Intrinsic gain and gain degradation modulated by excitation pulse width in a semiconducting conjugated polymer

Abstract

We have previously reported that substantially higher optical gain values can be achieved in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) through use of transient excitation conditions. In the present paper, we report on a systematic investigation of this behavior to elucidate the physical mechanisms involved, which enables us to distinguish between the fundamental intrinsic gain and an excitation induced degraded gain. Using pump laser pulses having temporal widths longer and shorter than the photoluminescence (PL) decay time of MEH-PPV, both quasi-steady-state (QSS) and transient excitation regimes are explored in our encapsulated waveguide heterostructures [Si(100)/SiO2/MEH-PPV/poly(methyl methacrylate)]. Under transient excitation (25ps pump pulses), extremely large optical gain is observed, reaching a value of 700cm−1 at a maximum pump energy density of 85µJ/cm2. However, under QSS conditions (8ns pulses), considerably lower gain coefficients are achieved with a maximum of ∼130cm−1 at an energy density of 2,000µJ/cm2; this factor of 5 decrease in optical gain performance is observed at the same excitation density as that for transient excitation using ps pulses. We have also employed unencapsulated waveguide structures [Si(100)/SiO2/MEH-PPV/air], which allows us to achieve additional insight on gain degradation under QSS conditions. It is clear that the gain measured under transient conditions is more representative of the intrinsic gain whereas that determined in the QSS regime is degraded by defect-mediated dissociation of emissive states due to localized thermal and oxidative damage to the films. It is in the QSS regime in which most optical gain measurements to date have been performed. These results suggest that further optimization of MEH-PPV – and most likely other conjugated polymers – as a robust optical gain medium can be achieved by consideration of the excitation pulse width.