Practical theory of spectral evolution in an organic laser diode

The spectrum of the light emitted by a microcavity OLED is strongly time dependent on the nanosecond timescale after switch-on of the pump mechanism. This is one reason why convincing confirmation of laser operation in such device is difficult. The simple theory here presented is helpful in interpreting observed light-emission behavior. One important finding is that the occurrence in the optical spectrum of a narrow Lorentzian on top of a spontaneous-emission pedestal is no guarantee for observed laser operation. Another finding is that reabsorption by the singlet excitons leads to the highest gain when the cavity is detuned to the red side of the emission spectrum. This is confirmed by experimental observations on an optically pumped organic laser diode.