Photonic crystals meet their match

17 Jun 2002

Danish scientists develop new medium for electromagnetic waveguiding.

Researchers in Denmark have discovered a material that could offer a route to integrated, all-optical systems using alternative materials to photonic crystals. (Physical Review Letters (86) 3008).

Photonic crystals propagate light at specific wavelengths using the photonic bandgap principle; they bring the promise of photonic circuits that could facilitate all-optical systems. However, they are difficult to fabricate. Now Sergey Bozhevolnyi from Aalborg University, and John Erland and colleagues from the Technical University of Denmark, have shown that a gold surface can be made to propagate surface plasmon polaritons (SPPs) - fleeting, electromagnetic signals that travel along a metal-dielectric interface - in a similar way.

"You can integrate various photonic-bandgap-based components within a few hundred micrometers," they said, "so we realized that we could use alternative two-dimensional waves, such as SPPs, for the same purpose."

Bozhevolnyi and colleagues discovered that SPPs also experience a band-gap effect if directed along a surface with aberrations that only allow certain wavelengths to proceed. By harnessing this phenomenon, the team found that they could make SPPs follow a charted course. The material could therefore offer an easier route to all-optical switching over very short distances.

The researchers fabricated a SPP bandgap structure by depositing a nanometer lattice of gold "scatterers" onto a gold surface and incorporating channels of different widths and orientations. On bombarding the surface with a laser to create SPPs of different wavelengths, the group noticed that the longer wavelengths didn't penetrate the lattice. This, they believe, is proof of the band gap effect.

They also observed that the shorter-wavelength SPPs travelled unhindered through the lattice, but only along the wider channels. "This propagation shows that line defects in the SPP bandgap structures can efficiently guide SPP fields," they said. "Unfortunately we didn't see the SPPs go round sharp corners."

The team plans further research into SPPs and hopes to develop components that are comparable to the photonic versions.