Femtosecond Pump and Probe Spectroscopy on Poly( Para -Phenylenes)
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ABSTRACT We studied the sub-picosecond dynamics of photoexcitations in methyl-substituted poly(pam--phenylene)-type ladder polymer (m-LPPP) films in a wide excitation density range up to values typical for the regime of narrow band emission by pump-probe spectroscopy. The singlet excitons relaxation dynamics, monitored at the stimulated emission (SE) peak at 2.53 eV, showed an intensity dependent ultrafast decay component, which occurs on a sub-picosecond time scale, ascribed to the onset of amplified spontaneous emission (ASE). In addition we identified photoinduced absorption (PA) bands of singlet excitons at 1.48 eV and polarons at 1.9 eV respectively. At high excitation density an additional absorption band becomes evident at 2.63 eV. INTRODUCTION The interest towards r-conjugated polymers has been constantly growing since their discovery in 1975, when polyacetylene was first doped (1]. Out of many possible applications of conducting polymers which have been envisaged and demonstrated the most promising in optoelectronic devices are light emitting diodes. LEDs with tunable, bright and durable emission are close to be commercial, while the new target is the construction of a "plastic" laser, i.e. a polymer based diode laser. As a consequence most of the fundamental studies, both theoretical and experimental, have been focused on optical properties of luminescent polymers. Methyl-substituted poly(para-phenylene)-type ladder polymer (m-LPPP) (2] is a very appealing candidate for understanding the photophysics of conjugated polymers because the high intrachain order and the narrow conjugation length distribution give rise to very sharp features in the optical spectra. m-LPPP is a stable, soluble and pure fluorescent material (3] which was used to fabricate electroluminescent devices in the blue spectral region with high luminance and efficiency (4] The observation that stimulated emission does not compete with photoinduced absorption [5] pointed to the possibility of using m-LPPP as optical amplifying medium. It was found that at high excitation density or in large gain length geometries a dramatic line-narrowing of the emission spectrum is observed [6]. The physical origin of this behaviour, which is common to other fluorescent conjugated polymers, is presently under debate. Stimulated emission and amplified spontaneous emission, superfluorescence, superradiance and exciton condensation have been proposed as possible explanation. Most of the experimental studies were focussed on photoluminescence experiments carried out using a varity of excitation sources, while transient photoinduced absorption data at high excitation density were so far not reported. In this work we report on sub-picosecond resolution differential-transmission AT/T experiments on m-LPPP films. Stimulated emission (SE) and photoinduced absorption (PA) are studied both at low excitation and at high excitation density where exciton-exciton interactions are important and line narrowing in the emission spectrum is taking place. The goal is to descri
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