Study Wide Spectrum Of Ultrafast Fiber Laser Properties Of Two-dimensional Material MoS₂

In the last ten years,two-dimensional（2D）materials have achieved rapid development and have become a promising new material for optocelectronics since the successful peeling of single atomic layers of graphene.Followed graphene,scientists have successfully prepared a lot of other 2D materials such as transition metal dichalogenides（TMDCs）,transition metal oxides,topological insulators,black phosphorus and so on.Along with rapid expansion of the 2D material families,2D materials have attracted more and more attention over the world.Here,the 2D materials are mainly refer to those whose thickness is of a single atomic layer or close to a single atomic layer,where the movement of electrons is confined in a near 2D plane and quantum localization effects play an important role on the properties and dynamics of electrons and consequently greatly influence the properties of materials.Owing to their unique structure and high surface-to-volume ratio,2D materials show excellent performance and have huge application potentials in wide areas,such as a new type of light source,optical detector,optical modulators,sensors,solar cells,lithium batteries,and so on.The success of graphene has greatly promoted the development of 2D materials.However,the zero bandgap feature of graphene limites its application in some areas such as high efficient light-emission sources and high extinction ratio modulators,etc.Fortunately,these defects of graphene can be made up by the newly emergent 2D materials-TMDCs.TMDCs have exhibited many unique optical and electrical properties,such as strong spin and/or valley polarization selection properties.After ten years of development,It had demonstrated excellent performance and broad application prospects in the fields of light sources,detectors,modulators,and other optoelectronic devices.However,there are still many novel areas required further exploration for 2D TMDCs based on their nonlinear optical properties,particularly about their nonlinear optoelectronic characteristics in the near infrared,mid-infrared and other long-wavelength regions.The study in these areas will not only help to make clear the new electronic and optoelectronic effects of 2D TMDCs but will also open up new application areas for 2D materials.This thesis focuses on the study of nonlinear photoelectronic properties of one of the2D TMDCs（MoS₂）.In order to better understand the special nature of 2D materials,this thesis first reviews the structure,material properties,and perparation methods of 2D materials.At the same time,the calculation method based on the first-principles is given,and a thorough and comprehensive description of the working mechanism for 2D MoS₂.Above of all,we give detailed exploration and discussion about the mechanism,application advantages and material requirements of 2D MoS₂ functioning as new nonlinear saturable absorbers in the ultrashort fiber lasers regime.After detailed and comprehensive analysis of related theories,we prepared solution type of MoS₂ material with different parameters,and the solution is then drop coated on high-reflection gold mirrors to achieve layered-MoS₂ saturable absorbers.Thereafter,we design various MoS₂ saturable absorbers（by changing the thickness and density of MoS₂）and fiber laser configurations.After many experimental measurement and analysis,optical linear and nonlinear absorption characteristics of 2D MoS₂ saturable absorbers from 1 to 3μm are successfully obtained.What’s more,we apply them in various fiber laser systems and successfully achieve mode-locked or Q-switched operation in the wide spectral spanning（from 1 to 3μm）.Last but not the least,2D MoS₂ saturable absorbers（according to their broad spectral features）are used to realize digital-wavelength mode-locking of fiber laser（in the 1-micron region）,which will find a new road for the application of 2D materials in digital communications,measurement,and etc.Detailed content of this thesis is divided into the following sections.Firstly,the structures,materials properties,the main application,and preparation methods of MoS₂ are introduced.Based on detailed comparative study,we develop one method to achieve high purity,few-layer MoS₂ material with excellent properties.At the same time,a theoretical tool based on the first-principles calculation method for study the photoelectronic characteristics of 2D MoS₂ is provided.Meanwhile,it proves that 2D MoS₂ is a suitable material for wide-band mode-locking after exploration their nonlinear properties in detail.Secondly,we design a simple linear laser cavity for realization of Q-switching and mode-locking of 1μm fiber lasers with the manufactured MoS₂ saturable absorber.In Q-switching operation,the laser output give maximum power of 243.4 mW,slope efficiency of 63.89%,laser pulse width of 1.53μs,and pulse energy of up to 5.9μJ.Thereafter,a fiber ring laser cavity is designed and optimized to carry out mode-locking operation of 1μm fiber lasers with 2D MoS₂ as the saturable absorber.The Mode-locked laser produces maximum output power of 2.6 mW,pulse width of³00 ps,and a mode-locking SNR greater than 50 dB.Thirdly,based on the heterostructure characteristics of MoS₂,linear laser cavity of thulium-doped fiber is constructed and optimized for realized mode-locking/Q-switching in the 2μm region.The Q-switched pulses have pulse width of 1.62μs,center wavelength of 1888.2 nm with bandwidth of 2.33 nm.In the mode-locking operation,the output laser pulses give a pulse width of⁷74 ps and laser wavelength of 1904.38 nm with 3dB bandwidth of 17.6 nm.Fourthly,based on the broadband response of MoS₂ and 1μm fiber laser system,we achieve digital-wavelength mode-locking through controlling the intracavity fiber and widely changing the polarization states of light with two polarization controller.After optimization of fiber length and adjusting the polarization controller（PC）,we achieve any combination of four output laser wavelengths（in the mode-locking state）at 1057nm,1063nm,1069nm,1075nm,etc.This is the first time to produce a mode-locking spectrum that can characterize 4 binary digits from 0 to 15 by using 2D materials.Finally,a 3μm pulsed fiber laser is designed based on the wide-spectrum features of MoS₂.With Er ZELBAN fiber as the gain medium,we achieve Q-switching operation at2754 nm.This is the first time to use 2D material MoS₂ as modulators realizing pulsing operation of fiber lasers in the 3μm band.Q-switched laser pulses give maximum output power of 140 mW,maximum pulse energy of²μJ,and pulse width of 806 ns.All these studies prove that 2D material MoS₂ can be efficiently employed to modulate laser light in the long wavelength regime.