Photoinduced Phase Transition Characteristics And Mechanism Of Vanadium Dioxide Films

As people step forward to the exploration of the unknown world,new demands and problems continue to emerge,and in order to support these needs and solve them,the development of various new functional materials is becoming increasingly urgent.Among them,vanadium dioxide（VO₂）,with a variety of phase structures and capable of producing semiconductor-metal phase transition under different external stimuli,especially its film materials have been attracting the attention of many researchers.The ultra-fast phase transition characteristics of VO₂ materials and their huge difference in optical and electrical properties before and after phase transitions enable them to have broad application prospects in many fields such as civil and military.However,the phase change mechanism of VO₂ has not yet been clarified and many problems caused by the imperfect performance of the material itself in practical applications need to be solved urgently to be solved.How to use more efficient preparation and research methods to explore the phase transition mechanism of VO₂ and promote its application in many fields is the main focus of this paper.In this paper,the laser induced phase transition characteristics of VO₂ films with different crystal phases are explored,and their applications in the field of laser protection are explored.The main contents include the following aspects:The first is the exploration of the preparation method.VO₂ films with different crystal phases and their multilayer films were grown on Al₂O₃ sapphire and SrTiO₃barium titanate（STO）substrates by magnetron sputtering and pulsed laser deposition,including M-phase and B-phase VO₂ singles and M-phase VO₂ based multilayer film,and we study the high-throughput preparation of thin film materials based on magnetron sputtering.The film thickness and doping concentration were used as variables to obtain film samoles with continuous gradient and interval gradient changes to improve material research efficiency.Then the laser-induced phase transition characteristics of VO₂（M）films were studied by time-resolved femtosecond pump laser test system,and the similarities and differences with thermochromic properties in phase transition mechanism were investigated.It is found that the two phase transitions can achieve the same phase transition effect.In terms of transmittance,differences before and after phase change of the two phase transition is the same;the laser induced phase transition time is within1 ps,which has the characteristics of ultrafast phase transition.The phase change time is unaffected by the doping and film thickness variations.However,it is found through calculation that the thermal effect of induced laser is far lower than the temperature change required for thermo-induced phase transition.At the same time,by comparing the phase transition properties of films with different thicknesses and doping concentrations,the variation of the two mechanisms is not exhaustive the same,it is concluded that in the VO₂ phase transition process,the stimulation of heat and laser has different effects on its structural phase transition and phase transition.The two phase transition induction mechanisms are very different in principle.There are different thresholds for the two changes.In order to expand the application of VO₂ film,the phase transition characteristics in low temperature environment were explored,and some basic physical properties of VO₂.VO₂（B）films were epitaxially grown on STO（100）substrates by PLD method,and their optical properties were investigated,including room temperature optical constants,high and low temperature transmittance changes and optical thermal hysteresis loops.The phase transition of VO₂（B）film and VO₂（M）film was induced by femtosecond pulsed laser at 80K low temperature.It was found that the temperature had a certain influence on the phase transition threshold,but it was much lower than the heat demand caused by temperature change,thus further confirming the difference.Formal induction causes a different mechanism of phase transition.Finally,using the limitation and resistance change characteristics of VO₂ for strong laser,as a phase change protection material and photoelectric switch,an intelligent laser protection structure capable of corresponding protection measures for different intensity lasers is proposed,which is expected to achieve all the strengths,according to the preliminary design of the VO₂-based multilayer film laser protection structure,the film layer was prepared by magnetron sputtering method,and the obtained protective film layer was tested for transmittance change,and the obtained film layer was changed in phase.The transmittance of the pre-specific band is over 80%,the transmittance after phase change is less than 1%,and the switching ratio is more than 100 times,which can effectively protect the blind laser of a specific band.