Excitation laser energy dependence of surface-enhanced fluorescence showing plasmon-induced ultrafast electronic dynamics in dye molecules

We find unique properties accompanying surface-enhanced fluorescence (SEF) from dye molecules adsorbed on Ag nanoparticle aggregates, which generate surface-enhanced Raman scattering. The properties are observed in excitation laser energy dependence of SEF after excluding plasmonic spectral modulation in SEF. The unique properties are large blue shifts of fluorescence spectra, deviation of ratios between anti-Stokes SEF intensity and Stokes from those of normal fluorescence, super-broadening of Stokes spectra, and returning to original fluorescence by lower energy excitation. We elucidate that these properties are induced by electromagnetic enhancement of radiative decay rates exceeding the vibrational relaxation rates within an electronic excited state, which suggests that molecular electronic dynamics in strong plasmonic fields can be largely deviated from that in free space.

Figure 1

(a) Conventional absorption and fluorescence transition of a molecule in a free space. $S_0$ and $S_1$ are electronic ground and excited states with vibrational states, respectively. ${\omega }_{\mathrm{ex}}$ and ${\omega }_{\mathrm{em}}$ are excitation and emission angular frequencies, respectively. Envelopes of three red bands and three blue bands correspond to fluorescence and absorption spectra, respectively. (b) Distribution of $|E_{\mathrm{loc}}{|}^2/{\left|E_{\mathrm{in}}\right|}^2$ around an Ag NP dimer calculated by finite-difference time-domain method and enlarged image of a crevasse of the Ag NP dimer. (c) Excitation laser line (blue lines) dependence of SDSEF transition (red lines). Envelope of three red bands corresponds to SDSEF spectrum (red lines). (d) Excitation laser line (blue lines) dependence of FDSEF transition (red lines). Envelope of three red bands corresponds to FDSEF spectrum (red lines).

Figure 2

(a1, b1, and c1) SEM images of three Ag dimers showing SERS and SEF activity. (a2 and b2) SEF spectra excited with 2.33 eV (532 nm, red lines) and plasmon resonance spectra (blue lines) from the Ag dimers in (a1 and b1). (c2 and c3) SEF spectra excited with 2.33 eV (532 nm, red lines in c2) and 1.96 eV (633 nm, red lines in c3) and common plasmon resonance spectra (blue lines in c2 and c3) from the Ag dimer in (c1). (d) SEM image of a portion of large Ag NP aggregates. (e) Focused (left panels) and defocused (right panels) SEF images excited with 2.41 eV (514 nm, upper panel) and 2.21 eV (561 nm, lower panel). (f) SEF spectra excited with 2.41 eV (green line) and 2.21 eV (red line), and elastic light-scattering spectra (blue lines) of large Ag NP aggregates. Scale bars are as follows. (a1), (b1), and (c1): 100 nm; (d): 1 μm; and (e): 10 μm.

Figure 3

(a–f) Excitation laser energy dependence of SEF spectra of CV from large Ag NP aggregates excited with (a) 2.71, (b) 2.54, (c) 2.41, (d) 2.33, (e) 2.21, and (f) 1.96 eV (red lines) and absorption spectrum of CV molecules in a free space (black lines). Detailed structures around excitation laser lines are SERS bands. (g) Absorption (black line) and fluorescence (red lines) spectrum of CV molecules in aqueous solution (∼10${}^{-6}$ M).

Figure 4

(a–c) Excitation laser energy dependence of SEF spectra of R123 (left panels), R6G (center panels), and Rb (right panels) from large Ag NP aggregates and absorption spectra of each molecule in aqueous solution (∼10${}^{-6}$ M) (black lines). Detailed structures around excitation laser lines are SERS bands. (d) Excitation photon energy dependence of cutoff energy of SEF spectra for CV (○), R123 (□), R6G (Δ), and Rb (∇) molecules. (e) Excitation photon energy dependence of ratio of anti-Stokes (with shift from −7.0 to −3.0 meV) to Stokes intensity (with shift from 3.0 to 7.0 meV) of SEF spectra for CV (○), R123 (□), R6G (Δ), and Rb (∇) molecules. Inset is excitation photon energy dependence of ratio of anti-Stokes to Stokes intensity of fluorescence spectra for CV (○), R123 (□), R6G (Δ), and Rb (∇) molecules of dye aqueous solution (∼10${}^{-6}$ M).

Figure 5

Ag NP dimer-by-dimer variations in (a1–a5) SEF spectra of R6G excited with 2.33 eV, (b1–b5) plasmon resonance spectra. (c1–c5) Ag NP dimer-by-dimer variations in SEF spectra of R6G divided by the plasmon resonance ones. Dotted lines in (c1–c5) are fluorescence spectra of R6G in aqueous solution (10${}^{-6}$ M).