Slow Light Mach-Zehnder Interferometer for Optical Label-free Biosensing
This work presents the design, fabrication, and characterization of a small footprint Mach-Zehnder interferometer (MZI) that possesses strong light-matter interaction. The demonstrated MZI structure is 400 times smaller than traditional MZI devices and exhibits a one order of magnitude higher detection sensitivity for label-free biosensing applications. The integration of a 1D photonic crystal with a MZI on a silicon-on-insulator platform, performed in this work, leads to the reduced footprint of the MZI (on the order of microns) as well as a high sensitivity of the MZI output spectrum towards refractive index changes based on the slow light effect. In order to achieve the proper design to support the slow light effect, various topologies and sizes of 1D photonic crystals were investigated using finite-difference time-domain analysis. Both traditional 1D photonic crystals and 1D photonic crystals with multiple defect holes structures for biosensing were studied. The highest simulated sensitivity was found to be 223,000 rad/RIU-cm, which is 15 times higher than that of traditional MZI biosensors. The slow light MZI-based sensor devices with the most promising designs were fabricated using standard silicon microelectronics and lab-on-chip microfluidics processing techniques. A bulk refractive index detection sensitivity of 170,000 rad/RIU-cm was experimentally measured using different concentrations of NaCl solution, which is 10 times larger than previously reported results. Specific detection of 16mer DNA molecules was also demonstrated using the slow light MZI platform. Accordingly, slow light MZI-based devices hold great potential for integrated, lab-on-chip sensors with compact form factors and high molecular detection sensitivities.