Scalable optical annealing of microfluidic droplets via whispering gallery mode geometry and infrared illumination.

This work presents a solution to limitations on scalability in traditional on-chip optofluidic polymerase chain reaction (PCR) methods that are based on infrared annealing and droplet-based microfluidics. The scalability in these PCR optofluidic methods is limited by the optical penetration depth of light in a fluid droplet. Traditionally, such an implementation has minimal absorption when the droplet diameter is scaled well below the optical penetration depth due to the small interaction length. In the presented whispering gallery mode (WGM) optofluidic method, a WGM wave is created through total internal reflection, where light is trapped within a droplet. The effect of the trapped light can extend the interaction length beyond the penetration depth, even for small diameter droplets. Thus, this WGM wave permits the use of droplets with diameters scaled below the penetration depth of the light. A theoretical analysis of traditional optical annealing and of the WGM optofluidic method is conducted using finite-difference time-domain analyses. The WGM wave optofluidic method is also demonstrated experimentally, providing higher annealing temperatures than traditional optical annealing. It is envisioned that the presented work will allow for scalable PCR devices implemented on-chip.