Influence of Silver Film Quality on the Threshold of Plasmonic Nanowire Lasers

In recent years, the field of plasmonics has become dependent on the ability to maximize the quality of metal films. In particular, plasmonic lasers will require a metal loss as low as possible in order to minimize their operational threshold if they are to find applications. In this article, the findings of recent studies, which report reduced thresholds in plasmonic lasers incorporating high quality metal films with both ultraflat surfaces and high crystallinity, are reassessed. In particular, the role of deposition rate and substrate temperature on silver film growth for improving the performance of hybrid plasmonic zinc oxide nanowire lasers is studied. High substrate temperatures and high deposition rates allow near-epitaxial silver growth on mica. Even though a 2 fold reduction in metal loss for the cystalline films is demonstrated, the presented plasmonic lasers show only a modest reduction in threshold, compared to noncrystalline films. This finding is consistent with our observation that the plasmonic cavity loss is comparable to the absorption of the unexcited semiconductor. Laser threshold is thus only partially dependent on reducing metal film loss. This work increases the fundamental understanding of metal film quality in plasmonics and gives new insights into the threshold characteristics of metal-based lasers.