Charge Density-Versus Time-Controlled Pulse Anodization in the Production of PAA-Based DBRs for MIR Spectral Region

• Main Authors: Ewelina Białek, Grzegorz Szwachta, Miron Kaliszewski, Małgorzata Norek
• Format: Article
• Language: English
• Published: MDPI AG 2021-08-01
• Series: Energies
• Subjects: distributed bragg reflector (DBR); porous photonic crystals; porous anodic alumina (PAA); mid-infrared (MIR); structural engineering; transmission spectra
• Online Access: https://www.mdpi.com/1996-1073/14/16/5149

A robust and reliable method for fabricating porous anodic alumina (PAA)-based distributed Bragg reflectors (DBRs), operating in mid-infrared (MIR) spectral region, is presented. The method relies on application of high (U_H) and low (U_L) voltage pulse sequence repeated in cycles. PAA-based DBR consists of alternating high-(d_H) and low-porosity (d_L) layers translated directly into periodically varied refractive index. Two anodization modes were used: time- and charge density-controlled mode. The former generated d_H + d_L pairs with non-uniform thickness (∆<i>d</i>) and effective refractive index (∆<i>n_eff</i>). It is supposed, that owing to a compensation effect between the ∆<i>d</i> and ∆n<i>_eff</i>, the photonic stopbands (PSBs) were symmetrical and intensive (transmittance close to zero). Under the charge density-controlled mode d_H + d_L pairs of uniform thickness were formed. However, the remaining ∆<i>n_eff</i> provided an asymmetrical broadening of PSBs. Furthermore, it is demonstrated that the spectral position of the PSBs can be precisely tuned in the 3500–5500 nm range by changing duration of voltage pulses, the amount of charge passing under subsequent U_H and U_L pulses, and by pore broadening after the electrochemical synthesis. The material can be considered to be used as one-dimensional transparent photonic crystal heat mirrors for solar thermal applications.