Scanning with golden bow ties

Detectors would operate in terahertz region.

By Phil Dooley

https://cosmosmagazine.com/physics/scanning-with-golden-bow-ties-1

Australian and British physicists have unveiled their design for a high-precision detector they say could enable a new generation of safe compact scanners.

As described in a paper in the journal Science, it is based around tiny “bow ties”, each comprising two triangles of solid gold connected by two nanowires.

This design allows it to operate in the terahertz region of the electromagnetic spectrum, between microwaves and infrared. Terahertz scanning offers a safer low-energy alternative to X-rays: it is not powerful enough to ionise materials.

However, it still penetrates materials such as plastics, wood and paper, is absorbed by water, and is reflected by metals, giving the technology the capability to analyse a wide range of samples.

The bow ties also are able to detect the polarisation of the terahertz radiation, which adds another dimension to the detector’s versatility.

“The polarisation gives you much more useful information, especially about biological molecules, for example their chirality,” says Chennupati Jagadish from the Australian National University (ANU).

“Complex molecules have their own terahertz fingerprints, so this technology can be used for finding cancer biomarkers, locating explosives or measuring moisture levels in crops.”

The device is the result of a collaboration between ANU and Oxford University in England and Scotland’s Strathclyde University.

Importantly, the researchers say, it overcomes a limitation in the resolution, or detail, of conventional terahertz imaging, which is linked to its millimetre-scale wavelength – a million times larger than X-rays, with nanometre-scale wavelengths.

The design gets around this limitation with the microscopic scale of the bow ties. The pair of nanowires at their heart are indium phosphide wires one hundredth the size of a human hair: around 280 nanometres in diameter and ten micrometres long.

Although each detector is much smaller than the terahertz waves (around 300 microns), an array of bow ties can be used to create a near-field image that bypasses the diffraction limit of the terahertz radiation’s wavelength.

To detect the polarisation of the radiation, the team combined two bow ties, set at right angles to each other, with their central nanowires crossing but not in contact – one bow tie is set slightly above the other.

Although a simplistic-sounding design, the vertically offset configuration took three years of collaboration to devise and manufacture.

The nanowires were created at ANU, the triangles were added at Oxford as antennae to boost the signal level (gold being the obvious choice due to its high conductivity), then the devices were assembled at Strathclyde.

The team is now developing nano-scale electronics to connect to the detector, so the whole device can be built onto a single chip, in contrast with existing bulky terahertz scanners.

Metamaterials: a new degree of freedom in physics

MY NOTE: THERE HAVE BEEN A NUMBER OF BLOG POSTS HERE ON THE TOPIC OF METAMATERIALS.  THIS PROVIDES THE LAYMAN LIKE MYSELF WITH SOME INTRODUCTORY IDEAS ABOUT THE FIELD, AND HENCE I'M REPOSTING IT. AS ALWAYS I'M LOOKING FOR ORIGINAL COMMENTARY BY ANYONE KNOWLEDGEABLE IN THE FIELD. PLEASE SEND ME STORIES ABOUT YOUR WORK IN THz, THOUGHTS OR COMMENTS ON THE STATE OF THE EMERGING INDUSTRY, OR COMMENTS ON THE BLOG POSTS. THANKS! knudson.randy@gmail.com

http://www.ruslanharuna.co.cc/editorial-metamaterials-a-new-degree-of-freedom-in-physics.php

Mikhail Lapine,
Nonlinear Physics Centre, Australian National University, Canberra
Coordinating Editor, Metamaterials

Turning back to the decade that passed since metamaterials were introduced in electromagnetics, we can feel confident to see how the field evolved into a hot topic encompassing a wide span across physics, from radio-frequency through microwave and terahertz range into optics, and still delivering a driving force to other areas, such as acoustics.

Metamaterials are artificial media, where man-made structural units play the role of atoms. Thus, the complex phenomena in metamaterials result from the characteristics of the individual elements as well as from the way they are arranged in a lattice. In other words, metamaterials gain emerging properties – such that were not available in their constitutive elements alone. This provides enormous flexibility in tailoring the response of metamaterials to external waves or fields, and secured the dominant application area of metamaterials: unusual or extreme properties.

The first task to metamaterials was the practical realisation of negative refraction – an impressive phenomenon theoretically predicted in the middle of XX century to occur in media having electric permittivity and magnetic permeability simultaneously negative (for a historical perspective, try a mini-review by Ekaterina Shamonina and Laszlo Solymar). Such material parameters turned out to be achievable with metamaterials, and the appeal of apparent success was so strong, that many researchers still associate the negative refractive index with the very definition of metamaterials.

However, metamaterials are much more than that. The key factor which defines the great success of metamaterial idea is that we are free to design the entire structure – from the tiny details of the individual “atoms” to the general symmetry of their arrangement and shape of the entire sample – with a particular feature or application in mind. We can therefore influence the overall properties of a metamaterial at several levels of the structural organisation.

In theoretical description of metamaterials, deriving effective material parameters is of particular importance and of great challenge. Strong spatial dispersion, typically present in metamaterials apart from a remarkable frequency dispersion, requires a lot of effort and care to be adequately addressed. In Metamaterials, we regularly publish on this topic, making this journal a perfect destination for those interested in the development and achievements along this fundamental direction: browse through the papers by Vladimir M. Agranovich, Ross McPhedran, Constantin R. Simovski, Alexey Vinogradov, and Mario G. Silveirinha, as well as several other articles across the journal, to have a reliable starting point.

As the feasibility of metamaterial idea spans over a huge range of wavelengths, specific attention is given to fabrication peculiarities. For radio and microwave frequencies, the design and manufacturing is relatively easy with the use of various split-ring resonators, wire media, or, alternatively, loaded transmission line meshes (see the review by George V. Eleftheriades). An advance into infra-red and optical range requires a different strategy, as the conventional metamaterial elements are not only difficult to make at such a small scale, but also loose the performance because of the fundamental limitations in physics. To approach optics, a different strategy is required, using the designs related to photonic crystals, involving cut-wire pairs, nanopillars, nanosphere arrays and fishnet-like meshes. Challenges and advances in the optical fabrication were recently reviewed by Alexandra Boltasseva and Vladimir M. Shalaev.

Clearly, metamaterials were claimed for a number of exciting applications, some relying on the unusual material properties while other benefiting from the design flexibility and diversity. Negative refraction (also useful in plasmonics, as discussed by Tamara A. Leskova and Alex A. Maradudin) offered a route to achieve a subwavelength resolution with a famous super-lens suggested by John B. Pendry; further freedom in material parameters might provide a possibility of electromagnetic cloaking (consult a few articles in one of the recent Special Issues). Most of the practical applications are so far found in the microwave range, with the design of filters, couplers, waveguides, transducers, frequency-selective surfaces, switches; as well as tunable, reconfigurable, nonlinear metamaterials. Another specific highlight is the use of metamaterials to aid with medical Magnetic Resonance Imaging (MRI), where various types of metastructures can be used to improve spatial resolution, detection quality or ease of image acquisition (see the articles from the groups of Richard R.A. Syms or Christophe Craye).

Metamaterials are now receiving attention at a number of international conferences, with the dedicated sessions or specialised events occurring several times per year. Meanwhile, Metamaterials journal provides an excellent publishing platform to highlight the key advances in the field, addressed with individual invited papers or special issues. Submissions to Metamaterials benefit from a careful editorial attention, quick and efficient review system, and high visibility of the papers immediately upon acceptance, while high selection criteria ensure a quality content greatly appreciated by our readers.