Coupling between semiconductor quantum dots and two-dimensional surface plasmons

When $\mathrm{Cd}\mathrm{Se}∕\mathrm{Cd}\mathrm{S}$ core/shell quantum dots (QDs) are dispersed onto a two-dimensional (2D) textured Ag grating, their luminescent behavior is dramatically modified. Excited QDs can resonantly excite surface plasmon polaritons (SPPs) propagating along the modulated metallic surface. The measurements of PL spectra and time-resolved PL spectra clearly indicate the interaction between QDs and 2D SPPs. The electromagnetic interaction between QDs and the 2D corrugated metal surface makes the luminescence be enhanced, polarized, and well directed.

Figure 1

The SEM image of sample $A$. The left inset shows the schematic of the surface and defines the direction $\psi$. The right inset gives the absorption and PL spectra of the QDs used in the experiment. The PL peak is at $\sim 610\mathrm{nm}$ with a full width at half maximum of $\sim 30\mathrm{nm}$, indicating a good size distribution of the QD assemble.

Figure 2

The ratio of $p$- over $s$-polarized emission intensity measured at $610\mathrm{nm}$ for samples $B$ and $C$, respectively, and the ratio of $p$- over $s$-polarized integrated intensity for sample $A$.

Figure 3

The PL decays of sample $C$ with the emission angles at 25 and 30°, respectively. $P$-25 and $P$-30: lifetimes measured at 25° and 30° for $p$-polarized light, respectively; $S$-30: lifetime measured at 30° for $s$-polarized light. For comparison, the PL decay of QDs on a smooth silica surface is shown in the inset. These PL decay curves can be fitted well by a biexponential function in the form of $A_1\exp (-t∕{\tau }_1)+A_2\exp (-t∕{\tau }_2)$, where the values of ${\tau }_1$ and ${\tau }_2$ are in nanosecond time scale. Colloidal II-VI semiconductor QDs usually exhibit multiple PL decay routes as a result of the complex energy structure near their energy band edges (Ref. 6). The average time $\left({\tau }_{\mathrm{av}}\right)$, which is defined as $(A_1\tau _1{}^2+A_2\tau _2{}^2)∕(A_1{\tau }_1+A_2{\tau }_2)$ (Ref. 15), is used here to label the radiative lifetime.

Figure 4

The $s$- and $p$-polarized emission spectra from sample $A$ with the emission angle of 75° and the direction $\psi$ of 0°. The two spectra are plotted according to their relative intensities. The inset shows the radiative decay monitored at 610 and $596\mathrm{nm}$, respectively.