We demonstrate that exceptional points exist in fully transparent, optically ”effectively” biaxial, anisotropic micro-cavities, fabricated using an uniaxial cavity material with its axis inclined to the Bragg mirror growth direction. This is similar to the existence of singular (optic) axes in absorbing biaxial crystals, but the lack of time reversal symmetry is mediated by the mode broadening, i.e. the photon escape from the – in principle – open cavity system. As a consequence the eigenmodes are generally elliptically polarized, and completely circularly polarized eigenmodes are expected in certain directions. Via geometric and chemical composition design degrees of freedom, the spectral and angular position of these chiral modes can be rationally designed. Possible applications arise from the use of such directions for circularly polarized emission without the use of spin injection or internal or external magnetic fields. Also the coupling of such modes to excitons, adding oscillator strength to the system, seems a promising avenue of research.
An optically pumped ZnO distributed feedback laser operating at 383 nm has been designed, fabricated and
characterized. Single mode operation was observed for a wide temperature range between 10 and 270 K. In order to
avoid technologically difficult etching of ZnO, a 3rd order diffraction grating was dry-etched into an additional 120 nm-thick
Si<sub>3</sub>N<sub>4</sub> layer deposited on the ZnO active region. The spectral linewidth of the laser emission was 0.4 nm, whereas
an optical pump threshold intensity of 0.12 MW/cm<sup>2</sup> and a peak output power of 14 mW were seen. The temperature
tuning coefficient of the ZnO refractive index was determined from wavelength vs. temperature measurements; a value
of 9 × 10<sup>-5</sup> K<sup>-1</sup> was found, in good agreement with literature values.
Lateral confinement for cylindrical micro-resonator light emitters improves the ratio of the number of the axial resonant modes to the number of the spontaneous emitting lateral modes. We have observed resonator-behaviour of cylindrical micro-structures, whose lateral surfaces were coated with coaxial MgO/ZrO<sub>2</sub> and a-Si/SiO<sub>χ</sub> Braggre flectors. Glass rods with circularly shaped basal planes and ZnO wires with hexagonally shaped basal planes were used as cavity material. Bragg-reflectors were deposited using pulsed laser deposition and plasma enhanced chemical vapour deposition at the lateral surface of the ZnO wires and the glass rods. The optical properties of the Bragg-reflectors were investigated using a confocal micro-reflectometer, spatially resolved spectroscopic ellipsometry technique, and spatially resolved cathodoluminescence measurements.