Two diffractive optical elements are used to create a compact raster THz scanning setup in reflective configuration. The first one focuses the radiation into the small focal spot on the sample, while the second one collects reflected radiation and focuses it on the detector. To assure small size of the setup and large apertures of optical elements, structures work in the off-axis geometry. Thus, the focal spot is formed 100 mm after and 60 mm below the optical axis of the element, which measures 75 mm in diameter. The designed iterative algorithm allows further minimization of these values.

Diffractive optical elements (DOEs) are strictly related to the design wavelength due to the fact that they must introduce particular phase delay of the wavefront propagating through the structure. Mostly the attenuation of the material is not taken into account. In this article we propose to optimize thickness of the DOE by reducing introduced phase retardation but also attenuation. The efficiency of DOEs is determined by the method of coding phase distribution and can be easily measured by using diffraction orders of corresponding diffraction grating. Here, we analyze binary phase diffraction gratings with assumed attenuation. Full Text: PDF ReferencesJ.-L. Coutaz, Optoélectronique térahertz (Les Ulis CEDEX A, France, EDP Sciences 2012). DirectLink D. Headland, Y. Monnai, D. Abbott, C. Fumeaux,and W. Withayachumnankul, "Tutorial: Terahertz beamforming, from concepts to realizations", APL Photonics 3, 5 (2018). CrossRef S. F. Busch, M. Weidenbach, M. Frey, F. Schäfer, T. Probst, nd M. Koch, "A 3D-Printable Polymer-Metal Soft-Magnetic Functional Composite—Development and Characterization", Journal of Infrared, Millimeter, and Terahertz Waves 35, 12 (2014) CrossRef A. Siemion, P. Kostrowiecki-Lopata, A. Pindur, P. Zagrajek, M. Sypek, "Paper on Designing Costless THz Paper Optics", Advances in Materials Science and Engineering 2016, 9615698 (2016). CrossRef A. Siemion, A. Siemion, M. Makowski, J. Suszek, J. Bomba, A. Czerwinski, F. Garet, J.-L. Coutaz, and M. Sypek, "Diffractive paper lens for terahertz optics", Opt. Lett. 37, 4320–4322 (2012). CrossRef J.-L. Coutaz, F. Garet, E. Bonnet, A. V. Tishchenko, O. Parriaux, and M. Nazarov, "Grating Diffraction Effects in the THz Domain", Acta Phys. Pol. A 107, 26-37 (2005). CrossRef M. S. Heimbeck, P. J. Reardon, J. Callahan, and H. O. Everitt, "Transmissive quasi-optical Ronchi phase grating for terahertz frequencies", Opt. Lett. 35, 21 (2010). CrossRef D. Li, S. Shu, F. Li, G. Ma, Y. Dai, and H. Ma, "Anomalous transmission of terahertz wave through one-dimensional lamellar metallic grating", Opt. Commun. 284, 10-11 (2011). CrossRef X. Li, and S. F. Yu, "Diffraction Characteristics of Concentric Circular Metal Grating Operating at Terahertz Regime", IEEE Journal of Quantum Electronics 46, 6 (2010). CrossRef B. Nöhammer, C. David, J. Gobrecht, and H. P. Herzig, "Optimized staircase profiles for diffractive optical devices made from absorbing materials", Opt. Lett. 28(13), 1087-1089 (2003). CrossRef V. Deuter, M. Grochowicz, S. Brose, J. Biller, S. Danylyuk, T. Taubner, D. Grutzmacher, and L. Juschkin, "Holographic masks for computational proximity lithography with EUV radiation", International Conference on Extreme Ultraviolet Lithography 2018 10809, 108091A (2018). CrossRef J. W. Goodman, Introduction to Fourier optics (Greenwood Village, USA, Roberts & Company Publishers 2005). DirectLink W. B. Veldkamp, "Optimized staircase profiles for diffractive optical devices made from absorbing materials", Appl. Opt. 21(17), 3209-3212W (1982). CrossRef W. B. Veldkamp, and C. J. Kastner, "Beam profile shaping for laser radars that use detector arrays", Appl. Opt. 21(2), 345-356 (1982). CrossRef https://www.mcortechnologies.com/de/3d-drucker/mcor-iris/ DirectLinkM. Sypek, M. Makowski, E. Hérault, A. Siemion, A. Siemion, J. Suszek, F. Garet, and J.-L. Coutaz, "Highly efficient broadband double-sided Fresnel lens for THz range", Opt. Lett. 37, 12 (2012). CrossRef

An advanced optical structure such as a synthetic hologram (also called a computer-generated hologram) is designed for sub-terahertz radiation. The detailed design process is carried out using the ping-pong method, which is based on the modified iterative Gerchberg–Saxton algorithm. The novelty lies in designing and manufacturing a single hologram structure creating two different images at two distances. The hologram area is small in relation to the wavelength used (the largest hologram dimension is equivalent to around 57 wavelengths). Thus, it consists of a small amount of coded information, but despite this fact, the reconstruction is successful. Moreover, one of the reconstructed images is larger than the hologram area. Good accordance between numerical simulations and experimental evaluation was obtained.

Recently, one of the most commonly discussed applications of terahertz radiation is wireless telecommunication. It is believed that the future 6G systems will utilize this frequency range. Although the exact technology of future telecommunication systems is not yet known, it is certain that methods for increasing their bandwidth should be investigated in advance. In this paper, we present the diffractive optical elements for the frequency division multiplexing of terahertz waves. The structures have been designed as a combination of a binary phase grating and a converging diffractive lens. The grating allows for differentiating the frequencies, while the lens assures separation and focusing at the finite distance. Designed structures have been manufactured from polyamide PA12 using the SLS 3D printer and verified experimentally. Simulations and experimental results are shown for different focal lengths. Moreover, parallel data transmission is shown for two channels of different carrier frequencies propagating in the same optical path. The designed structure allowed for detecting both signals independently without observable crosstalk. The proposed diffractive elements can work in a wide range of terahertz and sub-terahertz frequencies, depending on the design assumptions. Therefore, they can be considered as an appealing solution, regardless of the band finally used by the future telecommunication systems.

The redistribution of an incoming radiation into several beams is necessary in telecommunication to demultiplex data signals. In the terahertz spectral range, it can be realized by easy-to-manufacture diffractive optical elements (DOEs) allowing to focus the radiation into multiple focal spots in a single plane. In this article, we present diffractive optical elements focusing THz radiation into three focal spots. Different focal spot distributions (symmetric and asymmetric) are designed using an iterative algorithm. The phase distribution forming asymmetric focal spots can be realized by iterative design, which is a novel approach, to our knowledge. Then, the structures are manufactured using a sintering-based 3D-printing method from polyamide 12 (PA 12) and measured in an experimental setup for 150 GHz frequency. A novel approach based on neural networks (NNs) is proposed to optimize the phase delay maps of the structures to further improve their performance – the higher efficiency and the lower unwanted background noise.