Weibull analysis of atmospheric pressure plasma generation and evidence for field emission in microwave split-ring resonators

The generation of atmospheric pressure microplasmas using microwave resonators is promising for many applications due to the possibility of high electron densities and low electrode degradation. In particular, such plasmas may help enable reconfigurable metamaterials operating from GHz to THz. Since plasma metamaterials may require the generation of tens to hundreds of plasmas, it is important to find ways to reduce the power required for plasma breakdown. Here, we study gold and silver microwave split-ring resonators (SRRs) with a variety of materials near the interelectrode gap (Cu, CuO nanowires, aluminum oxide). We focus on those fabricated using a traditional thick film technique, screen-printing, and using fs- and ns-laser ablation. The use of laser ablation allows us to explore small interelectrode gap sizes (7-100 μm) and the use of different lasers and laser parameters enables us to produce a variety of microstructures. We utilize Weibull statistics to examine breakdown in atmospheric pressure Ar with and without deep ultraviolet illumination of SRRs. Fabrication methods and materials are shown to influence both Q-factor of the SRRs and breakdown voltage independently. It is found that superior performance in terms of breakdown voltage and consistency in breakdown is related to Weibull modulus. The power requirement for breakdown varied as widely as an order of magnitude depending on fabrication method and material used for the SRRs. Furthermore, we consider the performance differences seen between various resonators and relate this to microstructure/material which suggests that field-emission may play a role in providing the seed electrons required for breakdown. This need for seed electrons appears to be especially important for gap sizes of 40 μm and smaller.