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We investigate a high finesse organic microcavity consisting of 42 al-
ternating layers of TiO2 and SiO2 and a host-guest composite of tris-(8-
hydroxy quinoline) aluminium and 4-(dicyanomethylene)-2-methyl-6-(p-
dimethylaminostyryl)-4H-pyran (Alq3 :DCM) as an active layer, with a
DCM concentration of 2% by weight. Lasing is achieved in both planar
and wedged organic microcavities under optical pumping using femtosec-
ond pulses. While planar structures serve as a model system and can
be thoroughly modeled using a standard rate equation model, wedged
structures (the divergence angle of the cavity layer is 6.7 × 10-5 rad) al-
low systematic investigations of lasing characteristics as a function of the
mode volume. We show that the lasing saturation occurs in a small mode volume organic microcavity due to the limited number of excitable molecules per mode. We use different pumping regimes for investigating this behavior. On the one hand, a pumping wavelength of 400 nm is used to excite Alq3 molecules with a subsequent transfer of the energy to DCM molecules via Foerster-transfer. On the other hand, we pump the laser dye DCM directly to its absorption band (around 500 nm). In this case, a saturation of the output power is noticed due to the finite number of DCM molecules in the mode volume. In contrast, the saturation of the output power occurs at much higher excitation levels if Alq3 is pumped since the number of molecules is 50 times larger. A modified set of rate equations is applied to obtain a theoretical description of the lasing dynamics. These equations provide a basis to describe the occurence of saturation at high excitation levels in depen- dence of the excitation pulse width, the number of excited molecules in the mode volume and the spontaneous emission factor beta. |
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