Fractal Zone Plates for Terahertz Focusing and Imaging

We present novel designs for difiractive lenses with low chromatic aberration to be used in terahertz focusing and imaging. Conventional difiractive lenses (Fresnel zone plates) have been successfully applied in this area due to their low, attenuation, thickness, and weigth. However, the strong chromatic aberration of these difiractive lenses limit their application only to systems with narrowband sources. We propose Fractal zone plates (FrZP) as a special type of zone plate where the pattern of the zones is fractal along the square of the radial coordinate. As a result, these lenses produce multiple foci with a self-similar structure. The main lobes of the difierent foci provided by a FrZP coincide in the axial position with those obtained with a conventional Fresnel zone plate, but the subsidiary foci of the FrZP provide an extended depth of focus for each wavelength that partially overlap with the other one, creating an overall extended depth of focus which is less sensitive to the chromatic aberration. We have tested the THz imaging capabilities of FrZPs with broadband sources and compared their performance against a conventional Fresnel zone plate of the same main focal distance.