Grooves guide terahertz surface plasmon polaritons to subwavelength confinement
Texturing the surface of a metal wire with a series of tiny grooves could be a simple way to obtain ‘superfocusing’ and subwavelength confinement of surface plasmon polaritons (SPPs) at terahertz frequencies, according to scientists from the UK and Spain1.
Research into SPPs — electromagnetic waves that are guided at the surface of a conductor — enables energy and information to be guided at the subwavelength scale, which is highly attractive for applications in nanophotonics. In particular, it promises near-field imaging and single-molecule sensing with unprecedented sensitivity.
However, there's a catch. Subwavelength localization and guiding of SPPs only occurs at frequencies close to the plasma frequencies of conductors, which for most metals is ultraviolet. At much longer wavelengths, the guidance is weak and easily disrupted.
Computer simulations performed by researchers at universities in Bath, Madrid and Zaragoza now show that periodic corrugations on the surface of a perfect conductor can control the propagation behaviour of SPPs and thus allow guidance and control at terahertz frequencies.
Perhaps most interestingly the team has shown that by gradually reducing the diameter of the wire at the same time as gradually increasing the depth of the surface grooves, it should be possible to obtain superfocusing of the SPPs at the tip of a cone-shaped metal conductor. Such metal tips could be very useful for delivering terahertz radiation to micrometre-scale volumes for applications in near-field imaging, spectroscopy and sensing.
Importantly, the researchers say that the scale of the surface structures is within the limits of state-of-the-art laser machining and should be feasible to fabricate. For example, grooves that have dimensions and spacing on the order of a few tens of micrometres will be sufficient. Now someone just has to go and make them and check out these exciting predictions.
Maier, S., Andrews, S., Martin-Moreno, L. & Garcia-Vidal, F. J. Terahertz surface plasmon-polariton propagation and focusing on periodically corrugated metal wires. Phys. Rev. Lett. 97, 176805 10.1103/PhysRevLett.97.176805 (2006).
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Graydon, O. Terahertz triumph. Nature Photon (2006). https://doi.org/10.1038/nphoton.2006.48