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

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.