冯云松 FENG Yun-song’s scientific contributions

To investigate the phase transformation characteristics of vanadium dioxide thin film irradiated by infrared laser, the experiment was carried out based on pump probe method. First, three groups of VO2 monocrystalline epitaxial thin films with thickness of 20 nm, 40 nm and 60 nm were prepared by oxy-molecular beam epitaxy, moreover, the CO2 continuous laser with 10.6 μm was used as the pump beam, nanosecond pulse laser with 1 064 nm and 3 459 nm wavelength were used as probe light respectively, and the three groups of films were irradiated respectively. It was found that the mean values of transmittance reduction of 1 064 nm probe laser after phase transformation of the three groups of films were 5.26%, 6.2% and 8.92%, respectively, and the reflectivity reduction was 3.09%, 6.56% and 4.93%, respectively; for the 3 459 nm probe laser, the mean values of transmittance reduction were 28.4%, 47.78%, 55.13%, and reflectance increase were 6.65%, 17.87% and 7.49% respectively. The results show that the incident laser before and after the phase transformation of VO2 thin films prepared by molecular beam epitaxy were both mirror reflected; the phase transition properties of 1 064 nm probe were not as significant as that of 3 459 nm probe; the increase of film thickness will reduce the transmittance before the phase change, while the decrease after the phase change is more obvious; before and after the phase transition, the film keeps almost opaque to CO2 laser. The investigation will provide a reference for the application of film.

In order to explore the factors that affect the phase transition response time of VO2 thin films irradiated by pulse laser, we simulated the irradiation process based on COMSOL. The physical model of irradiation VO2 thin films was established first, and the boundary conditions of the model were set, the infrared pulse laser irradiated three groups of VO2 thin films prepared by molecular beam epitaxy, and the average absorptivity of the film was obtained indirectly, the absorptivity experimental data was brought into the calculation model. The influence of laser power density, substrate thickness and initial temperature of VO2 thin films on the simulation results were considered in the simulation. Experimental results show that: increasing the laser power density and initial temperature, decreasing the base thickness can shorten the phase transition time of the film irradiation center. What's more, the phase transition time and the laser power density show exponential decay trend. The 5 000 W/mm² laser irradiated three groups of VO2 thin films, respectively, their base thickness are 0.15 mm, 0.3 mm, 0.5 mm, the time of phase change is 157 ns, 250 ns and 455 ns respectively. The phase transition time decreases linearly with the initial temperature of the film. When the incident laser power level is not specified, we can apply a bias temperature which is close to the phase transition point on the VO2 film, and control the film substrate thickness to shorten the phase transition time. This study has certain guiding significance for the study of phase change response time in the VO2 thin films defensing laser jamming.

As the color ratio of radiation intensity of a target at different wavelengths will change along with the change of transmission distance, the color ratio of radiation intensity contains distance information. According the principle mentioned above, this paper proposed a scheme based on colorimetric method to range the target using its own infrared radiation. The relationship between the color ratio of radiation intensity for the target and its temperature, radiation transmission distance, atmosphere attenuation coefficient was deduced. By measuring the color ratio of radiation intensity at different wavelengths, the target was ranged when the atmosphere attenuation was certain and the infrared ranging equations of three-color ratio were deduced. According to a calculation model of radiation atmosphere transmission attenuation, three wavelengths of 8.5, 9.0, 9.5 μm were selected as color-ratio wavelengths, a 300 K blackbody 2 km away was used as a ranging target, and a color-ratio ranging figure was drawn. Analysis shows that the temperature and the distance of the target gotten from the figure are coincident with those of the target. Furthermore, it needs only once measurement to get the distance and temperature of the target by using this method.