Metamaterial shows its magnetic side (IST/OP)
08 May 2012 by Aurèle AdamAurèle Adam (IST/OP) and colleague researchers from TU Delft and Boston University have for the first time unveil the magnetic side of Metamaterial in their measurements published in their paper entitled "THz near-field Faraday imaging in hybrid metamaterials”. Below you can read the abstract.
Metamaterials are novel artificial materials with optical properties that don't exist in nature. Recently, they were used for the first time to demonstrate “cloaking”, a way to manipulate light around an object to make it appear invisible at certain wavelengths. A popular version of this metamaterial consists of an array of tiny coils, also-called split-ring resonators, covering the surface of a material. Although they are very similar to the electromagnets found in loudspeakers, they are much-much smaller and they’re not driven by electrical power supplies.
Instead they respond to the oscillating electric field of the incident light. However, similar to the case of a conventional electromagnet, this results in an electrical current in the ring which then creates a magnetic field. This magnetic field, which changes sign some 200 billion times per second, is essential to understand how metamaterials work. Just as two electromagnets can attract or repel each other, so can two split ring resonators influence each other through their magnetic field. The main difference being the extraordinarily high frequency at which the split ring magnetic field changes sign. Unfortunately, until recently, there existed no method to directly measure the magnetic field in the split-ring.
Researchers of the University of Technology Delft, together with colleagues from Boston University have now, for the first time, developed a technique capable of directly measuring the magnetic field of terahertz light with unprecedented detail. To achieve this, the researchers first needed to solve the problem that most techniques capable of measuring high frequency magnetic fields simply cannot see sharp enough.
The problem, in technical terms called the "diffraction limit", is that the sharpest features that one theoretically can see have to be larger than about half-of-a-wavelength of the light used. For the THz light used, this means that features smaller than about 1 mm could not be seen whereas the size of the split rings was about 50 micrometers, which roughly corresponds to the diameter of a hair.
The researchers solved this problem by using a focused laser beam, having a much shorter wavelength, to "read" the THz magnetic field using a so-called magneto-optic material. Using this trick, they could “see” hundreds of times sharper, allowing them, for the first time to create a detailed map of the THz magnetic fields in and around the small split-rings.
A comparison with a computer model of the split-ring yielded a surprise: they deduced that in some locations, the magnetic field was 200 times stronger than the magnetic field of the THz light without the split-ring! This result highlights the importance of being able to measure this magnetic field and illustrates that it may lead to a new and improved understanding of metamaterials.
Picture: The incident electric field (blue), induces current (green arrows) in the resonator magnet through the gap. It then produces the magnetic field (red lines).
More information
"THz near-field Faraday imaging in hybrid metamaterials", Opt. Express 20, 11277-11287 (2012).
Go to http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-10-11277 for more information or contact Aurèle Adam by e-mail a.j.l.adam@tudelft.nl
