Optical Resonances in Short-Range Ordered Nanoholes in Ultrathin Aluminum/Aluminum Nitride Multilayers

Nanoholes with short-range ordering were fabricated in ultrathin aluminum/aluminum nitride multilayer films where each layer is as thin as a few nanometers. Optical resonances of the trilayer system with a single metallic layer and five-layer system with two metallic layers were successfully tuned in the visible-near-infrared (vis/NIR) range. The resonance wavelength as well as the width can be predicted and designed by solving the dispersion relation and comparing with the lateral dimension of the short-range ordering. To solve the dispersion relation, we developed a general formulation for multilayer systems. The thermal stability of the fabricated nanoholes in ultrathin multilayers was also tested by vacuum annealing the samples up to 400 degrees C. While no structural change of the nanohole or the multilayer surface has been observed, the optical property showed almost no change in the resonance confirming no structural change but emergence of the interband transition around the wavelength of 900 nm. It means crystallinity improvement without grain growth by thermal annealing, which is consistent with the previous crystallographic studies on the same multilayer systems. The fabricated sensor revealed comparable refractive index sensitivity to the gold based sensors even with the top protective AlN layer. The chemical sensing test using the nanohole sensor with a bare aluminum top surface confirmed the applicability to the monitoring of aluminum surface reactions.