladding area with relatively low refractive index,which could confine the light propagation within small regions of orders of micrometers or sub-micron,leading to the enhancement of light intensity in the waveguide cavity.Furthermore,it enhances the optical properties of original bulk material,such as the laser performance of the laser gain medium,the nonlinear optical properties of the nonlinear material,and so on.Based on the structure of optical waveguide,some basic components for integrated circuit could be fabricated with compact structure,high integration,diverse functions,and excellent performance,such as waveguide splitter,waveguide coupler,waveguide laser and so on.Metal nanoparticle is the particle with the size less than 100 nm at least one dimension and has size-dependent optical properties.Under the excitation of incident light at a specific band,the surface plasmon resonance(SPR)effect is generated,and the surrounding local electromagnetic field is significantly enhanced,so that the optical properties of the substrate could be enhanced by several orders of magnitude such as the fluorescence and Raman spectral intensity,the nonlinear optical absorption coefficient.Therefore,it is possible for metal nanoparticles to improve the optical performance of the substrate,achieving signal amplification and reduced excitation conditions.In the field of integrated optics,it can be used as a saturable absorber with high integration and high stability to realize the generation of pulsed waveguide lasers.Also,it can be applied for micro/nano-scale integration as a high-precision sensor.As an ideal substrate material for micro/nano optical structure,the crystal medium,which has prominent optical properties,is widely used in various domains such as solid-state laser,optical amplification,nonlinear optics,electro-optical switches,etc.Among them,the laser crystal is often used as a gain medium material in solid-state lasers due to its excellent characteristics such as low laser threshold,high hardness,high gain,high thermal conductivity,etc.;nonlinear crystals can be used for laser frequency conversion to realize laser generation in specific wavelength by frequency multiplication,frequency mixing and optical parametric oscillation.The electro-optic crystal,whose refractive index could be controlled by applied electric field intensity,has broad application in the fields of optical modulation switches and high-speed optical communication.Combined with the functional crystalline materials,the micro/nano optical structure not only could be integrated in micro/nano-scale,but also has diverse functions as the optical devices.By this way,the overall performance of integrated optical system could be greatly improved.Multi-functional and high-performance micro/nano optical structures could be fabricated by various methods,including femtosecond laser micro-fabrication,focused ion beam(FIB)technology,photolithography,ion implantation and ion irradiation,ion exchange and proton exchange,plasma etching.In this paper,femtosecond laser micro-fabrication and ion implantation are used to fabricate optical waveguide microstructures and metal nanoparticles in functional crystal materials.According to designed pattern,femtosecond laser micro-fabrication uses a focused femtosecond pulsed laser to construct a three-dimensional micro/nano structure in a transparent medium such as a crystal,which has the advantages of high precision,three-dimensionality,low thermal effect,no pollution and so on.By the interaction of femtosecond laser and the crystalline dielectric material,the refractive index of the irradiated region could be changed to form an optical waveguide microstructure.During the ion implantation process,the charged ions with a high concentration lose their energy and stop in the substrate material through the interaction with the crystal target material,and finally synthesize the nanoparticles.Compared with other methods,ion implantation can freely control the type,size,depth and concentration of nanoparticles,and could not be influenced by solid solubility and thermodynamic equilibrium conditions.Thus,the embedded nanoparticles in solid materials have the advantages of high temperature resistance,high purity and high stability.The research work in this thesis is mainly based on crystal materials,and mainly studies two kinds of integrated micro/nano optical structures:femtosecond-laser-written optical waveguide microstructures and ion-implanted metal nanoparticles from the aspect of the fabrication and synthesis methods,basic characterization and the applications of laser and nonlinear properties.According to the different types of micro/nano structure and the function of substrate materials,the main research contents and results of this thesis can be summarized as follows:Based on the laser crystals Nd:YAP with different crystal orientation,two kinds of channel optical waveguides are fabricated by femtosecond laser writing including photonic-lattice-like waveguide and cladding waveguide.Under these waveguide microstructures,dual-wavelength waveguide lasers have been realized.Based on the sensitivities to crystal orientation and polarization in Nd:YAP crystal,channel optical waveguide microstructures are fabricated in a-cut and b-cut crystals,respectively.By changing the polarization of femtosecond laser and the polarization of waveguide pumping laser,the characteristics of optical waveguide are studied separately.In a-cut Nd:YAP crystal,a set of optical-lattice-like channel microstructures is produced by periodic arrangement of femtosecond laser induced tracks.With the polarization direction change of femtosecond laser,the waveguiding core would be varied in the optical-lattice-like microstructure,locating in either the regions between the neighbored tracks or the central zone surrounded by a few tracks as outer cladding.By confocal fluorescence microscope,the change in the light guiding properties caused by the polarization of different femtosecond lasers have been discussed from the perspective of refractive index change.Under the laser pumping at 812 nm,dual-wavelength laser near 1 ?m has been realized from waveguide micro-structure.In the b-cut Nd:YAP crystal,the low-loss cladding waveguide microstracture is fabricated by femtosecond laser writing.And pumiped by 812 nm laser,1064 nm and 1079 nm dual-wavelength waveguide lasers have been realized with polarization-maintaining output laser.By changing the polarization of the 812 nm pump light,the intensity ratio of dual-wavelength waveguide laser changes periodically,but the polarization direction of the waveguide laser remains unchanged in the TM direction.The maximum output total laser power is 200 mW,and the slope efficiency is 33.4%.Based on the laser crystal Nd:YVO4,the fabrication of the surface and buried cladding optical waveguide microstracture has been realized by femtosecond laser inscription.By using the phase-change material VO2,2D materials graphene and WS2,nanomaterial Au nanorods as the saturable absorbers,Q-switched waveguide lasers and Q-switched mode-locked waveguide laser are realized with the surface and buried cladding waveguide system,respectively.The VO2 exhibits reverse saturable absorption features in both insulating and metallic phases but exhibits the contradictory effect of the saturable absorption characteristics during the phase transition from the insulating state to the metallic state,with low saturable intensity,high modulation depth and short response time,which provides the potential for high-performance ultra-fast pulse laser generation.Under the excitation of 808 nm pump laser,Q-switched waveguide lasers based on the surface cladding waveguide platform have been compared with different saturable absorber:the phase-change material VO2,2D materials graphene and WS2.The results show that the VO2-based Q-switched waveguide laser has the shortest pulse duration(690 ps),the minimum laser threshold(18.8 mW)and the maximum peak power(33.1 W),while the graphene-based Q-switched waveguide laser has the largest repetition frequency(7.8 MHz),WS2-based Q-switched waveguide laser has the largest average output power(161.9 mW);Based on the buried cladding waveguide system,the high-performance Q-switched mode-locked waveguide laser has been realized by using the saturable absorber of Au nanorods,with the pulse duration of 27 ps and the repetition rate of 6.54 GHz.Based on the nonlinear crystal KTP,a group of optical-lattice-like waveguide microstructures with the functions of light tailoring or mode control are fabricated by femtosecond laser writing.And the second harmonic generation(SHG)in the visible light region is realized from the microstructures.According to the design,one could introduce additional defect tracks in appropriate positions of crystal to form the microstructure units with different positions or areas of the light guiding core.And by the combination of these microstructure units,different functions of integrated photonics device could be realized:laser splitting or mode control.Combined with the nonlinear optical properties of crystal,SHG from 1064 nm to 532 um has been achieved successfully through type ? phase matching,providing experimental basis for the fabrication of compact and multi-functional frequency conversion devices for integrated optics in the visible band.The results show that by changing the position of light guiding center,the femtosecond-laser-written KTP optical-lattice-like microstructures could realize one to four spatial splitter of the beam by two ways,and the normalized conversion efficiency of SHG in the splitter microstructures are 3.6%/W/cm2 and 2.4%/W/cm2,respectively;By changing the area of light guiding center,a hybrid taper-like photonic structure consists of two microstructure unites with large to small core diameter.The structure not only can realize nearly single mode output of the 532 nm laser,similar to waveguide microstructure with small core diameter,but can obtain a higher normalized conversion efficiency(1.1%/W/cm2),which is comparable to the large channel waveguide microstructure(1.2%/W/cm2).Finally,the structures could enable single-mode propagation at selected wavelength regime on purpose during second harmonic generation.Based on the crystals Nd:YAG and Pr:CaF2,embedded gold(Au)and silver(Ag)nanoparticles are synthesized by ion implantation.Through visible light excitation,the surface plasmon resonance effect of nanoparticles enhances the surrounding electromagnetic field,which improves the crystal nonlinearity and spectral characteristics,which is beneficial to pulsed laser integration with excellent performance and weak signal amplification.The results show that ion implantation can realize the nanoparticles size and concentration control by changing the implantation dose,leading to the linear absorption spectra variation of metal nanoparticles.In Nd:YAG crystal,gold nanoparticles with 2.7-nm average diameter are synthesized by subsequent annealing technique,and the experimentally measured surface plasmon absorption peak is 561 nm.Under the excitation of femtosecond laser at 515 nm,due to the existence of embedded nanoparticles,both nonlinear absorption coefficient and nonlinear refractive index of the crystal are improved by nearly 5 and 4 orders of magnitude,respectively,and exhibit saturable absorption and self-focusing effect.With the sample embedded with nanoparticles as saturable absor'ber,pulsed laser at 639 nm is realized with Pr:LuLiF4 crystal as gain medium.In Pr:CaF2 crystal,silver nanoparticles are synthesized with the average diameter of 5.3 nm.The measured resonance absorption peak is located at 417 nm.Under 473-nm continuous-wave laser excitation,the fluorescence intensity of the sample embedded with silver nanoparticles are enhanced by 60%compared with the original crystal.Combined with results of fluorescence lifetime,the photoluminescence enhancement is induced by the increased excitation efficiency of rare earth ions,resulting from the enhanced electromagnetic field around the nanoparticles.In addition,the sample embedded with silver nanoparticles exhibites surface enhanced Raman scattering effect excited by 532 nm with an enhancement factor of 31 at 312 cm-1.