title: High-sensitivity optical biochemical sensors
Creators: name: Cheng, Weiqing
Publisher: name: University of Glasgow
Year: 2024

description: Label-free biochemical sensors enable direct detection of biomolecules and analytes based on their physical properties. In addition to their applications in solutions and gas detection, these sensors can serve as affinity-based biosensors, with receptors attached to the sensor surface that interact with analytes. Emerging applications encompass point-of-care blood tests, disease diagnostics, drug testing, environmental monitoring, and food security. Pointof-care systems can be developed to process clinical samples with various biomarkers in diverse working environments.
In this thesis, I have designed, simulated, fabricated, and measured integrated optical labelfree biochemical sensors. Four distinct compact sensor designs have been proposed to address sensitivity limitations and the free spectral range (FSR) effect in microring resonators (MRRs), which have significantly advanced the field. I have measured the transmission losses of fabricated optical waveguides, including strip and slot waveguides. Experimental verification of sensitivity performance has been conducted across various parameters.
One of the key contributions is the demonstration of the inner-wall grating double slot micro ring resonator (IG-DSMRR) with a 6.72 µm radius, based on the silicon-on-insulator (SOI) platform. This sensor has demonstrated a high measured refractive index (RI) sensitivity in glucose solutions (563 nm/RIU) with a limit of detection (LOD) value of 3.7×10-6 RIU. The achieved bulk sensitivity is more than two times that of optimized designed single strip SOIbased quasi-transverse magnetic (TM) mode MRRs. The concentration sensitivity for sodium chloride solutions reaches 981 pm/%, with a minimum concentration detection limit of 0.02%. The detection range has been significantly extended to 72.62 nm, with a measured Q-factor of 1.6×104 . Transmission losses for straight strip, slot, and double slot waveguides are 0.9 dB/cm, 5.2 dB/cm, and 20.2 dB/cm, respectively.
Another significant contribution is the novel sidewall grating slot microring resonator (SGSMRR) with a compact size (5 µm centre radius) on the silicon-on-insulator (SOI) platform. This sensor demonstrates a measured refractive index (RI) sensitivity of 620 nm/RIU and a LOD value of 1.4×10-4 RIU. The concentration sensitivity and minimum concentration detection limit are 1120 pm/% and 0.05%, respectively. Notably, the detection range, based on a grating structure, has been significantly enlarged to 85.8 nm, four times the free spectral range of conventional slot MRRs, with a measured Q-factor of 5.2×103 . The subwavelength grating cascaded microring resonator (SWG-CMRR) structure has been developed, demonstrating high measured sensitivity at 810 nm/RIU, with a LOD value of 2.04×10-4 and a measured Q-factor of 7.7×103 .
Additionally, the double slot microring resonator (DSMRR) structure has been fabricated and verified in this project, showcasing a measured RI sensitivity of 600 nm/RIU, a LOD value of 1.15×10-6 , and a measured Q-factor of 2.6×104 .
Furthermore, I have undertaken the design, simulation, and fabrication of surface plasmonic sensing structures, such as nanodots and nano rings. These structures enable the detection of hybrid photons and mapping of anapole-like electronic modes using the Polarization Indirect Microscopic Imaging (PIMI) technique.