Objective Metal films have lots of excellent characteristics such as higher mechanical strength and damage threshold, better toughness, and thermal conductivity. They are widely used in modern optical industry. Moreover, copper films are used as infrared stealth material because the emissivity of copper is very low in mid-infrared band. This property can reduce the detection efficiency of passive mid infrared detector. The characteristic has extensive applications in military and many groups have performed research about it. However, they only focused on production of high quality copper film. However, the methods to destroy this film are ignored, which is presented in this paper. Laser-induced periodic surface structure (LIPSS) is special surface grating structures which is induced by polarized pulses that appear on nearly all kinds of solid materials. The period and direction of the grating only depend on the wavelength and polarized direction of laser. The structures can change the surface properties of materials such as super hydrophilic/hydrophobic, suppress the growth of the miscellaneous bacteria, as well as high emissivity. LIPSS has drawn attentions of many researchers. Lots of new materials with special characters have been produced by inducing LIPSS on the surface of the materials. However, the study of changing emission characteristics within mid-infrared band of metal films is lacking. Some researches are performed about the effect of LIPSS on infrared emission characteristics of copper films in this paper.
Methods First, the production of LIPSS on copper films is investigated using the surface interference between plasmons and incident laser model (Sipe model). The Sipe model involves two processing: softening and migration of materials. A two-temperature model is used to illustrate the copper-softening process. The theory of the two-temperature absorption of metals can be applied to all types of incident lasers because the processing is performed by the distribution of a large number of free electrons on the material surface, which is different from other types of materials. Therefore, a linear pulse at a center wavelength of 1064 nm with a pulse duration of 100 fs and energy density of 5 J/m 2 is used in the simulation experiment. Then, LIPSS is induced on a copper film that covered a quartz substrate using nanosecond linear pulses at a center wavelength of 1064 nm. Additionally, a simulation model is established according to the surface topography of the sample induced in the experiment. The emissivity is within the 1--5 μm band.
Results and Discussions The results of the two-temperature model experiment show that the temperature of the copper free electrons reaches 7073 K after the pulse introduction is finished, which is very much higher than the melting point of copper (1375.8 K). The high temperature softens the target, which means that materials can be rearranged under a periodic space electromagnetic field according to the Sipe model. Then, the temperature of the electronic system quickly decreases, whereas that of the lattice system gradually increases. At 12 ps, the temperatures of the two systems are balanced at 1017 K, which is less than the melting point (Fig. 2), indicating that classical heat damage does not occur. LIPSS is induced by linearly polarized nanosecond pulses (Fig. 4). The direction of the gratings is perpendicular to the polarization direction of the laser. The results show that lowsurface-frequency LIPSS (LSFL) is induced on the film. The electric field distribution of the reflected and transmitted fields of the simulation model show that the laser is modulated by the gratings (Fig. 6). Therefore, the emissivity of the model can reach 0.365 and 0.119 when the laser wavelength is 3 μm and 5 μm, respectively (Fig. 7), which is very much higher than that of smooth copper (Fig. 1). The results show that LIPSS can improve the emissivity of copper films.
Conclusions This paper has presented three main studies. The first one is a brief explanation of how LIPSS can be induced by pulses lower than the damage threshold of materials. The Sipe model is used to describe the process of inducing LIPSS on metals. The material-softening step due to the pulses, whose influence is lower than the damage threshold, is achieved using a two-temperature simulation model. The second study induces LIPSS on a copper film over a quartz basement. The LIPSS is of LSFL type. The results agree with those in the previous studies. The third one proves that LIPSS can improve the emissivity of copper films through a simulation experiment. The results show that a much higher emissivity of films is achieved with LIPSS because the space electromagnetic field is periodically modulated by the micro gratings. The effect on the improvement is significant. These results show that inducing LIPSS on the surface of copper films is a feasible technique to destroy the stealth characteristics of a material in the mid-infrared band.
