Tom Michalsky

Absorptive lasing mode suppression in ZnO nano- and microcavities

We conclusively explain the different lasing mode energies in ZnO nano- and microcavities observed by us and reported in literature. The limited penetration depth of usually used excitation lasers results in an inhomogeneous spatial gain region depending on the structure size and geometry. Hence, weakly or even nonexcited areas remain present after excitation, where modes are instantaneously suppressed by excitonic absorption. We compare the effects for ZnO microwires, nanowires, and tetrapod-like structures at room temperature and demonstrate that the corresponding mode selective effect is most pronounced for whispering-gallery modes in microwires with a hexagonal cross section. Furthermore, the absorptive lasing mode suppression will be demonstrated by correlating the spot size of the excitation laser and the lasing mode characteristic of a single ZnO nanowire.

Phonon-assisted lasing in ZnO microwires at room temperature

We report on room temperature phonon-assisted whispering gallery mode (WGM) lasing in ZnO microwires. For WGM laser action on the basis of the low gain phonon scattering process high quality resonators with sharp corners and smooth facets are prerequisite. Above the excitation threshold power

Maxwell consideration of polaritonic quasi-particle Hamiltonians in multi-level systems

P_{\textit{Th}}

Cavity polariton condensate in a disordered environment

of typically

Exceptional points in anisotropic planar microcavities

100\,kW/cm^2

Coexistence of strong and weak coupling in ZnO nanowire cavities

, the recombination of free excitons under emission of two longitudinal optical phonons provides sufficient gain to overcome all losses in the microresonator and to result in laser oscillation. This threshold behavior is accompanied by a distinct change of the far and near field emission patterns, revealing the WGM related nature of the lasing modes. The spectral evolution as well as the characteristic behavior of the integrated photoluminescence intensity versus the excitation power unambiguously prove laser operation. Polarization-resolved measurements show that the laser emission is linear polarized perpendicular to the microwire axis (TE).

Spatiotemporal Evolution of Coherent Polariton Modes in ZnO Microwire Cavities at Room Temperature

We address the problem of the correct description of light-matter coupling for excitons and cavity photons in the case of systems with multiple photon modes or excitons, respectively. In the literature, two different approaches for the phenomenological coupling Hamiltonian can be found: Either one single Hamiltonian with a basis whose dimension equals the sum of photonic modes and excitonic resonances is used. Or a set of independent Hamiltonians, one for each photon mode, is chosen. Both are usually used equivalently for the same kind of multi-photonic systems which cannot be correct. However, identifying the suitable Hamiltonian is difficult when modeling experimental data. By means of numerical transfer matrix calculations, we demonstrate the scope of application of each approach: The first one holds only for the coupling of a single photon state to several excitons, while in the case of multiple photon modes, separate Hamiltonians must be used for each photon mode.

Exceptional points in anisotropic photonic structures: from non-Hermitian physics to possible device applications

A.J. is supported by the Leipzig School of Natural Sciences BuildMoNa. This work was supported by Deutsche Forschungsgemeinschaft through project GR 1011/20-2, by Deutscher Akademischer Austauschdienst within the project PPP Spain (ID 57050448) and by Spanish MINECO Projects No. MAT2011-22997 and No. MAT2014-53119-C2-1-R

Improving the Optical Properties of Self-Catalyzed GaN Microrods toward Whispering Gallery Mode Lasing

Planar microcavities allow the control and manipulation of spin-polarization, manifested in phenomena like the optical spin Hall effect due to the intrinsic polarization mode splitting. Here, we study a transparent microcavity with broken rotational symmetry, realized by aligning the optical axis of a uniaxial cavity material in the cavity plane. We demonstrate that the in-plane optical anisotropy gives rise to exceptional points in the dispersion relation, which occur pair-wise, are circularly polarized, and are cores of polarization vortices. These exceptional points are a result of the non-Hermitian character of the system, and are in close relationship to singular optical axes in absorptive biaxial systems.

Carrier density driven lasing dynamics in ZnO nanowires

We present a high quality two-dimensional cavity structure based on ZnO nanowires coated with concentrical Bragg reflectors. The spatial mode distribution leads to the simultaneous appearance of the weak and strong coupling regime even at room temperature. Photoluminescence (PL) measurements agree with finite difference time domain (FDTD) simulations. Furthermore the ZnO core nanowires allow for the observation of middle polariton branches between the A- and B-exciton ground state resonances. Further, lasing emission up to room temperature is detected in excitation dependent photoluminescence measurements.