Group delay and dispersion in adiabatic plasmonic nanofocusing

We study the decrease in group velocity of broadband surface plasmon polariton propagation on a conical tip, using femtosecond time-domain interferometry. The group delay of (9 ± 3) fs measured corresponds to a group velocity at the apex of less than 0.2c. The result agrees in general with the prediction from adiabatic plasmonic nanofocusing theory, yet is sensitive with respect to the exact taper geometry near the apex. This, together with the sub 25 fs2 second-order dispersion observed, provides the fundamental basis for the use of plasmons for broadband slow-light applications. © 2013 Optical Society of America.