PRL 104, 203901 (2010) PHYSICAL REVIEW LETTERS
week ending 21 MAY 2010
Resonant Metalenses for Breaking the Diffraction Barrier
Fabrice Lemoult, Geoffroy Lerosey,* Julien de Rosny, and Mathias Fink
"We introduce the resonant metalens, a cluster of coupled subwavelength resonators. Dispersion allows the conversion of subwavelength wave fields into temporal signatures while the Purcell effect permits an efficient radiation of this information in the far field. The study of an array of resonant wires using microwaves provides a physical understanding of the underlying mechanism. We experimentally demonstrate imaging and focusing from the far field with resolutions far below the diffraction limit. This concept is realizable at any frequency where subwavelength resonators can be designed. Within all areas of wave physics it is commonly believed that a subwavelength wave field cannot propagate in the far field. This restriction arises from the fact that details with physical dimensions much smaller than the wavelength are carried by waves whose phase velocity exceeds that of light in free space, which forbids their propagation. Such waves, usually referred to as evanescent waves, possess an exponentially decreasing amplitude from the surface of an object [1]. Numerous works have been devoted to overcome this diffraction limit, starting from the early 20th century and the proposal by Synge of the first near field imaging method [2]. Since this seminal work, near field microscopes have been demonstrated from radio frequencies up to optical wavelengths, achieving resolutions well below the diffraction limit [3–8]. Fluorescence based imaging methods have also been proposed, which allow deep subwavelength imaging of living tissues [9]. Such concepts, however, employ several measurements of the same sample in order to beat the diffraction limit through image reconstruction procedures. Finally, various new concepts have been proposed such as far field superlens and hyperlens [10–12], demonstrating moderate subdiffraction imaging down to a quarter of the optical wavelength. In this Letter, we introduce the concept of resonant metalens, a lens composed of strongly coupled subwavelength resonators, and prove that it permits subdiffraction imaging and focusing from the far field using a single illumination. Our concept resides in exploiting time-coded far field signals for spacial subwavelength resolution [13,14]. Studying the specific case of an array of resonant wires, we explain theoretically, prove numerically, and demonstrate experimentally how this lens converts the subwavelength spatial profile of an object into a temporal signature and allows efficient propagation of this information towards the far field.We achieve far field imaging and focusing experiments with resolutions of, respectively, 1/80k and 1/25k, well below the diffraction limit. ..."