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Trapping of zero-dimensional polaritons in sub-micron regions in a microcavity

Subject Area Experimental Condensed Matter Physics
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2016 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 280523588
 
Final Report Year 2021

Final Report Abstract

With the combined expertise on photonic simulation, semiconductor/organic microcavity fabrications, and optical characterizations, the three groups aim to reach a common goal, i.e., the deliberate creation and trapping of polaritons in sub-micron regions through adiabatically structured inorganic/organic microcavities. This new type of devices may find important applications in low threshold polariton lasing and BEC of exciton polaritons. The group of F. Ding and O. G. Schmidt focused on the demonstration of strong/weak couplings between single semiconductor quantum dots and local nanocavities. It is based on a novel concept of introducing adiabatic photonic wells in conventional planar DBR microcavities (US Patent). Our works started by optimizing the MBE growth conditions for semiconductor DBR cavities. Then, four different fabrication approaches were developed to realize Gaussian-shaped nanoholes/defects inside the semiconductor DBR cavities. The first approach used focused ion beam milling to create arrays of nanoholes with designed structural parameters on the GaAs substrates. However, during the subsequent MBE growth, it was challenging to maintain the shape of nanoholes. Therefore, the second approach was developed by using in-situ local droplet etching technique. This approach is compatible with the subsequent growth of quantum dots. However, it was noticed that the hole diameters were too small for an efficient confinement of light, thus hindering the generation of exciton polaritons. The third approach based on the technique of nano-scale structuring with thermal scanning probe. The fourth approach relies on the reflow of resist to create Gaussian-shape lens instead of holes on GaAs substrates. In order to guarantee both spatial and spectral overlapping between the single quantum dots and the single local nanocavities, the fabrication procedures were carefully designed and calibrated. The first result from the device (that based on the resist reflow approach) demonstrated the weak coupling and a moderate Purcell enhancement, paving the way to realizing trapped exciton polariton in adiabatically confined structures. The Leo group has concentrated on the fundamental research of optical properties and various optical phenomena in highly confined organic microresonator systems in strong and weak coupling regimes. In particular, we found that the presence of coherently scattered waveguide modes in strongly coupled organic microcavity systems significantly perturbs phases and amplitudes of the transmitted light through the polariton branches. This leads to complex Fano-type interference phenomena. The coherent interplay between these modes shows significant enhancement of the density of modes at the frequencies where the transmitted signal is resonantly enhanced. This allows visualization of polariton branches that are usually hard to detect and typically applies to the higher polariton branches in inhomogeneously broadened organic systems with several excitonic transitions. Observation of Fano-type enhancement of the density of modes at some specific and nontrivial k values could be very important for future research on polariton lasers as it can potentially modify lasing properties such as emission directionality or lasing threshold.

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