Research Of VO₂ Thin Films Used In Laser Directional Protection

With the rapid development of laser weapons,infrared guidance weapons are facing a huge threat.So the study of directional laser protection technology must be put on the agenda.Among many laser protection materials,the advantage of VO₂ is more outstanding.It is a thermotropic phase change material and it has a high reaction speed and better phase change properties.Therefore,the research topic of this paper is the VO₂ film which used in directional laser protection.We researched the phase transition mechanism,and the preparation method and parameters of VO₂ films.The focus of our research is phase transition and laser damage characteristics of VO₂ films under laser irradiation.At first,we reviewed the research status,the preparation and phase change methods,and the phase transition mechanism of VO₂ films.Then in order to find out its optimum magnetron sputtering parameters on Al₂O₃ substrate,we designed 16orthogonal experiments.Eventually,we found the optimum parameters are 220W sputtering power,0.8Pa operating pressure,300?substrate temperature,and 0.5/25argon oxygen ratio.Through some test we found that the samples which were prepared under these conditions have good electrical and optical phase transition properties.The change rate of the block resistance is close to three orders of magnitude,and the variation range of the infrared transmittance at 3.4?m wavelength is more than 50%.In order to research the phase transition properties of VO₂ films under laser irradiation,we use ANSYS to simulate its horizontal and vertical temperature field distribution.It is found that the laser facula size,power density and other laser parameters may have a great impact on the horizontal temperature field but the vertical distribution is basically not be effected.What's more,the horizontal phase transition region cannot increase infinitely.It will eventually stabilize at about 31%of the facula radius.Next,we designed two sets of experiments to test its phase change characteristics on practice.Firstly,the pump-probe method was used to research the phase change response and recovery properties of VO₂ films under near-infrared nanosecond laser.It is shown that the minimum transition time of VO₂ is about 14ns.Its phase recovery time increases exponentially with the laser energy density,and it is related to the parameters of substrate material and nanosecond laser parameters.To research the cumulative effects of temperature rise of VO₂ thin films under the irradiation of mid-infrared laser and to overcome the low power difficulty,we designed a kind of external heat source method.It was shown that the laser power density,which can reach phase transition temperature within 100s,decreases gradually with the increase of external heat source temperature.In order to research the laser damage characteristics of VO₂ thin films,we tested its1-on-1 laser damage threshold according to the test standard.The 1-on-1zero-probability damage threshold of VO₂ thin films is 0.13J/cm² under nanosecond laser and 10.2mJ/cm² under femtosecond laser.By analyzing the damage morphology and its evolution,we found that the damage area increases with the the energy density of nanosecond laser,but not with the number of pulses.As to the femtosecond laser,the damage mechanism of the film is affected by different laser energy density,and then its ablation morphology is affected.On this basis,we explored how to improve the damage threshold furthermore.It is found that the laser damage threshold of VO₂ thin films can be increased about 31%by laser pretreatment with 60%-70%of the threshold damage laser energy.Finally,we discussed the feasibility of VO₂ thin films used on laser protection in many aspects.So it could make some experimental and theoretical contributions to the practical application of VO₂ thin films.