An ultrasensitive optical label-free polymeric biosensor based on concentric triple microring resonators with a central microdisk resonator

•We propose an optical label-free biosensor based on concentric coupled resonators.•The external radius of outer ring resonator is only 5 μm.•Transmission notch depth of sensor at rest is −48.7 dB.•Figure of merit for the proposed sensor is 43,000 RIU−1.

In this paper we propose an optical label-free biosensor based on a polymeric platform. Label-free biosensors have not the drawbacks and stability problems of commercialized devices which are used for detection of labeled molecules. In addition, we choose polymeric platform, due to simple and low cost fabrication process and also high biocompatibility properties. The suggested structure consists of concentric triple ring resonators along with a disk resonator which offers deeper notches, higher sensitivity and vaster detection area with respect to other similar configurations such as single ring resonator, double concentric ring resonators, etc. Our numerical simulations based on the finite difference time domain (FDTD) method, show that in optimized structure, a transmission notch depth of −48.7 dB for sensor at rest and a free spectral range of 56 nm are achievable. In addition, resonance wavelength sensitivity and output power sensitivity of sensor are 1000 nm/RIU and 1.8×104 dB/RIU, respectively. The external radius of outer ring resonator is only 5 μm, and detection area of the sensor is 40.37 μm2. With this small size, to the best of our knowledge, the obtained notch depth and sensitivity parameters are one of the highest values in ring resonator-based biosensors reported to date.