| Ultrafast lasers have extremely narrow pulse widths,ultra-high peak powers,and ultrawide spectral ranges,and have broad application prospects in advanced scientific research,precision laser manufacturing,laser medicine,military defense,and other fields.They are an important research direction in the field of laser technology and have long-term support from national major research plans and industrial policies,representing a national strategic direction and technological frontier.Passive mode-locking technology is currently the most effective and commonly used technique for achieving ultrafast lasers.The saturable absorber(SA)is key optical components in passive mode-locked ultrafast lasers,which performance directly determine the power,pulse width,and stability of mode-locked ultrafast lasers.Traditional SAs,such as Semiconductor Saturable Absorber Mirrors(SESAMs),have narrow operating bandwidths,complex fabrication processes,high integration difficulties,and high costs.Therefore,it is urgent to develop novel SA materials and devices,explore the mechanism of saturable absorption and characteristic parameter modulation,and fabricate high-reliability solid-state mode-locking key materials and devices to provide key components for the development of high-stability solid-state mode-locked laser technology.In 2009,graphene was successfully applied as SA in fiber mode-locked ultrafast lasers,triggering a research boom in exploring the optical modulation properties of novel twodimensional materials.The unique properties of two-dimensional(2D)materials,such as force,heat,light,electricity,and easy controllability,have made them a hot research topic in the fields of nonlinear optics and ultrafast laser technology.Currently,two-dimensional materials widely used in all-solid-state mode-locked lasers include graphene,transition metal sulfides,and black phosphorus,each of which still has drawbacks such as weak optical absorption,high saturation intensity,and easy oxidation.Moreover,once the device is fabricated,its modulation parameters(modulation depth,bandwidth,response rate,saturation intensity,etc.)cannot be adjusted,which greatly limits the improvement of mode-locked ultrafast laser performance.This paper aims to address the demands for high-performance solid-state mode-locked femtosecond lasers by focusing on the modulation of two-dimensional materials and the enhancement of solid-state mode-locked ultrafast laser performance.A systematic study was conducted on the selection of novel two-dimensional materials,preparation of SA,performance modulation,and solid-state mode-locked femtosecond ultrafast laser technology.Various methods such as defect engineering,surface plasmon polaritons,external electric fields,and heterojunction interfaces were studied to investigate the photo-generated carrier dynamics and nonlinear absorption parameters of two-dimensional materials,and explored the effects of external modulation methods on the mode-locked laser parameters(pulse width,average power,threshold power).The development of two-dimensional material solid-state mode-locked ultrafast laser technology was achieved,enabling higher-performance femtosecond laser output.This research provides important theoretical and data support for the design and development of ultrafast photonics devices based on two-dimensional materials and the development of solid-state mode-locked ultrafast laser technology.The main research contents of this paper are summarized as follows:1.Two-dimensional bismuth oxy-selenide(Bi2O2Se)exhibits superior features such as high electron mobility,moderate bandgap(～0.8 eV),strong spin-orbit coupling,easy modulation,and high stability,which make it a promising candidate for logic,thermoelectric,sensing,and optoelectronic devices.This paper conducted a systematic and in-depth study on defect modulation of the properties of 2D Bi2O2Se SA and the enhancement of solid-state modelocked femtosecond laser performance.Large-area and high-quality two-dimensional Bi2O2Se was prepared using chemical vapor deposition,and a Yb:KYW solid-state mode-locked laser based on two-dimensional Bi2O2Se was designed,achieving mode-locked laser output with a pulse width of～587 fs and an average power of～421 mW.To further improve the mode-locked laser performance,argon plasma irradiation was used to precisely control the defect density in two-dimensional Bi2O2Se,and pump-probe and Z-scan techniques were employed to investigate the defect-mediated carrier dynamics and nonlinear absorption characteristics in detail.A physically consistent theoretical model was established,and the mechanism of the SA saturation absorption parameters modulation via defect engineering in two-dimensional Bi2O2Se was analyzed in detail,achieving an enhancement of the solid-state mode-locked laser power(665 mW)and pulse width compression(～266 fs).2.Tungsten oxide with oxygen vacancies(WO3-x)as a heavily doped semiconductor with metallic properties exhibits strong and tunable local surface plasmon resonance(LSPR),which can enhance the light absorption properties and ultrafast optoelectronic dynamics of materials.This paper conducted a systematic study on the preparation of WO3-x/Bi2O2Se LSPR structures and the modulation of nonlinear saturable absorption properties,focusing on the issues of large non-saturable absorption loss and high mode-locking threshold of defect-modulated 2D Bi2O2Se SA.A WO3-x/Bi2O2Se surface plasmon structure was prepared using thermal evaporation,and the electromagnetic field distribution of the WO3-x/Bi2O2Se surface plasmon structure was theoretically simulated.The periodically arranged WO3-x nanoscale structure was found to have a local field enhancement effect,which effectively regulated the carrier recombination channels of photo-generated carriers in two-dimensional Bi2O2Se and accelerated their carrier recombination rate.The LSPR effect induced by the WO3-x nanostructure significantly enhanced the nonlinear absorption coefficient of two-dimensional Bi2O2Se in the 1050 nm wavelength range,from-(507±4)cm MW-1 to-(1587±14)cm MW-1,and the saturation intensity(from 3.64±0.20 MW cm-2 to 13.25±0.59 MW cm-2)and modulation depth(10.5%-16.6%)of two-dimensional Bi2O2Se were also greatly optimized.3.Two-dimensional SA is "passive" optical modulators based on saturable absorption effect.This paper focused on the active optical modulation of 2D SA and conducted a systematic study on the 2D PtTe2 electro-optic modulator,achieving precise control of the nonlinear optical properties through an external electric field.A back-gate electro-optic modulator had been fabricated based on 2D PtTe2 nanosheets.The study systematically investigated the mechanism of external electric field modulation of the photogenerated carrier recombination channel in PtTe2 nanosheets.The results demonstrated that the external electric field could modulate the photogenerated carrier recombination rate and flexibly control the saturation absorption parameters of PtTe2 nanosheets within the selected spectral range.Specifically,for the optical communication band,the linear transmittance could be tuned from 87.5%to 94.5%,while the modulation depth could vary from 8.8%to 2.4%.This research study provides a coherent and well-formed scientific description of the fabrication and characterization of the PtTe2 nanosheets-based electro-optic modulator,and contributes to the understanding of the external electric field control mechanism of photogenerated carrier recombination channels in two-dimensional materials.4.Stacking two different 2D materials to form a heterostructure can fully utilize the optical and electrical advantages of each material,potentially achieving significant breakthroughs in artificial structural material design,physical properties,and device performance research.Detailed exploration of the ultrafast carrier dynamics and nonlinear absorption properties of the heterojunction structure G/Nb2SiTe4 with graphene as an electron transfer channel and Nb2SiTe4 of different thicknesses,had demonstrated that graphene effectively shortened the lifetime of photogenerated carriers in Nb2SiTe4.Additionally,the heterojunction structure of thinner Nb2SiTe4(2 nm)and graphene exhibited a high nonlinear absorption coefficient(4.6×103 cm MW-1)and moderate saturation absorption parameters due to its high surface state density and strong electron-phonon coupling effect.A solid-state mode-locked laser resonator based on the saturable absorption of G/Nb2SiTe4 was designed,resulting in an output of a fully solid-state mode-locked pulse laser with an average power of 498 mW and pulse width of～415 fs. |
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