Conjugated luminescent polymers have been shown to exhibit stimulated emission under optical pumping. Dilution in
polymethylmethacrylate (PMMA) leads to an enlargement, towards longer wavelengths, of the gain region. In addition,
ultrafast optical switching is observed when a second pulse is applied during excited state lifetime. These two properties
are of high importance for optical communications and can bring to the plastic optical fibres (POFs) the equivalent of
erbium amplification in silica fibres. In this communication we report on the gain and switching properties of PMMA
films and POFs doped with conjugated oligomers and polymers, prepared by blending or copolymerisation. The dopants
were functionalised with polar side groups to improve their solubility in MMA and/or with reactive methacrylate groups
so they can be copolymerised with MMA.
We haven been exploring polymethylmethacrylate, PMMA, doped with conjugated luminescent polymers for
applications in plastic optical fibers (POFs) showing gain. In order to control loading and dispersion of the conjugated
polymers in the PMMA matrix, new polyfluorene- PMMA copolymers were synthesised. In this communication we
report on the optical properties of these copolymers, both in solution and in solid state. Furthermore, the properties of
POFs fabricated with such copolymers are presented and compared with the properties of POFs based on PMMApolyfluorene
We fabricated polymer optical fiber (POF) amplifiers operating between 440 and 480 nm, using POFs doped with a series of fluorene oligomers, including tri-, penta-(9,9-dioctylfluorene) and hepta-(9,9-dihexylfluorene). The gain properties of pure oligofluorene films demonstrate gain coefficients on the order of 250 dB/cm and amplified spontaneous emission thresholds between 1 and 8 µJ cm-2, significantly lower than other fluorene gain media. The optical and morphological characteristics of PMMA thin films doped with the oligomers demonstrate that the oligomers are largely isolated within the PMMA. The optical and gain properties of POFs produced using an adapted preform-drawing technique and doped with the oligofluorenes provide gain values on the order of 0.07 dB for 2 mm of doped POF. The oligofluorenes are largely isolated within the POFs, paving the way for all optical gain-switching.
Three fluorene-based copolymers carrying reactive side groups (oxetanes) were synthesised and characterised. Their optical properties are essentially controlled by the introduction of either fluorene or lower energy gap (benzothiadiazole or terthiophene) comonomers, randomly distributed along the polymer chain. We show that, in the presence of a small amount of a photoacid dispersed in the polymer films, these are converted into insoluble polymeric networks upon UV irradiation and heating. Absorption and emission spectra of the polymeric networks are similar to those of the starting polymers, showing that the cross-linking, due to the polymerisation of the oxetane groups, does not interfere. This ability to induce their insolubility upon UV irradiation is used to fabricate multi-layer light-emitting diodes. Using a transmission electron microscope grid as a shadow mask we show that micrometer-size patterns can be created.