Two-Dimensional Drexhage Experiment for Electric- and Magnetic-Dipole Sources on Plasmonic Interfaces

Fifty years ago, Drexhage et al. showed how photon emission from an electric dipole can be modified by a nearby mirror. Here, we study the two-dimensional analog for surface plasmon polaritons (SPPs). We print ${\mathrm{Eu}}^{3+}$-doped nanoparticles, which act as both electric- and magnetic-dipole sources of SPPs, near plasmonic reflectors on flat Ag films. We measure modified SPP radiation patterns and emission rates as a function of reflector distance and source symmetry. The results, which agree with an analytical self-interference model, provide simple strategies to control SPP radiation in plasmonic devices.

Figure 1

Experimental setup. (a) SPP source on plasmonic Ag surface structured with 480-nm-tall reflector and quarter-circular outscatterer. (b) Close-up of the source consisting of a printed line of ${\mathrm{NaYF}}_4:{\mathrm{Eu}}^{3+}$ nanocrystals. (c)–(e) Nanocrystals (highlighted with black rectangle) placed $10\mu \mathrm{m}$ from a quarter-circular outscatterer in absence of a plasmonic reflector. SPPs scattered into photons at the quarter-circle upon laser excitation of the nanocrystals collected as (d) real-space fluorescence image and (e) spectrally dispersed along the horizontal direction. (f)–(h) The corresponding experiment with a plasmonic reflector at $d=544\mathrm{nm}$ from the nanocrystals. (a)–(c),(f) Electron micrographs.

Figure 2

(a) Electric-field-amplitude patterns of SPPs emitted by the dipole components $p_z$, $m_x$, and $m_y$ on a plasmonic substrate (xy plane) in absence of a reflector. Schematic representation of self-interference of the SPP emission from (b) a $z$-oriented ED and (c) a $y$-oriented MD in front of a plasmonic reflector.

Figure 3

(a)–(c) SPP radiation patterns of isotropic ED (blue) and MD sources (red) at different distances $d$ from a plasmonic reflector. (Dots) Experimental data; (Lines) Calculated patterns. (d) Experimental and (e) calculated angular ED and MD branching ratios as color maps. Analytically calculated self-interference maxima for $p_z$ (dashed), $m_x$ (dotted), and $m_y$ (dash-dotted) are overlaid.

Figure 4

Experimental (dots) and calculated (lines) MD branching ratios as function of emitter-reflector separation $d$, integrated over all SPP emission angles (red). The narrow-angle branching ratio depicted in black only considers emission angles $\phi$ smaller than 10°.