Polarization cross-coupling in a polymer microlaser upon double-pulse excitation

We have studied the coupling of the two polarization modes of a polymer microlaser by pumping its gain medium with two mutually delayed orthogonally polarized femtosecond pump pulses, that have a variable interpulse delay. Because the dominant anisotropy in the laser is induced by the pump and the various time scales associated with this setup are well separated, insight into the dynamics of the system is obtained by this method. Both time-resolved and time-integrated measurements of the output of the microlaser demonstrate strong cross-coupling and memory effects between the polarization modes. These can be assigned to the interplay between the optical fields in the cavity and the inversion. The most remarkable result is that the dominant output polarization switches direction when the interpulse delay is varied. Using a simple model, both for the polarization properties of the polymer and for the polymer laser, we discuss the underlying physics of the polarization cross-coupling. An attractive aspect of our gain material, i.e., a light-emitting polymer in solution, is that it has a very long memory for the polarization anisotropy induced by the pump; such a material therefore provides a transparant model system.