The meta the better: Improving IR and molecular spectroscopy

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  • Published: Dec 1, 2014
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: The meta the better: Improving IR and molecular spectroscopy

Polaritons

US researchers have found that confined surface phonon polaritons within the material hexagonal boron nitride give rise to metamaterial properties that might one day be exploited in enhanced infrared and molecular spectroscopy. (Photo: U.S. Naval Research Laboratory)

US researchers have found that confined surface phonon polaritons - quasiparticles generated by electromagnetic interactions - within the material hexagonal boron nitride (hBN) endow the material with metamaterial properties that might one day be exploited in enhanced detectors for infrared and molecular spectroscopy as well as novel nanoscale optical devices for use in optical telecommunications, super high-resolution imaging and improved infrared cameras.

Researchers from the US Naval Research Laboratory (NRL) working with colleagues at the University of Manchester and Imperial College London in the UK and teams at the University of California San Diego, USA and the National Institute of Material Science (NIMS), in Japan, have made periodic arrays of cone-shaped hBN nanoantennae. These, the team explains confine hyperbolic polaritons in all three dimensions. Probing this novel optical property of the material demonstrates the highly directional, low loss confinement pointing the way to their use as building blocks for nanophotonic devices in the mid-infrared to terahertz (THz) spectral range.

Metamaterials are well known as artificial composites of various materials, often with structural features that are above the scale of the bulk atomic, or molecular level. They are being keenly investigated in many fields for their potential to manipulate and modulate electromagnetic radiation at different wavelengths, in ways that are unusual and not found in nature, with perhaps the exception of iridescent butterfly wings and related "colourful" materials that lack molecular level pigments.

Metallic and dielectric

One property of metamaterials that is of great interest is the property of hyperbolicity, wherein a material exhibits both metallic and dielectric type optical responses at the same time but along different axes of its "crystal" structure. The hyperbolic metamaterials are the basis for many potential applications such as 'hyperlenses,' used for imaging of nanoscale objects that are otherwise not observable because of the diffraction limitation of conventional light optics in which resolving objects that are smaller than about 200 nanometres is not possible.

"Our examination into the characteristics of hBN reveal the first experimental observation of sub-diffractional guided waves confined in all three dimensions, using a natural hyperbolic material," explains Joshua Caldwell of the Electronics Science and Technology Division, in the Power Electronics Branch. "This may, in turn, lead to the development of disruptive technologies such as the nanoscale equivalent of an optical fibre due to the volume-bound confinement of sub-diffractional modes within hBN." The research also shows that optical phonons, or crystal vibrations that can be excited with infrared light, can also be used to confine light to dimensions much smaller than the wavelength of light, while maintaining high efficiencies. These surface phonon polaritons are analogous to electron oscillations, plasmons, in metals or doped-semiconductors, but offer the benefit of low losses and operation in the infrared to terahertz spectral regions, which bodes well for IR and THz detectors.

A hex on your material

Hexagonal boron nitride is a van der Waals crystal, not dissimilar structurally to graphite or multi-layered graphene, with a hundred times the efficiency of previous hyperbolic metamaterials. Moreover, unlike metallic/dielectric hyperbolic metamaterials, hBN also provides the additional functionality of both types of hyperbolicity, allowing both the in-plane and out-of-plane crystal axes to behave in a metallic (reflective) or dielectric-like (transparent) manner, simply by changing the wavelength of the exciting light. Such mixing of both types of hyperbolic behaviour in the same metamaterials has not been demonstrated before and thus allows both regimes to be compared and contrasted simultaneously.

In addition to working with infrared and terahertz frequencies, the team was also able to demonstrate the natural hyperbolic behaviour of hBN with optical antennae, confining at up to 86 times smaller than the wavelength of light. This allowed confinement of 6.8 micrometre light in a 0.08 micrometre high antenna at high efficiency. Intriguingly, the resonance wavelength of the hyperbolic polaritons confined within these antennae was independent of the actual size and shape of the cones and depends instead on the height to diameter aspect ratio. The team explains that this means the antennae might be defined for a given application simply by controlling the aspect ratio, which means they could be constructed to fit various device form factors, whether nanophotonic circuit or infrared imaging sensor.

There is yet another interesting property of these arrays. The team found that the resonance response exhibits not just one mode, but four separate series, which Caldwell explains means that they can isolate each series by changing the wavelength and/or the angle of the incoming light with respect to the sample surface and so obtain the first complete description of these novel, three-dimensionally confined hyperbolic polariton modes.

Related Links

arXiv preprint 2014, online: "Sub-diffractional, volume-confined polaritons in a natural hyperbolic material: hexagonal boron nitride"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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