Random laser emission at dual wavelengths in a donor-acceptor dye mixture solution

The work was aimed to generate random laser emissions simultaneously at two wavelengths in a weakly scattering system containing mixture of binary dyes, rhodamine-B (Rh-B) and oxazine-170 (O-170) dispersed with ZnO nano-particles serving as scattering centres. Random lasing performances for individual Rh-B dye were extensively studied for varying small signal gain/scatterer density and we found lasing threshold to significantly depend upon number density of dispersed nano-particles. In spite of inefficient pumping, we demonstrated possibility of random lasing in O-170 dye solution on account of resonance energy transfer from Rh-B dye which served as donor. At optimum concentrations of fluorophores and scatterer in dye mixture solution, incoherent random lasing was effectively attained simultaneously at two wavelengths centered 90 nm apart. Dual-emission intensities, lasing thresholds and rate of amplifications could be controlled and made equivalent for both donor and acceptor in dye mixture solution by appropriate choice of concentrations of dyes and scatterers. Keywords: Random lasing, Energy transfer, Rhodamine-B, Oxazine-170, Zinc oxid

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

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