Dielectric Meta-Reflectarray for Broadband Linear Polarization Conversion and Optical Vortex Generation Yuanmu Yang, † Wenyi Wang, ‡ Parikshit Moitra, † Ivan I. Kravchenko, § Dayrl P. Briggs, § and Jason Valentine* ,∥ † Interdisciplinary Materials Science Program and ‡ Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, Tennessee 37212, United States § Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Department of Mechanical Engineering, Vanderbilt University, Nashville, Tennessee 37212, United States * S Supporting Information ABSTRACT: Plasmonic metasurfaces have recently attracted much attention due to their ability to abruptly change the phase of light, allowing subwavelength optical elements for polarization and wavefront control. However, most previously demonstrated metasurface designs suffer from low coupling efficiency and are based on metallic resonators, leading to ohmic loss. Here, we present an alternative approach to plasmonic metasurfaces by replacing the metallic resonators with high-refractive-index silicon cut-wires in combination with a silver ground plane. We experimentally demonstrate that this meta-reflectarray can be used to realize linear polarization conversion with more than 98% conversion efficiency over a 200 nm bandwidth in the short- wavelength infrared band. We also demonstrate optical vortex beam generation using a meta-reflectarray with an azimuthally varied phase profile. The vortex beam generation is shown to have high efficiency over a wavelength range from 1500 to 1600 nm. The use of dielectric resonators in place of their plasmonic counterparts could pave the way for ultraefficient metasurface- based devices at high frequencies. KEYWORDS: Metamaterial, dielectric antenna, polarization conversion, vortex beam A chieving full control over light propagation is an ever present issue in modern day optics. In order to realize such control it is necessary to create devices that allow 0 to 2π phase modulation and/or devices that allow control over the amplitude of light. In conventional optical elements such as birefringent waveplates and lenses, a significant propagation distance is needed to acquire disparate phase accumulation for different polarizations or spatial regions of the beam, thus requiring thick materials that are difficult to integrate into compact platforms. 1 One solution to this issue is the use of reflect and transmit-arrays, originally developed at radio frequencies, that allow one to control the amplitude and phase of light using a single, or several, layers of ultrathin antennae. 2,3 By changing the geometry of the antenna as a function of position, these arrays allow spatial control over the phase of light. Recently, similar materials, deemed metasurfaces, have been developed at optical frequencies. 4 Metasurfaces typically utilize asymmetric electric dipole resonances to allow 0 to 2π phase control for the cross-polarized scattered light. As in transmitarrays, varying the geometry of the resonator as a function of position allows arbitrary control of the phase front of light using a subwavelength-thin film and has led to demonstrations including anomalous refraction, 4−8 quarter and half waveplates, 9,10 lensing, 11−13 and manipulation of orbital angular momentum. 14,15 One of the drawbacks of plasmonic metasurfaces is that they typically suffer from low efficiency due to weak coupling between the incident and cross-polarized fields. Methods to increase efficiency include the use of overlapped electric and magnetic resonances 16 as well as the use of thicker metasurface sheets, 17 though both of these proposals require materials with increased complexity. One can also employ antennae working away from their resonance positions to realize quarter- waveplates with polarization conversion efficiencies of close to 50% over large bandwidths. 18 In another approach, it was shown that an array of metallic antennae in combination with a reflective ground plane can achieve efficiencies of up to 80% for anomalous reflection 7,13,19 and linear polarization conversion 10 by introducing multiple reflections within the film. While this design avoids complexity, the use of metallic antennae still limits metasurfaces from achieving unity efficiency at optical wavelengths due to the ohmic losses in metal. 7 Resonant dielectric metamaterials offer one potential solution to the issue of loss. Formed from high refractive index resonators, dielectric metamaterial unit cells can support electric and magnetic dipole responses due to Mie Received: December 1, 2013 Revised: February 14, 2014 Published: February 18, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 1394 dx.doi.org/10.1021/nl4044482 | Nano Lett. 2014, 14, 1394−1399