Porous surfaces on glasses have been proved to be effective in suppressing light reflection due to the continuous variation in the refractive index with thickness. The porous structures were fabricated on BK7 glass by neutral-solution leaching process, and broadband transmittance was measured by a spectrometer. An optical model was applied to determine gradient refractive index profiles of porous glasses using a genetic algorithm. Scanning electron microscopy (SEM) analysis of the nanostructure variants will be shown, along with spectral transmittance that is matched to theoretical models. This model has potential applications in tracking optical properties according to the depth of nanostructures measured by SEM, or obtaining gradient refractive index profiles of porous glasses by the measured transmittance. Therefore, it is useful to optimize experimental condition for special optical properties of porous glass
Since the application of conventional antireflective (AR) coatings is limited by the availability of proper materials, the gradient-index antireflective (GIAR) nanostructures have attracted an increasing interest, and offer an alternative to thin-film coatings for high power laser systems [1]. These advanced diffractive optical elements are investigated from the view points of manufacture [2], [3], [4], [5], [6] and [7] and design not only in the performance of antireflection for wide range of spectrum and wide incident angle range [2] but also in high laser damage threshold [8], [9], [10] and [11]. GIAR nanostructures were first found in the cornea of night-flying moths, and so was called the “moth-eye” effect [12]. The GIAR surfaces have the gradient refractive index from the incident medium to the substrate, shown schematically in Fig. 1[13]. The different distribution functions have been introduced to describe the graded-index profile, including linear, parabolic, cubic, Gaussian, quintic, exponential, exponential-sine, and Klopfenstein [14], [15], [16], [17], [18] and [19]. Porous surface is one form of GIAR structures by introducing porosity to glass or other substrate surface. Among various preparation of GIAR nanostructures, neutral-solution leaching process [6] is the low-cost and non-toxic method. Thus, this technique is applied in the preparation of GIAR porous glass. However, to the best of our knowledge, few people have considered combining the graded index (GRIN) and textured surface structures [20], and gradient refractive index profiles of porous glasses have not been studied before.In this paper, porous glasses were made by simply immersing borosilicate glasses in a hot neutral-solution. Different solution concentration and leaching time caused different surface porosity and optical property. Then, a theoretical model is used to study the refractive index profile of GIAR nanostructure by simulated as a sequence of sub-layers, which is based on the effective medium theory (EMT). The effective refractive index and thickness of each sub-layer are calculated using a genetic algorithm, and then the gradient refractive index profiles of porous glasses are determined. This approach has potential applications in tracking optical properties according to the depth of nanostructures measured by SEM, or obtaining its depth information by the transmittance measured, and this property may be useful to optimize experiment condition for special optical properties of porous glass.
By fabricating GIAR structures on borosilicate glasses using neutral-solution leaching process, we have found that porous glasses show broadband antireflection property and the peak transmittance was greater than 99.7%. The peak transmittance wavelength depends on the graded porosity and leaching depth, which can be controlled by leaching time and solution concentration. An optical model was developed to provide accurate replications of the gradient refractive index profile, and we simulated porous structure to be a stack of sub-layers. The simulated porous structure depth had a good agreement with the SEM measured results. The results showed that the effective refractive index approximated a smooth transition from the glass substrate to the air when porous structures were formed after leaching process. This model can be used to track optical properties according to the depth of nanostructures measured by SEM, or obtain its depth information by the transmittance, and this property may be useful to optimize experiment condition (leaching time and solution concentration) for special optical properties of porous glass. Future work should focus on reducing scattering while enhancing transmittance of the porous glasses. Durability of porous glasses will also be tested in our future work.
