Ultra-tall sub-wavelength gold nano pillars for high sensitive LSPR sensors

Free standing gold pillars with sub wavelength pitch have successfully demonstrated the sensing functionality by utilizing the local surface plasmonic resonance in longitudinal mode along the height. So far, the reported pillar height is not beyond 1 μm, limiting the further improvement in the sensitivity of refractive indices. Our numerical simulation by the method of finite-difference time-domain shows that the sensitivity can be enhanced by either increasing the height or narrowing down the gap between the pillars. Nanofabrication of gold pillar arrays with the heights from 1 μm up to 2.6 μm, as well as the gaps from 200 nm down to 50 nm were successfully carried out by electron beam lithography combined with electroplating. The surface plasmonic resonance localized in such tall pillar array was both numerically simulated and experimentally characterized. The highest sensitivity as refractive index unit in Au nano-pillar arrays was achieved. Our initial results show clear sensing behavior of refractive indices in such tall pillar arrays with 50 nm gaps. The advanced nanofabrication technique established in this work for gold nano-pillar arrays should find broad applications not only in plasmonic biosensors, but also in novel metamaterials for perfect imaging and anti-Doppler effect as well as X-ray source, etc.