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.
The application of organic materials as solid state lasers critically relies on a low lasing threshold. We investigate
the characteristics of emission from an organic vertical cavity surface emitting laser. The microcavity studied here
consists of two highly reflective distributed Bragg reflectors enclosing a wedge-shaped active layer of Alq3:DCM.
Lasing of the DCM molecules is induced via two different pump regimes, either exciting Alq3 at a wavelength
of 400 nm or pumping directly into the absorption band of DCM at 532 nm. By a variation of the pump beam
position with respect to the microcavity surface, we demonstrate a continuous wavelength tuning in the organic
microcavities in a range of 55 nm. The continuously variable cavity thickness allows us to study the thickness
dependence of the input-output characteristics in a single sample. These data are obtained at a certain emission
wavelength, λ, close to the maximum of the gain spectrum, for a number of cavity thicknesses, which correspond
to different multiples of λ/2. For a decreasing thickness of the active layer, one-dimensional optical confinement
is expected to result in an increased spontaneous emission factor. On the other hand, the loss rate through the
mirrors increases with decreasing thickness resulting in a minimum threshold value for an active layer thickness
of approximately 3/2 λ. This lower threshold limit is set by nonradiative losses as well as residual absorption.
×The lasing threshold of a microcavity is mainly determined by the spontaneous emission factor β, which is
inversely proportional to the mode volume Vc. We demonstrate an experimental way to decrease the mode
volume via lateral structuring of the microcavity. This redistributes both number and density of the transversal
cavity modes, which increases the amplitude of the internal electromagnetic field. Our samples are microcavities
with an active layer of variable thickness (0.2 to 2 μm) made of tris-(8-hydroxyquinoline) aluminium (Alq3) doped
with 4-(dicyanomethylene)-2- methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). With thermal coevaporation
through a shadow mask, this layer is structured into an array of photonic boxes square-shaped microcavities with
an area of 55 μm2. Using a microscope objective, we record the spatial distribution of the cavity transmission
spectra with submicron resolution. The modes of the photonic boxes show a clear discretization, which is due
to the multidimensional optical confinement. Under selective excitation of the DCM molecules via a focused
pulsed laser (532 nm, 1.5 ns, 2 kHz , &diameter; ≈ 3μm), we record the spatially and spectrally resolved emission of
single photonic boxes. The laser pulse energy is varied to obtain input-output curves of the cavity modes. At
an excitation energy of ~30 pJ, we observe superlinear growth as well as a spectral narrowing of the emission
from the lowest energy mode of a single photonic box. For this lasing transition, we determine a spontaneous
emission factor β of ≈0.01.
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