Role of Spectral Resonance Features and Surface Chemistry in the Optical Sensitivity of Light-Confining Nanoporous Photonic Crystals

Nanoporous anodic alumina optical microcavities (NAA-mu QVs) with spectrally tunable resonance band and surface chemistry are used as model light-confining photonic crystal (PC) platforms to elucidate the combined effect of spectral light confinement features and surface chemistry on optical sensitivity. These model nanoporous PCs show well-resolved, spectrally tunable resonance bands (RBs), the central wavelength of which is engineered from similar to 400 to 800 nm by the period of the input anodization profile. The optical sensitivity of the as-produced (hydrophilic) and dichlorodimethylsilane-functionalized (hydrophobic) NAA-mu QVs is studied by monitoring dynamic spectral shifts of their RB upon infiltration with organic- and aqueous-based analytical solutions of equally varying refractive index, from 1.333 to 1.345 RIU. Our findings demonstrate that hydrophilic NAA-mu QVs show similar to 81 and 35% superior sensitivity to their hydrophobic counterparts for organic- and aqueous-based analytical solutions, respectively. Interestingly, the sensitivity of hydrophilic NAA-mu QVs per unit of spectral shift is more than 3-fold higher in organic than in aqueous matrices upon equal change of refractive index, with values of 0.347 +/- 0.002 and 0.109 +/- 0.001 (nm RIU-1) nm(-1), respectively. Conversely, hydrophobic NAA-mu QVs are found to be slightly more sensitive toward changes of refractive index in aqueous medium, with sensitivities of 0.072 +/- 0.002 and 0.066 +/- 0.006 (nm RIU-1) nm(-1) in water- and organic-based analytical solutions, respectively. Our advances provide insights into critical factors determining optical sensitivity in light-confining nanoporous PC structures, with implications across optical sensing applications, and other photonic technologies.

Automated summary

The study on the optical sensitivity of nanoporous anodic alumina optical microcavities revealed that hydrophilic samples exhibit higher sensitivity in both organic and aqueous analytical solutions. In contrast, hydrophobic samples are more sensitive to changes in refractive index in an aqueous medium.