Ultimate Light Trapping in a Free-Form Plasmonic Waveguide

``Slow'' light is important for accessing various optical phenomena, due to the strong interaction with matter, but its realization in a free-form plasmonic waveguide requires consideration. The authors determine the optimized geometry for ultimate light trapping in a plasmonic cavity, with a quality factor near the theoretical limit, at an unusually short length. This saturation of quality factor enables the design of light-trapping devices of extremely small footprint. The ratio of quality factor to footprint is found to be comparable to that of state-of-the-art photonic resonators.