Designing porous photonic crystals for MIR spectral region-a deeper insight into the anodic alumina layer thickness versus charge density relation

Ewelina Białek; Maksymilian Włodarski; Małgorzata Norek

doi:10.1088/1361-6528/aca546

Designing porous photonic crystals for MIR spectral region-a deeper insight into the anodic alumina layer thickness versus charge density relation

Nanotechnology. 2023 Jan 13;34(12). doi: 10.1088/1361-6528/aca546.

Authors

Ewelina Białek¹, Maksymilian Włodarski², Małgorzata Norek¹

Affiliations

¹ Institute of Materials Science and Engineering, Faculty of Advanced Technologies and Chemistry, Military University of Technology, Str. gen Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland.
² Institute of Optoelectronics, Military University of Technology, Str. gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland.

PMID: 36595263
DOI: 10.1088/1361-6528/aca546

Abstract

The mid-infrared region (MIR) is crucial for many applications in security and industry, in chemical and biomolecular sensing, since it contains strong characteristic vibrational transitions of many important molecules and gases (e.g. CO₂, CH₄, CO). Despite its great potential, the optical systems operating in this spectral domain are still under development. The situation is caused mainly by the lack of inexpensive and adequate optical materials which show no absorption in the MIR. In this work, we present an easy and affordable way to develop 1D photonic crystals (PCs) based on porous anodic alumina for MIR region. The porous PCs were produced by the pulse anodization of aluminum using charge-controlled mode. The first order photonic stopbands (λ₁) were located within ca. 3.5-6.5μm. Annealing of the material at 1100 °C for an hour has allowed to recover the wavelength range from around 5.8 to 7.5μm owing to the decomposition of the absorption centers (oxalate anions) present in the anodic oxide framework while maintaining the PC structural stability. The spectral position and the shape of the resonances were regulated by the charge passing under high (U_H) and low (U_L) voltage pulses, porosity of the correspondingd_Handd_Lsegments, and dura tion of the process (t_tot). The thickness of thed_Handd_Llayers was proportional to the charge passing under respective pulses, with the proportionality coefficient increasing with the applied voltage. Despite the constant charge (2500 mC cm^-2) applied during the anodization, the thickness of anodic alumina (d) increased with applied voltage (10-60 V) and anodizing temperature (5 °C-30 °C). This behavior was ascribed to the different kinetics of the anodic alumina formation prompted by the variable electrochemical conditions. The photonic material can be used in portable nondispersive gas sensors as an enhancement layer operating up to around 9μm.

Keywords: MIR spectral region; anodic alumina; charge-controlled anodization; porous photonic crystal; reflectance and transmittance spectra.

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