Oblique angle lasing in a periodically pumped organic microcavity

We investigate a planar organic microcavity under spatially periodic optical excitation. The host:guest system of Alq3:DCM is the emitting layer embedded in between two dielectric mirrors. Excitation by an interference field of two femtosecond laser pulses generates an array of lasers spaced by few microns. The far field of the cavity response shows conventional stimulated emission at k=0 and, in addition, two stripes of laser emission at oblique angles. The excitation pattern generates a periodic modification of the optical properties of the cavity, a dynamic diffraction grating with a period of few microns. This enhances the spontaneous emission in the direction of the Bragg angle, which depends on the distance of the interference stripes. Via the angle of incidence of the excitation beams, we can optically tune output angle and the wavelength of lasing. Measurements are confirmed by simulations of the mode dynamics inside a lossy cavity with small excitation spot sizes, where the local gain exceeds the total mirror and absorptive losses. We find that adjacent cavity quasimodes couple out of phase at certain separation distances, which critically depend on the quasimode radius and, thus, on the residual absorption. Thus, we gain insight into the development of coherence and mode-locking in microcavities.