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Bu çalışmada güneş hücrelerinde, akıllı saat ve dokunmatik ekran gibi alanlarda kullanılmak üzere çeşitli saydam antenler tasarlanmıştır. 5.8 GHz çalışma frekansında saydam alttaşlı yama anten CST Studio Suite ve Ansys HFSS benzetim programlarında tasarlanarak anten karakteristik özellikleri hesaplamalı olarak elde edildikten sonra Ankara Üniversitesi Fizik Mühendisliği Mikrodalga İleri Araştırma Laboratuvarında fotolitografi yöntemiyle üretilmiştir. Üretilen antenin deneysel ölçümleri alınarak simülasyon sonuçlarıyla karşılaştırılmıştır. Ayrıca bakır yerine ITO (Indium Tin Oxide) kullanılarak 5.9 GHz frekansında optiksel saydam yama anten tasarlanmış ve elektron demeti püskürtme yöntemiyle büyütülerek üretilmiştir. Üretilen antenin geri dönüş kaybı ve voltaj dalga oranı ölçümü alınarak simülasyon sonuçlarıyla karşılaştırılmıştır. Daha sonra UV-VIS-NIR Spektrofotometre ile optiksel geçirgenlik analizi yapılmıştır. Diğer bir saydam anten çalışması olarak, 3 GHz çalışma frekansında çeyrek dalga dönüşümlü mikroşerit yama antenin bakır yama düzlemi metalik ızgaralar haline getirilerek % 95 optiksel saydam elde edilmiştir. Aynı şeffaf alttaş (akrilik) ve boyutta olan sıradan mikroşerit yama anten ve optiksel saydam ızgara anten, CST Studio Suite ve Ansys HFSS benzetim programlarında geri dönüş kaybı, bant genişliği, voltaj dalga oranı, kazanç, verimlilik gibi anten karakteristikleri simüle edilerek karşılaştırılmıştır. Daha sonra 5.8 GHz çalışma frekansında çalışan 200 nm ITO kalınlıklı kuvars saydam alttaşlı tek katmanlı saydam anten tasarlanmıştır. Gümüş kalınlığı sabit kalmak suretiyle farklı ITO kalınlıklarda çok katmanlı ITO-10 nm gümüş-ITO yapısı tasarlanarak CST Studio Suite programındaanten karakteristik sonuçlarına, OptiLayer yazılım programında ise geçirgenlik, yansıma, soğurmagibi optik karakteristiklerine bakılarak ITO için en uygun kalınlık belirlenmiştir. 40 nm ITO kalınlığı uygun görülerek tek katmanlı saydam anten ile anten ve optik karakteristikleri karşılaştırılmıştır. Saydam antenlerin anten ve optik karakteristik sonuş grafikleri incelenmiştir. In this study, various transparent antennas are designed to be used in solar cells, smart watch and touch screen areas. Transparent substrate patch antenna at 5.8 GHz operating frequency is designed in CST Studio Suite and Ansys HFSS simulation programs and the antenna characteristics are calculated computatively and then produced by photolithography method in the Microwave Advanced Research Laboratory of the University of Ankara. Experimental measurements of the produced antenna are compared with the simulation results. In addition, ITO (Indium Tin Oxide) instead of copper optically transparent patch antenna is designed and produced by electron beam spraying method. The return loss and voltage wave ratio measurement of the produced antenna is measured and compared with the simulation results. Then, the optical transmittance analysis is performed with UV-VIS-NIR Spectrophotometer. As another transparent antenna study, the copper patch plane of the quarter wave transform microstrip patch antenna at 3 GHz operating frequency is transformed into metallic grids to obtain 95% optical transparency. Ordinary microstrip patch antenna and optical transparent grid antenna, which are the same transparent substrates (acrylic) and size, are compared by simulating antenna characteristics such as return loss, bandwidth, voltage standing wave ratio, gain, efficiency in CST Studio Suite and Ansys HFSS simulation programs. Then, a single layer transparent antenna with 200 nm ITO thickness quartz transparent substrate is designed which operates at 5.8 GHz operating frequency. The ITO-10 nm silver-ITO structure of different ITO thicknesses are designed by keeping the silver thickness constant and the optimum characteristics for ITO are determined by considering antenna characteristic results in CST Studio Suite program and optical characteristics such as transmittance, reflection, absorption in OptiLayer software program. The ITO thickness of 40 nm is considered appropriate and the single layer transparent antenna was compared with the antenna and optical characteristics. The antenna and optical characteristic result graphs of the transparent antennas is examined. 99

Terahertz (THz) radiation is in powerful region of electromagnetic spectrum because of prosperous application areas yet deficiency still exists about sources and detectors in despite of improvements of the research field in this range. This gap can be filled by focusing on development of THz detectors. Therefore, bolometers were preferred through many detectors due to detection sensitivity above 1 THz. In this study, Bi2Sr2CaCu2O8+δ (Bi2212) single crystals were used to fabricate THz bolometric detector. Bi2212 single crystals were transferred on sapphire substrate by cleavage process and e-beam lithography and ion beam etching were used to fabricate the microchip clean room facilities. Customdesigned cryogenic cryostat was used for a-b axis electrical and THz response measurements with liquid nitrogen cooled system. After electrical measurements, Bi2212 microchips detected the signals using Stefan-Boltzmann Lamp and response time were calculated. This study have shown with our experimental results that Bi2212 single crystals are potential candidates for THz bolometric detectors., Ministry of Science, Industry and Technology (SANTEZ 1386.STZ.2012-1); ASELSAN

Elektromanyetik spektrumda, terahertz (THz) radyasyonu mikrodalgalar ve uzak IR arasındaki bölgede yeralır. Süperiletken malzemelerin benzersiz özellikleri, THz aygıt uygulamaları için çeşitli şekillerde kullanılmasına olanak sağlar. Bu tezin ilk bölümünde, titanyum, bakır ve indiyum kalay oksit ince filmlerinden uv litografisi ile fused silika alttaş üzerine metal-örgü şekilli üretilen THz bant geçiren filtrelerin spektral performansı araştırılmıştır. Metamalzeme filtre üretimi için, Bi2Sr2CaCu2O8+x ince filmleri, safir ve MgO altaşlarına DC ve RF magnetron saçtırma sistemi ile kaplanmıştır. İnce film karakterizasyonları sonrasında, filmlerin tek fazda olmadıkları ve filtre üretimi için kullanılamayacağı anlaşılmıştır. Yeni, özgün, dört köşeli THz metamalzeme filtre yapısı tasarlanmış ve hem altın ince filmlerden hem de YBa2Cu3O7-d yüksek Tc süperiletken ince filmlerden üretilmiştir. Tasarlanan dörtlü çapraz şekilli dikdörtgen filtre yapısı, şerit hatların kenarlarına yerleştirildiği periyodik metalik halka yapıdan oluşur. CST Mikrodalga Stüdyosu, metamalzeme filtre yapılarını tasarlamak ve optimize etmek için kullanılmıştır. Sıcaklığa bağlı rezonans davranışını araştırmak ve ölçümleri doğrulamak için, simülasyonlar Tc'nin üstünde ve altında sıcaklıklarda gerçekleştirilmiştir. Dörtlü çapraz metamalzeme filtreler, e-demeti litografisi ve iyon demeti aşındırma teknikleri kullanılarak üretilmiştir. Geçirgenlik spektrumlarında rezonans anahtarlaması, sıcaklığı kritik geçiş sıcaklığının altına düşürerek incelenmiştir. Bu özgün dörtlü çapraz dikdörtgen filtre, görüntüleme, algılama ve spektroskopi uygulamalarında aktif, ayarlanabilir ve düşük kayıplı THz cihazları için büyük potansiyel taşımaktadır. In the electromagnetic spectrum, terahertz (THz) radiation falls into the region among microwaves and the far-IR. The unique properties of superconducting materials allow them to be utilized in a number of ways for THz device applications. In the first part of this thesis, the spectral performance of THz bandpass filters were produced from titanium, copper and indium tin oxide thin films with a metal-mesh shape on fused silica substrates by uv lithography were investigated. For metamaterial filter fabrication, Bi2Sr2CaCu2O8+x thin films were deposited by DC and RF magnetron sputtering system on saphirre and MgO substrates. After thin film characterizations, it is obtained that the films were not in single phase and could not be used for filter fabrication. A new, unique fourcross shaped metamaterial THz filter was designed and fabricated from both gold thin films and YBa2Cu3O7-d high Tc superconducting thin films. The designed fourcross shaped rectangular filter structure consists of periodic metallic rings where strip lines are located at the sides of the ring. CST Microwave Studio, is used to design and optimize the metamaterial filter structures. To investigate the temperature-dependent resonance behavior and confirm the measurements, simulations are carried out at temperatures above and well below Tc. Fourcross metamaterial filters are fabricated by using e-beam lithography and ion beam etching techniques. The resonance switching of the transmission spectra was investigated by lowering the temperature below the critical transition temperature. This novel fourcross filter holds great potential for active, tunable and lowloss THz devices for imaging, sensing, and detection applications. 110

Bebek formülleri, bebeklerin yaşamlarının ilk ayları boyunca anne sütünün yetersiz olduğu ya da verilemesinin uygun olmadığı durumlarda içerik olarak anne sütüne yakın formüle edilmiş özel beslenme ihtiyacını karşılayan ürünlerdir. Bir çok gıda ürününde olduğu gibi bebek maması da, üretim aşamasında ya da sonrasında sağlığı tehdit eden patojen mikroorganizmalar açısından risk taşımaktadır. Bebek mamasında, yenidoğanlar için risk oluşturan patojen mikroorganizmaların başında Cronobacter sakazakii gelmektedir. C. sakazakii, su aktivitesi (aw=0.2) oldukça düşük olan toz bebek mamasında uzun süre canlı kalabilmesinden dolayı, bağışıklığı henüz gelişmekte olan bebekler tarafından tüketildiğinde, ölüm ile sonuçlanan ciddi sonuçlara neden olabilmektedir. Dolayısıyla, C. sakazakii ile kontamine bebek mamasında, bu mikroorganizmanın hızlı ve güvenilir bir yöntem ile tespit edilmesi, gerekli önlem ve aksiyonların alınabilmesi açısından önem taşımaktadır. Bu tez çalışmasında; C. sakazakii'nin bulunduğu tek ve çoklu rekabetçi kültür ortamları ve farklı kirletme seviyeleri oluşturularak, bebek maması yapay olarak kirletilmiş, bu örnekler referans ve hızlı metot ile analiz edilerek, iki metodun sonuçları arasındaki uyum değerlendirilmiştir. Bu amaç doğrultusunda, C. sakazakii (ATCC 29544) ve rekabetçi florayı oluşturmak için E. coli (ATCC 25922), S. epidermidis (ATCC 12228), C. freundii (ATCC 8090) ve S. Enteritidis (ATCC 13076) referans kültürleri kullanılmıştır. Tek rekabetçi kültür için; C. sakazakii (ATCC 29544) ve E. coli (ATCC 25922) ve çoklu rekabetçi kültür için; Cronobacter sakazakii (ATCC 29544), E. coli (ATCC 25922), C. freundii (ATCC 8090), S. Enteritidis (ATCC 13076), S. epidermidis (ATCC 12228) referans kültürleri kullanılmış ve bebek mamaları 1-5 kob/25 g, 5-10 kob/25g ve 100 kob/25 g olacak şekinde kirletilmiştir. Deneysel sonuçlar şunu göstermiştirki; hızlı metodun tek rekabetçi kültür için uyum dereceleri rölatif doğruluk (% RD) 97,1, rölatif hassasiyet (% RH) 98,6 ve rölatif özgüllük (% RÖ) 93,8; çoklu rekabetçi kültür için rölatif doğruluk (% RD) 85,9, rölatif hassasiyet (% RH) 91,5 ve rölatif özgüllük (% RÖ) 77,4 seviyelerindedir. Aynı zamanda rölatif tespit limiti (RTL) çalışması için 1-5 kob/25 g kirletme düzeyinde her iki yöntem ile % 84,6 pozitif sonuç alınırken; çoklu rekabetçi kültür için sırasıyla % 75 (hızlı metot) ve % 60 (referans metot) pozitif sonuç alınmış ve RTL değeri 1-5 kob/25g olarak belirlenmiştir. Bu sonuçlar göstermektedir ki hızlı metot ile oldukça güvenilir ve kabul edilebilir hassasiyette sonuçlar elde edilebilmektedir. Hızlı metodun, referans metoda oranla oldukça kısa sürece sonuç veriyor olmasıda kullanıcı için ek bir avantaj olarak karşımıza çıkmaktadır. Bunun yanında, rekabetçi florayı oluşturan mikroorganizma sayısının artması, hedef bakterinin tespitini zorlaştırmakta ve her iki yönteminde tespit hassasiyetini düşürmektedir. Infant formula, is a food formula which is given to babies in their first days/weeks of their life in the cases that the mother's breast milk is not sufficient or is not suitable for the baby (allergy, intolerance etc.). They are special nutrients which are formulated in a very close substitute to the human breast milk. Like many other foods, infant formulas are also a risk factor for foodborne pathogens. Cronobacter sakazakii is one of the leading pathogens risk originating from infant formula, for newborns. Because, C. sakazakii, can survive in infant formula which has significantly low level of water activity (aw=0.2) for a certain period of time, it can cause severe infections in newborn babies with death in several cases. Thus, realible and rapid detection of C. sakazakii in contaminated formulas is very important for planning preventive/corrective actions. In this thessis, various levels of C. sakazakii accompanied with single and multiple microorganisms to form competetive flora, were used for spiking infant formula samples. The artifically spikes samples were, detected in paralel with reference clasical cultural method and rapid PCR method and the agreement of the results of these two methods, was evaluated. For this purpose, besides C. sakazakii (ATCC 29544), E. coli (ATCC 25922), S. epidermidis (ATCC 12228), C. freundii (ATCC 8090) and S. Enteritidis (ATCC 13076) reference cultures were used to prepare competetive flora. For single competetive flora; C. sakazakii (ATCC 29544) and E. coli (ATCC 25922), and for multiple competetive flora Cronobacter sakazakii (ATCC 29544), E. coli (ATCC 25922), C. freundii (ATCC 8090), S. Enteritidis (ATCC 13076) and S. epidermidis (ATCC 12228) reference cultures were used to spike samples at levels of 1-5 cfu/25 g, 5-10 cfu/25g and 100 cfu/25 g. The experimental results indicated that, the agreement degrees of the two method is as fallows; for single competetive flora, relative accurarcy (RA %) is 97.1, relative sensitivity (RS %) is 98.6 and relative specificity (RS %) is 93.8 and for multiple competetive flora; relative accurarcy (RA %) is 85.9, relative sensitivity (RS %) is 91.5 and relative specificity (RS %) is 77.4. Additionally, the relative limit of detection (RLoD) was determined as 1-5 cfu/25 g; as positive detection at this spiking levels could be obtained at the ratios 84.6 % with both methods in samples containing single competive flora and 75 % (for rapid method) and 60 % (for reference method) in samples containing multiple competetive flora. These results showed that, rapid method provides reliable and relatively sensitive results in short detection period. The faster resulting capability is an other additional advantage for the user, besides accurate detection. In addition, our results proved that, the range of the type of bacteria in the competetive flora, makes the detection of target bacteria more difficult and the sensitivity of both methods decreases. 104

30th URSI General Assembly and Scientific Symposium, URSIGASS 2011; Istanbul; Turkey; 13 August 2011 through 20 August 2011, We work on a frequency tunable solid state device to meet the needs of continuous, coherent, powerful terahertz emission sources that fill practically important terahertz gap. High temperature superconducting (HTS) coherently oscillating Josephson junctions in Bi2Sr2CaCu 2O8+δ (Bi2212) crystal make this approach very promising. Since doping dependence of Bi2212 is an important parameter, Bi2212 crystals are annealed in vacuum or purified argon gas flow at 425°C. For further processing we pattern both single and double rectangular mesa structures by using electron beam lithography on the cleaved surface of the crystal. Resistance-temperature (R-T), and current-voltage behavior (I-V) measurements achieved. © 2011 IEEE., Scientific and Technical Council of Turkey project no. 110T248

Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2011, Includes bibliographical references (leaves: 81-87), Text in English; Abstract: Turkish and English, xii, 87 leaves, Terahertz radiation is part of the electromagnetic spectrum lying between microwaves and the far-IR. This region has frequencies ranging from 0.1 - 10 THz and wavelengths from 3 mm to 0.03 mm. We work on a frequency tunable solid state device to meet the needs of continuous, coherent, powerful terahertz emission sources that fill practically important terahertz gap. Among the cuprates, single crystal of Bi2Sr2CaCu2O8+ (Bi2212) is a potential candidate of compact solid state devices designed for generating electromagnetic waves in terahertz frequency range. Bi2212 crystals are consisting of natural arrays in the form of identical layers called intrinsic Josephson junctions (IJJ). In this study, under optimized doping conditions we aimed to investigate powerful terahertz emission. THz emission in the μW range can be obtained fabricating rectangular-shaped mesa structures on the Bi2212 crystal. In the experimental procedure, in order to obtain various doping levels, we annealed the high temperature superconducting Bi2212 single crystals at various temperatures in vacuum or under argon flow. By using the thermal evaporation, optic and electron beam lithography, ion beam etching techniques single and double mesa structures were fabricated. For the e-beam lithography process, we optimized and improved the required parameters. After the mesa fabrication, the exact dimensions of the mesas were obtained using surface profilometer and atomic force microscope. In order to characterize the Bi2212 mesas,by three probe contact c-axis resistance versus temperature (R-T), and current-voltage behavior (I -V) were measured in a He flow cryostat. Some of the hysteretic quasiparticle branches are seen in the I-V characteristics of Bi2212 crystals.

Vanadium oxide (VOx) thin film has been widely used for IR detectors and it is one of the promising materials for THz detectors due to its high temperature coefficient of resistance (TCR) values. VOx films with proper TCR values have also high resistance and it restricts bolometer performance especially for uncooled bolometers. To overcome this problem, deposition at elevated temperatures or annealing approach has been accepted and used but gold co-deposition approach has been proposed recently. In this study, vanadium oxide films were fabricated on high resistivity silicon substrates by reactive direct current magnetron sputtering in different O2/Ar atmosphere at room temperature. We investigated influence of oxygen partial pressure during deposition process and fabricated VOx thin films with sufficient TCR values for bolometer applications. In order to decrease resistivity of the deposited films, post annealing and gold doping approaches were performed separately. Effect of both post annealing process and gold doping process on structural and electrical properties of VOx thin films deposited at room temperature were investigated and detailed comparison between these methods were presented. We obtained the best possible approach to obtain optimum conditions for the highly reproducible VOx thin films which have the best resistivity and suitable TCR value for bolometer applications., TUBITAK (Scientific and Technological Research Council of Turkey); 115F549

In 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., TUBITAK (114F091); METU research office (BAP-08-011-2016-053); BAGEP Award of the Science Academy in Turkey; Turkish Academy of Sciences

5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS); FEB 27-MAR 01, 2017; Porto, Portugal, Patterned 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.

Vanadium dioxide (VO2) shows metal-insulator phase transition at nearly 68 °C. This metal-insulator transition (MIT) in VO2 leads to a significant change in near-infrared transmittance and an abrupt change in the resistivity of VO2. Due to these characteristics, VO2 plays an important role on optic and electronic devices, such as thermochromic windows, meta-materials with tunable frequency, uncooled bolometers and switching devices. In this work, VO2 thin films were fabricated by reactive direct current magnetron sputtering in O2/Ar atmosphere on sapphire substrates without any further post annealing processes. The effect of sputtering parameters on optical characteristics and structural properties of grown thin films was investigated by SEM, XRD, Raman and UV/VIS spectrophotometer measurements. Patterning process of VO2 thin films was realized by e-beam lithography technique to monitor the temperature dependent electrical characterization. Electrical properties of VO2 samples were characterized using microprobe station in a vacuum system. MIT with hysteresis behavior was observed for the unpatterned square samples at around 68 °C. By four orders of magnitude of resistivity change was measured for the deposited VO2 thin films at transition temperature. After e-beam lithography process, substantial results in patterned VO2 thin films were observed. In this stage, for patterned VO2 thin films as stripes, the change in resistivity of VO2 was reduced by a factor of 10. As a consequence of electrical resistivity measurements, MIT temperature was shifted from 68 °C to 50 °C. The influence of e-beam process on the properties of VO2 thin films and the mechanism of the effects are discussed. The presented results contribute to the achievement of VO2 based thermochromic windows and bolometer applications., TUBITAK (113F349); University Research Foundation (BAP) 2015-IYTE- 42

In 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. ©, Scientific and Technological Research Council of Turkey TUBITAK (114F091); Middle East Technical University-Northern Cyprus Campus (METU-NCC) Grant BAP-FEN-16-K-8; BAGEP Award of the Science Academy in Turkey

In this study, we developed a microbolometer chip fabricated from high temperature superconducting Bi2Sr2CaCu2O8+δ (Bi2212) single crystals for the terahertz (THz) detection. For the manufacturing of the microbolometer chips, Bi2212 single crystals were transferred on substrate in the thin film like form and electron beam lithography, ion beam etching techniques were used. Resistance versus temperature behavior of the bolometer chips were performed by four probe technique in liquid nitrogen cryostat. Bi2212 microchips were integrated and characterized using in our custom-designed cryogenic bolometer system instead of expensive and massive cooling systems. The fabricated microchips significantly detected signals from the Stefan-Boltzmann lamp which includes a portion of THz radiation. The detected power and response time were studied for Bi2212 thin film like microbolometer chips. Our results demonstrated the feasibility of improved Bi2212 microchips could be used for bolometric detection for THz applications., Ministry of Science, Industry and Technology (SANTEZ 1386.STZ.2012-1)

In 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., TUBITAK (114F091)

The rapid increase in applications of terahertz waves requires new techniques to obtain continuous wave terahertz sources. Mesa structures fabricated from high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal have been observed as an intense, coherent, continuous electromagnetic wave source in the terahertz (THz) frequency region. However, in order to produce coherent radiation with high applicable power, we need large mesa structures that enter a collective electromagnetic state in which their oscillations are largely synchronized in phase. On the other hand, large mesa structures cause a heating problem. In this study, we report on the critical current density dependence of mesa area and the crystal inhomogeneity to understand heating problems in large area mesas for terahertz radiation. Since the doping dependence of Bi2212 is an important parameter, the as-grown Bi2212 crystals were heat-treated at various temperatures under vacuum conditions. We have fabricated triple mesa structures from Bi2212 single crystal using e-beam lithography and argon ion beam etching techniques with same area and with different area on the same chip. We investigated and compared characteristics of triple mesas which are on the same chip and next to each other. In this way, we searched the crystal inhomogeneity in triple mesa structures and studied the critical current density dependence of mesa area to obtain high emission power for the THz radiation. Our experimental results clearly show that the Josephson critical current density is decreasing when the area of mesa is increasing. © 2015 Elsevier Ltd. All rights reserved., TUBITAK (110T248); SANTEZ (1386.STZ.2012-1)

The effect of electromagnetic wave irradiation on the phase dynamics of intrinsic Josephson junctions in high temperature superconductors is investigated. We predict three novel effects by variation of the radiation amplitude and frequency: changing of the longitudinal plasma wavelength at parametric resonance; double resonance of the Josephson oscillations with radiation and longitudinal plasma wave; charging of superconducting layers in the current interval corresponding to the Shapiro step. The "bump" structure in IVC recently observed experimentally is demonstrated. We also observe ragged Shapiro steps at double resonance.

We fabricated rectangular mesa structures of superconducting Bi 2Sr 2CaCu 2O 8+d (Bi2212) using e-beam lithography and Ar ion beam etching techniques for terahertz (THz) emission. c-axis resistance versus temperature (R-T), current-voltage (I-V) characteristics and bolometric THz power measurements were performed to characterize Bi2212 mesas. The emission frequency of mesas was determined using a Michelson interferometer setup which also demonstrates polarized emission. Interference patterns of THz radiation from Bi2212 mesas were detected by various detectors such as a liquid helium cooled silicon composite bolometer, a Golay cell and a pyroelectric detector. An emitted power as high as 0.06mW was detected from Bi2212 mesas. For the first time, most of the pumped power was extracted as THz emission from a Bi2212 mesa. The radiation at 0.54THz was detected using the Michelson interferometric setup., TUBITAK (110T248); Alexander von Humboldt foundation

30th URSI General Assembly and Scientific Symposium, URSIGASS 2011; Istanbul; Turkey; 13 August 2011 through 20 August 201, Rectangular Bi2Sr2CaCu2O 8+δ (Bi2212) mesa structures were fabricated on as-grown Bi2212 single crystal superconductors using standard photolithography and Ar ion beam etching techniques. We have performed c-axis resistance versus temperature (R-T), current-voltage (I-V) characteristics and bolometer measurements. Furthermore, in contrast to previous studies, the emission frequency was determined using interferometer set up instead of FTIR. The interference patterns were detected outside the cryostat after traveling long way through ambient space. The emission frequency calculated by Fourier transform of interference data is consistent with Josephson frequency-voltage relation. © 2011 IEEE., Scientific and Technical Council of Turkey project no. 110T248