Daniel Yves Oberli

Periodic photonic structures have attracted much interest due to their versatility for controlling light propagation. The dispersion of photon modes in photonic lattices exhibits an energy band structure analogous to the electronic one in crystals. An important consequence of the photonic band structure is the localization of light in a band gap, which can be engineered by breaking the translational symmetry introducing a defect in the lattice. Here, we demonstrate experimentally the localization of light in a two-dimensional periodic system of polaritons confined in a patterned microcavity. We generate a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the photon lasing regime we show the existence of confined modes that can be controlled by the shape of the generated defect. We demonstrate single on-site localization with lasing mode at the edge of the Brillouin zone similar to a gap soliton. Our results pave the way for useful tools to optically control localization and propagation of light towards the generation of microlasers.

Carrier localisation and transport were studied in GaAs/AlGaAs heterostructures grown by metal organic chemical vapour deposition on patterned substrates using cathodoluminescence (CL). In inverted-pyramid quantum dot structures carrier recombination in the quantum dot, quantum wires and quantum wells is observed. Using CL wavelength dispersive images, bandstructure variations in these structures can be observed. We find indications for efficient carrier coupling from a Ga rich vertical quantum wire to the quantum dot. We show that tapered quantum wires form at the intersection of the pyramid sidewalls and discuss their possible use as exciton accelerators. Tapered quantum wires can also be generated by growth on V-grooves with lateral variation of the groove width. When the transition zone between different groove segments is abrupt, carriers are localised in QWR segments. On the other hand, variable width V-groove systems with long transition zones are candidates for efficient carrier coupling between different QWR zones.

We demonstrate the occurrence of spatial multistability using laterally confined microcavity exciton-polaritons. By coherently exciting with a blue detuned laser a series of confined polariton modes, we investigate the effects of multistability on the transmitted laser beam as a function of the excitation power. At each threshold of the hysteresis loop, a switching of the mode profile of the laser beam is associated with a significant energy jump of each of the confined polariton modes in the mesa. A simulation of this behavior is achieved with a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross interaction between polaritons in different modes.

Using femtosecond pump-probe spectroscopy, we identify excitation-induced dephasing as a major mechanism responsible for the breakdown of the strong coupling between excitons and photons in a semiconductor microcavity. The effects of dephasing are observed on the transmitted probe-pulse spectrum as a density-dependent broadening of the exciton-polariton resonances and the emergence of a third resonance at high excitation density. A striking asymmetry in the energy shift between the upper and the lower polaritons is also evidenced. Using the excitonic Bloch equations, we quantify the respective contributions to the energy shift of many-body effects associated with Coulomb fermion exchange and photon assisted exchange processes and the contribution to collisional broadening.

High symmetry epitaxial quantum dots (QDs) with three or more symmetry planes provide a very promising route for the generation of entangled photons for quantum information applications. The great challenge to fabricate nanoscopic high symmetry QDs is further complicated by the lack of structural characterization techniques able to resolve small symmetry breaking. In this work, we present an approach for identifying and analyzing the signatures of symmetry breaking in the optical spectra of QDs. Exciton complexes in InGaAs/AlGaAs QDs grown along the [111]B crystalline axis in inverted tetrahedral pyramids are studied by polarization resolved photoluminescence spectroscopy combined with lattice temperature dependence, excitation power dependence and temporal photon correlation measurements. By combining such a systematic experimental approach with a simple theoretical approach based on a point-group symmetry analysis of the polarized emission patterns of each exciton complex, we demonstrate that it is possible to achieve a strict and coherent identification of all the observable spectral patterns of numerous exciton complexes and a quantitative determination of the fine structure splittings of their quantum states. This analysis is found to be particularly powerful for selecting QDs with the highest degree of symmetry (C_3v and ) for potential applications of these QDs as polarization entangled photon sources. We exhibit the optical spectra when evolving towards asymmetrical QDs, and show the higher sensitivity of certain exciton complexes to symmetry breaking.

We investigate polariton-polariton interactions in a semiconductor microcavity through two-dimensional Fourier transform (2DFT) spectroscopy. We observe, in addition to the lower-lower and the upper-upper polariton self-interactions, a lower-upper cross interaction. This appears as separated peaks in the on-diagonal and off-diagonal parts of 2DFT spectra. Moreover, we elucidate the role of the polariton dispersion through a fine structure in the 2DFT spectrum. Simulations, based on lower-upper polariton basis Gross-Pitaevskii equations including both self- and cross interactions, result in a 2DFT spectra in qualitative agreement with experiments.

We review the research done in recent years in our group on the effects of nanostructure symmetry, and outline its relevance both for nanostructure physics and for computations of their electronic and optical properties. The exemples of C3v and C2v quantum dots are used. A number of surprises and non-trivial aspects are outlined, and a few symmetry-based tools for computing and analysis are shortly presented.