Lasing in optofluidic ring resonators with aqueous quantum dots as their gain medium and the prospects of such lasers for biochemical sensing

Abstract

We achieved four types of laser emissions with quantum dots (QDs) using the same high-Q-factor optofluidic ring resonator (OFRR) platform. In the first type, 2 μM QDs dissolved in toluene that filled the entire OFRR cavity volume were employed as the gain medium. The lasing threshold was 15-22 μJ/mm 2 . In the second type, 2 μM aqueous QDs were in bulk buffer solution that filled the entire OFRR cavity volume. The lasing threshold was 0.1 μJ/mm 2 , over 3 orders of magnitude lower than the state-of-the-art. In the third type, the aqueous QDs were immobilized as a single layer on the interface between the OFRR inner wall and buffer solution with a surface density as low as 3 × 10 9 - 10 10 cm -2 . The lasing threshold of 60 μJ/mm 2 was achieved. In the fourth type, we achieved optofluidic fluorescence resonance energy transfer (FRET) lasing using aqueous QDs as FRET donors and Cy5 dye molecules as acceptors. We observed lasing from Cy5 emission band in QD-Cy5 pair when excited at QD absorption band, far away from Cy5 absorption maximum. We also report a comprehensive theoretical analysis of optofluidic FRET lasers that was performed based on a Fabry-Perot microcavity using a rate equation model. By comparing FRET lasing based sensors with conventional sensors using FRET signals obtained by spontaneous fluorescence emission, we show that for optimal pump fluence and FRET pair concentration, FRET lasing can lead to more than 20-fold enhancement in detection sensitivities of conformation changes for linker lengths in the Förster radius range. We also study the dependence of the sensitivity enhancement on the cavity Q-factor. We show that the highest enhancements can be obtained for Q-factors between 10 4 -10 6 , and enhancement values decrease for Q-factors above 106 due to the radiative energy transfer in the cavity.