Nasim Mohammadi Estakhri

By applying the optical nanocircuit concepts to metasurfaces, we propose an effective route to locally control light transmission over a deeply subwavelength scale. This concept realizes the optical equivalent of a transmit-array, whose use is demonstrated for light bending and focusing with unprecedented efficiency over a subwavelength distance, with crucial benefits for nano-optics applications. These findings may lead to large improvements in the manipulation of optical transmission and processing of nanoscale optical signals over conformal and Si-compatible substrates.

Metastructures hold the potential to bring a new twist to the field of spatial-domain optical analog computing: migrating from free-space and bulky systems into conceptually wavelength-sized elements. We introduce a metamaterial platform capable of solving integral equations using monochromatic electromagnetic fields. For an arbitrary wave as the input function to an equation associated with a prescribed integral operator, the solution of such an equation is generated as a complex-valued output electromagnetic field. Our approach is experimentally demonstrated at microwave frequencies through solving a generic integral equation and using a set of waveguides as the input and output to the designed metastructures. By exploiting subwavelength-scale light-matter interactions in a metamaterial platform, our wave-based, material-based analog computer may provide a route to achieve chip-scale, fast, and integrable computing elements.

Metasurfaces are engineered systems that enable advanced control of electromagnetic waves over deeply subwavelength thicknesses. Researchers make a careful study of the use of metasurfaces to transform the impinging optical wave front.

Recent advances in metasurfaces, i.e., artificial arrays of engineered inclusions assembled over a thin surface, have opened promising venues to control electromagnetic waves in unique and unprecedented ways, by means of locally engineering their near-field wave–matter interactions. Gradient or locally nonperiodic metasurfaces are one of the most exciting recent advances in nano-optics, due to the promise of enabling ultimate light molding, in both the near-field and the far-field, with large efficiency and a minimal footprint. These artificial surfaces are characterized by a transverse variation of their surface properties and lack of local periodicity, distinguishing them from conventional frequency selective surfaces and optical gratings. In this paper, we review recent work in the area of gradient metasurfaces, aimed at arbitrary wave shaping. The significant recent progress and novel applications achieved through optical metasurfaces, including ultrathin invisibility cloaks and polarization-dependent light splitting, are discussed, outlining the typical challenges and their outstanding prospects in integrated nanophotonic devices. Following our discussion on metasurface design approaches, we then revisit the problem of controlling the distribution of energy between multiple diffraction orders by means of gradient metasurfaces. Our discussions reveal that Huygens-based designs hold the promise of overcoming the low conversion efficiency issues associated with other techniques.

The negative refraction and evanescent-wave canalization effects supported by a layered metamaterial structure obtained by alternating dielectric and plasmonic layers is theoretically analyzed. By using a transmission-line analysis, we formulate a way to rapidly analyze the negative refraction operation for given available materials over a broad range of frequencies and design parameters, and we apply it to broaden the bandwidth of negative refraction. Our analytical model is also applied to explore the possibility of employing active layers for loss compensation. Nonlinear dielectrics can also be considered within this approach, and they are explored in order to add tunability to the optical response, realizing positive-to-zero-to-negative refraction at the same frequency, as a function of the input intensity. Our findings may lead to a better physical understanding and improvement of the performance of negative refraction and subwavelength imaging in layered metamaterials, paving the way towards the design of gain-assisted hyperlenses and tunable nonlinear imaging devices.