Changes in fluorescence lifetimes induced by variable optical environments

Changes in fluorescent lifetimes induced by variation of the optical environment of active centers (${\mathrm{Eu}}^{3+}$ ions) were observed in three new types of experiments with about 30-nm-thick evaporated europium benzoyltrifluoroacetone chelate (EuBTF) layers. The decay of their fluorescence after excitation with uv light (${\lambda }_{\mathrm{uv}}=366\mathrm{}$ nm) in the emission bands at $\lambda =612\mathrm{} \mathrm{and} 592$ nm was measured. (1) For EuBTF layers deposited—in one and the same coating run—on a number of substrates $S_1$ with different refractive indices $n_1$, the $\frac{1}{e}$ decay time $t_e$ decreases with increasing index $n_1$; we found, e.g., $t_e=620\mathrm{}$ μs on BK7 glass ($n_1=1.52\mathrm{}$), $t_e=545\mathrm{}$ μs on SF6 glass ($n_1=1.80\mathrm{}$), and $t_e=460\mathrm{}$ μs on a SrTi${\mathrm{O}}_3$ crystal ($n_1=2.40\mathrm{}$). (2) For an EuBTF layer on a substrate $S_1$, the fluorescence lifetime is changed by varying, either (a) spatially or (b) as a function of time, the width $d$ of the air gap between the layer and a second plate $S_2$ between $d\gg \lambda$ and $d=0\mathrm{}$. For a layer on a fused silica (Si${\mathrm{O}}_2$) substrate $S_1$, the decay time $t_e=620\mathrm{}$ μs for $d\gg \lambda$ was reduced to $t_{}^{\prime }{}_e{}^{}=415\mathrm{}$ μs in optical contact ($d=0\mathrm{}$) with a SrTi${\mathrm{O}}_3$ crystal plate $S_2$. With the optical-contact experiments (2b) we have experimentally demonstrated for the first time that spontaneous emission rates can be reversibly changed for one and the same fluorescent center by varying its optical environment. The experiments permit us to determine the quantum efficiency (QE) of the emitting state; this QE has to be distinguished from the fluorescence quantum yield. Analytical expressions are presented for the spontaneous emission rate of electric and magnetic dipole transitions in active centers embedded in a very thin layer (with optical thickness $n_0{d}_0\ll \frac{\lambda }{8}$) between two adjoining loss-free dielectric media. With this theory we obtained the approximate value ${\eta }_{\infty }\simeq 70\%$ for the QE of the ${}_{}{}^5{}_{}{}^{}D_0^{}{}_{}{}^{}$ level of the ${\mathrm{Eu}}^{3+}$ ion. In our experiments, fluorescence intensity and decay times were also measured as functions of the angle of observation for both states of polarization of the emitted light. This permits us to distinguish between electric and magnetic dipole transitions and yields information about the orientation of the transition dipole moments. We found that the fluorescence at $\lambda =612\mathrm{}$ nm is emitted in electric dipole transitions and that the orientation of the transition dipole moment of each ${\mathrm{Eu}}^{3+}$ ion fluctuates randomly in time intervals much smaller than the decay time.