Surface plasmons limit the efficiency of organic light‐emitting diodes (OLEDs). Up to 40 % of the radiative power may be lost to surface plasmon (SP) modes associated with the metallic cathodes of OLEDs. Calculations show the importance of material selection and device geometry in this process. Experiments reveal the presence of the SP modes and suggest ways in which this power may be recovered using periodic nanostructure.

The emission of light by sources in close proximity to a thin metallic film is dominated by surface plasmon-polariton modes supported by that film. We explore the nature of the modes and examine how the energy lost to such modes can be recovered. Both cross-coupled and coupled SPPs are presented as a means of transferring energy across a thin metal film. These modes are then scattered and thereby coupled to light by a wavelength scale grating type microstructure. We show that the photoluminescence emission from a structure containing a microstructured thin metal film that supports coupled SPPs is over 50 times greater than that from a similar planar structure. Similar strong photoluminescence emission is also exhibited by a sample that contains a planar metal film coated with a microstructured dielectric overlayer.

We report strong photoluminescence from a top-emitting organic light-emitting structure where emission takes place through a thin ͑55 nm͒ silver film. We show that this emission is mediated via coupled surface plasmon-polariton modes. Our results show that the addition of a dielectric grating to otherwise planar structures, such as surface-emitting organic light-emitting diodes, may offer a way to increase the external efficiency of top-emitting organic light-emitting diodes. have two advantages over substrate-emitting structures. First, the drive electronics may be integrated into an opaque silicon substrate and second, losses associated with guided modes in the substrate are eliminated. However, a top-emitting OLED does present other problems, notably losses to the surface plasmon-polariton (SPP) modes associated with the metallic cathode. Calculations show that up to 40% of the power that would otherwise be emitted may be lost via this decay channel, 4,5 thus limiting the external quantum efficiency. If some way could be found to recover some of the power lost to these SPPs to light, device efficiency could be improved. One method of coupling SPP modes to light has been the introduction of a periodic microstructure into the metal film, allowing the SPPs to Bragg scatter, 6-8 however, such an approach is demanding in terms of fabrication. Here, we present an alternative in which a microstructured dielectric overlayer is superimposed onto the completed device.A metallic cathode supports two SPP modes, one associated with each metal surface. For a given frequency, these two modes have different in-plane wave vectors (momenta) and so do not interact. An emitter in the organic layer couples more strongly to the SPP associated with the metal/ organic interface than with the SPP associated with the metal/air interface. 9 The energy coupled into the metal/ organic SPP thus needs to be transported across the metal if it is to emerge as light. One way is to use grating-mediated SPP cross coupling; an appropriate microstructure enables the momentum mismatch between the SPP modes to be overcome, allowing them to interact providing a route to transport energy across the film.9,10 However, emission mediated via SPP cross-coupling occurs only over a narrow range of emission angles and wavelengths. 9 An alternative method of coupling the SPPs is to ensure that the materials on either side of the metal have the same refractive index, the SPP wave vectors are then degenerate and may couple.11 It is this coupled geometry that we employ here, a dielectric overlayer added to the metal acts to match the effective refractive index sampled by SPP fields. The coupled SPPs are then scattered to light by a microstructure imposed on the top surface of the dielectric layer.Our experimental structure consisted of a silica substrate spin coated with a 160 nm film of a polymer poly(methylmethacrylate) (PMMA) doped with tris(8-hydroxyquinoline)aluminium ͑Alq 3 ͒ (3% Alq 3 by weight). A 55 nm thick silver film was added by thermal evaporation...

We present experimental results showing electroluminescence from a dendrimer based organic light-emitting diode ͑OLED͒ mediated via surface plasmon polariton ͑SPP͒ modes. A combination of angle dependent electroluminescence, photoluminescence, and reflectance measurements is used to identify emission originating from the guided modes of the device. It is found that the SPP modes, which are usually nonradiative, are coupled to light by a wavelength scale periodic microstructure. It is demonstrated that the necessary microstructure can be readily fabricated by solvent-assisted micromoulding. Our results indicate that such an approach may offer a means to increase the efficiency of dendrimer based OLEDs. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2193795͔While organic light-emitting diodes ͑OLEDs͒ have shown much promise in display applications, their device architecture is such that excitons created within the organic emissive layer may relax via a number of decay routes. These routes may include some or all of the following: surface plasmon-polariton ͑SPP͒ modes associated with the metallic cathode/organic interface, waveguided modes within the substrate and organic emissive layers, and Joule heating of the cathode. As a consequence of these decay routes, the intensity of the emitted light from these devices into the far field may be significantly impaired. 1,2 Improving the efficiency with which light is extracted from these structures is thus a key concern.One class of material that has shown great promise as highly efficient organic emitters are phosphorescent iridium ͑III͒ complex cored dendrimers. 3,4 Dendrimers allow fine tuning of photophysical properties by independent variation of core, dendrons and dendrimer generation. [5][6][7] The combination of high dendrimer light-emitting diode ͑DLED͒ efficiency, molecular level engineering of the physical properties, and their suitability for solution processing makes these materials extremely attractive for industrial fabrication processes. However, DLEDs using phosphorescent dendrimer emitters also suffer losses to SPP and waveguided modes, thus diminishing the overall device efficiency. 2 The very high DLED internal quantum efficiency ͑ϳ80% ͒ of the best dendrimer materials indicates that significant increases in light output from these OLEDs must be sought via increases in the optical outcoupling efficiency of these devices, and it is just such an approach that we pursue here. In particular, we concentrate upon recovering energy lost from the emissive layer to SPP modes as useful light in the far field by modifying the device geometry, an aspect of DLED devices that has not so far been investigated experimentally.SPPs are guided transverse magnetic ͑TM͒ polarized electromagnetic surface modes that occur at the interface between a metal and a dielectric. SPP modes have electromagnetic fields that decay exponentially into both the bounding metal and dielectric media and, as they possess a greater momentum than free space photons of the same frequency...

The optical reflectivity of a metal-dielectric-metal microcavity in which the upper layer is periodically perforated by narrow slits is explored. Complete characterization of the observed modes in terms of their resonant electromagnetic fields is achieved by comparison of the experimental data to the predictions of a finite-element model. In particular, we demonstrate that the slits provide efficient diffractive coupling to a surface plasmon mode buried within the microcavity whose propagation is strongly confined to the dielectric layer. DOI: 10.1103/PhysRevB.74.073408 PACS number͑s͒: 78.20.Ci, 42.25.Bs, 42.79.Dj, 73.20.Mf The texturing of metal surfaces on a subwavelength scale is known to strongly modify their electromagnetic ͑EM͒ response. Excitation of surface plasmons ͑SPs͒ at these interfaces plays a crucial role in light-matter interactions ͑see Refs. 1 and 2, and references therein͒. One of the reasons why the field of plasmonics has received so much attention in recent years is the localization of EM fields associated with these modes, together with their ability to guide energy along interfaces. In addition, metallic microcavity structures have also been explored to provide enhanced performance in a range of optoelectronic devices ͑e.g., light-emitting diodes 3 ͒. Furthermore, light-guiding structures that allow subwavelength confinement of the optical mode are critical for the achievement of compact photonic components.4,5 The design and fabrication of metallic nanostructures combining surface wave properties with microcavity resonant behavior opens up substantial new device potential and has already seen application as ultrathin electromagnetic attenuators in the microwave regime. 6 In this paper, we study the visible EM response of a metal-dielectric-metal microcavity where the illuminated metal film is perforated with periodically spaced identical slits ͓Fig. 1͑a͔͒. Both the width of these slits and the thickness of the dielectric core are ϳ /10 ͑where is the incident wavelength͒, the latter being small enough to permit coupling between SPs of the two metal surfaces. The excitation of these coupled-SP modes 7 using radiation polarized orthogonal to the slits results in resonant absorption of the incident power via Joule heating of the metal. In addition, it is found that single-surface SPs are excited at the illuminated face of the slit array. By recording the positions of the resonances in the reflectivity spectra as a function of the angle of incidence, the band structure of the modes supported is elaborated. The predictions of a finite-element method ͑FEM͒ model 8 are then fitted to the experimental data with good agreement being obtained. Each of the modes supported by the structure is identified by studying the numerically predicted EM field distributions.To fabricate the sample, a Ag/ SiO x / Ag trilayer structure was deposited by vacuum evaporation onto a clean glass slide. The first silver layer is optically thick ͑Ͼ100 nm͒, with the thickness of the SiO x dielectric core ͑t c ͒ and up...