Analog Computing Using Reflective Plasmonic Metasurfaces

Abstract

Motivated by the recent renewed interest in compact analog computing using light and metasurfaces (Silva, A. et al., Science 2014, 343, 160-163), we suggest a practical approach to its realization that involves reflective metasurfaces consisting of arrayed gold nanobricks atop a subwavelength-thin dielectric spacer and optically-thick gold film, a configuration that supports gap-surface plasmon resonances. Using well established numerical routines, we demonstrate that these metasurfaces enable independent control of the light phase and amplitude, and design differentiator and integrator metasurfaces featuring realistic system parameters. Proofof-principle experiments are reported along with the successful realization of a high-quality poor-man's integrator metasurface operating at the wavelength of 800 nm.Keywords: Metasurfaces, plasmonics, analog computing, metamaterials, gap surface plasmons * To whom correspondence should be addressed † In the quest to fully control light at the nanoscale, the year 2000 marks a new epoch in studying light-matter interactions, as researchers, fascinated by the experimental verification of negative refraction 1 and the theoretical work on perfect lensing, 2 in copious amounts ventured into the field of man-made materials, i.e., metamaterials. More than a decade later, several groundbreaking applications have been suggested and verified, such as super-resolution imaging, 3 invisibility cloaks, 4 and metamaterial nanocircuits. 5 In any case, however, and especially at optical frequencies, the general usage of metamaterials seem hindered by difficulties in fabrication and too high losses.As a way to circumvent the drawbacks of metamaterials, the two-dimensional analog, known as metasurfaces, have attracted increasing attention in recent years. 6 Metasurfaces are characterized by a subwavelength thickness in the direction of propagation, while the transverse plane typically consists of an array of metallic scatterers with subwavelength periodicity. Generally speaking, metasurfaces function as interface discontinuities which, depending on size, shape and composition of scatterers, allow for an abrupt change in the amplitude and/or phase of the impinging light. 7 It should be noted that a single layer of scatterers (due to their Lorentzian-shaped polarizability) only allow for full 2π-phase control of the cross-polarized light component, 8 meaning that such metasurfaces have a theoretical efficiency of maximum 25%, 9 though most realizations show efficiencies of a few percent. 10,11 In order to improve the efficiency of plasmonic metasurfaces, the low-frequency concept of transmit-and reflectarrays has been generalized and adopted to the visible and infrared regimes, where metasurfaces working in transmission consist of several layers in order to reach full phase control and proper impedance matching with surroundings. 9,12 Accordingly, such metasurfaces are quite complex to fabricate at near-infrared and visible frequencies, with a moderate efficiency of ∼ 20 − 50% due to ...

Principles of electromagnetic waves in metasurfaces

“…More recently, researchers reported a host of metasurfaces comprised of various kinds of nano-antennas to modulate the phase [20,30,41]. In particular, reflective metasurfaces have shown their strong ability since higher energy efficiency can be obtained [60,79,80]. By combining the MLRR with spectral dispersion engineering, broadband response could be achieved.…”

Advances in Full Control of Electromagnetic Waves with Metasurfaces

“…[159][160][161][162] Different achievements contribute immensely towards the growth of metasurfaces. Some fascinating stories continue to be put on the stage, such as 2D analogue of Cherenkov radiation (Figure 6 c), [ 163 ] mathematical operation ( Figure 6 d), [ 164,165 ] nonlinear metasurface (Figure 6 e) [ 166 ] and remote quantum interference (Figure 6 f). [ 167 ] Beside the spatial gradient, a transverse temporal gradient can also be taken into account to break the reciprocity symmetry, which shows important application in nonreciprocal devices, such as optical diode.…”

Design, concepts, and applications of electromagnetic metasurfaces

“…This includes metasurfaces producing beams possessing angular orbital momentum [57] or vortex waves [58][59][60][61][62][63], holograms [64,65], and stable beam traction [66]. Additionally, nonreciprocal transformations [67][68][69][70][71], nonlinear interactions [72][73][74], analog computing [75,76], and spatial filtering [77][78][79] have also been reported.…”