5 March 2004
Experiments May Yield Materials With Negative Refractive Index
by Kate Melville
British and American scientists have demonstrated an artificially made material that can provide a magnetic response to Terahertz frequency radiation, bringing the realisation and development of novel 'T-ray' devices a step closer.
The advance, reported in the journal Science, suggests novel applications in biological and security imaging, biomolecular fingerprinting, remote sensing and guidance in zero visibility weather conditions, say the authors.
Theorist John Pendry of Imperial College London, co-author of the paper, hailed the making of the material as a feat of technological virtuosity, and looked forward to some incredible applications.
"This was terra incognita, but we just pushed on to higher frequencies," said Professor Pendry. "This is the first material to show a Terahertz frequency magnetic response; it's the proof of concept experiment. We've shown we can do it, and that sends a powerful message out to the community of researchers."
Terahertz frequencies sit in a largely unexplored region of the electromagnetic spectrum between infra-red and microwaves, known as far infra-red radiation. The frequency of a terahertz is 1 trillion cycles per second and Terahertz radiation has a wavelength between 0.1 and 1 millimetre. It is thought to be safe, as it is non-ionising and does not have DNA-damaging effects.
The authors from the University of California Los Angeles, University of California San Diego and Imperial College London, are collectively looking to build materials that respond magnetically to THz, infra-red, and visible radiation as there is an almost total absence of naturally occurring materials with magnetic responses to these frequencies.
Their quest to build such artificial materials, or metamaterials, is motivated by their desire to explore a strange and intriguing property, named 'negative refractive index', which is found only in this new class of materials.
Conventional optical devices are limited in resolution by the wavelength of radiation employed (eg light or X-rays), but in a series of papers building on work by Russian physicist Victor Veselago from 1968, Professor Pendry predicted the existence of devices capable of focusing features smaller than the wavelength of light.
Referred to as 'perfect lenses', these revolutionary lenses break the wavelength barrier and achieve resolution limited only by the quality of the materials from which they are constructed.
Perfect lenses rely on a phenomenon theorised by Veselago who made a theoretical investigation of novel electromagnetic materials in which the normal response to both electric and magnetic fields is reversed. He referred to these materials as 'left handed' because the inverted response reverses the energy flow associated with a ray of light.
Amongst many strange properties of left handed materials, he found that when light is refracted from air into a left handed medium, it bends the opposite way to light entering a normal medium such as water or glass, making a chevron shape at the surface as it bends back on itself inside the left handed medium. This strange effect has subsequently been interpreted as a negative refractive index. Left handed materials are triply negative: in response to electric and magnetic fields, and also in response to a ray of light. The problem Veselago faced was that there are no such materials found in nature and this field of research was abandoned for almost thirty years.
In 1999, Professor Pendry's Condensed Matter Theory group at Imperial College were collaborating with scientists from the Marconi Company on the new class of metamaterial. In normal materials the constituent atoms and molecules determine electrical and magnetic properties; they are much smaller than the wavelength of light so only the average response of the atoms matters. In the new materials an intermediate or meta-structure is engineered on a scale somewhere between atomic dimensions and the wavelength of radiation. The properties of Metamaterials are not limited by the periodic table and scientists can now engineer a huge range of electromagnetic responses that can be tailored to anything allowed by the laws of electromagnetism, says Professor Pendry.
The Imperial/Marconi team proposed the first design for a magnetic metamaterial, known as a 'Split Ring' structure. "A simple, plain ring of metal gives a magnetic response, but in the wrong direction," says Professor Pendry, "By cutting the ring the flow of current is interrupted by capacitance across the gap which, together with the inductance of the ring, makes a tuned circuit whose resonant frequency is determined by the inductance and capacitance. It is well known that a resonant structure responds with opposite signs on either side of the resonant frequency. Hence by tuning through the resonance the desired negative magnetic response is obtained: positive or negative."
A Split Ring viewed from above looks like a small letter 'C' inside a larger letter 'C', with the smaller C turned to face the opposite direction. A single Split Ring is the metamaterial equivalent of a magnetic atom; many Split Rings brought together in organised 2D or 3D grids form a magnetic metamaterial.
The original Split Rings were designed to operate at Gigahertz, or microwave, frequencies: orders of magnitude or hundreds or thousands of times below the Terahertz range. To get a magnetic response at Terahertz frequencies, the resonant frequency of the rings has to be raised, requiring researchers to build metamaterials with a much smaller size and spacing of the elements. The microstructure must always be much smaller than the wavelength so that radiation sees only average properties of the structure.
The key technical achievement by the authors at UCLA and UCSD was to fabricate the Terahertz-responding Split Rings using a special 'photo-proliferated process' that deposited the 3 micrometer-wide (0.003 mm) copper rings on a quartz base.
"This is a technological advance by the virtuosi of their craft," said Professor Pendry of the work by his colleagues at UCLA and UCSD.
"Looking to higher frequencies, in the optical region of the spectrum, magnetism just does not at present figure in our thinking because almost all materials are magnetically inert at these frequencies.
Optical properties are almost entirely due to the electrical response of materials to one of the two available fields - the electric field. Professor Pendry likens controlling light in this way to driving a motorbike with one hand - it's possible, but gives you only a fraction of the possible control and subtlety of resolution available in imaging. By bringing the magnetic field into play, he suggests, we may be able to harness a vastly more powerful imaging technology. "Now we are all on notice to include the possibility of optical magnetism when discussing new devices," he adds.
"We want to push the limits of frequency and produce structures that work in the infra red and ultimately in the visible. The march of magnetism towards the visible will enhance our power to control and use electromagnetic radiation in these frequency ranges." he said.
"So far we have only seen negative refraction at microwave or GHz frequencies but some of the most exciting applications in sensing, communication, and data storage would be at higher frequencies," he said. "But I believe that the really valuable applications have yet to be dreamt of. Think back to when the first lasers were made, the reaction was that they were just incredible, but what the hell would we do with them?" said Professor Pendry.