Terahertz Sieves

[EN] Imaging at terahertz (THz) frequencies offers a great potential for applications including security screening, telecommunications biodetection, and spectroscopy. Some of these applications need specially designed lenses with customized characteristics that are not commercially available. In this letter, we present the THz sieves as a new kind of THz lenses. We demonstrate that these lenses improve the resolution of conventional zone plates constructed with the same level of detail. Amplitude and phase THz sieves were three-dimensional printed and tested experimentally. Excellent agreement was obtained between the experimental and calculated results.This work was supported in part by the Ministerio de Economia y Competitividad and FEDER, Spain, under Grant DPI2015-71256-R, in part by the Generalitat Valenciana under Grant PROMETEOII-2014-072, Spain, and in part by the National Center for Research and Development in Poland under Grant LIDER/020/319/L-5/13/NCBR/2014.Machado-Olivares, FJ.; Zagrajek, P.; Monsoriu Serra, JA.; Furlan, WD. (2018). Terahertz Sieves. IEEE Transactions on Terahertz Science and Technology. 8(1):140-143. https://doi.org/10.1109/TTHZ.2017.2762292S1401438

Multiplexing THz Vortex Beams With a Single Diffractive 3-D Printed Lens

[EN] We present a novel method for experimentally generating multiplexed THz vortex beams by using a single three-dimensional printed element that combines a set of radially distributed spiral phase plates, and a binary focusing Fresnel lens. With this element, we have experimentally demonstrated that THz multiplexing can be tailored to fit within a small space on an optical bench. Results are presented beside numerical simulations, demonstrating the robust nature of the experimental method.This work was supported in part by the Ministerio de Economia y Competitividad, Spain, under Grant DPI2015-71256-R, in part by the Generalitat Valenciana, Spain, under Grant PROMETEO II-2014-072, and in part by the National Center for Research and Development in Poland under Grant LIDER/020/319/L-5/13/NCBR/2014.Machado-Olivares, FJ.; PRZEMYSLAW ZAGRAJEK; Ferrando Martín, V.; Monsoriu Serra, JA.; WALTER DANIEL FURLAN (2019). Multiplexing THz Vortex Beams With a Single Diffractive 3-D Printed Lens. IEEE Transactions on Terahertz Science and Technology. 9(1):63-66. https://doi.org/10.1109/TTHZ.2018.2883831S63669

Sub-Terahertz Computer Generated Hologram with Two Image Planes

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

3D printed diffractive terahertz lenses

[EN] A 3D printer was used to realize custom-made diffractive THz lenses. After testing several materials, phase binary lenses with periodic and aperiodic radial profiles were designed and constructed in polyamide material to work at 0.625 THz. The nonconventional focusing properties of such lenses were assessed by computing and measuring their axial point spread function (PSF). Our results demonstrate that inexpensive 3D printed THz diffractive lenses can be reliably used in focusing and imaging THz systems. Diffractive THz lenses with unprecedented features, such as extended depth of focus or bifocalization, have been demonstrated.Ministerio de Economia y Competitividad (MINECO) (FIS2011-23175); Generalitat Valenciana (PROMETEOII/2014/072); National Center for Research and Development in Poland (LIDER/020/319/L-5/13/NCBR/2014).Furlan, WD.; Ferrando Martín, V.; Monsoriu Serra, JA.; Zagrajek, P.; Czerwinska, E.; Szustakowski, M. (2016). 3D printed diffractive terahertz lenses. Optics Letters. 41(8):1748-1751. https://doi.org/10.1364/OL.41.001748S1748175141

Time Resolution and Dynamic Range of Field-Effect Transistor–Based Terahertz Detectors

We studied time resolution and response power dependence of three terahertz detectors based on significantly different types of field-effect transistors. We analyzed the photoresponse of custom-made Si junctionless FETs, Si-MOSFETs, and GaAs-based high-electron-mobility transistor detectors. Applying monochromatic radiation of a high-power, pulsed, line-tunable molecular THz laser, which operated at frequencies in the range from 0.6 to 3.3 THz, we demonstrated that all these detectors have at least nanosecond response time. We showed that detectors yield a linear response in a wide range of radiation power. At high powers, the response saturates varying with radiation power P as U = R0P/(1 + P/P-s), where R-0 is the low-power responsivity and P-s is the saturation power. We demonstrated that the linear part response decreases with radiation frequency increase as R-0 proportional to f(-3), whereas the power at which signal saturates increases as P-s proportional to f(3). We discussed the observed dependencies in the framework of the Dyakonov-Shur mechanism and detector-antenna impedance matching. Our study showed that FET transistors can be used as ultrafast room temperature detectors of THz radiation and that their dynamic range extends over many orders of magnitude of power of incoming THz radiation. Therefore, when embedded with current driven read-out electronics, they are very well adopted for operation with high power pulsed sources

Field Effect Transistors Based Terahertz Detectors 25 Years History, State of the Art and Future Directions

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3-D-Printed Flat Optics for THz Linear Scanners

THz beam shaping via a single diffractive optical element is used to convert a divergent beam into a focal line segment perpendicular to the optical axis. The novel structure was designed for narrowband applications as a kinoform element and we successfully applied it in active, high-speed, THz linear scanners. The theoretical approach and experimental results are presented