Nanoscale ultrasensing using a nonbonding plasmon resonance

Placing a plasmonic nanorod near the termination of a plasmonic nanowire dramatically changes the reflection of the wire's guided mode. By carefully choosing the length of the nanorod, the reflectivity at the wire termination nearly vanishes due to destructive interference between the directly reflected wire mode and the infinite sum of the partial transmissions back into the wire of the reflected modes inside the nanorod. We show that this near-zero reflection condition corresponds to the so far overlooked nonbonding resonance which corresponds to a minimal coupling condition with extreme sensitivity to any changes in the nanorod's local environment. We explicitly quantify the sensitivity of the nonbonding condition towards small local and global perturbations of the refractive index and outline a method to exploit the nonbonding condition for near-field ultrasensing.