Abstract
In this study, we present an advanced nanofabrication approach, so-called ‘heterogeneous pulse anodization’ (HPA), in which galvanostatic stepwise and apodized sinusoidal pulse anodizations are combined in a single process. This novel anodization method enables the precise optical engineering of the characteristic photonic stopbands (PSBs) of nanoporous anodic alumina photonic crystals (NAA-PCs). The resulting structures are hybrid PCs (Hy-NAA-PCs) composed of distributed Bragg reflectors (DBRs) and apodized gradient-index filters (APO-GIFs) embedded within the same PC structure. The modification of various anodization parameters such as anodization period, relative and total anodization time, structural arrangement of PCs within Hy-NAA-PCs, and pore widening time allows the fine-tuning of the PSBs’ features (i.e. number, position and bandwidth of central wavelength) across the spectral regions. The effects of these fabrication parameters are systematically assessed, revealing that the positions of the characteristic transmission bands of Hy-NAA-PCs are highly controllable. Our study provides a comprehensive rationale towards the development of unique Hy-NAA-PCs with controllable optical properties, which could open new opportunities for a plethora of applications.
Highlights
‘Hybrid’ PC structures can be defined as the combination of several individual PCs within a single complex PC structure, the spectrum of which corresponds to the combination of the spectra of each individual PC composing the hybrid structure
nanoporous anodic alumina (NAA)-distributed Bragg reflectors (DBRs) feature a stepwise modulation of the pore diameter in depth that follows with precision the stepwise current density profile applied during the stepwise pulse anodization (STPA) process (Fig. 1a)
In the case of APO-NAA-GIFs, the nanoporous structure is composed of stacked layers of NAA featuring a sinusoidally modified porosity in depth produced during apodized sinusoidal pulse anodization (ASPA), where the higher the anodization current density the higher the level of porosity following a logarithmic negative apodization function with time-dependent current density amplitude (Fig. 1b)
Summary
We present an advanced nanofabrication approach, so-called ‘heterogeneous pulse anodization’ (HPA), in which galvanostatic stepwise and apodized sinusoidal pulse anodizations are combined in a single process. We have recently proved that stacked NAA-bandpass filters, which consist of layers of NAA produced with different pseudo-stepwise pulse anodization periods, lead to the generation of NAA-PCs with unprecedented broad transmission bands across the UV-visible-NIR spectrum[34] We envisage this nanofabrication concept for the generation of hybrid nanoporous anodic alumina photonic crystals (Hy-NAA-PCs) with unique optical properties. We present a novel nanofabrication approach that enables the production of Hy-NAA-PCs composed of different combinations of DBRs and APO-GIFs in a single PC structure This one-step anodization method (‘heterogeneous pulse anodization’–HPA) is a combination of logarithmic negative apodized sinusoidal and stepwise pulse anodizations under mild conditions to attain a better controllability over the porosity and growth rate of the anodic oxide (Fig. 1). These Hy-NAA-PCs are fabricated with either two or three individual PCs featuring different geometries, demonstrating that the number, the position, and the bandwidth of the characteristic PSBs for each PC embedded within the hybrid PC structures can be precisely engineered to produce a set of unique Hy-NAA-PCs with outstanding optical properties
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