Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Modulation of terahertz waves by VO2 based metamaterials(01. Izmir Institute of Technology, 2023) Noori, Aileen; Aygün Özyüzer, GülnurTerahertz (THz) waves, being a form of electromagnetic radiation have frequen-cies ranging from 0.1 THz to 10 THz. Due to the lack of suitable radiation sources and detectors, this region is not well known. Interaction with THz waves becomes more effective by introducing the metamaterials (MM) and metasurfaces (MS) (3D and 2D, respectively), which are made up of artificially subwavelength compositions arranged in periodic arrays. MMs provide a unique control on the propagation of (EM) waves and their geometries determine their properties. Recently, coding MM has made it possible to regulate the far-field scattering pattern of EM waves. In other words, it is possible to shape the THz wavefront by changing the sequence of the coding unit cells into a 2D surface pattern. The reflection phase of the two types of unit cells in 1-bit coding MM is 0 and π. In this thesis, two different types of coding MMs were designed and fabricated. One type is hard-coded (metal-based), while the other is based on VO2 thin film. Both types of samples share a similar structure, which includes a sapphire substrate, a gold patch, a PET layer serving as a dielectric spacer, and a ground gold layer. However, there is an additional layer of VO2 beneath the gold patch and on top of the sapphire substrate in one of the fabricated MMs. The coding MM consists of two identical unit cells, with the only distinction being the size of the gold patch's side. This size determines whether the unit cell is considered as 0-bit or 1-bit. When the side size is 90 µm, the unit cell is 0-bit. On the other hand, when the side sizes are 60 µm and 70 µm (for different samples), the unit cell is 1-bit. Two different sets of hard-coded MM were fabricated. One set is composed of the 60-90 µm unit cells arranged in the form of checkerboard and stripe designs. The other set is made of 70-90 µm unit cells arranged in the form of checkerboard and stripe designs. The samples were measured with a custom-built setup and the THz full spectrum (0.50-0.75 THz) was obtained at each reflection angle. The results indicate that the checkerboard samples' reflection angle for each frequency has a good consistency with the calculations. Since the detector was obstructing the incoming beam, the measurable angle range begins at 23 degrees from normal incidence. This issue limits the ability to obtain the maximum scattering pattern of the strip design samples. At the final section of the thesis, the VO2-based MM, was fabricated and mea-sured. VO2 layer was used in this structure, due to its phase-changing characteristic. It undergoes a reversible transformation from an insulator to a metallic state at about 68◦C. The initial concept was to design and fabricate the MM just entirely out of 1-bit (60 µm) unit cells. After that, using a CW laser pump and a digital micromirror (DMD), convert this 1-bit into the 0-bit unit cell by modifying the conductivity of the VO2 layer of each individual unit cell. Using this idea, it was possible to develop a tunable digital MM. However, the CST simulation results demonstrated that the proposed MM is ineffective due to the significant amplitude difference between the 0 and 1-bit unit cells. Due to that, using the VO2-based unit cells (0 and 1-bit) the striped and checkerboarded pattern of MMs was designed and fabricated. The VO2 conductivity was modulated using a CW 915 nm laser beam. The measurement results show that VO2-based MM can be used for THz beam splitting at room temperature, and the scattering pattern weakens when the laser is illuminated over the sample, causing the VO2 layer to turn conductive. CST Studio Suite simulation software was used to determine the unit cells' geo-metrical dimensions, amplitudes, and phases. The analytical calculations were performed using MATLAB. The investigated MM has the potential to be used in THz communica-tions.Doctoral Thesis Single-Photon Generation From Defects and Manipulation With Nanostructures(Izmir Institute of Technology, 2019) Özçeri İyikanat, Elif; Aygün, Gülnur; Tarhan, Enver; Tarhan, Enver; Aygün Özyüzer, GülnurSingle-photon sources are essential components for several applications in the field of quantum information technologies, such as quantum cryptology and quantum computation. To this aim, efficient generation and detection of single-photons are the crucial to be achieved. Among single-photon sources that are extensively studied in the literature, defect centers in solid are very promising due to their room temperature operation and their stability. The aim of this thesis is to generate single photons at room temperature and control their optical properties by nanostructures. Single-photon emission from TMDCs originates from localized weakly bound excitons at cryogenic temperatures due to their small exciton binding energies. However, room temperature SP emission from WS2 can be obtained by creatingWO3 defects. In our study, room temperature emission from defects in WO3 was investigated. Density functional theory calculations showed that the source of the emission can be oxygen defects. Additionally, the emission was brightened by plasmonic gold nanoparticles. Furthermore, defects in two-dimensional (2D) hexagonal boron nitride (hBN) is offered as an efficient room temperature SPS. HBN is a wide bandgap 2D material, in which defect centers create discrete energy level to generate single photons. In our study, reversible single-photon emission control from defects in hBN was demonstrated by Förster-like resonance energy transfer between the single-photon emitter and a graphene layer. To this aim an ionic liquid based device structure was used.Doctoral Thesis Magnetron Sputter Grown Metal Doped Vanadium Oxide Thin Films for Terahertz Bolometers(Izmir Institute of Technology, 2018) Alaboz, Hakan; Aygün Özyüzer, GülnurTerahertz (THz) studies and hence technological improvements have increased and that caused expansion of application of THz waves. Applications of THz region have been expanded in many areas such as security, medical imaging, detection of explosives, nondestructive tests and wireless communication recently. THz radiation passes through many plastic materials, clothing but it reflects from metals and it is used in the detection of a lot of well known explosive materials. In spite of mentioned advantages and a wide range of application area, constructing a detector which is low cost, compact and uncooled is difficult and this causes the industry to improve slowly. Now, detectors which are widely used in THz region are pyroelectric, Schottky barrier diodes, field effect transistors and they have disadvantages such as low sensitivity, hard to construct an array and low speed. Instead of these detectors, superconducting bolometers are presented but they require liquid helium cooling. It is thought that VOx will be a premium technology for THz region due to its success in the infrared region. In this thesis, VOx:Au thin films were produced by DC magnetron sputtering and properties of these films were optimized for uncooled bolometer that operates at THz region. Polycrystalline VOx:Au thin films which have -1.7 % K-1 temperature coefficient resistance (TCR) and 0.07 Ωcm resistivity values were obtained. Increasing TCR values mostly depend on sputtering parameters such as gas and Au dope rates. These parameters were changed until the best TCR value was achieved and with Au doping, high resistance values of the films decreased to acceptable levels hence it decreased Johnson noise of the bolometer. Originally, these thin films which were sputtered on high resistivity silicon wafers and doped with Au, provided the bolometer to operate in THz region efficiently. The antenna design on the device was made by CST Microwave Studio, the antenna resonance was arranged to 0.6 THz which is related to our THz source operating frequency.Doctoral Thesis Magnetron Sputtering Growth of Azo/Zno Multilayers for Cu2znsns4 Thin Film Solar Cells: Material and Device Characterization(Izmir Institute of Technology, 2017) Köseoğlu, Fulya; Özyüzer, Lütfi; Aygün Özyüzer, GülnurCu2ZnSnS4 (CZTS) absorber layer attracts so much attention in photovoltaic industry since it contains earth abundant, low cost and non-toxic elements contrary to other chalcogenide based solar cells such as CuInGa(S,Se)2 (CIGS) and CdTe. Although, CZTS studies have been newly started, recently 9.4 % efficiency has been achieved. In the present thesis, all layers used in the CZTS device structure were investigated using energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), scanning electron microscopy (SEM), spectrophotometry and Raman spectroscopy. For CZTS absorber layer, CZTS films based on a stacked precursor (Cu/Sn/Zn/Cu) were prepared. The effect of sulfurization time and the thickness of top and bottom Cu layer in precursors on the properties of CZTS thin films were investigated. We addressed the importance of Cu layer thickness adjacent to Sn to avoid developing detrimental phases and to get complete formation of kesterite CZTS absorber layer. We also addressed the importance of sulfurization time to restrict the Sn and Zn losses, formation of oxides such as SnO2 and ZnO, formation of MoS2 and voids between Mo/CZTS interface. Effect of the sulfur concentration on the properties of Zn(O,S) thin films were investigated. We showed that key parameters such as energy gap and crystal structure of the Zn(O,S) thin films can be tuned by changing the sulfur concentrations of the films. We succeed substitute conventionally used CdS buffer layer with environmentally friendly alternative Zn(O,S) buffer layer in CZTS solar cells. Effect of substrate position and rotation speed during the deposition of AZO thin films were investigated. We addressed that stress on the films can be significantly reduced by off-center deposition and rotating the sample holder during the deposition. In this way, high transmission in the visible range and metal like resistivity were achieved simultaneously at room temperature. We observed strong dependence of device performances on both sulfurization time and the thickness of Cu layer adjacent to Sn in CZTS absorber. The best device was based on CZTS films sulfurized for 30 minutes and having thicker Cu layer adjacent to Sn layer in precursors.