Collimating Truncated Virtual Image Lens

The Luneburg lens is of significant interest for microwave and optical devices due to its wide-angle focusing properties. However, the inhomogeneous refractive index of the lens is restrictive and can be difficult to realize, especially at high frequency (typically millimetre-waves and above). Here, we present an inhomogeneous lens referred to as the collimating truncated virtual image lens, which is derived as a combination of the recently proposed virtual image lens and a conventional extended hemispherical lens. We investigate the operation of the proposed lens and we demonstrate that it provides similar focusing properties as the Luneburg lens, but with a flexible refractive index profile. This flexibility can be used to alleviate the strict manufacturing constraints typically associated with the Luneburg lens. We validate the properties of the proposed lens with a demonstrator at millimetre-wave frequencies showing that an antenna based on the proposed lens can obtain similar directivity and side lobe levels to those obtained in an ideal Luneburg lens antenna, while the proposed lens is easier to realize. The collimating truncated virtual image lens is attractive for instruments and antennas at high frequency.