Effect of Förster resonance energy transfer efficiency and pump wavelength absorption on the acceptor’s amplified spontaneous emission in an on-chip droplet system

Abstract

We demonstrate efficient amplified spontaneous emission (ASE) of an acceptor from a sophisticated on-chip dynamic droplet Förster resonance energy transfer (FRET) system. The effect of changing the FRET efficiency and the pump wavelength absorption by the donor molecules is studied using two FRET pairs having dissimilar donors [Rhodamine 6G (R6G) and Rhodamine B (RB)] but a common acceptor [Nile blue (NB)], in microdroplets of different sizes and shapes ranging from spherical to squashed cylindrical. We show that the threshold of acceptor ASE depends on the extent of resonance of pump wavelength with the absorption maximum of the donor molecule, whereas this has no effect on the FRET efficiency, in agreement with the existing finding that FRET efficiency is independent of the excitation wavelength. We also demonstrate a tunability of 18 nm in the acceptor ASE by tweaking the size and shape of the generated droplets from spherical to squashed cylindrical. The reason for tunability is attributed to a change in the effective concentration of the dye molecules in different droplets.