The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using transient four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donorbound A exciton (D 0 XA) at temperature of 1.8 K we evaluate optical coherence times T2 = 33−50 ps corresponding to homogeneous linewidths of 13 − 19 µeV, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time T1 = 30 − 40 ps, while pure dephasing is negligible in the studied high quality samples even for strong optical excitation. Temperature increase leads to a significant shortening of T2 due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D 0 XB) is significantly faster (T2 = 3.6 ps) and governed by pure dephasing processes.

For many applications in photonics, e.g., free-space telecommunication, efficient UV sources are needed. However, optical excitation of such sources requires photons of even higher energies, which are difficult to integrate into photonic circuits. Here, we present photonic crystal devices based on zinc oxide (ZnO) that allow excitation using highly abundant sources in the near-infrared (NIR). These devices offer control of generating tailored photonic modes in the UV range via higher order nonlinear processes by combining the wide electronic band gap and pronounced nonlinear effects in ZnO with the adjustable properties of photonic crystal (PhC) membranes. Two different techniques for fabricating such ZnO-based PhC membranes are discussed, including the presentation of a novel bottom-up approach. Furthermore, dispersive theoretical simulations are introduced to determine the size and position of the photonic band gap, leading to an optimized cavity with only one dominant mode. This is followed by an evaluation of dominant loss channels, comparing cavities for both fabrication techniques, where we implemented a semianalytical model to determine scattering losses at imperfections of the PhCs. Additionally, energetic fine-tuning of such a mode as well as for other photonic modes that are formed by different cavity types is demonstrated. Ultimately, we validate that both linear one-photon and nonlinear three-photon excitation is possible with the presented devices, which renders them potential candidates for efficient UV light emitters that are powered by IR or NIR light sources.

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Due to its strong nonlinear susceptibility tensor components, zinc oxide (ZnO) provides highly efficient frequency conversion when excited with near-infrared pulses. Three-photon absorption offers an alternative to conventional ultraviolet sources for sub-bandgap excitation of ZnO. In this work, plasmonic nanoantennas are used to enhance coupling of infrared light into photonic ZnO resonators. The nanoantennas provide a strong field localization, which allows for a more efficient second- and third-harmonic generation within the ZnO film, as well as an immensely increased spontaneous photoluminescence emission due to interband absorption. The results demonstrate that the combination of plasmonic nanoantennas with photonic microresonators leads to a strongly enhanced nonlinear light-matter-interaction in thin ZnO films.