PHYSICAL REVIEW APPLIED 12, 054045 (2019) Parallel-Coupled Dual SiO x N y Racetrack Resonators as Biosensors with High Improved Intrinsic Limit of Detection F. Khozeymeh 1,2 and M. Razaghi 2, * 1 Department of Physics, University of Kurdistan, P.O. Box 66177-15175, Sanandaj, Iran 2 Department of Electrical Eng., School of Engineering, University of Kurdistan, P.O. Box 66177-15175, Sanandaj, Iran (Received 23 May 2019; revised manuscript received 8 August 2019; published 19 November 2019) This Paper introduces analyzing and modeling dual silicon-oxy-nitride [SiO x N y ] racetrack resonators (RRs) for biosensing applications. The geometric configuration of the resonators and design of alterna- tive small sensing windows result in the realization of optical biosensors with a highly improved intrinsic limit of detection (ILOD). A small sensing window, located above the coupling region of dual RRs, can be used to maximize the total quality factor (minimized ILOD). The best characteristics for each optical component in the parallel-coupled RR-based biosensors are single-mode-channel waveguides with dimen- sions of 35 × 1 μm 2 ; the radius of curvature 90 μm and quality factors up to approximately 5.53 × 10 6 for RRs. The biosensing performance of our proposed configuration is compared with other experimental and simulated biosensor devices. Our structure shows an ILOD of approximately 1.81 × 10 -6 refractive index units (r.i.u.), which significantly improved 100 times compared with the traditional single SiO x N y RR biosensor. This ultra-improved ILOD parameter is found in the dynamic refractive index range of 1.332–1.350 r.i.u. It shows that the proposed biosensor could be a promising diagnostic candidate for ana- lyzing different components of blood samples, which is especially important in the early detection of some critical diseases such as prostate cancer. DOI: 10.1103/PhysRevApplied.12.054045 I. INTRODUCTION Bio and chemical optical sensors are gaining consid- erable attention because of the increasing demand for sensing applications in health care, defense, security, environment, and food quality control [1–6]. In particu- lar, the development and integration of microfluidic and photonic technologies in so-called lab-on-a-chip (LOC) microsystems, allow sensing performance to be signifi- cantly enhanced in terms of sensitivity (S) and intrinsic limit of detection (ILOD). In the framework of reach- ing LOC microsystems, optical bio and chemical sensing by guided-wave devices has been gaining considerable attention. In these microsystems, all functionalities (fluid handling, sample preparation, target detection, transducer readout, and signal processing) can be integrated into one chip. This chip, which is a miniaturized analytical device, should include easy-to-use, portable, and robust real-time diagnostic instruments. Different types of integrated opti- cal bio and chemical sensors including Mach-Zehnder interferometer devices [1], surface plasmon resonance sen- sors [2], spherical resonator-based sensors [7,8], cylin- drical resonator-based biosensors [9–11], and single- or * m.razaghi@uok.ac.ir cascaded-ring resonator-based biosensors [12–14] have been intensively studied and demonstrated. In particular, resonator-based optical sensors like the ring and race- track geometries, have been widely studied due to their positive features such as their performance, miniaturiza- tion capability, and potential for high-volume fabrication. Their performance principle is based on the change of the refractive index (RI) by capturing analyte molecules. An important component in these biosensors is the trans- ducer part. The transducer part of an optical biosensor transforms an RI change in its surrounding environment to a measurable change in its optical transmission. Many research groups have focused on improving the perfor- mance of the transducer part. The parameter showing the performance of the transducer part is ILOD. This param- eter depends only on the characteristics of the transducer (total quality factor of Q t , and resonance wavelength of λ Res ) and S of the biosensor. The ILOD is a fair metric for comparison among the biosensors relying on different sensing technologies. This parameter is used throughout this Paper when discussing detection limits. Indeed, with finding high-Q t transducers (improved ILOD transducers), the performance of the biosensors can be improved. In the literature, there are different types of transducers on SOI that use a variety of methods to achieve an improved 2331-7019/19/12(5)/054045(12) 054045-1 © 2019 American Physical Society