Versatile sensing platform using silicon photonic microring resonators

Abstract

Measurement of refractive indices of liquids and thin films can play an important role for chemical analysis in the fields of healthcare and biomedical research. There is a requirement of miniaturized refractive index sensor platforms that have high sensitivity, low detection limits and scalable for high throughput label free bio-sensing. Silicon photonic sensors are emerging as the key solution that can satisfy all of the aforementioned criteria. These optical sensing platforms can be fabricated on a silicon wafer using the same processes employed for manufacturing of CMOS integrated circuits, which provides the advantages of low cost and high volume production. However, the cost advantages of these miniaturized sensors are often negated by the requirement of expensive optical interrogation equipment such as a tunable laser and a spectrum analyzer. In our research work, we have demonstrated new sensor con figuration based on silicon photonic microring that is capable of low-cost refractive index sensing. We have also extended the microring resonator platform to measure thermo-optic coeffcients of liquids in small volumes. The first part of the thesis research focuses on the development of tunable cascaded Silicon microring resonators for refractive index shift sensing. This configuration uses two microring resonators in series cascade with one of the two rings probing the analyte liquid (called sensor) while the second microring functions as a spectral filter. By implementing thermo-optic tunability in the fi lter ring, one can track the shifts in the spectrum of the sensor. At the output, a single photodetector is used to capture variations in the intensity. This arrangement is used to translate spectral shifts of sensor microring, caused by analyte index variation, into equivalent changes in the position of intensity peak at the output of the cascade. In our experiments, we used a broadband source (1550 nm) for the input and a single photodetector for measuring optical intensity variation at the output port. For proof of concept studies, we emulated the analyte index shift on sensor microring using thermo-optic effect. The total detection range of the 1550 nm operating device was estimated to be about 0.0241 refractive index units (RIU), with a detection limit of 4:6 10ð€€€5 RIU. In the second part of our research we focused on improvement of the detection limit of the tunable cascaded microring device. The precision with which shifts in the intensity peak is tracked was enhanced by the use of lock-in ampli fier assisted harmonic ratio detection. Speci cally, we compute the ratio of the second harmonic to the fundamental frequency of modulation signal provided to the filter ring microheater. Prior to performing experiments, we analyzed the method with theoretical models and simulations to understand the effect of variations in the modulation signals provided by lock-in amplifi er. Experimental results with the 1550 nm cascaded microring devices showed a substantial reduction (a factor of 1330) in the width of harmonic ratio peak compared to that of the unprocessed intensity curve. The detection limit of the device was improved to 8:6 10ð€€€6 RIU, now limited only by the performance of electrical equipment providing power to microheaters. Lastly, we have demonstrated a method to measure thermo-optic coeffcient of small volume of liquids using silicon microring resonators. This effort can help in multiparameter analysis of bio fluids and also for correcting errors in refractive index measurements by silicon microrings. For this experiment, we measured the wavelength shifts of analyte covered mircoring resonators as a function of controlled increments in chip temperature. Using theoretical models and simulated parameters, we calculated the thermo-optic coeffcients of standard liquids and obtained a good match with values reported in literature. In summary, we have explored new methods of using silicon photonic microring resonators for reduced cost refractive index sensing and thermo-optic coeffcient measurements.