Aberrations of the large aperture attenuating THz lenses

Abstract

The elements for manipulating THz beams can be designed both on the basis of the microwave and on the optical technology. Similarly to typical optical components, refractive lenses and diffractive structures can be used in the THz range. For many practical applications the passive THz systems require sophisticated optical elements with a large numerical aperture (NA). The expected resolution of the optical setup is close to the diffractive limit. Therefore the aperture diameter of such optical elements is mostly in the range between 100 and 250 mm or even more and their focal length is often equal to the diameter. A standard refractive high NA spherical lens for the THz range exhibits high signal attenuation due to significant thickness. For typical converging lenses their attenuation is higher near the optical axis (low geometrical aberrations) than in the peripheral regions (high geometrical aberrations). This additionally boosts overall geometrical aberrations of the lens. Here we propose a sophisticated Fresnel-type structure. It should be thin enough to provide low attenuation and thick enough (high order kinoform) to avoid chromatic aberration. Due to special design process the spherical aberration of the structure can be significantly decreased. Computer modeling and experimental results are presented.