1-D nanoporous anodic alumina rugate filters by means of small current variations for real-time sensing applications
© Macias et al.; licensee Springer. 2014
Received: 2 May 2014
Accepted: 29 May 2014
Published: 25 June 2014
A rugate filter based on nanoporous anodic alumina was fabricated using an innovative sinusoidal current profile with small current variation. The resulting structure consisted of highly parallel pores with modulations of the pore diameter along the pore axis and with no branching. The effect of the period time and the pore widening post-treatment was studied. From reflectance measurements, it was seen that the position of the reflection band can be tuned by adjusting the period time and the width by pore-widening post-treatments. We tested one of the rugate filters by infiltrating the structure with EtOH and water in order to evaluate its sensing capabilities. This method allows the fabrication of complex in-depth modulated nanoporous anodic alumina structures that open up the possibility of new kinds of alumina-based optical sensing devices.
KeywordsNanoporous anodic alumina Rugate filter Photonic crystal Fabrication Nanostructuring Current control Sensing
Rugate filters are one-dimensional photonic crystals based on a smooth variation of the refractive index along the depth of the structure which results in a photonic bandgap (PBG) . Unlike distributed Bragg reflectors (DBR), rugate filters display a single reflectivity band without harmonics or sidelobes. Thanks to this feature, rugate filters with complex 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 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 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 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. Finally, we tested the sensing capabilities of the NAA rugate filters by real-time monitoring the shift of the central wavelength in ethanol and deionized water.
Aluminum (Al) foil (thickness = 250 μm, purity = 99.999%) was purchased from Goodfellow (Huntingdon, UK). Oxalic acid (H2C2O4), ethanol (C2H5OH), acetone ((CH3)2CO), perchloric acid (HClO4), hydrochloric acid (HCl), and copper chloride (CuCl) were purchased from Sigma-Aldrich (Madrid, Spain). Double deionized (DI) water (18.6 MΩ, Purelab Option-Q, Elga, Marlow, UK) was used for all the solutions unless otherwise specified.
Scanning electron microscope (SEM) micrographs used for structural characterization of the NAA rugate filters were taken on SEM FEI Quanta 600 (FEI, Hillsboro, OR, USA). The optical characterization of the rugate filters was performed on a PerkinElmer UV/vis/NIR Lambda 950 spectrophotometer (PerkinElmer, Waltham, MA, USA). For the reflectance measurements, the spectrophotometer was coupled with the universal reflectance accessory (URA).
Real-time measurements for the sensing experiments were performed in a custom-made flow cell. Reflectance spectra of the NAA rugate filter were obtained using a halogen light source and a CCD spectrometer (Avantes, Apeldoorn, The Netherlands). Light was directed to the surface at a normal angle through a fiber optic cable consisting of six illuminating waveguides and one reading waveguide coupled to an optical lens which focused the light on top of the NAA rugate filter. The light reflected by the rugate filter sample was collected by the reading waveguide and directed to the CCD spectrometer, which recorded a spectrum every 10 s.
Results and discussion
Central wavelength calibration
Effect of porosity
NAA rugate filters were fabricated using a current control method based on a sinusoidal current profile with a maximum amplitude of just 1.45 mA cm−2. Thanks to this small current peak-to-peak value, the voltage was contained within 40 ± 5 V. The position of the band can be accurately tuned by varying the period time of the current profile. This process allows the fabrication of highly reflective bands with just 50 periods. Moreover, for as-produced rugate filters, the reflectance bands were narrow (less than 30 nm) which is an important feature for the development of highly sensitive chemical and biochemical sensors based on the monitorization of the position of the reflectance band. As a proof of concept, we performed a sensing experiment in a flow cell in order to determine the sensing possibilities of the structure and found out that changes in refractive index of 0.031 can be readily monitored with high sensitivity (48.8 nm/RIU) and low noise level (<0.04 nm).
distributed Bragg reflector
nanoporous anodic alumina
photonic band gap
refractive index unit.
This research was supported by the Spanish Ministerio de Economía y Competitividad through the grant number TEC2012-34397 and the Generalitat de Catalunya through the grant number 2014-SGR-1344.
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