Spoof breakthrough for sensing

Optical materials

February 8, 2008

Artistic rendering of a metamaterial layer for spoof SPPs. (Courtesy of Stefan A. Maier, Imperial College London.)

Specially designed artificial materials – metamaterials – have intriguing properties but currently lack tangible applications.

This could change thanks to a recent report making use of a metamaterial to propagate and confine terahertz (THz) electromagnetic surface modes [Williams et al., Nat. Photon. (2008) doi: 10.1038/nphoton.2007.301]. This region of the electromagnetic spectrum – the far infrared – and particularly the ‘terahertz gap’ of 0.1-3 THz is important in sensing.

The metamaterial created by UK and Spanish researchers consists of planar Cu-coated polymer films decorated with two-dimensional arrays of subwavelength periodicity square pits.

“By putting dimples in our surface, with a period smaller than the wavelength, we have created an effective surface layer with a plasma frequency in the THz part of the spectrum,” explains Stefan A. Maier of Imperial College London.

This enables the surface to maintain tight control of electromagnetic modes, so-called surface plasmon polaritons (SPPs). “Hence, we can now guide SPPs with much higher confinement,” he says.

The plasmonic metamaterials have surface waves that mimic the properties of SPPs, even in the limit of a perfect conductor, and are known as ‘spoof SPPs’.

Using THz time-domain spectroscopy, the researchers directly observe SPP propagation on the Cu metamaterials and the strong confinement at the surface in line with predictions. This approach could be exciting for surface enhanced sensing, particularly of biomolecules, in the THz regime.

“However, before metamaterials can be employed for such sensing purposes, the bandwidth over which high-confinement guiding occurs has to be improved,” cautions Maier.

Cordelia Sealy