Strong coupling between surface plasmon-polaritons and organic molecules in subwavelength hole arrays

The interaction of a $J$-aggregate and surface plasmon polariton modes of a subwavelength hole array have been studied in detail. By measuring the effects of hole array period, angular dispersion and concentration of the $J$-aggregate on the transmission of the array, the existence of a strong coupling regime is demonstrated with a Rabi splitting of 250 meV. This large splitting is explained not only by the high oscillator strength of the dye but also by the high local field amplitudes generated by surface plasmons of the metallic structure.

Figure 1

Scanning electron image of a typical hole array milled in a silver film ($\text{thickness}=370\mathrm{nm}$; $\text{period}=380\mathrm{nm}$; $\text{diameter}=150\mathrm{nm}$).

Figure 2

Normal incidence transmission spectrum of a silver hole array $(\text{period}=380\mathrm{nm})$ covered with a 300-nm-thick PVA film (black), and absorption spectrum of a PVA film doped with the cyanine dyes (dashed gray line). Markers indicate the wavelengths of the different SPP modes estimated from Eq. (1) and identified by the indices $(i,j)$.

Figure 3

Normal incidence transmission spectra of a hole array $(P=380\mathrm{nm})$ covered with undoped PVA (dotted line) and with $J$-aggregate doped PVA (solid line). The vertical line at 693 nm indicates the peak of the $J$-aggregate absorption band.

Figure 4

(Color) Energy dispersion curves for undoped samples (white) and $J$-aggregate doped samples (red and blue) determined by varying the period of the hole arrays. The blue and red dots correspond to the two peaks shown in Fig. 3 with an extended range of period (from 290 to 450 nm), and the red dashed line to the exciton absorption energy.

Figure 5

(Color) SPP dispersion curves, for a silver/PVA (black dotted) and silver/$J$-aggregate doped PVA interface (blue curves). These plots were done using Eqs. (1, 2); the dielectric constant of silver was taken from Ref. 29 and that of the dye was estimated from a simple Drude-Lorentz model using its known absorption properties. The period $P$, used for these calculations, was chosen such that the (1,0) modes of the PVA/silver interface cross the dispersionless exciton band (red line) at normal incidence.

Figure 6

(Color) Measured transmittance of $p$-polarized light as a function of angle of incidence and photon energy for different hole arrays with period 330 (a), 380 (b), and 430 (c) nm. The data are presented using a grey-intensity scale (white stands for high transmission while black for low transmission). White dotted lines represent the theoretical dispersion curves of the SPP mode propagating at the PVA/silver interface in the absence of dye; the blue and red curves are only guides for the eye to show the two upper and lower polariton branches (UB and LB). Note that the data were mainly collected for positive angles and the symmetrical representation is only used for clarity.

Figure 7

Variation of the Rabi splitting as a function of the square root of the absorbance of the $J$-aggregate/PVA film.