Technical Papers and Presentations

Optical biosensors are powerful analytical tools for detecting and quantifying biological molecules and particles with high sensitivity and specificity. These devices operate on the principle of detecting changes in optical signals. For example, surface plasmon resonance (SPR) biosensors and localized surface plasmon resonance (LSPR) biosensors detect changes in refractive index due to changes in the analyte concentration. Fluorescence-based biosensors (down-conversion or up-conversion) detect luminescence signals from the fluorescent labeling. The advances of nanostructures have tackled the limitations of current optical biosensors in terms of sensitivity and miniaturization by enhancing the interaction between light and matter. However, the fabrication of nanophotonic devices requires significant time and resources, which can make it a challenging and costly process. Therefore, computational modeling is crucial both in comprehending the operational principles of such nanophotonic biosensors at detailed level, but also in enabling effective design, optimization, and fabrication of the devices.

This work utilizes the Wave Optics Module in COMSOL Multiphysics® to design, optimize, and analyze two different nanophotonic biosensors: plasmon-enhanced upconversion (PEUC) biosensor and SPR biosensor. In the PEUC biosensor, a grating structure is optimized to enhance the excitation rate and emission efficiency of upconversion nanoparticles. A 980-nm excitation model and a 540-nm emission model are developed to optimize and analyze the enhancement of the 540-nm upconversion signal from the biosensor. The SPR biosensor is based on grating-coupled surface plasmon polaritons. The grating is designed for the tunable laser working from 1530 nm to 1570 nm. The structure of the grating is optimized to maximize the sensitivity and the figure of merit, defined as the ratio between the sensitivity and the full width at half maximum of SPR dip. Both nanophotonic biosensors were based on gold which was simulated by Brendel-Bormann model taken from the COMSOL Material Library. The models were set up for one unit cell of the grating with periodic ports. Built-in Floquet boundary conditions were used to describe the periodicity. The models were meshed by the built-in physics-controlled mesh at extremely fine element size. The boundary between gold and the liquid solution was highly resolved within a range of one-fifth of the wavelength. Diffraction efficiencies for the transmitted and reflected waves were computed to optimize the structure.

The PEUC biosensor was fabricated, characterized, and tested. This biosensor experimentally achieved a few hundred-fold enhancement of upconversion signal. Furthermore, there was a strong agreement between computational and experimental results of the PEUC biosensor. While the simulated sensitivity of the SPR biosensor was 1200 nm/RIU with a high figure of merit of 444. The SPR sensor is predicted to have high performance in sensing very low physiological analyte concentration. Both designed nanophotonic biosensors can be applied for sensing different analytes with high sensitivity. Furthermore, this work has demonstrated the efficiency and robustness of the COMSOL Multiphysics® software in designing, optimizing, and analyzing the performance of nanophotonic biosensors. This accelerates the development of the nanophotonic biosensors with diverse applications in fields such as medical diagnostics, biomedical research, and drug discovery.