Abstract

We observe spontaneous emission lifetime shortening and enhanced lasing gain in 10 μm diameter liquid droplets. Nearly transparent microdroplets act as high Q resonators; the feedback provided by totally internally reflected light waves at the droplet–air interface that fold back on themselves. Purcell’s formula predicts significant enhancement (>10⁶) of molecular emission in droplets of this size. Such large values are not easily achieved in direct lifetime measurements because typical broad fluorophor emission bands generally encompass spectral regions of inhibition as well as enhancement. We observe lifetime shortening of 2.5× in the emission lifetime of [Eu(BTF)₄P] in DMF/alcohol solution droplets. Lasing gain measurements, however, have a spectral advantage (inherently narrow linewidth) and yield enhancements that approach the Purcell value. We observe¹ enhancements in excess of 100 for Rhodamine 6G in ethanol solution droplets when mode Q values are only 3 × 10³. First, we identify the various modes observed lasing since specific mode order determines Q which can vary from 10³ to 10⁸. Next, the concentration N, of the dye is reduced until a point is reached where the specific mode of interest fails to lase. The cavity loss per unit length, estimated from Lorenz-Mie theory, may then be equated to gain per unit length to yield an enhancement, η [i.e., Nησ_cm ≈ 2πzmx/Q]. Here x is k times the droplet radius, σ_cm the noncavity emission cross section, and m the index refraction.

© 1991 Optical Society of America