Investigation of Serially Coupled Double Microsphere Sensor for Detection of Biomolecules

Microsphere sensors have attracted increasing attention in recent years especially in label-free detection of biomolecules thanks to their very high quality factors which causes to have the high sensitivity and low detection limit. In this paper, two novel structures for microsphere sensors based on serially coupling of two microspheres are proposed. They are modeled using directional coupling theory while considering the loss condition in biosensing applications. The proposed structures are simulated for sensing rodlike bacteria and their key sensing parameters are compared with a single-microsphere sensor. It is demonstrated that by using serially coupled double microsphere sensor, the frequency spacing between adjacent resonances observed at the output spectrum of the sensor is expanded considerably compared to the single-microsphere sensor and therefore it is much easier to track the resonance frequencies variation due to changes in the surrounding medium while the sensor's quality factor remains high.