Abstract
Fluorescence and lasing emission that are produced separately in time during excitation laser pulse for an mm-sized Rhodamine 6G dye-water droplet are reported. The droplet acts as a quasi-spherical closed optical resonator and due to multiple internal reflections, the resonant amplified emission is delayed with respect to fluorescence emission. Measurements of the temporal evolution of the droplet’s emission were performed by varying the signal acquisition gate width and gate delay with respect to the pumping pulse. The droplet emission spectra are structured in two bands which appear one after the other in time: first, the fluorescence emission band which follows pumping laser pulse time shape and then a second band, the lasing band, placed at shorter wavelengths and formed in time after the peak of the pumping laser pulse intensity, on the pulse tail. The lasing threshold pumping intensity is much lower than those for typical dye lasers.
Highlights
Droplets are a subject of increased interest, from the point of view of their microfluidic and optical properties
Optical phenomena specific to droplets always represented an attractive field for basic research and for applications in biology [13,14] or chemical sensing [15,16]. Due to their quasi-spherical shape and to the ratio between the refractive index of droplet’s material and the surrounding medium, electromagnetic waves may be trapped in the sphere by multiple total internal reflections which lead to optical resonances described as whispering gallery modes (WGM) and reported typically for droplets of hundreds of nanometers in diameter [4,17]
In [18], we studied the evolution from pulse to pulse of the emission of a 1-μL Rhodamine 6G (Rh6G) droplet when excited with a sequence of laser pulses
Summary
Droplets are a subject of increased interest, from the point of view of their microfluidic and optical properties. Optical phenomena specific to droplets always represented an attractive field for basic research and for applications in biology [13,14] or chemical sensing [15,16]. Due to their quasi-spherical shape and to the ratio between the refractive index of droplet’s material and the surrounding medium, electromagnetic waves may be trapped in the sphere by multiple total internal reflections which lead to optical resonances described as whispering gallery modes (WGM) and reported typically for droplets of hundreds of nanometers in diameter [4,17]. They emit fluorescence radiation similar to a higher volume bulk sample (mL), but may emit lasing radiation that makes them, at the limit, micro-lasers [2]
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