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

Abstract

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 represent the intrinsic net gain of MEH-PPV than prior measurements in the quasi-steady-state regime.