Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Subject: search.filters.subject.Terahertz spectroscopy

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Now showing 1 - 4 of 4
  • Master Thesis
    Terahertz Wave Sensitive Superconducting Bolometric Detector
    (Izmir Institute of Technology, 2015) Semerci, Tuğçe; Özyüzer, Lütfi; Özyüzer, Lütfi; Özdemir Köklü, Mehtap; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    Terahertz (THz) waves has varied properties than other part of the electromagnetic spectrum which between microwave and infrared and has 0.3-10 THz frequency range and 3-0.03 mm wavelength. There are many areas to utilize from THz radiation that defense industry particularly security part such as detecting of unknown materials, poison or explosive matters. Additionally, THz waves can pass through leather, fabric and paper despite of metal and water and there is no enough energy to ionize the atoms in THz radiation. That’s why, THz radiation is not hazardous for human. Many detectors are developed to use these benefit of THz radiation. Bolometric detector is preferred between these detectors to detect the radiation above 1 THz frequency. Bi2Sr2CaCu2O8+δ (Bi2212) single crystal was used in literature for the first time to detect the THz waves with superconducting bolometers with our study. In this study, epoxy was used to transfer Bi2212 single crystal onto the sapphire substrate. Intended thickness was acquired by scotch tape via cleaved crystal mechanically. Edge of crystals were cut pyramid shape to provide each layer to contact with gold, crystal and silver epoxy. 150-200 nm crystals were shadowed by aluminum foil to deposit roughly 200 nm gold. Then clean room process was implemented to prepare our samples to electron beam lithography. Log-periodic antenna and four point contact structure was placed onto the crystals then UV light, developer and ion beam etching system were performed. Cryostat was designed and produced to measure electrical and bolometric measurements. Resistance-temperature in a-b axis gave the around 90 K as critical temperature. Bolometric measurement was done via superconducting transition region to decide the selected temperature to measure the change of resistance by sending signal from Stefan-Boltzmann Lamp in specific seconds. Change of resistance was observed from resistance-time measurement for bolometric measurement. Response time was calculated approaching 825 ms from the result of bolometric measurement. Our measurement set-up has limitation to read right data. Lock-in-amplifier may use to more accurate result.
  • Master Thesis
    Spin Polarized Tunneling in Large Area Mesas of Superconducting Bi2sr2cacu2o8+d for Terahertz Emission
    (Izmir Institute of Technology, 2010) Türkoğlu, Fulya; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There is an increasing interest in science and technology of electromagnetic waves in terahertz frequency range (0.1-10 THz) because of their emerging application areas including physics, biology, chemistry, astronomy, medicine etc. The observation on generation of THz radiation emitted from lateral dimension of high temperature superconductor (HTS) Bi2Sr2CaCu2O8+.. (Bi2212) and responses to THz waves increase the importance of these HTSs. Single crystal of HTS Bi2212 forms natural superconductor-insulator-superconductor (SIS) layered junctions, which are called intrinsic Josephson junctions (IJJ). The stacks of IJJs in Bi2212 can be used such a voltage-frequency converter and their large energy gap allows the emissions at THz frequency range. Recently, it has been demonstrated that rectangular IJJ mesa structures of Bi2212 can be used as a source of continuous, coherent and polarized THz radiation. It was shown that all THz emitting mesas are below a certain underdoped level, which has relatively small critical current in contrast to optimally doped and overdoped Bi2212. In this work, rectangular Au/Co/Au/Bi2212 mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals using standard photolithograph and Ar ion beam etching techniques. In order to characterize the mesas, c-axis resistance versus temperature (R-T) and current-voltage (I-V) characteristics were investigated. During I-V characterization, Si composite bolometer was used to detect the emission. We obtained small critical current from as-grown mesas due to injection of spin polarized current. We observed THz emission peak for one of the mesas which has low quasiparticle conductivity and low dissipation due to its small critical current density. It means that the adjustment of doping level can be eliminated for THz emission by the injection of spin polarized current through the c-axes of the asgrown mesas.
  • Master Thesis
    Area Dependence of Josephson Critical Current Density in Superconducting Bi2sr2cacu2o8+d Mesas for Terahertz Emission
    (Izmir Institute of Technology, 2013) Sağlam, Hilal; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    There are numerous application fields of terahertz waves such as airport screening of passengers for weapons, explosives, drugs, secure wireless communications, cancer detection, etc. high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal has been observed as an intense, coherent, continuous electromagnetic wave source in terahertz frequency region. Bi2212, which is highly anisotropic high-Tc superconductor, is considered as a stack of intrinsic Josephson junctions (IJJs) on atomic scale. In this study, we have fabricated triple mesa structures on a same chip with various mesa areas (300x50, 200x50, 100x50 μm2). Firstly, single crystal of Bi2212 is glued onto a sapphire substrate from its smooth a-b surface by silver epoxy. After deposition of 100 nm Au layer, rectangular mesa structures were fabricated on the surface of an under-doped Bi2212 crystal by using e-beam lithography and Ar-ion etching step by step. On account of the difficulties in making a contact on small area of the mesa, CaF2 insulating layer deposition was performed. After that, a gold stripe with the width of 30 μm was created by lift-off technique on the mesa and CaF2 layer. Finally, three gold probe wires were connected to the two contact paths and mesa by silver epoxy. After the mesa fabrication, the exact dimensions of the mesas were obtained using atomic force microscope. To obtain the electrical characterization, c-axis resistance versus temperature (R-T), and current-voltage behavior (I-V) were measured. From I-V characteristics, critical current of each mesa structure having different dimension was obtained, after that, we have calculated the critical current densities of each mesa structure and then we have studied change in Josephson critical current density of mesas with different dimensions. We can conclude from the I-V measurements of the mesas that the Josephson critical current density is decreasing when the area of mesa is increasing. Furthermore, the backbending voltage points are increasing since heating effects dominate for the large areas of the mesa structures.
  • Master Thesis
    Fabrication of Double Mesa Structures From Superconducting Bi2sr2cacu2o8+d by E-Beam Lithography for Terahertz Emission
    (Izmir Institute of Technology, 2011) Demirhan, Yasemin; Demirhan, Yasemin; Özyüzer, Lütfi; Özyüzer, Lütfi; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    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.