Whispering gallery mode emission generated in tunable quantum dot doped glycerol/water and ionic liquid/water microdroplets formed on a superhydrophobic coating

Abstract

Emission spectra from microdroplets doped with CdSe/ZnS quantum dots (QDs) have been recorded on superhydrophobic coatings (water contact angle > 170°). Whispering gallery modes (WGMs) with Q-factors as high as 4.0 × 10³ were discernible. Excitation parameters for optimal microdroplet WGM response illumination and acquisition are also presented. Fluorescent QDs provide a robust WGM reporting mechanism under extreme continuous wave microdroplet excitation (465.5 μW) for periods greater than 15 minutes. In this format droplets could be optically tuned on demand using combinations of QDs. Ionic liquid QD-doped droplet emission was found to be spectrally stable: a >75% improvement in WGM blue-shift was recorded (due to droplet evaporation) compared to QD-doped glycerol/water droplet emission. Theoretical analysis of emission spectra confirmed the observed emission response curves correspond to Mie theory suggesting the droplets are extremely close to spherical on the surface. This versatile liquid resonator system has direct implications in high performance room-temperature laser development, telecommunications research and lab-on-a-chip based diagnostics.