Harnessing the Magnetic Nature of Light for Terahertz-driven Devices

https://incompliancemag.com/harnessing-the-magnetic-nature-of-light-for-terahertz-driven-devices/

BY LAUREN SACCONE

Scientists from The University of New South Wales Sydney and The University of Sydney have demonstrated a new way to enhance magnetic responses in a dipole emitter by a sub-wavelength fiver. This research, and additional studies on dielectric sub-wavelength particles have provided scientists with a new platform with which to work on magnetic light.

Researchers experimentally demonstrated an enhanced magnetic dipole source in the terahertz frequency range. The fiber is placed next to a hole in a metal screen, which enhances the radiation power more than one order of magnitude. This is due to the excitation of the Mie-type resonances found in the aforementioned fiber. The scientists when on to demonstrate that the system was roughly equivalent to a double-fiber system when excited by a magnetic source.

The optical fiber system is coupled with magnetic dipole. When combined, it can be construed as a unit cell of metasurfaces. The metasurfaces can then be used for the manipulation of terahertz radiation. This also provides proof-of-concept regarding the possibility to achieve enhanced radiation of a dipole source, once it it in proximity of a sub-wavelength fiber. Additionally, the unit cell can be easily scaled down to optical frequencies. This could provide a huge boon to scientists working on developing integrated nanophotonic devices including nanoantennas and lasers on fiber.

Further research proved that the aperture-fiber system created by the scientists was equivalent to that of an MD-double fiber system. This also confirmed the theory that the experimentally measured enhancement ratio was a fair approximation of the THz Purcell enhancement of an MD source in the vicinity of subwavelength fibers. Scientist were able to approximate field uniformity in the aperture as well as the representation of the aperture as an MD source.

Enhancing the forward emission relies on two distinct factors. These are the loss of the fiber material, as well as refractive index. A lower material loss and higher refractive index could also be used to enhance the emission further. Because of this, the ideal material in the experiments would be silicon. Silicon has lower losses and a high refractive index, especially when compared to the soft glass used in the scientist’s work. Unfortunately, the diameter required for such an enhancement has proven extremely difficult to achieve so far — a problem researchers continue to struggle with.

Despite some setbacks, scientists are optimistic about the future applications of their discoveries. Nanotechnologies in particular will significantly benefit from these advancements.

OT-Invisibility cloaks closer thanks to 'digital metamaterials'

    Now you see him … Eric Tastad/Flickr, CC BY-NC-SA

  

by Penny Orbell

http://phys.org/news/2014-09-invisibility-cloaks-closer-digital-metamaterials.html

The concept of "digital metamaterials" – a simple way of designing metamaterials with bizarre optical properties that could hasten the development of devices such as invisibility cloaks and superlenses – is reported in a paper published today in Nature Materials.
Metamaterials are artificially engineered out of microscopic subunits – such as glass, metal or plastic – arranged in a repeating fashion. Once assembled, these metamaterials possess unique properties, such as interacting with light in unusual ways, which aren't often seen in natural materials.

"The idea behind metamaterials is to mimic the way atoms interact with light, but with artificial structures much smaller than the wavelength of light itself," said Boris Kuhlmey, associate professor of photonics and optics at the University of Sydney.

"This way, optical properties are no longer restricted to those of the constituent materials, and can be designed almost arbitrarily."

The material world goes digital

The researchers of the Nature Materials paper, from the University of Pennsylvania, were inspired to develop digital metamaterials by the binary numeral system of Boolean algebra.

The binary system is used internally by most digital electronic devices, such as computers and smartphones. Complex digital devices have their digital information simply encoded as a string of 1s and 0s called "bits".

The proposed method for digital metamaterials is a simplified way of building metamaterials, yet still allows for complex and diverse properties to be achieved.

"The beauty of the new method is its simplicity," said Min Gu, professor of optoelectronics at Swinburne University of Technology.

Through the use of simulations in two-dimensional space, the researchers explored the possibility of creating metamaterials with only two specially chosen component parts, called metamaterial bits – analogous to the 1 and 0 "bits" of binary computer code. The arrangement of metamaterial bits represents the "digitising" of metamaterials.

In their study, the researchers chose to use nano-sized pieces of silver and silica (glass) as their repeating metamaterial bits. These are materials that interact with light in very different ways on an individual level. Once they were "digitised", the resulting metamaterial had its own unique properties, very different to those of its constituent parts.

"The components of the material work together to generate effects or give rise to phenomena that you wouldn't observe if they weren't arranged together in 3D (or in this case, 2D) space as an ordered assembly," said Tiffany Walsh, professor of bionanotechnology at Deakin University.

Sourcing material parts in order to achieve unusual properties of a metamaterial can be time consuming and expensive. This new way of thinking about the design of metamaterials may allow researchers to produce the optical properties they want from the metamaterial using only two component parts.

"What this [research] really does is put a new spin on the idea that with only two set materials arranged with the right portions – one metal, one insulator, here silver and silica – almost any optical property can be achieved," said Associate Professor Kuhlmey.

Professor Walsh said: "This is like the concept of turning sound waves from analog into digital – and they've pushed it into a new realm of physics.

"They've been able to take the permittivity – the response of the material when it's exposed to radiation – and digitised this. They've turned it into something that is more readily manipulated."

Waves and matter collide

One of the key applications for metamaterials lies in their ability to manipulate light.

"We already have knowledge about how to manipulate radiation (such as light) – we can use lenses, like a magnifying glass, for example, which focus light down on a spot; we can use mirrors to reflect light and change its direction," Professor Walsh said.

"But what these [metamaterials] can do is something more sophisticated: they're able to bend light, to scatter it, to manipulate it in unusual ways."

Using their digital method, the researchers showed that it is possible to create certain metamaterials with very low permittivity, which are rarely found in nature. Having control over these properties may open doors to more advanced technological applications, such as invisibility cloaking devices.

"It would be interesting in future to see if such a digital design method can facilitate the construction of optical, or invisibility, cloaks," said Professor Gu.

"With varying changes of silver/glass ratios (structured at the nanoscale) it is then in principle possible to make flat lenses and other tiny optical elements," Associate Professor Kuhlmey said.

"The authors […] showed in simulations that nano-patterned glass/silver structures can then bend light, which is also the principle behind invisibility cloaking."

He added that fabricating the proposed structures would be challenging but not impossible.

"[It would] require structuring glass and metal with a precision of a few atoms in thickness only – but thinking of metamaterials as binary structures may help devise new nano-patterning lithography (printing) techniques that take advantage of this," he said.

 Explore further: Array of perfect nanometer-scale spheres that can control the flow of visible light may find use in invisibility cloaks

Journal reference: Nature Materials  

More on the University of Sydney microscopes: Metamaterial lens promises clinical diagnoses

http://optics.org/news/4/10/51

Terahertz device produced by University of Sydney team could lead to new microscopes 'within two or three years'.

Sydney team A research team in Australia says that a new type of lens based on metamaterials could open up an entirely new field of medical diagnostics within just three years. Critically, the device is far smaller than any previous attempts at making metamaterial lenses, and was produced using the industrial method of fiber drawing – suggesting that it has a good chance of transitioning to commercial use. The University of Sydney group, whose work has just been published in the journal Nature Communications, says that the lens, which operates in the terahertz spectral region, could be used for early cancer diagnosis or to monitor drug delivery to cells. According to lead author Alessandro Tuniz and his colleagues at Sydney, the lens has ten times the resolution of any current equivalent, making it a potentially very powerful tool for biological sciences and medicine. In a release accompanying the paper’s publication, Tuniz explained: “This advance means we can unlock previously inaccessible information on the structure of molecules, their chemical make-up and the presence of certain proteins.” “This opens up an entirely new tool for biological studies,” he predicted. “It could allow earlier skin cancer diagnosis, because smaller melanomas can be recognized. For breast cancer, it can also be used to more accurately check that all traces of a tumor have been cut out during surgery.” Industrial-style production Based on a combination of metal and plastic materials, the lens operates in the terahertz region – situated between the far-infrared and microwave regions of the electromagnetic spectrum. That should enable physicians to see phenomena that until now have been invisible. The lens was made using the well-known fiber-drawing method, and based on a stack of more than 450 tapered indium wires arranged hexagonally and surrounded by a polymer with a high terahertz transmission. In focus: the Sydney team's lens The stack of indium wires was then heated and drawn to produce a fiber containing a long, continuous array of metal microwires that can be chopped into a large number of smaller devices – indicating the method's scalability. “The wire-based hyperlenses we demonstrate here are three orders of magnitude smaller than previous experiments in the microwave,” writes the team in its paper. Manufacturing challenge While metamaterials has become a key new area of photonics research, the difficulty of making technologically useful devices has, until now, restricted any potential industrial impact. Boris Kuhlmey from the Sydney team commented: “We know of only two or three other cases worldwide, including for wireless internet and MRI applications, where metamaterials could also be put into practice in the next couple of years.” He explains that a key challenge has been to make metamaterials on a useful scale. “This is one of the first times a metamaterial with a real-world application, quickly able to be realized, has been feasible,” Kuhlmey said. “Within the next two to three years, new terahertz microscopes that are ten times more powerful than current ones will be possible using our metamaterial.” Beating the optical diffraction limit In their paper, the Sydney researchers describe their adoption of wire array metamaterials as the key to the breakthrough. Because of their extreme anisotropy, they are able to beat the conventional diffraction limit that typically restricts optical microscopy techniques to a resolution of half the wavelength of light being used to interrogate a specimen. But until now they had only been demonstrated at microwave frequencies, and even then were restricted to extremely short propagation lengths that would not be clinically useful. In the abstract of the paper, Tuniz and colleagues write: “We demonstrate imaging through straight and tapered wire arrays operating in the terahertz spectrum, with unprecedented propagation of near-field information over hundreds of wavelengths and focusing down to 1/28 of the wavelength with a net increase in power density.” “Applications could include in vivo terahertz endoscopes with resolution compatible with imaging individual cells." Although some existing techniques are already able to beat the optical diffraction limit, they are typically complex and expensive in nature, and their use has so far been restricted to central technology development laboratories rather than clinical application. Tuniz also highlights the potential spectroscopic usefulness of terahertz technology beyond that of conventional X-ray imaging, saying: “The metamaterial lens would allow us not only to see through some opaque materials, but also to gather information on their chemical composition, and even information on interaction between certain molecules, without the danger of X-rays.” That could result in the lens being used to analyze the delivery of drugs to individual cells, something that may prove revolutionary in future medical research.

A new way of seeing - meta-material lens with ten times more power

http://sydney.edu.au/news/physics/1737.html?newsstoryid=12568

School of Physics researchers have developed a meta-material lens with ten times the resolution of any current lens, making it a powerful new tool for the biological sciences. The results were published in Nature Communications on October 29 2013

Dr Alessandro Tuniz and his team have developed a meta-material lens, a powerful tool for biological science.

"This advance means we can unlock previously inaccessible information on the structure of molecules, their chemical make-up and the presence of certain proteins," said lead author Dr Alessandro Tuniz.

Dr Tuniz, a postdoctoral associate at the University, said, "This opens up an entirely new tool for biological studies. It could allow earlier skin cancer diagnosis, because smaller melanomas can be recognised. For breast cancer, it can also be used to more accurately check that all traces of a tumour have been cut out during surgery."

The four member research team from the University's School of Physics, including Alessandro Tuniz, are all authors on the paper. They created the lens using fibre optic manufacturing technology.

The lens is a metamaterial - a material with completely new properties not found in nature.

The meta-material fibre consisting of a long continuous array of metal microwires

Making the lens was not a matter of making a better form of the lenses already in existence but of making a lens which uses light waves in a way not previously possible.

"Creating metamaterials is a cutting-edge area of science with a massive range of potential uses from aerospace to solar power, telecommunications to defence," said team member Dr Boris Kuhlmey.

"The major challenge is making these materials on a scale that is useful. This is one of the first times a metamaterial with a real world application, quickly able to be realised, has been feasible. Within the next two to three years, new terahertz microscopes that are ten times more powerful than current ones will be possible using our metamaterial.

"We know of only two or three other cases worldwide, including for wireless internet and MRI applications, where metamaterials could also be put into practice in the next couple of years."

The potential to create a new high power lens, able to see much finer details than using conventional lenses was spotted almost a decade ago. It has taken until now to make the lens on a useful scale, a thousand times smaller than the early experimental models.

"The difficulty was making large quantities of matter structured on a micrometric scale," said Dr Tuniz.

The new lens, made of plastic and metal, uses terahertz waves, electromagnetic waves with frequencies higher than microwaves but lower than infrared radiation and visible light. It operates in a region of the spectrum where very few other optical tools are available and all of them have limitations, in particular in terms of resolution.

"If we think of this in comparison to an X-ray which allows us to see inside objects at a high resolution but with associated danger from radiation, by contrast our metamaterial lens allows us not only to see through some opaque materials, but also to gather information on their chemical composition, and even information on interaction between certain molecules, without the danger of X-rays," said Dr Tuniz.

This means the lens is perfectly suited to analysing the delivery of drugs to cells, which is crucial to medical research.

This research was undertaken with the Freiburg Materials Research Centre from the University of Freiburg and supported by the Australian Research Council, and the Australian National Fabrication Facility using commonwealth and NSW state government funding.

Read the article published in Nature Communications here

Contact: Tom Gordon

Phone: 02 93513201

Email: tom.gordon@sydney.edu.au