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Label-free optical biosensing technologies have superior abilities of quantitative analysis, unmodified targets, and ultrasmall sample volume, compared to conventional fluorescence-label-based sensing techniques, in detecting various biomolecules. In this review article, we introduce our recent results in the field of evanescent-wavebased refractive index sensing and surface enhanced Raman scattering (SERS)-based sensing, both of which are promising platforms for label-free optical biosensors. First, silicon-on-insulator (SOI) nanowire waveguide and metallic surface plasmon resonance (SPR)-based refractive index sensing are discussed. In order to improve the detection limit, phase interrogation techniques are introduced to these types of sensors based on prism-coupled SPR and SOI microring resonators. A detection limit in the order of 10−6 refractive index unit is achieved. Detection of 16.7 pM anti-IgG is also demonstrated based on the SPR devices. Second, SERS substrates based on various nanometallic structures are discussed. Metallic nanowire arrays and inverted nanopyramids and grooves with a thin metal surface are fabricated based on anisotropic wetetching of silicon substrates. Both structures have demonstrated a Raman signal enhancement on the order of 107. In order to improve the extraction efficiency of the Raman signal at a high wave number, a nano-bowtie array substrate is fabricated, which exhibits double resonances at both the excitation wavelength and the desired Raman scattering wavelength. Experimental results have shown that this double-resonance structure can further enhance the received Raman signal, as compared to conventional SERS substrates with only one resonance at the excitation wavelength.

Optical integrated chips with micro and nanostructures for refractive index and SERS-based optical label-free sensing