Scopus İndeksli Yayınlar Koleksiyonu / Scopus Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7148
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Article Citation - WoS: 4Citation - Scopus: 4Vo<sub>2</Sub>-based Dynamic Coding Metamaterials for Terahertz Wavefront Engineering(Springer, 2025) Akyurek, Bora; Noori, Aileen; Demirhan, Yasemin; Ozyuzer, Lutfi; Guven, Kaan; Altan, Hakan; Aygun, GulnurDigital coding metasurfaces (DCMS) offer a promising alternative to conventional metasurface designs for achieving common functionalities by controlling the phase of reflected or transmitted electromagnetic waves. Their simple unit cell designs allow for scalability across the THz spectrum and facilitate large-area fabrication. The true potential of DCMS lies in dynamical coding, which enables real-time reconfigurability through a tuning and/or switching mechanism. In this study, metasurfaces that achieve 1-bit dynamic coding of unit cells via thermally induced metal-insulator transition of VO2 layers are designed and fabricated. We investigate experimentally the beam splitting functionality at certain frequencies in the 0.50-0.75 THz range reflected from the stripe- and checkerboard patterned metasurface samples, and demonstrate the switching of this functionality under thermal illumination.Article Citation - WoS: 4Citation - Scopus: 5Investigation of Resonant Properties of Metamaterial Thz Filters Fabricated From Vanadium Dioxide Thin Films(WORLD SCIENTIFIC PUBL CO PTE LTD, 2023) Zeybek, Şehriban; Demirhan, Yasemin; Noori, Aileen; Tugay, Halime; Altan, Hakan; Sabah, Cumali; Aygun, GülnurVanadium dioxide (VO2) is a promising candidate for electronic and optical switching applications in the terahertz frequency range due to the metal to insulator transition (MIT). In this study, the use of VO2 patterned as a metamaterial surface or coupled as a homogeneous layer with a metallic metamaterial surface on top is investigated in terms of performance. High-quality VO2 thin films were deposited on c-cut sapphire substrates by using the dc magnetron sputtering technique. A change in resistivity by a factor of 104 MIT in VO2 was observed allowing to investigate its use as a controllable metamaterial. The layer was patterned using a unique geometry (four-cross shaped) that operates in the THz frequency range. To understand its performance as a tunable THz filter, the four-cross structure fabricated from VO2 is compared to one fabricated from Au on VO2 bare film using UV lithography and ion beam etching techniques. The spectral performances of metamaterials were assessed using THz-Time Domain Spectroscopy (THz-TDS) and results were compared with simulations based on CST Microwave Studio. Absence of the resonant effects in the purely developed VO2 device, while clear observation of the MIT behavior shows the strong dependency of the inductive and/or capacitive effects of the four-cross structure on conductivity of the surface metamaterial, which is clearly observable for the Au-based device. In the latter case, the resonant transmittance of the filter can be effectively modulated by change in temperature.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.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.Book Part Citation - Scopus: 1Thz Sources and Detectors Fabricated From High Temperature Superconductors(Springer, 2021) Demirhan, Yasemin; Türkoğlu, Fulya; Altan, Hakan; Sabah, Cumali; Özyüzer, LütfiHigh temperature superconductors have unique properties that can be useful in the THz region, single crystal constituted from superconducting CuO2-layers could sustain high voltages across the junctions and they are coupled through the intrinsic Josephson Effect this maintains the potential for very intense, coherent radiation which spreads over the THz gap. We investigated various experimental techniques to fabricate THz sources, bolometers and filters for efficient THz emission and detection. Rectangular mesa structures were fabricated on Bi2Sr2CaCu2O8-x (Bi2212) single crystal superconductors using standard e-beam lithography and Ar ion beam etching systems and an emitted power as high as 60 ?W at frequencies up to 0.85 THz was detected from micron sized continuous wave terahertz sources. We also fabricated bolometric microchips for THz detection purpose from Bi2212 single crystals. Bi2212 microchips detected the signals and response time were calculated, our results have clearly shown that Bi2212 single crystals are potential candidates for THz detection. The detection properties and sensitivity of bolometer chips can be further improved by integrating an antenna and filter structures. In order to investigate this we have fabricated metamaterial THz filters based on metals and YBa2Cu3Ox superconducting thin films with metal-mesh shape and a unique fourcross shape pattern. Both a THz time domain spectrometer and a Fourier transform infrared spectrometer (FTIR) were used to investigate the performance of these filters and the results were compared with simulations done with a commercially available electromagnetic simulation software. © Springer Nature B.V. 2021.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.Conference Object Citation - Scopus: 1Cleo®/Europe - Eqec 2015, an Indium Tin Oxide Metamaterial Filter for the Terahertz Regime: Design, Fabrication and Characterization(Optical Society of America (OSA), 2015) Takan, Taylan; Nebioğlu, Mehmet Ali; Kurt, Metin; Demirhan, Yasemin; Özyüzer, Lütfi; Sabah, Cumali; Altan, Hakan[No abstract available]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.