Ben Wood
Research
Metamaterials
I am currently working with John Pendry on metamaterials. These are structures designed to control the way light behaves, fabricated on a scale much smaller than the light wavelength. The structured material acts like an effective medium: the light doesn't "see" the details of the structure, only an averaged picture. By modifying the structure, we can tune the electric and magnetic response of the metamaterial; this gives us an unprecedented level of control and flexibility, and access to properties not found in natural materials.
The way a material (or metamaterial) responds to electric and magnetic fields is parameterized by the dielectric permittivity and the magnetic permeability, respectively. One of the most intriguing ideas is the possibility that both of these quantities could take negative values. This gives a negative index of refraction, so that light entering a negative index medium is refracted "the wrong way". We can use this property to make a superlens: a very unusual lens that can beat the diffraction limit for resolution.
For those seeking more information and an overview of the subject, John's web page has a list of some recent papers, including reviews covering negative refraction and metamaterials at various levels.
More recently, metamaterials have been in the spotlight again; they have been used to create an invisibility cloak. A powerful technique called transform optics allows us to mimic the effect of warping space by simply modulating the permeability and permittivity. We start with light in empty space, then imagine squashing, squeezing and twisting space so that the light goes where we want it to go; the theory provides us with a formula for the electric and magnetic properties required to reproduce the effect, without the need to physically deform space (which would be very hard to do!).
Transform optics therefore provides us with a recipe for designing devices. In the case of the invisibility cloak, we would like to mimic the effect of ripping a hole in space so that we can hide things inside. The recipe provided by transform optics is quite demanding: both the permeability and permittivity are required to be anisotropic, and to take on a range of values. The cloak would therefore be impossible to make using only natural materials; fortunately, metamaterials are able to come to the rescue, and a simplified version of the cloak, which works for microwave frequencies, has been built and successfully tested by our collaborators at Duke University.
I have recently been working on a related topic. At very low frequencies, the electric and magnetic fields effectively decouple, and it becomes sensible to consider producing a cloak to shield only magnetic fields (or only electric fields). Designing metamaterials to work in this regime is challenging: there are extra restrictions on the permittivity and permeability, and no resonances to exploit. A cloak for magnetic fields requires an anisotropic magnetic material that looks diamagnetic in one direction but paramagnetic in the other two. (A diamagnet repels magnetic fields, while a paramagnet attracts them.) The hard part is diamagnetism; we have shown that chopping a superconductor into small cubes gives us a controllable diamagnetic response, and flattening the cubes into plates restricts this response to one direction. We are currently planning to make and test a version of this zero-frequency metamaterial.
Previously, I investigated a system of very thin alternating layers of metal and dielectric. Over a small frequency range, this system can act like a superlens for electric fields. I have been investigating another regime, where fields propagate through the system in a preferred direction.
Quantum Monte Carlo simulations
During my PhD, I investigated ways to improve Quantum Monte Carlo techniques for studying surface systems. My PhD thesis is available in PDF form; my PhD supervisor was Professor Matthew Foulkes.
Publications
On metamaterials:- Directed sub-wavelength imaging using a layered silver structure, Phys. Rev. B 74 (2006) 115116
- Metamaterials at zero frequency, J. Phys.: Condens. Matter 19 (2007) 076208
- Improved many-electron wave functions from plasmon normal modes, J. Phys.: Condens. Matter 18 (2006) 2305-2326
- Coulomb finite-size effects in quasi-2D systems, J. Phys.: Condens. Matter, 16, (2004) 891-902
- Quantum Monte Carlo calculations of the jellium surface energy, submitted.
Notes
I have written some notes concerning the fields emitted by simple cylindrical current sources, and the expansion of these fields in terms of plane waves.