Dog Bone Triplet Metamaterial Wave Plate
  Metamaterials are artificially made sub-wavelength structures arranged in periodic arrays. They can be designed to interact with electromagnetic radiation in many different and interesting ways such as allowing radiation to experience a negative refractive index (NRI). We have used this technique to design and build a quasi-optical Half Wave Plate (HWP) that exhibits a large birefringence by virtue of having a positive refractive index in one axis and a NRI in the other. Previous implementations of such NRI-HWP have been narrow band ($\sim$1-3%) due to the inherent reliance on needing a resonance to create the NRI region. We manufacture a W-band prototype of a novel HWP that uses the Pancharatnam method to extend the bandwidth (up to more than twice) of a usual NRI-HWP. Our simulated and experimentally obtained results despite their differences show that a broadening of a flat region of the phase difference is possible even with the initially steep gradient for a single plate. 
Imran Mohamed, Giampaolo Pisano, Ming Wah Ng, Vic Haynes, and Bruno   Maffei
astro-ph.IM physics.ins-det physics.optics