DBRs are dielectric multilayer structures [17–20] with a periodic variation of the refractive index in the direction perpendicular to the surface. This gives rise to photonic stop bands for light incident in a direction parallel to the pore axes. The central wavelength of such stop bands depends on the effective refractive index and P5091 price on the optical thickness of each of the cycles, while the width of the bands is directly related with the contrast of the refractive index variations. Ideal

photonic stop bands are achieved for infinite periodic structures [21, 22]. However, DBR structures are finite and consequently, the characteristics of the photonic stop band depend on the number of cycles they contain. NAA-based DBR can be achieved by taking

advantage of the fact that a wet etching applied after the anodization to enlarge the pore diameter (pore-widening step) has a different rate find more depending on the used anodization voltage [23]. Thus, by combining a cyclic anodization voltage with a subsequent pore-widening step, tunable in-depth modulation of the pore diameter and effective refractive index variations are obtained. Other authors have reported on the fabrication of DBR structures by applying a cyclic anodization voltage [19, 20, 24] although they did not stress the importance of the pore-widening step in order to obtain the photonic stop bands. Temperature is also a key factor in the fabrication of NAA structures [25, 26], as it is directly influencing the reaction speed. By lowering adequately the temperature, an increase in anodization voltage is possible so that hard-anodization Tobramycin NAA can be obtained without the need of an initial protective layer [25]. The

color of the NAA can also be influenced by temperature [26]. In this work, we study the influence of the number of cycles and of the anodization temperature on the optical properties of NAA-based DBR. We also study how the pore-widening step (Natural Product Library clinical trial necessary to obtain the well-defined photonic stop bands) can be combined with these parameters in order to adjust the stop band position of the fabricated structures. Methods For the synthesis of NAA-based DBR, we have used high-purity Al substrates (99.99%) of 500-μm thickness from Sigma-Aldrich (St. Louis, MO, USA). A pretreatment is required to meliorate the physical properties of the commercial Al substrate: first, the Al substrates were rinsed in deionized water, then cleaned with ethanol and rinsed in deionized water again, then dried with N2 and stored in a dry environment.