Surface plasmon polaritons, propagating bound oscillations of electrons and light at a metal surface, have great potential as information carriers for next-generation, highly integrated nanophotonic devices [1,2]. Since the term 'active plasmonics' was coined in 2004 , a number of techniques for controlling the propagation of guided surface plasmon polariton signals have been demonstrated [4-7]. However, with sub-microsecond or nanosecond response times at best, these techniques are likely to be too slow for future applications in such fields as data transport and processing. Here we report that femtosecond optical frequency plasmon pulses can propagate along a metal-dielectric waveguide and that they can be modulated on the femtosecond timescale by direct ultrafast optical excitation of the metal, thereby offering unprecedented terahertz modulation bandwidth - a speed at least five orders of magnitude faster than existing technologies
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