We have studied spectra and angular distribution of emission of Rhodamine 6G dye in Fabry-Perot cavities in weak and strong coupling regimes and demonstrated control of the strong coupling with the pumping intensity.
We have studied spectra and angular distribution of emission in Fabry–Perot cavities formed by two silver mirrors separated by a layer of poly (methyl methacrylate) polymer doped with rhodamine 6G (R6G) dye in low ( 20 g / l ) and high ( 200 g / l ) concentrations. The frequency of emission radiated to a cavity mode was larger at large outcoupling angles—the “rainbow” effect. At the same time, the angle of the strongest emission was also determined by the cavity size: the larger the cavity, the larger the angle. The angular distribution of emission is commonly dominated by two symmetrical lobes (located at the intersection of the three-dimensional emission cone with a horizontal plane) pointing to the left and to the right of the normal to the sample. Despite the strong Stokes shift in R6G dye, the branch of the cavity dispersion curve obtained in the emission experiment is positioned above the one obtained in the reflection (extinction) experiment. Some dye molecules are poorly coupled to cavity modes. Their emission has very broad angular distribution with the maximum at θ = 0 ∘ . The signatures of strong cavity–exciton coupling were observed at high dye concentration ( 200 g / l ) but not at low concentration ( 20 g / l ). The evidence of the effect of strong coupling on emission is exemplified by a strong difference in the angular distribution of emission in two almost identical cavities, one with and another without strong coupling. Most importantly, we have demonstrated the possibility to control the ground state concentration, the coupling strength, and the dye emission spectra with Q-switched laser pulses.
Coupling of macroscopic ensembles of highly concentrated R6G molecules with aluminum and silver Fabry–Perot cavities of different sizes, in which one or two standing wave modes are in resonance with the absorption transition(s) of the dye molecules, have been studied. (i) It has been found that strong coupling of the cavity with one molecular eigenstate (S0→S1 transition) does not perturb the energies of the other molecular states. (ii) The S0→S2 absorption transition is too weak to strongly couple to the resonant cavity (“λ” standing wave mode). However, the resonant cavity causes a blue shift of the S0→S2 excitation band. (iii) The splitting of the dispersion curve is theoretically predicted and experimentally observed at strong coupling of the cavity resonance (“λ” standing wave mode) with the material resonances of Ag, including bound electron and “epsilon equal zero” transitions.
In this study, we demonstrate that the fluorescence of fluorescein dye molecules can be significantly modified in both the weak and strong coupling regimes by placing the molecules inside a perturbative or non-perturbative Fabry-Pérot cavity. By controlling the cavity thickness and dye concentration, we selectively altered the frequency and number of cavity transmission peaks, thereby establishing conditions for weak or strong coupling for fluorescein molecules. Using steady-state fluorescence spectroscopy, we confirmed that both the excitation and emission spectra were substantially tuned in these coupling regimes, enabling tunable and multicolor fluorescence from a single type of dye molecule, fluorescein.