Transparent all-dielectric gradient index waveplates with compact profiles

Purely dielectric waveplates overcome problems typically associated with metals, e.g., corrosion. However, they are often bulky and/or lossy, substantially reducing their applicability. As the operating frequency shifts towards lower frequencies where compactness is a sought after quality, these problems become even more severe. In this letter, we theoretically demonstrate how to combine axially varying gradient index materials with form birefringence to realize nearly transparent and compact dielectric waveplates. The waveplate is subsequently studied considering a discrete distribution of indexes of refraction. Our results indicate that the number of discretization levels required to perform as the continuous gradient index case is small, which would simplify significantly the fabrication process. As an example, a 0.65λ-thick quarterplate with less than 0.1 dB insertion loss at ∼34 GHz is designed and compared with full-wave simulations, proving an excellent agreement.