| Nonlinear optical devices such as optical switches,photodiodes and optical modulators are important components of optical signal processing technology and play an important role in high-speed optical communication,optical sensing,biomedicine,and national defence.Along with the demand for ultrafast,broadband,and easily tunable nonlinear optoelectronic devices,the exploration of nonlinear optical materials with high stability,ultrafast response,broadband operation,and easily tunable physical performance has become a popular research topic in optoelectronics and materials science.Vanadium dioxide（VO2）,a strongly correlated oxide,which can undergo reversible phase transition induced by external conditions（temperature,light,electric field,mechanical stress,electrochemistry,etc.）.Vanadium dioxide has a nonlinear optical response with increasing incident light intensity and shows great potential for applications such as light limiting and light modulators.However,most of the current nonlinear research on vanadium dioxide has focused on the near-infrared band,with less research in the visible and mid-infrared bands,and its photoelectric properties modulation and applications need to be further studied and expanded.In this thesis,the broadband,tunable nonlinear optical device applications are demanded.The broadband third-order nonlinear optical properties of low-dimensional vanadium dioxide are studied,its broadband nonlinear optical parameters are obtained,and its broadband phase modulation,ultrafast pulsed laser generation and thermal-optical switching are experimentally implemented to verify its application potential in broadband nonlinear optical devices.The main work of the thesis includes the following:（1）The nonlinear optical properties of low dimensional vanadium dioxide at visible wavelengths were studied based on the spatial self-phase modulation effect,and the nonlinear optical parameters of vanadium dioxide were obtained and experimentally implemented in all-optical devices such as optical logic gates and optical switches.The interaction between light and vanadium dioxide dispersions in the visible wavelength band was studied based on spatial self-phase modulation（SSPM）and spatial cross-phase modulation（SXPM）effects,and the nonlinear refractive index（n2）and third-order nonlinear polarizability(χ（3）)of vanadium dioxide dispersions were obtained in the visible wavelength band.The n2 andχ（3）of the vanadium dioxide dispersions were measured at 671 nm as 3.06×10-6 cm2/W and 1.68×10-4 esu,respectively,and at 532 nm as 5.17×10-6 cm2/W and 2.68×10-4 esu,respectively.The response time of vanadium dioxide dispersions to light was analyzed and the results show that the formation of SSPM and SXPM diffraction rings mainly originates from electronically coherent third-order nonlinear optical processes.Based on the excellent nonlinear optical properties of vanadium dioxide dispersions,optical logic or gates,steady-state and transient optical switches,and multichannel information transmission devices in the visible wavelength band have been experimentally realized.The results show that the vanadium dioxide dispersions exhibit excellent nonlinear optical response properties,laying the foundation for vanadium dioxide based optical device applications.（2）The broadband nonlinear optical properties of low dimensional vanadium dioxide core-shell heterostructures were studied,and saturable absorbers based on VO2/V2O5 core-shell heterostructures were prepared to achieve broadband pulsed laser outputs at 1μm,1.5μm,and 2.8μm.The VO2/V2O5 core-shell heterostructure with type Ⅱ staggered band arrangement was found to exhibit a broadband nonlinear optical response in the near-to mid-infrared spectral range,and modulation depths and saturation intensities of the heterostructure were experimentally verified to be 27%and42 GW/cm2 at 1064 nm,23%and 78 GW/cm2 at 1550 nm,and 16.5%and 63.9 GW/cm2 at 2800 nm,respectively.Stable ytterbium-doped and erbium-doped mode-locked fiber laser outputs with pulse widths as short as 310 ps and 633 fs,respectively,were achieved utilizing the VO2/V2O5 nonlinear light modulator.In addition,a stable passively Q-switched erbium-doped fluoride fiber laser output with a minimum pulse width of 680 ns was achieved.The experimental results show that the VO2/V2O5 core-shell heterostructures are nonlinear optical materials with excellent performance,which can support the development of broadband optoelectronic devices.（3）The temperature-dependent nonlinear optical properties of low-dimensional vanadium dioxide were studied,and it was found that the nonlinear absorption coefficient of the vanadium dioxide film after the thermally induced phase transition was about two orders of magnitude larger than that before the phase transition,and the modulation of near-infrared pulsed laser output by temperature changes was achieved.The nonlinear optical properties of thermally induced phase transition of vanadium dioxide thin films were experimentally measured,and the nonlinear absorption coefficient of vanadium dioxide thin films was found to be～10-7 m/W before the phase transition and～10-5 m/W after the thermally induced phase transition.The effect of the phase transition of vanadium dioxide on the output pulsed laser was studied,and it was found that the laser pulses can be turned on and off by temperature control.The effect of the temperature change of the vanadium dioxide film on the near-infrared Q-switched pulses was studied,and a wide range of pulse widths from 346 ns to 957 ns was achieved by the temperature change.The above results enrich the basic research of vanadium dioxide materials and broaden the nonlinear optical applications of vanadium dioxide materials. |
