Study On Phase Transition Characteristics Of VO₂ Thin Films Based On Multi-band Pulsed Laser Irradiation

The high concentration of laser energy of infrared jamming system poses a great threat to the infrared precision guidance weapons,the high energy laser irradiation makes the infrared detector oversaturated and ineffective.At present,western military powers have been equipped with a large number of laser interference equipment,so it is urgent to carry out research on laser directional protection.Vanadium oxide has a reversible phase transition from semiconductor phase to metal phase near the phase transition point,and with the phase transformation process will have a special photoelectric phenomenon.Of all the vanadium oxides,VO₂ has the most widely application due to its 68?phase transition temperature which is closest to the room temperature,and it has been put into practical use in many fields.VO₂ can realize the phase transition through various excitation modes,such as thermal,electrochemical and photonic phase transformation.Moreover,after the phase transformation,VO₂ reduces the transmissibility of infrared light,consequently,it has military application value.This paper is in such a background,and in combination with the special phase change properties of VO₂,as a laser protection material,we study its protective performance against directional infrared laser.This paper first introduces the background of the subject,and VO₂ is introduced in detail,including the theory of phase transition,several methods of inducing phase transition,typical applications and the research status at home and abroad.The preparation methods of magnetron sputtering and molecular beam epitaxy are introduced in detail from the aspects of principle,equipment and specific operation process.At the same time,XRD and AFM methods used in this paper are introduced.Multiple VO₂ nano-films with different thickness were prepared by using oxygen source molecular beam epitaxy on Al₂O₃ substrate,and the films were characterized by XRD,VU-Vis-IR spectrophotometer and AFM.The characterization results show that thin film XRD figure here are two obvious diffraction peaks appeared at 2?=40°and42°,which are VO₂ diffraction peaks?020?and Al₂O₃ diffraction peaks?0006?,respectively,this indicates that the VO₂ films prepared have relatively pure crystalline phase structure and preferred growth orientation;the results of AFM characterization show that the surface of the film is neat and compact arrangement of nanoclusters,and the root mean square value was only 0.38nm;ultraviolet-visible-infrared spectrophotometer in two temperature 25?and 90?condition for spectral scanning VO₂ thin film,the results showed that there is little difference in transmittances in the near ultraviolet and visible light at two temperatures,transmittance of thin film under the condition of 90?have obvious attenuation than that of 25?from 750 nm to 2400 nm range,it indicates that the prepared film has phase change characteristics.The film phase transition is assisted by external heat source,and finds the phase transition temperature of 30nm and 40nm are 45?and 49?respectively,this is lower than the general report phase transition temperature of 68?.The physical model of pulsed laser irradiated thin film is established,based on the COMSOL heat conduction module,the effects of the power density of the incident laser,the substrate thickness of the film and the initial temperature of the film on the time of the film reaching the phase transition point were calculated.The results show that increasing laser power density within a certain range can effectively shorten the time of film phase transition,but the two are not linear,but the phase change time and the power density are similar to exponential function attenuation;and the phase transition time and the initial temperature of the film show a linear attenuation relation;when the incident laser power density is 200 W/mm²,the time for the center temperature of the film to reach the phase transition point is in microseconds;when the incident laser power density increases to 5000 W/mm²,the time when the irradiating center of the film with base thickness of 0.15mm,0.3mm and 0.5mm reached the phase transition temperature was 157 ns,250 ns and 455 ns,respectively.Based on the pump probe method,the phase transformation characteristics of different thickness films were investigated.The 10.6?m continuous CO₂ laser with adjustable power between 8W and 30W was used as pump light,the near-infrared1064nm and medium-infrared 3459nm nanosecond pulsed laser were used as probe light,three groups of films with thickness of 20nm,40nm and 60nm were irradiated,it mainly explores the relationship between the changes of the transmittance and reflectivity of the probe and the film thickness,laser wavelength and irradiation time of the VO₂ film under laser irradiation.We have obtained followingexperimental conclusions:?1?The nano VO₂ films prepared by molecular beam epitaxy are smooth and compact,and the incident laser is mirror reflection before and after the phase transition;?2?The phase change characteristics of the film for the 1064 nm probe were not significant for the 3459 nm probe;?3?The increase of film thickness decreases the transmittance before the phase transition,but the decrease is more obvious after the phase transition,therefore,the increase of thickness within a certain range can improve its phase-change modulation characteristics;?4?The film remained almost opaque to the10.6?m continuous CO₂ laser before and after the phase transition.