Intrinsic optical gain in thin films of a conjugated polymer under picosecond excitation

A strong excitation pulse width dependence on optical gain is reported in thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV), which suggests that previously reported gain measurements have occurred in an excitation regime that cause damage to the polymer. Symmetric waveguides Si(100)/SiO2/MEH-PPV/poly(methyl methacrylate) are fabricated and optically pumped using laser pulses having temporal widths shorter and longer than the PL decay time, resulting in transient and quasi-steady-state excitation conditions, respectively. Under quasi-steady-state conditions (8 ns pulses), a maximum gain coefficient of ∼135 cm−1 is achieved at a fluence of 2250 μJ/cm2. However, extremely large optical gain is observed under transient pumping (25 ps), reaching 700 cm−1 at a fluence of only 85 μJ/cm2; this 5× improvement in optical gain performance is achieved at the same excitation density as that for ns pulses. It is clear that our ps gain measurements more accurately r...