Fluorescence lifetimes of Oriented Molecules in Microdroplets

Over the last several years there has been considerable interest in the properties of atomic resonance fluorescence in an optical cavity with a primary dimension comparable to the relevant transition wavelength.1 In particular, there has been great interest in the realization of strong atom-cavity coupling2 and the suppression of spontaneous emission into "free-space" modes. However, an important but poorly understood issue relevant to low- or zero-threshold condensed phase optical devices is the nonradiative coupling of the emitting species to a thermal bath. Unlike experiments involving dilute atomic beams where the transition is well defined and broadening is negligible, coupling to a thermal bath induces spectral broadening which is usually much larger than the cavity resonance width and may often exceed the cavity mode spacing. Interesting examples of such systems are solvated dyes whose condensed phase dynamics are well known and characterized. To date, several studies have been made on fluorescence properties of solvated dyes in microcavities,3,4,5 however additional complexities such as spatial and orientational averaging have obscured to some extent the connection between radiative and nonradiative processes in such systems. We discuss the observation of spontaneous emission rate modification (both enhancement and apparent suppression) for molecular species in a microcavity where both the molecular position and transition moment orientation are well defined.6 These experiments serve as an interesting test case of molecule-cavity systems in which the emitting species is nonradiatively coupled to a thermal bath.