ZnO nano-pillar resonators with coaxial Bragg reflectors
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ZnO nano-pillar resonators with coaxial Bragg reflectors Rüdiger Schmidt-Grund1, Annekatrin Hinkel1, Helena Hilmer1, Jesus Zúñiga-Pérez1,2, Chris Sturm1, Bernd Rheinländer1, and Marius Grundmann1 1 Universität Leipzig, Institut für Experimentelle Physik II, Linnéstr. 5, Leipzig, 04103, Germany. 2 CRHEA, CNRS, Rue Bernard Grégory, Sophia Antipolis, Valbonne, 06560, France ABSTRACT We demonstrate the growth of lateral concentric BR on ZnO nano-pillars. It opens the opportunity to be used for (i) the enhancement of the lateral confinement in classical pillar-resonators in order to increase the emission rates in the regime of weak exciton-photon coupling (Purcelleffect), (ii) to enhance the exciton-polariton coupling strength in the strong-coupling regime, and (iii) to be used for two-dimensional confinement in free-standing photonic wire resonators. Spatially resolved PL experiments in dependence on the pillar diameter and on the temperature provide strong hints for the ZnO nano-pillar resonator being in the strong-coupling regime. The coupling strength can be estimated to be V = 80 meV. INTRODUCTION A great deal of attention has been focused in the last years to optical micro-resonators, which confine light to a small volume, and which exhibit a wide variety of types and physical concepts [1,2]. Among these, Fabry-Perot (FP) resonators are of the most prominent. In FP resonators, the confinement or rather the quantization of the electromagnetic field inside a cavity is caused by reflection between oppositely arranged mirrors and can be in one (planar FP resonator, a so called photonic quantum-well), two (photonic-wire), or three (photonic dot) dimensions [3]. The lateral coating of laterally structured planar resonators and photonic wire resonators with highly reflective mirrors could considerably improve the performance of such resonators existing currently. Here we report on hexagonally shaped ZnO nano-pillars which are lateral-concentrically coated with high-reflective Bragg reflectors. In these structures, which are free-standing photonic wire resonators, we have found very strong hints for strong exciton-photon coupling. The confinement of the electromagnetic field inside a cavity leads to a long lifetime of photons, which enhances the coupling strength V to an electronic excitation with respect to those of non-confined photons. The lifetime, or the “temporal confinement” of photons inside the resonator is restricted due to losses and is described by the Quality factor Q. Q is related to the broadening of the resonator mode ∆ωc by Q = ω (∆ωc)-1, where ω is the frequency of the light, and depends on the development of the photonic band-gap. A further important parameter is the “spatial confinement” or rather the mode-volume Vmode. It is a measure of the electro-magnetic field density and can be controlled by the geometrical size of the cavity and the penetration depth of the electromagnetic field into the mirrors. The coupling strength V depends on the dipole moment of the electronic excitation and (
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