Compound Metaoptics for Lossless Amplitude and Phase Control of Wavefronts

Summary form only given. Lossless and reflectionless metasurfaces reported to date control radiation by imposing phase discontinuities onto a wavefront. As a result, a single metasurface of this type can control either the phase or the amplitude profile of a field at a particular plane. In this talk, compound metaoptics will be described that can independently manipulate the amplitude and phase profiles of a wavefront. The compound metaoptic consists of two passive metasurfaces that are lossless and reflectionless, and separated by a wavelength-scale distance. As shown in Figure 1, the first metasurface projects the correct power density onto the second metasurface, allowing beam shaping. The second metasurface provides a phase correction that allows beam steering. The proposed approach allows amplitude control without relying on loss (reflection, absorption, or polarization loss), as in earlier works. In designing the compound metaoptic, a phase retrieval/optimization algorithm is used to determine the field distribution between the metasurfaces, region II in Figure 1. This field links the power density of the incident wavefront (incident from region I) to the power density of the desired wavefront (transmitted to region III) at the metasurface boundaries. The field in region II, along with the incident and desired (overall transmitted) wavefronts determine the surface parameters of each transparent metasurface comprising the metaoptic. The two metasurfaces are, in general, bianisotropic and can be implemented as cascades of three electric impedance sheets: patterned metallic or dielectric surfaces. A metaoptic design will be presented at the conference that provides the amplitude and phase control needed to convert an incident Gaussian beam to a Dolph-Tchebyscheff radiation pattern. In addition, a second metaoptic design will be shown that forms a simple complex-valued hologram. Specifically, it recreates the field scattered by a collection of line scatterers. More recent efforts to implement polarization preserving, non-bianisotropic compound metaoptics at near infrared wavelengths will also be described. In this case, arrays of silicon pillars, embedded in a PDMS medium, are used to provide phase shifts and implement each metasurface.