Oblique angle lasing in a periodically pumped organic microcavity
2010
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.
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