Abstract
High capacity radio lines operating in the sub-THz and THz ranges often require very efficient optical elements with a focal length to an aperture diameter ratio—f-number—less than 1. Here, we propose a new type of double-sided sub-THz focusing diffractive optical element with f-number equal to 0.2, designed for quasi-monochromatic illumination with carrier frequency equal to 170 GHz. The element is manufactured by 3D printing technology. Its focal spot diameter defined as the Airy disc size is comparable to the used wavelength. In order to optimize numerically the phase distribution on the anterior side of the structure, we proposed a novel idea based on reversal of phase distribution in outer zones with additional constant phase factor (a method called free form phase distribution, FFPD). Moreover, we applied the modified numerical algorithm to obtain an additional phase correction in a form of a corrective kinoform placed on the posterior side of the diffractive system. The resulted diffractive structure, illuminated by a quasi-plane wave, forms an extremely small focal spot. The paper presents the technical and the theoretical backgrounds, the results of the computer simulations and finally the experimental results.
Highlights
The number of applications for sub-THz and THz radiation is constantly growing
The kinoform on the anterior side was designed in a novel form as the free form phase distribution (FFPD) element, limiting the shadow effect and appearing in outer parts of diffractive structures with large numerical apertures
We have analysed numerically and experimentally the diffractive structures with a very large numerical aperture corresponding to the f-number equal to 0.2, much smaller than that characterized structures described in Ref. [18] (f-number equal to 1)
Summary
The number of applications for sub-THz and THz radiation is constantly growing. It spreads from medical applications and the cancer analysis [1], through telecommunication [2, 3], drugs and explosives detection [4], non-destructive testing during production process (http://terasense.com/terahertz-technology/terasense/), to safety measures and agriculture [5]. It is estimated that the size of the THz application market will dynamically grow with each subsequent year (https://www.tematys.fr/reports/en/index.php?controller=attachment&id_ attachment=2). In the case of high capacity (high bit rate) radio lines, it is extremely important to achieve a very efficient concentration of THz radiation on the detector. It is necessary to concentrate the incoming radiation into a small spot with a size of the wavelength or smaller. Many methods have been proposed to overcome resolution limit and achieve focal spot with size varying from λ/5 [6] to even λ/38 [7]
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