Abstract
Materials which possess a high local density of states varying at a subwavelength scale theoretically permit the focusing of waves onto focal spots much smaller than the free space wavelength. To do so, metamaterials—manmade composite media exhibiting properties not available in nature—are usually considered. However, this approach is limited to narrow bandwidths due to their resonant nature. Here, we prove that it is possible to use a fractal resonator alongside time reversal to focus microwaves onto subwavelength focal spots from the far field, on extremely wide bandwidths. We first numerically prove that this approach can be realized using a multiple-channel time reversal mirror that utilizes all the degrees of freedom offered by the fractal resonator. Then, we experimentally demonstrate that this approach can be drastically simplified by coupling the fractal resonator to a complex medium, here a cavity, that efficiently converts its spatial degrees of freedom into temporal ones. This makes it possible to achieve deep subwavelength focusing of microwave radiation by time reversing a single channel. Our method can be generalized to other systems coupling complex media and fractal resonators.
- Received 4 March 2016
DOI:https://doi.org/10.1103/PhysRevB.93.180201
©2016 American Physical Society
Physics Subject Headings (PhySH)
Synopsis
Fractals for Sharper Vision
Published 5 May 2016
A material with fractal geometry can focus microwaves onto a spot 15 times smaller than their wavelength.
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