Position controlled lasing threshold of the acceptor emission in a dynamic twin droplet system made using a microfluidic chip

Abstract

We demonstrate low threshold acceptor lasing consequent to the efficient coupling and radiative energy transfer between the donor and acceptor droplets in a dynamic array of coupled droplets named twin droplets, made using a microfluidic chip. The ability of the microdroplet to focus the incoming plane wave internally along the principle diameter, by virtue of its spherical interface and higher refractive index than the background medium, is recognized as the prime cause of observed acceptor lasing in the coupled droplet system. This is confirmed by observing only acceptor fluorescence when the refractive index of the donor droplet falls below that of the background. By flipping the position of the acceptor in the twin droplets, we further show that the threshold of the acceptor lasing is dependent on the acceptor position and, hence, the coupling direction. Using the emission polarization measurements from the twin droplets, we also show that the acceptor emission is highly polarized and follows the pump laser polarization.