Broadband Field Localization, Density Of States, And Nonlinearity Enhancement In Nonreciprocal And Topological Hotspots

Enhancing light-matter interactions is necessary in a wide range of applications, such as sensing, nanophotonics, nonlinear and quantum optics. Nanoscale interactions are typically enhanced through localized resonances, resulting in a stringent trade-off between bandwidth, footprint, and overall enhancement factor. Here, we discuss how nonreciprocal electromagnetic hotspots, arising at the truncation of nonreciprocal and topological interfaces, can decouple these quantities, supporting extremely large and broadband field enhancements, leading to efficient nonlinear phenomena at the nanoscale and extreme broadband enhancement of local density of states. We discuss the impact of material nonlocality and surface roughness on these effects, providing closed-form expressions for the field enhancement in the presence of nonlocality and material loss, and demonstrating that many of these effects can be observed in several realistic scenarios. Finally, we outline how topological photonics can play a role in establishing these nonreciprocal hotspots. Our findings open opportunities to implement broadband nanophotonic platforms and metamaterials that exploit topological concepts to enable exotic light-matter interactions.