Slow guided surface plasmons at telecom frequencies

The phenomenon of slow light is interesting not only from a fundamental physics standpoint, but also because it introduces the possibility of new applications in telecommunications(1). For a practical slow-light device, the important features are bandwidth, range of wavelength tunability and size, rather than the absolute slowdown factor achieved(2). Slow light can be achieved in three main ways: through quantum interference effects(3-8), which can slow the speed of light down to several metres per second, albeit within a very narrow bandwidth; by using photonic crystals(9), which are able to slow fight over large bandwidths but with much smaller slowdown factors(10,11); and by using stimulated Brillouin or Raman scattering(12,13). Surface plasmon polaritons have the advantage that they can overcome the diffraction limit of fight in a microchip-sized device(14-17). Increases in the propagation lengths of surface plasmon polaritons(18) and the feasibility of all-optical wavelength tunability(19) have been reported. Here we report the observation of slow, femtosecond surface-plasmon-polariton wavepackets. We show that a highly compact (55 mu m length) plasmonic structure is able to achieve an effective slowdown factor of two over a 4 THz bandwidth. These results will increase the scope of photonic devices based on surface plasmon polaritons.