| Antireflective coatings have been widely applied in various optical systems due to their capability of reducing light reflection and increasing light transmission. The application of antireflective coatings not only increases the light efficiency, but also improves the optical quality of the optical system. The transmittance is an important performance parameter for the antireflective coatings, which include peak transmittance and band width. Recently antireflective coatings with near 100.0% transmittance can be produced. However, the antireflection band of the coating should be broaden, which means we need broadband antireflective coatings. The transmittance depends on both refractive index and coating thickness. In this paper, broadband antireflective coatings have been designed and prepared. Through the characterization of coating microstructures and coating properties, we attempt to study the relationship between the coating structure and optical properties. Three kinds of antireflective coatings, including single-layer antireflective coatings, multilayer antireflective coating and graded index antireflective coating have been prepared and studied.The transmission curves of the single-layer antireflective coating have been designed with TFCacl optical film software. The effects of refractive index and coating thickness on the transmission curve have been studied. The refractive index of the coating is not the larger the better, nor the smaller the better. The refractive index of the coating is best when it is matched with the refractive index of the substrate. The optical coating with refractive index of 1.20 is matched well with the substrate with refractive index of 1.44. For such coatings, the peak transmittance can reach 100.0%, with a broad antireflection band. But when the refractive index derivates from 1.20, the peak transmittance decreases and the antireflection band becomes narrow. Thus, in order to achieve broadband antireflection with single-layer antireflective coatings, the refractive index of the coating should match well with the refractive index of the substrate. When the refractive index of the coating is matched well with the refractive index of the substrate, the coating thickness does not affect the peak transmittance, but do affect the position of peak transmittance and the width of the transmission band. The single-layer antireflective coatings have been prepared on fused silica substrates using sol-gel method. The effect of the coating thickness on the transmittance has been studied in the experiment. The experimental results agree well with the theoretical results. The effect of preparation parameters on the coating thickness, refractive index and transmission curves have also studied. The withdrawal rates in the preparation procedure have a significant effect on coating thickness and ignorable effect on refractive index. Thus, the withdrawal rates does not influence the peak transmittance but have an effect on the position of peak transmittance and the width of the transmission band. The microstructures of the sol-gel coatings have been characterized using a variety of techniques. The coating is composed of silica nanoparticles. The coatings have a large amount of voids, thus shows low refractive index. Through studying the relationship among the packing model of nanoparticles, porosity and refractive index, we speculate that the silica nanoparticles in the coating stack in the simple cubic packing model. The porous coating is readily to adsorb contaminant in the environment, resulting in the decrease in transmittance and poor environmental stability. In order to improve the environmental stability of the coatings, FAS modified coatings have been developed by the addition of FAS into the sol during the preparation of the sol. The results show that the FAS modified silica coating exhibited good oil-resistant property.We have studied two-layer antireflective coating and three-layer antireflective coating in the multilayer antireflective coating research. The transmission of the multilayer antireflective coating depends on the refractive index and coating thickness. Thus the transmission of the coatings can be adjusted by varying the coating thickness and refractive index. Theoretically, the two-layer antireflective coating consists of inner layer and outer layer, the inner layer with refractive index of 1.26 and thickness of 98.2 nmn, the outer layer with refractive index of 1.14 and thickness of 106.4 nm. The two-layer coating is wide antireflective, with transmittance of higher than 98.0% from 300 nm to 1100 nm. The coating shows peak transmittance of 99.8% at 351 nm and 800 nm. The three-layer antireflective coating consists of inner layer, middle layer and outer layer, the inner layer with refractive index of 1.30 and thickness of 106 nm, the middle layer with refractive index of 1.20 and thickness of 115 nm, the outer layer with refractive index of 1.14 and thickness of 106.4 nm. The three-layer coating shows broader antireflection band than the two-layer coating. The three-layer coating shows transmittance of higher than 99.5% from 351 nm to 1100 nm, with peak transmittance of higher than 99.8% at 351 nm,527 nm and 1053 nm. But the experimental transmission curves do not agree well with the theoretical transmission curves. This is because that the prepared coating differs from the designed coating due to the complex procedure during the preparation of three-layer coatings. This suggests that it is challenge for the sol-gel method to prepare multilayer optical coatings. The transmission curves of the prepared two-layer coating agree well with that of the designed two-layer coating. However, the transmission curves of the prepared three-layer coating are different from that of designed three-layer coating. The experimental transmittance of three-layer coating at 351 nm is 99.2%, which is lower than the designed value of 100.0%, also lower than 99.8% for the two-layer coating. This suggests that number of nanoparticles in thicker coating is larger, which cause significant light scattering, resulting in the decrease in transmission. The multilayer coating is similar to the single-layer in the porous microstructure. Thus the multilayer coatings also adsorb contaminant and show poor environmental stability. In order to improve the environmental stability of the two-layer coatings, we modify the coating using combined chemical gas treatment. The treated coatings shows improved environmental stability. But the transmittance of the modified coating decrease slightly in the environment, which suggests that the adsorption of the porous coating can be decreased, but cannot eliminated.We prepared graded index coating using graded index preparation technique. This technique is described as follows. The thermal sensitive polymer is firstly introduced into the sol-gel coating and then decomposes after heating, leaving microporosity in the coating. The microporosity is the active site of the etching reaction. The etchant is used to modify the porosity structure in the coating, making the porosity graded distribution in the thickness direction. The theoretical design shows that the graded index coatings have transmittance of higher than 99.5%from 300 nm to 1100 nm. But the prepared coatings just show transmittance of higher than 98.5% from 400 nm to 1100 nm, with peak transmittance of 99.7% at 400 nm and peak transmittance of 99.8% at 900 nm. There is some difference between the transmittance of prepared coating and that of the designed coating. This difference is mainly due to the disagreement of refractive index between prepared coating and the designed coating, and the light scattering of the porous coatings. The designed coating is ideal coating, with perfect graded index, no light adsorption and scattering. We have systematically study the effect of various etching parameters on the transmission. The transmission can be controlled by varying the coating thickness and etching time. The thermal sensitive polymer has a great effect on the transmission. The coating cannot achieve broadband antireflection without the addition of the thermal sensitive polymer in the coating. We have characterized the microstructure and optical properties of the coating at different preparation steps. The micropores formed through the decomposition of thermal sensitive polymer after heating, but the micropores are so small that they cannot cause significant change in refractive index and transmission. The significant change of coating microstructure occurs at the etching process. Due to the selective etching reaction in the thickness direction, the etched coatings show graded porosity along the thickness, resulting in the graded refractive index along the thickness. As results, the graded index coating is broadband antireflective.Through the design, preparation and characterization of various antireflective coatings, we find that the transmission of the coating mainly depends on the coating thickness and refractive index, which are controlled by the microstructures of the coating. Adjusting the coating microstructures is the key to controlling the optical properties. |
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