A model for an Optical Ring Microresonator towards Nanoparticle Detection

F. Ahmeti, K. Teker
Istanbul Sehir University,

Keywords: microresonator, label-free detection, biosensors, modeling


Optical resonators have shown great potential as fundamental building blocks, in a wide range of photonic devices and applications including filters, optical multiplexers, logic gates, switches and sensors. A microresonator is defined by a micron-scale circular geometry (microdisk, microsphere, or microring) that confines light by total internal reflection. In the classical picture, optical resonances are coupled to the resonator when the length of the circular light path around the circumference matches an integer values of the wavelength. In a ring resonator, this resonance can be identified by a Lorentzian-shaped response in transmission spectra, typically obtained by delivering tunable laser light to the resonator via a tapered optical fiber positioned in the vicinity of the resonator. The resonance dips can be observed by measuring the transmission spectrum while sweeping the laser wavelength. As the wavelength of the laser matches the resonance wavelengths of the ring, light couples from the optical fiber to the microring, resulting in an intensity drop in Lorentzian line of transmission spectra monitored at the output of the waveguide. In this work, we have used an optical microring-based resonator, and developed a Multiphysics finite element model of this resonator for ultrasensitive biomolecule and nanoparticle detection. This model was developed using COMSOL Multiphysics v.5.2. We used an input tunable laser light at the input node of the waveguide at a wavelength of 1.55 μm and monitored at the transmitted light at the output node simulation. An illustration of the optical micro-ring resonator is given in Figure 1. Refractive index of the microring material was set to be at 1.5 for our simulations. During this process, we recorded the transmission spectra and located the exact position of the resonance wavelengths shifted towards higher wavelengths (red shift). Figure 2 illustrates the resonance wavelength value (transmission dip) by plotting the transmittance as a function of wavelength, for a ring radius of 83μm. We observed a linear correlation between increasing the radius of the microring and the red shift in the resonance wavelengths. In addition, we found that we can detect resonance shifts for very small changes in microring radius, as low as 10 nm. Figure 3 shows the resonance wavelength response with respect to the change in the microring radius. In summary, we have developed a model platform to simulate a microring resonator as a biosensor for biomolecule and nanoparticle detection based on the resonant frequency shift as a function of small microring size changes. In this respect, ring-based microresonator biosensors prospect to have a bright future in clinical diagnostics since they are label-free analytic detectors and also enabling the integration possibility on a chip-scale device.