Unlike distributed Bragg reflectors (DBR), rugate filters
display a single reflectivity band without harmonics or sidelobes. Thanks to this feature, rugate filters with complex selleck chemical optical response and multiple PBG can be fabricated by superimposing multiple refractive index profiles [1–3]. However, these filters are difficult to fabricate because the smooth variation of the refractive index is challenging and requires complex equipment. An interesting method for fabricating rugate filters is by means of electrochemically etched materials such as porous silicon (pSi). In porous materials, the refractive index depends on the porosity of the layer. Thus, pSi rugate filters have been fabricated thanks
to the this website ease of porosity modulation by adjusting the electrochemical etching conditions [4–6]. Thanks to the porous nature of the resulting pSi rugate filters, these optical devices have been exploited for the development of highly sensitive detectors [7–12]. Another interesting material for the development of highly sensitive optical sensors is nanoporous anodic alumina (NAA) [13–21]. NAA is a nanostructured material obtained from the electrochemical etching of high-purity aluminum foils that has attracted much interest in recent years thanks to its unique structural properties. NAA consists of highly uniform and parallel pores with no branching. The interpore distance can be easily tuned by adjusting the voltage applied during the electrochemical etching, and the pore diameter can be adjusted by wet chemical etching in phosphoric acid . Moreover, honeycomb structures of self-ordered pores can be obtained Parvulin by the two-step anodization procedure . However, porosity modulation with NAA has been challenging. One of the first techniques used for pore modulation during the anodization was pulse anodization [24–26]. This technique consisted in combining mild and hard anodization regimes by means of step voltage variations. This allowed great changes in the pore diameter along the pore axis, but despite the fact that no optical characterization was performed, the combination
of mild and hard anodization regimes would result in abrupt refractive index variations which are incompatible with the development of rugate filters. Another technique is cyclic anodization. This method was used to fabricate DBRs by applying a periodic voltage which resulted in well-defined layers with branched pores [27–29]. Lately, NAA photonic crystals fabricated with current check details control techniques have been reported [30, 31]. However, these structures also showed branched pores. In this work, we report a current control technique for the fabrication of NAA rugate filters. We have characterized the resulting structure and analyzed its optical response as a function of porosity by applying subsequent pore-widening processes.