Abstract
In this paper, we demonstrate FRET-based multicolor lasing within chemically open droplet cavities that allow online modulation of the gain medium composition. To do this, we generated monodisperse microfluidic droplets loaded with coumarin 102 (donor), where the spherical droplets acted as whispering gallery mode (WGM) optical cavities in which coumarin 102 lasing (~ 470 nm) was observed. The lasing color was switched from blue to orange by the introduction of a second dye (acceptor, rhodamine 6 G) into the flowing droplet cavities; subsequent lasing from rhodamine 6 G (~ 590 nm) was observed together with the complete absence of coumarin 102 emission. The ability to control color switching online within the same droplet cavity enables sequential detection of multiple target molecules within or around the cavity. As a demonstration of this concept, we show how the presence of FITC-Dextran and methylene blue (MB) in the medium surrounding the lasing droplets can be sequentially detected by the blue and orange laser respectively. The method is simple and can be extended to a range of water-soluble dyes, thus enabling a wide spectral range for the lasing with the use of a single pump laser source.
Highlights
In this paper, we demonstrate Förster energy transfer (FRET)-based multicolor lasing within chemically open droplet cavities that allow online modulation of the gain medium composition
We have demonstrated the incorporation of dye-based liquid lasers within or around flowing aqueous microfluidic droplets, which enables whispering gallery mode (WGM)-based ‘on drop’ sensing[15]
Benzyl alcohol was chosen as a solvent for coumarin 102 for two reasons: (i) its high refractive index contrast with water ensures efficient WGM-based light confinement at the boundary of the droplets[5,13], and (ii) it is a good solvent for many laser dyes, including rhodamine 6 G, which allows rapid diffusion of rhodamine 6 G from aqueous continuous phase into coumarin 102-loaded droplets
Summary
We demonstrate FRET-based multicolor lasing within chemically open droplet cavities that allow online modulation of the gain medium composition. We present the first demonstration of multi-color lasing within chemically open droplet cavities via the online introduction of a second gain medium into the droplets followed by a non-radiative Förster energy transfer (FRET) process. We demonstrate this by introducing a dye (acceptor, rhodamine 6 G) from the continuous phase into droplet cavities that initially contain a dye (donor, coumarin 102). Owing to the FRET pumping mechanism and chemically open droplet cavities, our method can be extended to a wide selection of water-soluble laser dyes, where an extended laser color switching range can be achieved via sequential energy transfer, instead of dyes being excited directly by a single pump pulse laser
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