Seamus Burns

Role of optical properties of metallic mirrors in microcavity structures

In thin metal films the phase change on reflection of incident light is dependent on the wavelength, the angle of incidence, the type of metal, and the metal thickness. These properties have been exploited to improve the performance of planar metal mirror microcavities. We model substantial alteration of peak emission wavelength and linewidth with mirror thickness. This allows the tuning of the cavity resonance wavelength by variation of metal mirror thickness. The dependence of the phase change on wavelength and angle of incidence can also be used to suppress the angular dependence of the cavity resonance wavelength. These effects are observed in silver-mirrored cavities containing the polymers poly(p-phenylene vinylene), (PPV), and a cyano-substituted derivative of PPV, MEH-CN-PPV.

Suppressed angular color dispersion in planar microcavities

We report an improved microcavity design which allows the suppression of the viewing angle dependence of the color emitted by a planar device. This is demonstrated for luminescent conjugated polymer based cavities, for which the wavelength change is reduced from ∼60 to 10 nm at an angle of 60°. We introduce the concept of cavity optical length dispersion and suggest structures for which the wavelength change with viewing angle is reduced to 5 nm at a viewing angle of 60° irrespective of the emissive material.

High finesse organic microcavities

Abstract We have constructed high-finesse light-emitting microcavities from the conjugated polymer poly(p-phenylene-vinylene), PPV, and high reflectivity distributed Bragg reflectors. The modifications to the photoluminescence and electroluminescence of the polymer arising from confinement of the photon field by these mirrors were investigated. Spontaneous emission enhancements of 2 orders of magnitude were found in the forward direction. The total integrated quantum yield for photoluminescence from PPV in such devices was 8.5 ± 1%. Stimulated emission from these devices was also investigated using short-pulse laser excitation. The emission line width narrowed by a factor of 2 above the lasing threshold. Short-term polarisation memory effects were observed as the life time of the excited states were reduced by coupling to stimulated emission. The lasing threshold for pulsed excitation was found to have been reduced by an order of magnitude, to 15 μJ/cm2, compared to previous studies on microcavities incorporating metal mirrors. Findings were complemented by simulations of the optical fields using multi-layer stack theory and transfer matrices.

Modelling of optical interference effects in conjugated polymer films and devices

In this paper, we model the effects of optical interference on the emission properties of conjugated polymer thin films and devices. The influence of dielectric interfaces and metal surfaces on the radiative lifetime of the singlet exciton is investigated in a number of multi-layer structures. We find that changing the structure can have a significant effect on the radiative lifetime, and we discuss the effect of this change on the emission spectrum as well as the measured photoluminescence efficiency. We compare the results of our model with the results of photoluminescence experiments using a number of structures.

L-4: Late-New Paper: Active-Matrix Operation of Electrophoretic Devices with Inkjet-Printed Polymer Thin Film Transistors

Active-matrix operation of an electrophoretic device has been achieved with all-polymer thin film transistors (TFTs). The source/drain, gate electrodes and gate lines of the TFTs were fabricated by inkjet printing with a conductive polymer, polyethylenedioxythiophene (PEDOT). Electrophoretic material was prepared from TiO2 particles suspended in dyed solution, and was microencapsulated in polymer membrane spheres. The microcapsules were sandwiched between the polymer active-matrix array and a counter electrode. The voltage of each pixel was independently changed by switching the printed polymer TFT, and contrast in the electrophoretic material was successfully obtained.

Interference phenomena in polymer light-emitting diodes: photoluminescence and modelling

Abstract We report an experimental and theoretical study of the effects of interference in polymeric light-emitting diodes (LEDs). These effects are due to the complex optical structures of the devices, which include many layers of materials with different refractive indices, and are of considerable importance since they affect spectral distribution and intensity of the absorption and emission in a significant way. By way of comparison, they can also provide a flexible, noninvasive optical probe of the electroluminescent processes, such as, for example, the spatial distribution of the recombination inside the LED. In this paper we analyse single-layer diodes with indium-tin oxide (ITO) and Al electrodes, where poly(p-phenylene vinylene) (PPV) is the luminescent polymer. We find that photo-induced excitation of the radiative species produce different spectral shapes depending on the excitation energy (and hence on the profile of excited chromophores) which we can describe in terms of interference phenomena. The theoretical analysis is conducted by means of multilayer stack theory and transfer matrix calculations, and takes into account additional quenching effects due to In contaminations from the ITO electrode. The theoretical results are in good agreement with the experiment.

High excitation density in light-emitting polymers

Properties and processes in poly (p-phenylenevinylene) are described in the context of lasing structures. In this communication we highlight the high-excitation-density regime, which seems to be the regime where new laser systems/technologies starts. We also illustrate, in the context of polarization memory, how device structure can be used to probe material properties.

Measurements of Photoluminescence Quantum Efficiencies in Conjugated Polymers: Implications for Polymer Photophysics and for Light-Emitting Diodes

Abstract The photoluminescence properties of conjugated polymers are currently of great interest, since the radiative decay of singlet excitons is the process by which light emission occurs in polymer light-emitting diodes. We describe here accurate measurements of photoluminescence efficiencies in solid films of conjugated polymers, using an integrating sphere to collect the emitted light. Values in excess of 40% are measured for cyano-substituted derivatives of poly(p-phenylenevinylene) (PPV). In PPV itself, measured photoluminescence efficiencies of 27%, combined with time-resolved measurements of the luminescence decay, suggest that the species produced by photoexcitation is predominantly the singlet exciton responsible for luminescence. The photoluminescence spectrum and the rate of radiative decay can be strongly affected by the presence of dielectric or metal interfaces. We model here the effects of optical interference in a number of multi-layer structures. We compare these results with measured p...

Microcavity optical mode structure measurements via absorption and emission of polymer thin films

Abstract Langmuir-Blodgett deposition was used to control the positions of luminescent thin films of the polymer substituted poly( p -phenylene) (SPPP) within metal mirror microcavities. In this way, the position of the absorbing/emitting layers were precisely defined, permitting the optical mode structure of the cavities to be probed by both the emission and absorption of the chromophore. It was demonstrated that the spontaneous emission from a thin film of chromophore varied as a function of the optical electric field intensity within the microcavity and was therefore dependant on its position within the cavity. The absorption of the chromophore film was investigated by coupling the ultraviolet excitation source to the cavity mode and was also found to vary as a function of the optical electric field intensity of the cavity mode.

Liquid-crystal displays using printed plastic TFT backplanes

— In this paper, we present results from a new liquid crystal over plastic printed thin-film-transistor (TFT) display. The display demonstrator shows that the processing incompatibilities between the plastic TFT backplane and the liquid-crystal materials can be addressed to make a stable twisted-nematic structure. New fabrication processes such as the photo-alignment of liquid crystals have made it possible to create a new generation of displays, which pave the way towards fully integrated plastic liquid-crystal-display technologies.