Using a periodic network of capacitors and inductors, it is possible to create a structure that has a negative index of refraction at microwave frequencies.
Currently, we are working on a free space negative refractive lens. The lens exhibits a broad backward-wave bandwidth. To the left is a picture of the lens within a parallel-plate waveguide. At 2.45 GHz the lens can focus electromagnetic waves to subwavelength dimensions. By image theory, the two parallel plates in this picture cause the lens to act as if it were infinite in height. To stream a video of the lens in action click here.
To the right is a picture of a structure which exhibits broadband negative permeability. A negative permeability slab can be used to amplify S-polarized evanescent waves, which usually decay exponentially away from the source. The periodically loaded capacitors, which give rise to negative permeability, can be seen on the top surface of the slab.
We are currently developing new devices which can manipulate and focus the electromagnetic near-field. We refer to these devices as near-field plates. Near-field plates are planar (grating-like) structures that can focus electromagnetic radiation to spots or lines of arbitrarily small subwavelength dimension. They can be tailored to give subwavelength focal patterns of various types.
A near-field plate can be thought of as an impedance sheet that has a modulated surface reactance. It focuses the field of a plane wave or finite source from one side of the sheet to the other side, with subwavelength resolution. The impedance sheet is composed of completely passive inductive and capacitive impedances. In practice, these inductive surface impedances could be implemented as metallic strips/wires and the capacitive surface impedances as metallic patches printed on a microwave substrate. At optical frequencies, nanofabricated plasmonic and dielectric elements could be used to fabricate these plates.
A general procedure for designing near-field plates has been outlined in the paper below. A practical design at microwave frequencies is given as well. Full-wave simulations clearly demonstrate the proposed near-field plate’s ability to overcome the diffraction limit.
A. Grbic and R. Merlin, “Near-field Focusing Plates and Their Design”, submitted to IEEE Trans. on Antennas and Propagation, submitted on June 2, 2007 (can be found at http://arxiv.org/abs/0708.0049).
Near-field plates (evanescent field lenses) were introduced in:
R. Merlin, "Radiationless Electromagnetic Interference: Evanescent-Field Lenses and Perfect Focusing," Science Express, 10.1126/science.1143884, July 12, 2007.
Photo Credit: Cyan James
A photograph showing the capacitive elements of the experimental nearfield plate that operates at 1.027 GHz. The plate consists of an array of 39 interdigitated capacitors (elements) printed on an electrically-thin dielectric substrate.