Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 3Citation - Scopus: 4Terahertz Wavefront Engineering Using a Hard-Coded Metasurface(Springer, 2023) Noori, Aileen; Akyürek, Bora; Demirhan, Yasemin; Özyüzer, Lütfi; Güven, Kaan; Altan, Hakan; Aygün, GülnurDuring the past few years, coding metamaterials (MM) drew significant attention, where the far-field scattering/transmission pattern of the electromagnetic wave (particularly in the THz regime) can be encoded into a single or few-bit digitized phase-response of the metasurface, thereby enabling a full digital control. Single-bit MMs contain two types of unit cells where the phase becomes 0 and 1 (in units of ?), respectively. By arranging these unit cells into a 2D surface pattern, the THz wavefront can be shaped. In this work, a novel hard-coded metasurface was designed, fabricated, and experimentally investigated for multi-beam reflection of incident THz beam. The design employs stripe and checkerboard patterns of bilayer MM unit cells consisting of square gold patches with a polymer spacing layer from a gold backplane. Experimental and simulation results show that the incident wave in the 0.500–0.750 THz range can be reflected with > 95% efficiency in uniform amplitude and 1-bit coded phase. For the checkerboard metasurface pattern, the measured and analytically calculated reflection angle shows good agreement. The metasurface design is suitable for large-scale fabrication and can potentially be used as a template in the development of actively coded metasurfaces. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.Research Project Süperiletken metamalzeme temelli terahertz band-geçirgen filtreleri(2016) Özyüzer, Lütfi; Altan, Hakan; Sabah, CumaliBu projede süperiletken filmlerinden terahertz dalgaboyunda çalışabilen metamalzeme filtreler yapılması amaçlanmıştır. Öncelikle nümerik simülasyonlarla Terahertz aralığında çalışan metal ve süperiletken metamalzeme filtreler tasarlanmıştır. Tasarlanan ve modellenen filtreler, fused silika alttaşlar üzerine büyütülmüş Au ve Cr ince filmler üzerinde şekillendirilmiştir ve İzmir Yüksek Teknoloji Enstitüsünde bulunan Fourier Dönüşümlü Kızılötesi Spektroskopisi (FTIR) ile karakterize edilmiştir. Daha sonra ODTÜ’de filtrelerin Terahertz zamana dayalı spektroskopisiyle ölçümleri alınmıştır. Bu çalışmalar raporda detaylı olarak verilmiştir. Filtre tasarım ve üretimine paralel olarak süperiletken Bi2Sr2CaCu2O8+d (Bi2212) ince filmlerinin büyütülme çalışmaları yapılmıştır. Bi2212 ince filmleri c düzlemli (0001) safir ve MgO alttaş üzerine (Bi2212) hedef stokiyometrik oranlardaki Bi2O3, SrCO3, CaCO3 ve CuO tozlarından kalsinasyon, presleme ve sinterleme işlemleriyle hazırlanmıştır.) prosedürleri detaylı bir şekilde belirtilen kendi hazırladığımız hedef ve satın alınan hedef ile DC ve RF güç kaynağı ile farklı parametrelerde büyütülmüştür. İki farklı hedef ile büyütülen örnekler karşılaştırılmıştır. Yapısal karakterizasyon için ince filmlerin EDX (Enerji Dağılımlı XIşınları Mikroanaliz Spektrometresi) ve XRD (X-ışını difraksiyon spektroskopisi) analizleri yapılmıştır. Bi2212 ince filmlerinin Taramalı Elektron Mikroskobu ile SEM görüntüleri alınmıştır. Üretilen hedef ve büyütülen filmlerin direnç sıcaklık ölçümleri alınmıştır. Bu çalışmalar da raporda detaylı olarak verilmiştir. Proje için özgün kuadkross metamalzeme filtre yapısı tasarlanmış ve hem altın hem YBCO ince film üzerine üretilmiştir. Kuadkross metamalzeme filtrelerin geçirgenlik özelliklerini THz zamana dayalı spektroskopisi ile kapalı He kroyostat sisteminde ölçülmüştür. Sonuçlar birbiri ile karşılaştırılmış, projede çalışan her üç araştırma grubunun öğrencileri ve araştırmacı öğretim üyeleri projenin devam ettiği 2 yıl boyunca düzenli olarak İzmir, Kıbrıs ve Ankara’da toplanmış ve projenin durumu ve sonuçlar hakkında fikir alışverişinde bulunmuştur.Conference Object Investigation of In-Gap Field Enhancement at Terahertz Frequencies for a Metasurface Enhanced Sensor(Institute of Electrical and Electronics Engineers, 2021) Tugay, Halime; Altan, Hakan; Demirhan, Yasemin; Özyüzer, Lütfi; Sabah, CumaliThe arrangements of subwavelength inclusions in a metasurface can serve as an effective absorber for the terahertz region. When such an absorber is combined with a unique material, the absorption can induce effects that can lead to a change in the materials electrical properties. Vanadium dioxide shows a passive and reversible change from monoclinic insulator phase to metallic tetragonal rutile structure by using external stimuli such as temperature (340K), photo excitation, electric field, mechanical strain or magnetic field [1,2]. Upon absorption of the THz radiation, the high electric fields that are generated inside the gaps of the metasurface can serve as trigger points, as was shown previously using kV strength THz E-fields.Conference Object Terahertz Transmission Through Patterened Vanadium Oxide Thin Films on Dielectric Substrates(SCITEPRESS, 2017) Akkaya, M.; Demirhan, Yasemin; Yüce, Hürriyet; Aygün, Gülnur; Özyüzer, Lütfi; Sabah, Cumali; Altan, HakanPatterned and unpatterned films of vanadium oxide grown on dielectric substrates such as fused silica and sapphire were grown and analysed by varying the temperature using terahertz time domain spectroscopy. After investigating the critical transition temperature near 340K, a well-known cross-shaped pattern was studied to observe any resonances upon transmission. Due to the poor conductivity of the films the frequency selective nature of the structure was not observed, however an etalon effect could be seen in the sapphire substrate as opposed to the fused silica substrate above the critical temperature. Dependence of the refractive index difference between substrates upon transmission of the THz pulse is likely in explaining this observed difference.Article Citation - WoS: 56Citation - Scopus: 62Cross-like terahertz metamaterial absorber for sensing applications(Springer Verlag, 2018) Sabah, Cumali; Mulla, Batuhan; Altan, Hakan; Özyüzer, LütfiIn this work, a new multiband terahertz metamaterial absorber is designed and characterised by numerical simulation method. In addition, the utilisation of the proposed absorber as a sensor is also investigated. The dielectric and thickness sensing characteristics are analysed. The proposed multiband metamaterial absorber has the ability for utilising the terahertz region up to 2 THz. According to the results, it is found that the proposed absorber is capable of sensing unknown materials and material thickness with any of its five absorption bands. The sensitivity of the proposed sensor is 6.57 GHz / unit sensitivity for dielectric sensing and 7.66GHz/μm for thickness sensing.Article Citation - WoS: 6Citation - Scopus: 7Thermally and Optically Tunable Sub-Terahertz Superconducting Fishnet Metamaterial(Elsevier Ltd., 2018) Sabah, Cumali; Mulla, Batuhan; Altan, Hakan; Özyüzer, LütfiIn this paper, a novel fishnet metamaterial structure is designed and analyzed under different material combinations and under different active controlling techniques. The results indicate that, the proposed fishnet metamaterial has a single resonance with double negativity at 0.39 THz when quartz substrate and aluminum is utilized in the design. Moreover, when the metallic parts are replaced with YBCO, the proposed design also exhibits double negativity with a stronger resonance and can be used as a switch between the double negative and single negative modes if the temperature is altered. In addition to these, when substrate (quartz) is replaced with MgO, the resonance shifts from 0.39 THz to 0.26 THz and shows double negativity. Moreover, switching properties under illumination can also be obtained when the silicon is utilized in the design (MgO-YBCO combination). According to these results, it is found that, in the case that the conductivity of silicon exceeds a certain value, the character of the resonance changes from double negative to the single negative mode.Article Citation - WoS: 13Citation - Scopus: 13Fourcross Shaped Metamaterial Filters Fabricated From High Temperature Superconducting Ybco and Au Thin Films for Terahertz Waves(IOP Publishing Ltd., 2017) Demirhan, Yasemin; Alaboz, Hakan; Nebioğlu, Mehmet Ali; Mulla, B.; Akkaya, M.; Altan, Hakan; Sabah, Cumali; Özyüzer, LütfiIn this study, we present a new, unique fourcross shaped metamaterial terahertz (THz) filter fabricated from both gold thin films and YBa2Cu3O7-δ high Tc superconducting thin films. A commercial electromagnetic simulation software, CST Microwave Studio, is used to design and optimize the metamaterial filter structures. The proposed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam exhing techniques. Terahertz time-domain spectroscopy measurements validated the design predictions for both the center frequencies and bandwidths of the resonances due to the fourcross structures. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This resonance switching effect is not observed in filters made up of metals. This novel fourcross rectangular resonator with a temperature-dependent resonance behavior holds great potential for active, tunable and low loss THz devices for imaging, sensing, and detection applications.Article Citation - WoS: 7Citation - Scopus: 7An Indium Tin Oxide Metasurface Filter for Terahertz Applications: Design, Fabrication, and Characterization(World Scientific Publishing Co. Pte Ltd, 2017) Nebioğlu, Mehmet Ali; Takan, Taylan; Altan, Hakan; Demirhan, Yasemin; Alaboz, Hakan; Özyüzer, Lütfi; Sabah, CumaliIn this paper, using a cross-shaped complementary Indium Tin Oxide (ITO)-based metasurface design, the transmission of THz radiation is shown to be filtered within the 3 dB level from maximum in the frequency range of interest (333 GHz). Various metasurface structures primarily composed of cross-shaped openings with a 400 micron unit cell size are patterned on top of 1750 micron thick fused silica substrates. They are patterned using UV lithography methods after the films were grown using DC sputtering. The fabricated structures were characterized using Terahertz Time Domain Spectroscopy (THz-TDS) measurement technique. The measured transmission agrees well with the simulation of the structure for four different samples with different geometries. These results suggest that metasurface and/or metamaterial patterns based on ITO in visibly transparent media can be utilized for filtering of frequencies in the long wavelength spectrum. These types of filters can be very useful in the near future for THz communication and security applications. ©Article Citation - WoS: 11Citation - Scopus: 13Metal Mesh Filters Based on Ti, Ito and Cu Thin Films for Terahertz Waves(Springer Verlag, 2016) Demirhan, Yasemin; Alaboz, Hakan; Özyüzer, Lütfi; Nebioğlu, Mehmet Ali; Takan, Taylan; Altan, Hakan; Sabah, CumaliIn this study, we have investigated the spectral performance of resonant terahertz (THz) bandpass filters which were produced from thin films with a metal-mesh shape. The aforementioned filters were fabricated from titanium, copper and indium tin oxide thin films on fused silica substrates by UV lithography with an array of cross-shaped apertures. Since the mesh period, cross-arm length and its width specify the spectral characteristics of the filters, we were able to reveal the performance of these filters experimentally using both a THz time domain spectrometer and a Fourier transform infrared spectrometer. A commercial electromagnetic simulation software, CST microwave studio, was used to verify the experimental data. The transmission of the filters are in the range 20–55 % at their relevant center frequencies. To our knowledge this study is the first to show that fabricated patterns based on ITO thin films can be used to filter THz radiation.