Nonlinear Optical Study Of Low-Dimensional Bismuth-Based Compounds And Its Application In Pulsed Laser

Infrared pulsed lasers in the wavelength range of 1.34 μm,1.55 μm,2 μm,and 2.3 μm have wide applications in fields such as laser communication,laser radar,biomedicine,and enviro nmental monitoring.Passive Q-switching or passive mode-locking pulsed lasers can be achieved by utilizing saturable absorbers,which is the most compact,efficient,stable,reliable,and cost-effective way.Nanomaterials,due to their small size and high specific surface area,exhibit unusual optoelectronic properties and can serve as saturable absorbers with high saturation absorption capacity,faster response time,and a wider range of working spectral range,which can further optimize laser performance.This paper investigated the nonlinear optical properties of bismuth-based low-dimensional compounds,which have high nonlinear optical performance and low linear absorption coefficients,making them suitable as excellent saturable absorbers for passive Q-switching and passive mode-locking pulse laser technology with enormous application potential.The study focused on the preparation,characterization,modification,theoretical research on the photoelectric properties and interaction between light and matter to enhance the nonlinear optical properties,the design and production of saturable absorber devices,the investigation of stable Q-switching and mode-locking pulse laser output characteristics,and the exploration of the evolution dynamics of mode-locked solitons,using nanocrystals of oxygen vacancydeficient monoclinic BiVO₄ and oxygen-deficient tetragonal-phase BiOCl as the main research objects.This article extends the application of low-dimensional nanomaterials in the field of pulsed lasers,provides new ideas for the design of novel pulsed lasers,and further enriches the theory and phenomena of soliton evolution in lasers,providing experimental and theoretical support for optimizing the stability and reliability of pulsed lasers.The primary focus of the research can be outlined as follows:1.First-principles calculation methods were used to study the band structures of m-BiVO₄ and BiOCl nanosheets.The direct band gap of m-BiVO₄ is about 2.45 eV,while the indirect band gap of BiOCl nanosheets is about 2.77 eV.The band gap of m-BiVO₄ with oxygen vacancies was also calculated,and the results show that defect energy levels are introduced into the band gap.2.The sol-gel method and solvothermal methods were used to prepare m-BiVO₄ nanoparticles,porous m-BiVO₄ nanoparticles,and BiOCl nanosheets.A range of characterization techniques were employed to examine and analyze their structures,surface morphology,and element distribution.The results showed that by adjusting the preparation process,oxygen vacancy defects can be effectively introduced,which provides a foundation for the study of their broadband nonlinear optical properties.3.The quartz substrate nanofilm of m-Bi VO₄ nanoparticles,porous m-BiVO₄,and BiOCl nanosheets were prepared by spin-coating method.By employing Z-scan and I-scan techniques,the broadband nonlinear optical（NLO）properties of m-BiVO were thoroughly investigated and analyzed.The study indicated that m-BiVO₄,when incorporating oxygen defects,displayed notable nonlinear optical（NLO）properties.Specifically,the modulation depth ΔT of oxygendeficient m-BiVO₄ nanoparticles was as high as 20.1%at 1.34 μm,and the nonlinear refractive index coefficient n2 of oxygen-deficient m-BiVO₄ nanoparticles was as high as 1.14×10^-4 cm² MW^-1 at 2 μm.Moreover,the absolute value of the effective nonlinear absorption coefficientβeff of oxygen-deficient porous m-BiVO₄ was as high as 7.7 cm MW^-1 at 2.3 μm.The NLO properties of BiOCl at 1.34 μm were also studied using Z-scan and I-scan techniques,and the absolute value of the effective nonlinear absorption coefficient βeff was approximately 0.97 cm MW^-1.These results indicate that oxygen-vacancy-rich m-BiVO₄ and BiOCl have potential as saturable absorber devices.4.Stable broadband passively Q-switched solid-state pulse laser outputs based on mBiVO₄ and BiOCl saturable absorbers were achieved.At 1.34 μm,the shortest pulse width was 355 ns,the maximum peak power was 1.42 W,and the upper limit of energy in a single pulse was 0.67 μJ;at 2 μm in the mid-infrared range,the maximum peak power is 3.7 W,and the highest recorded energy value for a single pulse is 2.3 μJ;at 2.3 μm in the mid-infrared range,with a pulse width of 349 ns,a repetition rate of 195 kHz,and a peak power of 3.4 W,the highest energy level attained by a single pulse is 1.18 μJ.The results demo nstrate that bismuthbased nanomaterials saturable absorbers can achieve stable narrow pulse width Q-switched pulsed laser output.5.Theoretical simulations were performed to analyze the mode field distribution of tapered microfibers,confirming the existence of a strong eVanescent field.Tapered microfibers were fabricated and coated with bismuth-based nanomaterials that exhibit saturable absorption properties,and their nonlinear optical absorption characteristics were studied to obtain their modulation depth.By using tapered microfibers coated with m-BiVO₄ nanoparticles and BiOCl nanosheets as saturable absorbers,stable Q-switched and mode-locked pulse laser outputs were achieved in Er-doped fiber lasers.For the Q-switched pulse laser,the minimum pulse width was 2.17 μs,and the maximum repetition rate was 57 kHz.For the mode-locked pulse laser,the central wavelength was 1563 nm,the pulse width was 994 fs,and the signal-to-noise ratio was 36 dB.The experimental results showed that bismuth-based nanomaterials with oxygen vacancy defects can achieve long-term stable and efficient operation in generating ultrafast pulses,making them important components for ultrafast photonics applications.