Silicon based On-chip Sub-Wavelength Grating Ring and Racetrack Resonator BioSensors
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Silicon based On-chip Sub-Wavelength Grating Ring and Racetrack Resonator BioSensors Hai Yan,1,† Lijun Huang,2,† Xiaochuan Xu,3,*,† Naimei Tang,3 Swapnajit Chakravarty,3 Huiping Tian,2 and Ray T. Chen1,3,* 1
Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, TX 78758, USA 2 State Key Laboratory of Information Photonics and Optical Communications, School of Information and Communication Engineering, Beijing University of Posts and Telecommunications, Beijing 100876, China 3 Omega Optics Inc., 8500 Shoal Creek Blvd., Bldg. 4, Suite 200, Austin, TX, 78757, USA *Corresponding author: [email protected];[email protected] † These authors contributed equally to this paper.
ABSTRACT In this paper, we experimentally study the unique surface sensing property and enhanced sensitivity in subwavelength grating (SWG) based microring resonator biosensors versus conventional ring resonator biosensors. In contrast to a conventional ring, the effective sensing region in the SWG microring resonator includes not only the top and side of the waveguide, but also the space between the silicon pillars on the propagation path of the optical mode. It leads to an unique property of thickness-independent surface sensitivity versus common evanescent wave sensors; in other words, the surface sensitivity remains constantly high with progressive attachment of biomolecules to the sensor surface. To increase the robustness of performance of ring shaped circular SWG biosensors, we experimentally demonstrate silicon SWG racetrack resonators. A quality factor of 9800 and bulk sensitivity (S) is ~429.7 nm/RIU (refractive index per unit) results in an intrinsic detection limit (iDL) 3.71×10-4 RIU in racetrack SWG biosensors while still retaining the accumulated surface thickness properties of circular rings. 1. INTRODUCTION Micro- and nano-scale photonic biosensors have become a fast growing research topic driven by the need of portable bio-detection systems with high sensitivity, high throughput, and real-time and label-free detection [1–3]. Various devices, including surface plasmon devices [4,5], microring resonators [6–8], silicon nanowires [9], nanoporous silicon waveguides [10], one-dimensional (1D) and two-dimensional (2D) photonic crystal (PC) microcavities [11–13], have been proposed and demonstrated. Most proposed structures are based on the interaction between the evanescent wave and the biomolecules that are adsorbed or immobilized on the sensor surface. In the silicon-on-insulator (SOI) platform, significant efforts have been made on various evanescent wave sensors focusing on increasing the sensitivity and lowering the detection limit [14–18]. However, in evanescent wave sensing, the sensitivity drops inevitably with increasing thickness of the surface layer accumulated on the sensor surface with reduced overlap of the optical mode with the analyte. In real applications, this layer includes necessary oxide and chemical layers generated by surface treatment, probe biomarkers, tar
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