Conjugated Polymers as Materials for Thin Film Solid State Lasers

Abstract

Optically pumped gain narrowing and lasing have been demonstrated in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. The dramatic collapse of the emission line width occurs at very low pump energy thresholds (~10 μJ/cm2). Gain narrowing is found in over a dozen different conjugated polymers representing a variety of molecular structures, including poly(p-phenylenevinylene), poly(p-phenylene) and polyfluorene derivatives; the emission wavelengths in these materials span the visible spectrum. The short gain lengths in conjugated polymers are attributed to the high density of chromophores, the large density of states associated with the interband (π-π*) transition in quasi-one-dimensional systems, and the Stokes shift which minimizes self-absorption and allows optical pumping to the excited state without simultaneously stimulating emission (thereby yielding population inversion). Lasing and gain narrowing are compared for a soluble poly(phenylene vinylene) derivative using two different resonant structures: planar waveguides and microcavities. In both cases, the gain narrowing threshold is at 0.05 - 0.1 μJ per 10 ns pulse focused to approximately 1.5 mm. Single mode microcavity lasers are obtained when a cavity resonance occurs at the wavelength where the gain of the polymer is a maximum. Low threshold lasing (threshold more than an order of magnitude below that observed in planar waveguides and microcavities) has also been demonstrated using distributed feedback in a planar chip film configuration.