G.J. Denton

Electronic excitations in luminescent conjugated polymers

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Lasing characteristics of PPV microcavities

Abstract We report lasing characteristics of poly (p-phenylenevinylene) (PPV) microcavity devices.

Electronic processes of conjugated polymers in semiconductor device structures

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved, and single-layer EL diodes achieve luminous efficiencies above 2 Lumens/W and peak brightnesses up to 90,000 cd/m 2 . We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments, and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Femtosecond transient photoinduced transmission measurements on a novel conjugated zinc porphyrin system

We present a comparative study on the ultrafast nonlinear optical response of a novel conjugated zinc porphyrin system. The linear optical absorption spectra of these molecules all show the Q‐band and B‐band transitions of the basic porphyrin unit. We have taken spectrally resolved ultrafast pump–probe measurements on monomer, dimer, and polymer solutions, which allows us to compare their excited state dynamics and relate these to their linear optical absorption. The spectra show several common features, but these features have markedly different decay dynamics. The bleaching is preferential in the Q band for the polymer and the B band for the monomer. The polymer Q‐band bleaching shows a two‐component decay, of approximately 700 fs and 170±50 ps time constants in a biexponential fit, which we attribute to both exciton–exciton annihilation and exciton diffusion to recombination centers on the polymer chain. The Q band of the dimer also has a two‐component decay with 13±5 and 1250±70 ps time constants whic...

Optical response of conjugated polymers excited at high intensity

The realisation of optically pumped lasing devices using poly(p-phenylenevinylene) has intensified interest in the optical response of conjugated polymers on ultra-short time-scales and at high optical injection density. We describe an investigation of these properties using time-resolved and time-integrated techniques. We demonstrate the importance of exciton-exciton annihilation at high injection densities and present evidence that this process produces photo-induced absorption (PA) through the generation of charges. We present a model which accounts for PA from both photoexcited singlet excitons and the charge pairs generated by annihilation. This model describes the time-evolution of the PA well and indicates that, in the presence of exciton-exciton annihilation, dissimilar stimulated emission and PA dynamics are consistent with a high quantum yield of excitons as deduced from PL lifetime and efficiency studies.

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.

Ultrafast field-induced dissociation of excitons in a conjugated porphyrin polymer

Abstract We present a study of field-induced modulation of the ultrafast transient photo-induced response in a conjugated zinc porphyrin polymer. This was in the form of aggregated strands in a host polymer matrix. The photobleaching of the polymer Q-band is significantly reduced by the application of an electric field. This field modulation occurs at very short times (

Laser ablation of conjugated polymers

Abstract The debris from laser ablation of conjugated polymers can be collected on a substrate to form thin films of the target material. We have deposited thin films of the conjugated polymer poly( p -phenylenevinylene), PPV, by laser ablation of a PPV target film. The photoluminescence (PL) spectra of films deposited in this way are very similar to those of pristine, spin-cast material. The UV-visible and infrared absorption spectra of the films produced by ablation also show features characteristic of PPV. We also observe gratings etched onto the surfaces of target films. These grating are due to interference effects and have the potential to be used in photonic device structures based on PPV or other conjugated polymers.

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.

Stimulated emission, lasing, and line narrowing in conjugated polymers

We present time-resolved and time-integrated measurements of optical gain and its effects in films and microcavities made from poly(p-phenylenevinylene) (PPV). We demonstrate that PPV films yield net gain due to stimulated emission with a lifetime of about 20 ps when excited with 200 fs pulses. The short duration of this gain is shown to result from exciton- exciton annihilation. We present results showing how this gain leads to spectral narrowing of the emission from PPV films and discuss why it is unlikely cooperative phenomena play a role in this phenomenon. We present evidence that it results from waveguiding of amplified spontaneous emission. Newly developed high-finesse microcavities are described, for which it is possible to obtain lasing thresholds below the onset of exciton-exciton annihilation. We present measurements of the far-field emission patterns from these structures and also demonstrate a measurement of the coherence time of their emission.