Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-7-12-4286
Timestamp: 2019-04-26 12:00:09+00:00

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Laser dyes, in particular, rhodamine 6G (R6G), play an important role in many proof-of-principle demonstrations in metamaterials, nanophotonics, plasmonics, and strong coupling. Despite the numerous experimental and theoretical studies, interpretation of many features in optical spectra of high-concentrated R6G dye is still a subject of controversy. In this work, we have measured and interpreted absorption, excitation, and emission spectra of polymeric (PMMA) films doped with R6G dye. In contrast to several reports, our results suggest that the ~495 nm shoulder in the absorption spectrum is chiefly not due to a dimer formation, but is likely owing to vibronic transitions.
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Fig. 1 Absorption spectra of R6G:PMMA films. Trace 1 – experiment; trace 2 – fit with the sum of two Gaussian functions, traces 3 and 4 – Gaussian functions corresponding to the main peak and the shoulder, respectively. (a) c = 2.2x10−4 mol/L, (b) c = 6.7x10−1 mol/L, (c) c = 8.9x10−1 mol/L.
Fig. 2 (a) Concentration dependence of the area under the Gaussian bands, representing the main peak (red circles) and the shoulder (blue squares) of the absorption spectrum (blue squares). (b) Dependence of the concentration of monomers (red trace) and dimers (blue trace) on the total concentration of molecules, predicted by the aggregation/dissociation model. The slopes of the curves at low and high molecular concentrations are shown in the figure.
Fig. 3 Emission and excitation spectra of R6G:PMMA: (a) c = 2.2x10−3 mol/L. Trace 1 – experimental emission spectrum; trace 2 – its fit with the sum of two Gaussian functions, traces 3 and 4 – Gaussian functions corresponding to the main peak and the shoulder of the emission band. Trace 1’ – experimental excitation spectrum; trace 2’ – its fit with the sum of two Gaussian functions; traces 3′ and 4’ – Gaussian functions corresponding to the main peak and the shoulder of the excitation band. (b) c = 6.7x10−1 mol/L. Traces 1,2-4 and 3′,4’: same as in Fig. 3(a). Emission spectra 1, 5 and 6 were collected when the samples were excited at 400 nm, 485 nm and 530 nm, respectively. Excitation spectra 7, 9 and 10 were collected when the emission was collected at 606 nm, 555 nm and 620 nm, respectively. Trace 8 – fit of trace 7 with the sum of two Gaussian functions.
Fig. 4 Comparison of the excitation and the absorbance spectra: Trace 1 – experimental excitation spectrum, trace 2 – its fit with the sum of two Gaussian functions, traces 3 and 4 – Gaussian functions corresponding to the main peak and the shoulder, respectively, trace 5 – experimental absorbance spectrum (a) c = 2.2x10−4 mol/L, (b) 6.7x10−1 mol/L. Insets: zoomed maxima of the excitation and absorbance bands.
Fig. 5 Normalized absorbance spectrum (trace 1), scaled excitation spectrum (trace 2), and their difference (trace 3), suggest that the molecules excited close to the long-wavelength edge of the absorption spectrum contribute to spontaneous emission less than the molecules excited close to the short-wavelength edge of the absorption spectrum. (a) c = 2.2x10−3 mol/L, (b) c = 6.7x10−1 mol/L.
Fig. 6 Energy level diagram of coupled molecules (the configuration coordinate is plotted on the horizontal axis). Parabola S0 represents the ground state. Parabola S1 represents the first excited state of uncoupled molecules. Parabolas S1- and S1+ represent the two branches of the split excited state of the strongly coupled molecules. The transition | 2P−〉→| 3P−〉 is parity forbidden. Correspondingly, the molecules excited at the transition | 1〉→| 2P−〉 do not emit. This explains the shift between the absorption and excitation bands observed in our experiment. (Adopted and modified from Ref .).

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