K-M Chang, Y-M Chen, K-J Lai, Y-X Zhang, C-H Lai, Y-L Lai, H-L Lai, F-C Lee, C-H Fan
National Chiao Tung University ; Chung Hua University,
Keywords: transfer, aluminum thin-film, plasmonic, bio-sensing
Summary:Since the high environmental sensitivity of the surface plasmon resonance, plasmonic metal materials have been rapidly applied in bio-sensing and related applications. The strong localized optical fields can effectively interact with bio-molecules and nanoparticles. Furthermore, due to the strong localized energy properties of the LSPRs, plasmonic nanostructures have been widely studied for the photothermal therapy and resulted in a good and rapid progress. In the article, we first introduce the transfer technique of ultra-smooth single-crystalline aluminum thin-film and may be applied as a sensor chip for the non-invasive plasmonic bio-molecular sensing. We report on the transfer result of high quality and ultra-smooth aluminum thin-film transferred onto an optical cover glass with root-mean-square roughness of 0.474nm. The transfer technique is based on the designed multi-epitaxial structures of Al, GaAs, AlGaAs, and GaAs substrate. Also, we use the properties of selective etching of citric acid and HF solutions for GaAs and AlGaAs. We finally obtain a high-quality aluminum thin film on the optical cover glass. This is significant because there has not been a well-established method to obtain single-crystalline aluminum by chemical synthesis. The common way to grow a single-crystalline aluminum film through molecular beam epitaxy (MBE) on a GaAs substrate will drastically affect the effective optical spectrum since the optical responses from GaAs itself. The transfer technique overcomes the drawbacks of GaAs substrate and the transferred high quality aluminum thin-film on the optical cover glass may work as the sensor chip for plasmonic bio-sensing. The most common way in plasmonic bio-sensing is based on the angular modulation of surface plasmon resonance (SPR) combined with fluorescence spectroscopy. The biomolecular recognition elements (BREs) are anchored on the metal surface for specific target molecules which are contained in a liquid sample flowing through the top of the metal film and BREs. The target molecules are labeled with fluorophores of specific absorption band matching with the wavelength of the excitation laser source. Another mechanism of the label-free detection depends on the sensitivity of SPPs resonances to the dielectric material and thus the refractive index changes can be measured optically as spectral shifts when the bio-molecules attach on the metal surface. When we couple the laser into surface plasmons, the extinction spectrum is of a minimum dip which indicates a surface plasmon resonance at specific incidence angle. When the bio-molecules or nanoparticles attach on the metal film and change the resonant condition, we can detect the existence of specific bio-molecules by analyzing the change of the dip positions. This work provides a way to obtain high-quality aluminum thin-film on the optical cover glass for the first time. We are looking forward to put high-quality aluminum thin-film into non-invasive bio-sensing and related applications, especially for ultraviolet (UV) spectral region.