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16 April 2008 Sub-nanojule threshold lasing in 5×5 μm2 organic photonic boxes
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×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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Maik Langner, Markas Sudzius, Susanne I. Hintschich, Hartmut Fröb, Vadim G. Lyssenko, and Karl Leo "Sub-nanojule threshold lasing in 5×5 μm2 organic photonic boxes", Proc. SPIE 6999, Organic Optoelectronics and Photonics III, 699902 (16 April 2008);

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