Resonance-regime behaviour of a Fِrster-transfer fluorescent dye couple dissolved in a chiral nematic liquid crystal

We have studied the photophysics of a Fِrster-transfer fluorescent dye couple (Coumarin `sensitizerʹ/Pyrromethene `emitterʹ) dissolved in a chiral nematic liquid crystal (CNLC). Fluorescence spectra were dominated by the (lower concentration) emitter although only the (higher concentration) sensitizer was excited. The reflection band of the CNLC was chosen to overlap with the fluorescence band of the emitter. Emission was modulated by the photonic stopband condition imposed by the CNLC in a similar manner as for single-dye systems dissolved in CNLCs. Using Fِrster-couples instead of single-dye systems may lead to lower threshold dye-doped CNLC lasers.