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Surface plasmon polaritons get active

29 Jan 2009

The first modulated femtosecond plasmon pulses open new possibilities for ultrafast data processing.

'Active plasmonics' are poised to follow their 'photonics' analogues thanks to the first experimental demonstration of femotsecond plasmon pulses in an integrated optical device.

Researchers at the Optoelectronics Research Centre (ORC) in Southampton, UK, were able to generate and modulate plasmon pulses with a switching time of 200 fs in a metal-dielectric waveguide. Using this new method of direct optical modulation of plasmon signals, the ORC team says that its result could lead to unprecedented terahertz modulation bandwidth–a speed at least five orders of magnitude faster than existing technologies(Nature Photonics 3 55).

"We have achieved a modulation speed several orders of magnitude faster than existing techniques and we have done so using a technique that does not require the introduction of any dedicated 'active' media to facilitate switching–the metal of the waveguide is itself the active medium," Kevin Macdonald, a member of the research group at ORC, told optics.org.

Surface plasmon polaritons (SPPs) are seen to have great potential as information carriers in next-generation, highly integrated devices for data transport and processing-offering to combine the small size of today's micro-/nano-electronic systems with the speed and bandwidth of photonic technologies.

For the first time, ideas from the fields of plasmonics and ultrafast nonlinear optics were combined in a metal-silica plasmon waveguide comprised of a thin aluminium layer on a silica substrate. Grating structures etched into the silica were used to couple and decouple light to the plasmon waves. By exploiting the nonlinearity of the metal rather than the dielectric waveguide, the group was able to eliminate the need for activated phase-change waveguide materials to control propagating SPPs.

The result was achieved using a pump-probe setup where degenerate beams were derived from a single pulsed laser source of nearly transform-limited 200 fs optical pulses at 780 nm–close to the interband absorption peak energy in aluminium.

The probe beam generated SPP waves which travelled along an unstructured region at the aluminium-silica interface between gratings.

Pump pulses incident on the unstructured region were found to prompt a nonlinear response between the propagating SPP and pulsed light in the skin layer of the metal surface along which the plasmon wave was propagating. The transient effect of the pulse excitation on the propagation of the SPP signal revealed a fast nonlinear component with a relaxation time shorter than the 200 fs pulse duration, five orders of magnitude shorter than the 40 ns switching time reported for a quantum dot device.

The fast nonlinear response was detected only when the linear polarisation direction of the pump field was in the plane of incidence containing the SPP propagation direction which is also predominantly in the direction of the electron oscillations in the SPP wave.

The group believes that the advantages of the present switching technology for plasmon-polariton signals include simplicity of both geometry and material composition, compatibility with existing CMOS fabrication techniques and an operational wavelength close to an important on-chip interconnect wavelength.

Caryl Richards is features editor of Optics & Laser Europe magazine.

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