Application of ring down measurement approach to micro-cavities for bio-sensing applications

Optical biosensors can detect biomarkers in the blood serum caused by either infections or exposure to toxins. Until now, most work on the micro-cavity biosensors has been based on measurement of the resonant frequency shift induced by binding of biomarkers to a cavity. However, frequency domain measurements are not precise for such high Q micro-cavities. We hypothesize that more accurate measurements and better noise tolerance can be achieved by the application of the ring down measurement approach to the micro-cavity in a biosensor. To test our hypothesis, we have developed a full vectorial finite element model of a silica toroidal micro-cavity immersed in water. Our modeling results show that a toroidal cavity with a major diameter of 70 mu m and a minor diameter of 6 mu m can achieve a sensitivity of 28.6 mu s/RIU refractive index units (RIU) at 580nm. Therefore, our sensor would achieve the resolution of 5 x 10(-8) RIU by employing a detector with picosecond resolution. Hence we propose a micro-cavity ring down biosensor with high sensitivity which will find wide applications in real time and label free bio-sensing.