Characterization and fabrication of horizontal slot microdisk resonator for biosensing application

Abstract

A subwavelength-scale low-refractive index slot structure sandwiched between high-refractive index regions has attracted a lot of attention due to strong light confinement in a slot region. Most of the recent works has been reported based on the vertical slot structure. However, etching such a thin, high aspect ratio slot is difficult and usually requires time consuming lithography techniques such as the e-beam lithography or focused ion-beam milling. Moreover, the surface roughness of slot side wall induced by the imperfection of fabrication restricts the quality factor of the demonstrated micro-resonator due to large scattering loss. Recently, we demonstrated a horizontal air slot with smooth sidewalls in atomic scale by using multilayer deposition and selective chemical etching. Using conventional photolithography, reactive ion etching, and selective wet-etching, horizontal slot microdisk resonators can be formed. The fundamental TM modes of the demonstrated horizontal slot microdisk resonators concentrate the electric fields strongly into the air slot with a thickness in the order of tens of nm. The strong field concentration enables to increase the sensitivity of bio-material sensing further than the conventional, evanescent-field based biosensors. In this thesis, we discuss on the performance of the horizontal slot microdisk as a biosensor theoretically and experimentally. Theoretical and numerical calculations show that, for resonator-based biosensors, the mode overlap is the limiting factor for the sensing performance. We note that our horizontal slot microdisk resonators offer about an order of magnitude enhanced surface sensitivity compared to conventional microdisks without slots. In addition, employing multiple slots, we can further enhance the surface sensitivity with a factor of 2. We demonstrated single-slot SiN microdisks as bio-sensors. Surface sensitivity and detection limit of streptavidin - biotin reaction, which is one of the materials used for biosensing, were measured statistically by fabricating a silicon nitride single-slot microdisk. Statistical measurements show that the surface sensitivity of the single-slot SiN microdisk can be 80 times higher than the previously reported Si waveguides in vitro. This sensitivity can increase further by factor 11.72 using optimized multislot structure. Creating a fully optimized multislot structure were difficult. We found feasible structure for biosensing and fabricate multislot microdisk resonator for biosensor. We experimentally demonstrate that the double slot microdisk resonator has improved surface sensitivity by about 18 % over the single slot disk resonator. We also integrated a waveguide with a horizontal slot microdisk on chip for reducing the measurement error and operating in water environment, and measured the transmission spectrum of the waveguide coupled resonator.