Single-molecular surface-enhanced resonance Raman scattering as a quantitative probe of local electromagnetic field: The case of strong coupling between plasmonic and excitonic resonance

We investigate electromagnetic coupling between plasmonic and molecular electronic resonances using single-molecular surface-enhanced resonance Raman scattering (SERRS) from single silver nanoparticle dimers. When dimers exhibit SERRS activity, their elastic light scattering spectra show two lines, which are temporally closing toward each other. The higher energy line eventually disappears at the time of SERRS quenching. A coupled-oscillator model composed of plasmonic and molecular electronic resonances consistently reproduces the above interesting results by decreasing coupling energy, indicating that SERRS can be a quantitative probe for strong coupling between the two resonances.

Figure 1

(a1) Experimental setup for simultaneous measurement of SERRS and elastic light scattering from single Ag NP dimers. A sample aqueous solution of NaCl and Ag NP dimers adsorbed with R6G were sandwiched between two glass plates. D is a dark-field condenser lens, L is a convex lenses, O1–O3 are objective lenses, N is a notch filter, Pin is a pinhole, and CCD1 and CCD2 are charge-coupled devices. (a2) Elastic light scattering and (a3) SERRS images of Ag NP aggregates dispersed on a common glass plate. Scale bars are 5 μm. (b1)–(b3) Elastic light scattering (red lines) and SERRS spectra (blue lines) of three Ag NP dimers. After the spectroscopic measurements, the whole sample, including the ITO glass plate, was sent to a scanning electron microscope (SEM; JSM-6700F, JEOL) for the identification of the morphologies of the nanostructures [17]. The shapes, sizes, and orientations of the nanostructures were read off the acquired SEM images. The detailed process to find common Ag NP dimers is described elsewhere [17]. Insets of (b1)–(b3) are their SEM images. Scale bars are 50 nm.

Figure 2

(a) Elastic light scattering spectra of dimers showing SERRS activity (red lines) and those after losing SERRS activity (black lines). (b) Plot of peak energy of lower energy lines [indicated by arrowsin (a)] against their line widths in elastic light scattering spectra of the dimers showing SERRS activity (red circle) and after losing SERRS activity (black circle). (c) Absorption spectrum (black line) and fluorescence spectrum (red line) of R6G in aqueous solution.

Figure 3

(a1) Temporal series of elastic light scattering spectra for single Ag NP dimers. (a2) Images of temporal changes in elastic light scattering spectra made from (a1). (b1) Temporal series of SERRS and background spectra for the common single Ag NP dimers. (b2) Images of temporal changes in SERRS and background spectra made from (b1). The gray bars indicate loss of light by a laser notch filter.

Figure 4

(a1), (b1), and (c1) Images of temporal changes in elastic light scattering spectra for three silver NP dimers. Lower and higher energy lines in the elastic light scattering spectra are indicated by circles. (a2), (b2), and (c2) Images of temporal changes in SERRS and background spectra for three Ag NP dimers. The maximum positions in (a1), (b1), and (c1) are indicated by circles. Dimers for (a1), (b1), and (c1) correspond to those for (a2), (b2), and (c2). The white bars indicate loss by a laser notch filter.

Figure 5

(a1), (b1), and (c1) Lower energy line intensity against irradiation time (○) and higher energy line intensity against laser irradiation time (◊). (a2), (b2), and (c2) Widths of lower energy lines against laser irradiation time. (a3), (b3), and (c3) SERRS intensity at 2.16 eV against irradiation time. The gray bars indicate SERRS quenching time, as shown by stepwise changes. (a4), (b4), and (c4) SERRS intensity at 2.16 eV against lower peak energy from (a1), (b1), and(c1).

Figure 6

(a1), (b1), (c1), (d1), and (e1) Calculated spectra of elastic light scattering using $\hslash g$ from 500 to 0 meV with an interval of 100 meV. The insets show widths of lower energy lines against coupling energy. The values of the original plasmonic resonance energy of (a1), (b1), (c1), (d1), and (e1) are 2.3, 2.2, 2.1, 2.0, and 1.9 eV, respectively. The value of the original plasmonic resonance line width is 220 meV. (a2), (b2), (c2), (d2), and (e2) Contour images of the changes in elastic light scattering. Peak positions of lower and higher energy lines are indicated by circles.

Figure 7

Relationships between peak energy and line width of calculated elastic light scattering spectra made from Fig. 6 in order to evaluate blue shifts and broadening in experimental elastic light scattering in Fig. 2. The values of original plasmonic resonance energy for $\hslash g=0$ of ◊, $\nabla$, $\Delta$, □, and ○ are 1.9, 2.0, 2.1, 2.2, 2.3 eV, respectively. The values of $\hslash g$ for each dimer are from 500 to 0 meV, with an interval of 100 meV. The arrows indicate the decrease in the values of $\hslash g$.