Stimulated Emission of Surface Plasmon Polaritons

We have observed laserlike emission of surface plasmon polaritons (SPPs) decoupled to the glass prism in an attenuated total reflection setup. SPPs were excited by optically pumped molecules in a polymeric film deposited on the top of a silver film. Stimulated emission was characterized by a distinct threshold in the input-output dependence and narrowing of the emission spectrum. The observed stimulated emission and corresponding compensation of the metallic absorption loss by gain enables many applications of metamaterials and nanoplasmonic devices.

Figure 1

(a) Experimental sample; excitation and decoupling of SPP. (b) Angular profile of emission of scattering-induced SPPs (trace 1) and the angular profile of reflectivity (trace 2); $\lambda =575\mathrm{nm}$. (c) Spectra of SPP spontaneous emission decoupled at different angles $\theta$; characters—experiment; solid lines—simulations; triangles and trace $1-\theta =67.17°$, circles and trace $2-\theta =66.14°$, squares and trace $3-\theta =65.62°$. The thickness of the silver film is 57 nm. (d) Angular profile of emission of scattering-induced SPPs (trace 1) and the angular profile of reflectivity (trace 2). $\lambda =632.8\mathrm{nm}$.

Figure 2

(a) Spectra of the SPP emission recorded at $\theta =68.7°$ at low pumping density $10.9\mathrm{mJ}/{\mathrm{cm}}^2$ (diamonds) and high pumping density $81.9\mathrm{mJ}/{\mathrm{cm}}^2$ (squares). (b) Input-output curves of SPP emission recorded at different angles $\theta$. The thickness of the silver film is 39 nm. The diameter of the pumped spot is 2.16 mm, $\lambda =602\mathrm{nm}$. Squares—$\theta =63.17°$, circles—$\theta =68.70°$, diamonds—$\theta =59.12°$, triangles—$\theta =70.35°$. Resonance angle—${\theta }_0=65.56°$. (c). Dependence of the SPP stimulated emission threshold vs $\theta$ (circles) and the reflectivity profile (squares) measured at $\lambda \approx 604\mathrm{nm}$.

Figure 3

(a) Calculated SPP spectra in the wave number domain. The values of gain in the SPP system (from high to low) correspond to $k_x^{\prime \prime }=-50,-45,\dots ,-10$, $-5$, $-1$, $0{\mathrm{cm}}^{-1}$. ${\epsilon }_0=n_0^2=3.18$, ${\epsilon }_1=-13.6+i0.75$, ${\epsilon }_2=2.25$; $\lambda =594.1\mathrm{nm}$). The thickness of the silver film 39 nm. (b) Input-output curves of SPP emission calculated above the threshold at $k_x^{\prime }=173660{\mathrm{cm}}^{-1}$ (close to the resonance)—diamonds, $173740{\mathrm{cm}}^{-1}$—squares, $173860{\mathrm{cm}}^{-1}$—triangles, and $174000{\mathrm{cm}}^{-1}$—circles.