June 08, 2010

Metalens can Image Down to 1/80th of a Wavelength

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(a) The experimental resonant metalens on the ground copper plane. Experiments are performed in an anechoic chamber. (b-c) Signals and spectra received in the far-field after emission from central monopole with the lens (blue) and without as a control curve (red). (d) Focal spot obtained after one channel Time Reversal of (b) from the far-field: a =25 width is demonstrated in the presence of the resonant metalens (blue), no focusing without the lens (red). (e) An imaging experiment. 16 monopoles generate a subwavelength phase and amplitude profile in the near field of the lens (black points). The far-field is acquired on 8 antennas. We plot the result of the image reconstruction: a true =80 resolved image of the initial pattern is reconstructed in the presence of the resonant metalens (blue) while it is impossible without (red).
Resonant metalenses can image down to 1/80th of a wavelength. The classical diffraction limit (which was thought to be impossible to exceed less than twenty years ago) was half of a wavelength.
* losses limited the experimental focal spot sizes to one 25th of a wavelength, but using other focusing techniques may shrink the spots even further
* we prove the imaging capabilities of the resonant metalens through a simple experiment: a subwavelength profile is generated at the input of the lens using simultaneously 16 monopoles, and the far-field recorded in the anechoic chamber.An inversion procedure with predesigned filters is used to reconstruct the profile, using the knowledge of each monopole temporal signature (See e in the image above). The subwavelength profile is perfectly reconstructed and an imaging resolution of about one 80th is demonstrated through this basic experiment.

Technology Review has coverage on the metalens.

Here's how it works. The object under study is bathed in light and the lens placed in the near field. Any subwavelength detail in the em field couples with the sub wavelength resonators, which also have modes that couple with larger details in the em field.

These resonances propagate through the lens until they are radiated again on the other side, reproducing the near field exactly (or as well as the losses within the system and its resolution allow).

Lemoult and co call this device a resonant metalens and have even built one to prove the principle in the microwave region of the spectrum. Their lens consists of a 20 x 20 array of copper wires, each 40 cm long and 3mm wide with a period of 1.2 cm.

One obvious question which they do not address is the theoretical link between resonant metalenses and other devices that also beat the diffraction limit, in particular, the superlenses described by John Pendry at Imperial College London, the leading theoretician in this field.

Pendry takes an entirely different approach to deriving the properties of his superlens but the end result is more or less identical. Which means there's bound to be a formal mathematical link between the two approaches.

This specific studied lens is scalable towards near-IR and in this range, the losses will increase, limiting the resolution. Using gain media in the matrix may counter this problem. More generally, we are currently working on a criterion linking the resolution achievable to the losses and typical size of the metalens. This lens presents degenerated modes because of the symmetry: adding some disorder in the spatial or resonant frequency distribution of the resonators, as well as in the matrix should lift this degeneracy and enhance dispersion. Finally, we would like to emphasize that the concept of resonant metalens should be realizable in any part of the electromagnetic spectrum, with any subwavelength resonator, such as split-rings, nanoparticles, resonant wires, and even bubbles in acoustics.

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