Strong Coupling between Surface Plasmon Polaritons and Molecular Vibrations

We report on the strong coupling of surface plasmon polaritons and molecular vibrations in an organic-inorganic plasmonic hybrid structure consisting of a ketone-based polymer deposited on top of a silver layer. Attenuated-total-reflection spectra of the hybrid reveal an anticrossing in the dispersion relation in the vicinity of the carbonyl stretch vibration of the polymer with an energy splitting of the upper and lower polariton branch up to 15 meV. The splitting is found to depend on the molecular layer thickness and saturates for micrometer-thick films. This new hybrid state holds a strong potential for application in chemistry and optoelectronics.

Figure 1

(a) Schematics of the ATR measurements in the Kretschmann-Raether configuration using a calcium fluoride hemicylindrical prism as the coupling medium. The in-plane wave vector $k=(\omega /c)\sqrt{{\epsilon }_{{\mathrm{CaF}}_2}}\sin \alpha$ is experimentally defined by the angle of incidence $\alpha$. (b) Transmission spectrum of a 1360-nm-thick layer of PVMK on silicon in TM polarization ($\alpha =45°$). Full black curves: Experiment. Dashed red curves: TRMA calculations. Inset 1: Enlargement of the transmission signal of the stretch vibration of the carbonyl group. Inset 2: Repeating unit of the PVMK polymer.

Figure 2

TM-polarized ATR spectra of a PVMK/silver hybrid structure in the strong coupling regime. The silver and PVMK layer thickness is 29 and 965 nm, respectively. (a) Experimental spectra. (b) Corresponding TRMA calculations. The angle of incidence changes for all curves from $\alpha =60°$ to 68° in steps of 2° (bottom to top). UB, upper branch; LB, lower branch. Dashed lines follow the ATR minima. The dotted line marks the molecular resonance frequency ${\omega }_0$. For parameters used in the TRMA computations, see the text.

Figure 3

Dispersion relation of a PVMK/silver hybrid structure in the strong coupling regime ($d=965\mathrm{nm}$). UB, upper branch (blue); LB, lower branch (red); middle branch (black). (a) Circles: Experimental ATR minima. Full curves: Dispersion ($\mathrm{Re}\omega$) calculated with the molecular resonance [${\epsilon }_1(\omega )={\epsilon }_m$]. Dashed curve: Dispersion ($\mathrm{Re}\omega$) calculated without the molecular resonance [${\epsilon }_1(\omega )={\epsilon }_{bm}$]. Dotted curves: Light line in air (black) and PVMK (red). Inset: The configuration analyzed theoretically. (b) The calculated dissipation rate ($\mathrm{Im}\omega$). Full curve: With the molecular resonance. Dashed curve: Without the molecular resonance. Dotted line: The molecular dissipation rate. (c) Calculated penetration depths into air ($1/\mathrm{Re}{\mathrm{q}}_0$). The dispersion relation is calculated from Eq. (4).

Figure 4

Dependence of the energy splitting on the molecular film thickness. Red circles: Experimental results. Dashed line: Calculated from Eq. (4). Dotted line: Splitting of a hybrid structure with an infinitely thick PVMK layer calculated from Eq. (4) with ${\epsilon }_1={\epsilon }_0={\epsilon }_m(\omega )$.