Advanced THz setups require high performance optical elements with large numerical apertures and small focal lengths. This is due to the high absorption of humid air and relatively low efficiency of commercially available detectors. Here, we propose a new type of double-sided sub-THz diffractive optical element with suppressed geometrical aberration for narrowband applications (0.3 THz). One side of the element is designed as thin structure in non-paraxial approach which is the exact method, but only for ideally flat elements. The second side will compensate phase distribution differences between ideal thin structure and real volume one. The computer-aided optimization algorithm is performed to design an additional phase distribution of correcting layer assuming volume designing of the first side of the element. The experimental evaluation of the proposed diffractive component created by 3D printing technique shows almost two times larger performance in comparison with uncorrected basic diffractive lens.

Fan-out elements create an array of beams radiating at particular angles along the propagation axis. Therefore, they are able to form a matrix of equidistant spots in the far-field diffraction region. In this work, we report on the first fan-out structures designed for the THz range of radiation. Two types of light-dividing fan-out structures are demonstrated: (i) the 3×1 matrix fan-out structure based on the optimized binary phase grating and (ii) the 3×3 fan-out structure designed on the basis of the well-known Dammann grating. The structures were generated numerically and manufactured using the 3D printing technique with polyamide PA12. To obtain equal powers and symmetry of diffracted beams, the computer-aided optimization algorithm was used. Diffractive optical elements designed for 140 and 282 GHz were evaluated experimentally at both these frequencies using illumination with the wavefront coming from the point-like source. Described fan-out elements formed uniform intensity and equidistant energy distribution in agreement with the numerical simulations.

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.

Analysis of optical structures that can work for broadband range of THz radiation is the aim of this article. Such structures can be designed as kinoforms of higher order or elements with extended depth of focus, like axicons or light sword elements. The theoretical and experimental comparison of different optical elements for three significantly different frequencies is performed. Full Text: PDF ReferencesM. C. Kemp, P.F. Taday, B.E. Cole, J.A. Cluff, A.J. Fitzgerald, W.R. Tribe, "Security applications of terahertz technology", International Society for Optics and Photonics, 5070, pp. 44-53 (2003). CrossRef D. Yavorskiy, J. Marczewski, K. Kucharski, et al., "THz Scanner Based on Planar Antenna-Supplied Silicon Field-Effect Transistors", Photonics Letters of Poland, 4(3), 100-102 (2012). CrossRef A.A. Angeluts, A.B. Gapeyev, M.N. Esaulkov, et al., "Study of terahertz-radiation-induced DNA damage in human blood leukocytes", Quantum Electronics, 44(3), 247 (2014). CrossRef Q. Sun, Y. He, K. Liu, S. Fan, E.P. Parrott, E. Pickwell-MacPherson, "Recent advances in terahertz technology for biomedical applications", Quantitative imaging in medicine and surgery, 7(3), 345 (2017). CrossRef J. Suszek, A. Siemion, M. Bieda, et al., "3-D-Printed Flat Optics for THz Linear Scanners", IEEE Thz Sci. T. 5, (2015). CrossRef M. Naftaly, R. Dudley, "Methodologies for determining the dynamic ranges and signal-to-noise ratios of terahertz time-domain spectrometers", Optics Letters, 34(8), 1213-1215 (2009). CrossRef C. Bruckner, G. Notnia, and A. Tünnermann, "Optimal arrangement of 90° off-axis parabolic mirrors in THz setups", Optik, 121, 1 (2010). CrossRef J. Richter, A. Hofmann, L.-P. Schmidt, "Dielectric Wide Angle Lenses for Millimeter-Wave Focal Plane Imaging", Proc. of the 31st European Microwave Conference, London, UK (2001). CrossRef E.D. Walsby, S. M. Durbin, D.R S. Cumming, R.J. Blaikie, "Analysis of silicon terahertz diffractive optics", Curr. App. Phys., 4, (2004). CrossRef J.A. Jordan Jr et al, Appl Opt., 9(8), 1883-1887 (1970) CrossRef J.C. Marron, D.K. Angell, A.M. Tai, "Higher-order kinoforms", International Society for Optics and Photonics, 1211, 62-67 (1990). CrossRef J. Suszek, A.M. Siemion, N. Błocki, M. Makowski, A. Czerwiński, J. Bomba, P. Zagrajek et al., "High order kinoforms as a broadband achromatic diffractive optics for terahertz beams", Optics Express, 22(3), 3137-3144 CrossRef J. Sochacki, A. Kołodziejczyk, Z. Jaroszewicz, S. Bara, "Nonparaxial design of generalized axicons", Applied Optics, 31(25), 5326-5330 (1992). CrossRef A. Kołodziejczyk, S. Bará, Z. Jaroszewicz, M. Sypek, "The Light Sword Optical Element—a New Diffraction Structure with Extended Depth of Focus", Journal of Modern Optics, 37(8), 1283-1286 (1990). CrossRef M. Sypek, "Light propagation in the Fresnel region. New numerical approach", Opt. Commun., 116, 43–48 (1995). CrossRef J.P. Kruth, X. Wang, T. Laoui, L. Froyen, "Lasers and materials in selective laser sintering", Assembly Automation, 23(4), 357-371 (2003). CrossRef

Thin and lightweight achromatic focusing elements with F-number close to 1 are desirable in many practical applications. We present the idea to use diffractive structures designed to work for the substantially increased THz frequency range. The paper analyses mono- and multi-focal lenses forming point-like foci as well as axicon and light sword optical elements focusing THz radiation into line segments located along the optical axis. We consider diffractive elements in a form of the first and the second order kinoforms having various thicknesses. Designed and fabricated elements were numerically and experimentally examined to verify their achromatic functioning. We present point spread functions (XY scans) and 2D energy maps (XZ scans) for different THz frequencies. Moreover, a diagram of chromatic aberration is created by registering energy distribution along the optical axis for different frequencies. The distance corresponding to the highest energy is chosen for each frequency. Therefore, we can compare broadband working of designed structures. The spherical lens coded as kinoform of the second order provides the best broadband functioning, however it is two times thicker than structures providing extended depth of focus (light sword and axicon) working with slightly smaller efficiency but being much thinner.