Light emission through a corrugated metal film: The role of cross-coupled surface plasmon polaritons

We examine the phenomenon of light emission through a thin metal film that takes place via surface plasmon polaritons. Surface plasmon polariton cross coupling has recently been invoked to explain sharp features observed in the angle dependent emission spectra obtained from surface-emitting (through cathode) organic light-emitting diode structures. We investigated whether such a cross-coupling process is needed to explain such observations. We undertook measurements on samples for a variety of metal film thicknesses. Our results are consistent with the mechanism of surface plasmon polariton cross coupling but also show that the processes underlying the emission from such structures can be rather subtle.

Figure 1

(a) Schematic representation of a dispersion map for a thin metal film bordered on one side by an organic light-emitting material and on the other side by air (inset). The shaded region labeled air light-cone represents the frequencies and wave vectors accessible to light propagating in air. (b) Schematic representation of a dispersion map for a corrugated thin metal film bordered on one side by an organic light-emitting material and on the other side by air (inset). The area enclosed within the dashed lines represents the range of frequencies and wave vectors presented in Fig. 4. [Note that for clarity the horizontal scale used in (b) not the same as (a).] (c). An SEM of the sample with a silver film thickness of $61\mathrm{nm}$. The image was obtained by first milling a trench in the sample using a focussed ion-beam milling machine (FEI DB2000). The image is taken at an angle of 52° from the normal, courtesy of E. Devaux and T. Ebbesen, ISIS, Strasbourg, France.

Figure 2

Calculated power dissipation spectrum on a logarithmic scale for an emitter located in the light-emitting at a distance of $20\mathrm{nm}$ from the metal surface (a). The large peak at a normalized in-plane wave vector of $\sim 1.7$ indicates power being lost to the metal/organic SPP, the inset is a expanded plot (linear scale) of the feature associated with the metal/air SPP. The power coupled to the metal/organic and metal/air SPP modes as a function of silver film thickness (b).

Figure 3

Photoluminescence spectra of light emitted through a corrugated $55\mathrm{nm}$ thick silver film, and a $55\mathrm{nm}$ thick planar control film.

Figure 4

Dispersion map constructed from photoluminescence (PL) emission spectra for a sample with a $55\mathrm{nm}$ thick corrugated silver film. The $z$ axis represents intensity and is on a log scale with dark regions indicating strong emission. The line labeled 26° corresponds to the PL emission spectrum measured at a polar emission angle $\left(\mathrm{\theta }\right)$ of 26°. In order to see the modal features the intensity at the crossing-point is overexposed.

Figure 5

Peak value of the photoluminescence emission associated with the SPP at (a) the Ag/air and (b) the $\mathrm{Ag}∕\mathrm{Al}{\mathrm{q}}_3$ interfaces as one tracks the dispersion of each feature. The data shown are for a sample with a silver thickness of $55\mathrm{nm}$.

Figure 6

The ratio of the observed intensity at the crossing point $I_C$ to the expected intensity $I_{\exp }$.

Figure 7

TM polarized transmittance data in the form a dispersion map. The sample had a silver film thickness of $55\mathrm{nm}$.

Figure 8

Relative intensity of the emission seen via the two SPP modes as a function of silver film thickness.