Causality of surface plasmon polariton emission processes

Fundamental physical quantum phenomena, such as surface plasmon polaritons (SPP’s), require specific conditions as described in their dispersion relation for excitation by photons. These conditions exactly define the $\mathbf{k}$ vector of the incident photons as well as the elastically reemitted, outbound photons that follow from the decay of the plasmon. Our conclusion demonstrates that these requisites are indeed fulfilled even on the introduction of an inelastic, wavelength-shifting process like fluorescence emission to the system. Such a process is established by placing fluorescent molecules into the strong evanescent field of the SPP. The analysis of the outbound photons shows an angular behavior, which can only be explained if the fluorescent photons also satisfy SPP conditions. This behavior enables the development of a model for the chronology and causality of surface plasmon polariton deexcitation mechanisms.

Figure 1

(Color online) The ATR-SPP setup (Kretschmann type). Note the differences in definition between angles $\alpha$, $\gamma$, and $\phi$.

Figure 2

(Color online) The ATR-SPP setup (Kretschmann type). Plasmon excitation at $514.5\mathrm{nm}$ on an evaporated silver film. The ATR SPP cone is projected onto a screen and is visible as a bright circle. The “spots” are a result of the unscattered strong laser light and its partial reflections, while the brightness of the circle in respect to the spots is a measure of the silver surface roughness. For the sake of illustrating the phenomenon an untypical rough silver film and a semispherical prism were used.

Figure 3

(Color online) (a) Sketch for the detection of elastically scattered photons within the ATR cone. (b) Sketch of the wavelengths distribution and its detection within the ATR cone.

Figure 4

(Color online) Measured angular dependence of the fluorescent emission with respect to its wavelength.

Figure 5

(Color online) Calculation of the SPP excitation with respect to the angle $\alpha$ for the selected three wavelengths.