Source: Electronics Weekly.
Researchers at Imperial College London have constrained THz radiation on a flat surface, allowing it to be manipulated by planar structures.
¡°We have created a metamaterial that guides terahertz radiation close to the surface in a surface wave,¡± researcher Dr Stephan Maier said.
Visible light can already be trapped as surface plasmon polaritons (SSPs) on a surface because, said Maier, the plasmon frequency of gold, silver and copper is similar the frequency of visible light. He added that radio ground waves sticking to the Earth¡¯s surface as Sommerfeld waves are SSPs on a grand scale.
¡°Red light at 600nm is trapped within 100 to 200nm of a gold surface enabling you to manipulate it,¡± he said. ¡°Metals are easy to process and have low losses, so it would be very nice to do this with THz radiation and even microwaves.¡±
Working with the Universities of Bath, Madrid, and Zaragoza, the Imperial team has perforated a metal surface with rectangular pits to create the metamaterial with a suitable plasmon frequency to trap 1THz radiation. ¡°The trick with metamaterials is that the surface layer looks to the radiation as it has an effective plasma frequency of 1THz just controlled by geometry.
Without the pits, 1THz radiation is loosely confined to the first 20mm above the metal. With appropriate pits this drops to a fraction of 1mm. ¡°It would be nice to increase this further by 10 times,¡± said Maier. ¡°To do this we will have to fill the holes with a high refractive index material such as silicon.¡±
Now the technique has been proven by experiment, the universities intend to manipulate the radiation by varying the pit size across the surface. Waveguides are one target and ¡°we should be able to construct a two-dimensional lens that will concentrate THZ radiation passing across a sheet around one point on that sheet¡±, said Maier.
THz spectrometry is a technique made possible by the recent invention of practical compact THz sources and detectors. Such a lens could increase the sensitivity of THz spectrometry on molecules dropped onto the surface.
For this kind of analysis, the surface will also need more bandwidth. ¡°Our metamaterial gives good confinement over a very narrow frequency band,¡± said Maier. ¡°We need a couple of THz and are looking at ways, but I can¡¯t comment on them at the moment.¡±