"Micro-optics of organic semiconductors"

 "Micro-optics of organic semiconductors"

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 [1]. 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 [3]. 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 [2]. Using suitable polymer blends with tunable fluorescence properties, efficient, long-range unidirectional photon energy transfer ("photon one-way street") can be demonstrated [3]. 

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 [4]. 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 [5].

[1] K. Tabata et al., Sci. Rep. 4, 5902 (2014).
[2] D. Braam et al., Sci. Rep. 6, 19635 (2016).
[3] S. Kushida et al., ACS Nano 10, 5543 (2016).
[4] A. Asaithambi et al., J. Phys. Chem. C 123, 5061 (2019).
[5] 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