Abstract
Relying on abrupt phase discontinuities, metasurfaces characterized by a transversely inhomogeneous surface impedance profile have been recently explored as an ultrathin platform to generate arbitrary wave fronts over subwavelength thicknesses. Here, we outline fundamental limitations of passive gradient metasurfaces in molding the impinging wave and show that local phase compensation is essentially insufficient to realize arbitrary wave manipulation, but full-wave designs should be considered. These findings represent a critical step towards realistic and highly efficient conformal wave manipulation beyond the scope of ray optics, enabling unprecedented nanoscale light molding.
- Received 9 July 2015
DOI:https://doi.org/10.1103/PhysRevX.6.041008
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Arbitrary control and molding of the impinging electromagnetic wave front has been a long-sought goal, inspiring the invention of various optical devices such as mirrors, lenses, and holograms. More recently, metasurfaces have demonstrated the possibility of achieving analogous degrees of control, yet over an ultrathin surface suitable for integration. Metasurfaces are artificially engineered structures that can be tailored to reflect, transmit, and absorb incident light in desired ways. The current literature on metasurface design mostly relies on local phase-correction recipes stemming from ray optics. It has been observed, however, that the performance of such metasurface designs significantly deteriorates for extreme wave manipulation and fails to provide reasonable efficiency. Here, we investigate fundamental limitations and potentials of passive metasurfaces for arbitrary wave manipulation.
Our study reveals inherent limitations of locally passive surfaces, even in the simplest scenario of steering the direction of propagation of the incident wave. We determine that a careful combination of balanced loss and gain regions over the metasurface is required to allow efficient reflection of an incident plane wave into another one. Based on our theory, we propose appropriate corrections to the ray optics approximations widely employed in the current literature on metasurfaces, making it possible to maximize the transformation efficiency as well as the purity of the scattered wave from a passive metasurface. These corrections, which take into account the impedance mismatch between impinging and desired wave fronts, become more important as the wave transformation becomes more extreme. We also employ our theory to design near-field lenses with deeply subwavelength hot spots, demonstrating the importance of the derived local impedance corrections.
Our findings pave the way for efficient wave-front manipulation with metasurfaces, offering interesting implications for compact nanophotonic devices and imaging systems.
