Influence of Photonic Crystal on Fluorescence Resonance Energy Transfer Efficiency between Laser Dyes

Abstract

Spontaneous emission by an excited molecule strongly depends upon the available density of states into which the molecule can decay. In a photonic crystal, the allowed local density of states depletes within the stop band and enhances at the band edge of the crystal. As a result, an emitter implanted in a photonic crystal is forced to redistribute its fluorescence energy within its emission spectral range, and its spontaneous emission spectrum thus gets modified. Here, we studied the influence of change in local density of states on energy transfer efficiency between a donor–acceptor pair embedded in a colloidal photonic crystal. Rhodamine-B and Rhodamine-800 dyes were chosen as the energy donor and energy acceptor, respectively. We observed an angle-dependent quenching in the emission intensity of the donor accompanied by enhancement in acceptor emission when both dyes were in the photonic crystal. This occurred owing to depletion in the allowed local density of states available to the donor. Reduction in the fluorescence lifetime of the donor in the presence of the acceptor confirmed fluorescence resonance energy transfer between the chromophores. In spite of marginal overlapping between the emission band of the donor and the absorption band of the acceptor, the energy transfer efficiency between the dyes was ∼80% in the photonic crystal environment. This enhancement resulted from forced proximity and hence reduced intermolecular distance between the donor and acceptor on being physically restricted within nanovoids of the photonic crystal.