This talk will report on recent experiments, performed on micron-scale optically active organic semiconductor structures.
Single microspheres of π-conjugated polymers have been investigated by photoluminescence in the visible region. The spectra exhibit sharp whispering gallery modes (WGMs) with Q-factors up to
10 000, superimposed upon a broad luminescence background . The high Q-factors, together with the properties of the polymer spheres, make it possible to study the evolution of WGMs, both inside a single sphere, as well as between optically coupled multi-sphere arrangements . Strong optical excitation leads to a slight oblate deformation of the resonators. This results in a splitting of the WGMs, as the high degeneracy of perfectly spherical confinement is lifted . Using suitable polymer blends with tunable fluorescence properties, efficient, long-range unidirectional photon energy transfer ("photon one-way street") can be demonstrated .
Microcrystals of a BOron-DIPYrromethene-based dye (BODIPY) exhibit distinct, green and red photoluminescence emission, which can be explained by crystalline polymorphism, with local variation of high (green) and low (red) HOMO–LUMO separation . Green-emitting microcrystals show a series of photoluminescence peaks, with emission energies 𝐸! that can very well be described by a Wannier/Rydberg series 𝐸n = 𝐸g – 𝐸0/𝑛2, where 𝐸g is the gap energy (HOMO–LUMO separation) and 𝐸0 is an effective Rydberg constant. Rydberg-like states with quantum numbers up to 𝑛 = 18 are observed. The origin of these resonances remains a mystery that will be discussed in the talk.
Carbazole dendrimers with a carbon-bridged oligo(phenylenevinylene) core (COPV2) were care- fully crystallized to yield luminescent microcrystals with sizes up to several 10 μm. The dendrons work as light-harvesting antennas that absorb non-polarized light and transfer it to the COPV2 core, from which a polarized luminescence radiates. The excellent crystalline quality and the high lumines- cent yield made it possible to use the crystals as optical micro-cavities. Upon strong optical pumping, these microcrystals display amplified spontaneous emission and lasing without noticeable mechanical and optical degradation .
 K. Tabata et al., Sci. Rep. 4, 5902 (2014).
 D. Braam et al., Sci. Rep. 6, 19635 (2016).
 S. Kushida et al., ACS Nano 10, 5543 (2016).
 A. Asaithambi et al., J. Phys. Chem. C 123, 5061 (2019).
 K. Iwai et al., Angew. Chem. Int. Ed. 59, 12674 (2020).
Transmisión en vivo de la videoconferencia por YouTube: http://bit.ly/YouTube_ICF
Participante: Professor Dr. Axel Lorke
Institución: Universität Duisburg-Essen, Alemania
Fecha y hora: Este evento terminó el Miércoles, 30 de Septiembre de 2020