Abstract
Engineering light-matter near-field interactions on the nanometer scale offers the possibility of devices with unique functions. Here we show that two metal nanostructures can be designed to exhibit far-field radiation only when their shapes are appropriately configured and when they are closely stacked. Such functionality is useful in ensuring product authentication or certification, where a system should work only when the two nanostructures match, just like a lock and key. We describe its operating principle by observing induced electric currents and their associated optical near-fields, and we show example nanostructures designed by numerical simulations.