Browsing by Author "Bardak C."

Now showing 1 - 8 of 8
  • No Thumbnail Available
    Item
    Microwave imaging with a time-reversed finite-difference time-domain technique
    (Taylor and Francis Ltd., 2014) Bardak C.; Saed M.
    A novel approach to microwave imaging in three dimensions based on time-reversed finite-difference time-domain method (FDTD-TR) is presented. The proposed computationally efficient technique allows the reconstruction of detailed images of scattering objects. Transmissions as well as reflection imaging modalities in the near-field and far-field were investigated. In this study, we implemented the FDTD-TR algorithm in three dimensions using time-reversed convolutional perfectly matching layers as boundaries to truncate the simulation domain. The method works for dielectric as well as conducting objects. The methods capabilities in imaging multiple objects and behind-the-wall imaging are also presented. © 2014 © 2014 Taylor & Francis.
  • No Thumbnail Available
    Item
    Imaging multiple objects with time-reversed FDTD method at microwave range
    (Institute of Electrical and Electronics Engineers Inc., 2015) Bardak C.; Saed M.; Tas̈kln S.
    A simulation technique is presented for imaging multiple objects in microwave region. The proposed technique is based on the time-reversed finite-difference time-domain method (FDTD-TR) in three dimensions. Using this technique, not only the locations of objects but also their detailed images can be obtained. The method is capable of imaging hidden objects with transmission as well as reflection imaging modalities in the nearfield and far-field. The proposed technique is computationally efficient due to using a much smaller data set compared to conventional FDTD-TR imaging techniques where the entire time span of the captured data is reversed. The method was validated using experimental results as well as simulations. © 2015 IEEE.
  • No Thumbnail Available
    Item
    Effect of the external electric field on the electronic structure, spectroscopic features, NLO properties, and interionic interactions in ionic liquids: A DFT approach
    (Elsevier B.V., 2019) Bardak C.; Atac A.; Bardak F.
    Ionic liquid (IL)s are a unique group of chemicals with extraordinary physical and chemical properties. Due to the advantage of the tunability of their features by varying the ion compositions and either IL/IL or IL/conventional solvent mixing ratios, uncountable functions of them have been discovered. In the field of electrochemistry and energy applications, the function of ILs under external field should also be understood well. The most fundamental way of gaining insight into the function of a substance is to study the structure. Therefore, this work focuses on the investigation of structural variations of an exemplary IL, 1-hexyl 3-methylimidazolium chloride (HmimCl), under external electric field (EEF) via ab initio quantum chemistry calculations. Field dependent geometry analysis show that the interionic bond and the molecular conformation can be modulated by the strength and the direction of the applied field, and the disassociation occurs at ~0.7 V/Å level. Mulliken, atomic polar tensor, and natural bond orbital charges were obtained within field dependency and their correlation was examined. Charge displacement curves were constructed to visualize the field inducement on the charge transfer. Interionic interactions were profiled as hydrogen, van der Walls, and steric effect by using reduced density gradient analysis and the breaking of the single hydrogen bond was determined to occur at ~0.6 V/Å level. The vibrational spectra (IR and Raman) were simulated and the critical peaks that are affected by the field inducement were interpreted. The analysis of the field dependent Nonlinear Optical (NLO) properties shows that the only important component is in the field direction for the field intensity >0.2 V/Å. Electronic transition properties determined via TD-DFT calculations with B3LYP/6-311G** level of theory indicate that the UV–Vis spectrum and underlying transitions are depended on the direction of the field, and experience a bathochromic shift with increasing inducement showing that the IL becomes more susceptible to excitation. © 2018 Elsevier B.V.
  • No Thumbnail Available
    Item
    Microstrip Patch Antenna Design Using the Superformula
    (Institute of Electrical and Electronics Engineers Inc., 2020) Ulu E.; Bardak C.
    In this study, the super-shaped microstrip patch antenna is presented to demonstrate application of naturalistic geometry in antenna design. The shape of the antenna structure is modelled according to Superformula of Gielis, which helps to improve the antenna characteristics. The modelling of naturelike shaped patches provides the minimization of the antenna size and better return loss and gain. We have modelled a leaflike of a palm tree by a special choice of the parameters of the Superformula. The designed antenna attains -56.9 dB return loss at 5.02 GHz resonance frequency with operational bandwidth from 4.37 GHz to 6.1 GHz for -10dB. The dimensions of the antenna are minimized as 15 text{mm} times 25 text{mm}. © 2020 IEEE.
  • No Thumbnail Available
    Item
    Anionic dependency of electronic and nonlinear optical properties of ionic liquids
    (Elsevier B.V., 2022) Bardak F.; Bardak C.; Karaca C.; Kose E.; Bilgili S.; Atac A.
    Nonlinear optical phenomena play significant roles in the bulk properties and responsive characteristics of ionic liquids, especially when used under strong electric fields. The variability of the anion-cation pair in ionic liquids makes them designer solvents; thus, the anion or cation dependency of the physicochemical properties should be understood in depth. Accordingly, the electric field-induced characteristics of eight ionic liquids with [Br]−, [BF4]−, [PF6]−, [Ac]−, [TFAc]−, [MS]−, [NTf2]−, and [Tos]− anions paired with the 1-butyl-3-methyl imidazolium cation were investigated using density functional theory modeling at four theoretical levels: B3LYP-6-31G(d), M06-2X 6-31G(d), M06-2X 6-311++G(d,p), and M06-2X aug-cc-PVTZ. The frontier molecular orbitals, electrostatic potential surface, and electron density difference maps were obtained to visualize the electrostatic characteristics. The permanent electric dipole moment, linear electric polarizability, and first-order and second-order hyperpolarizabilities were also determined. While the static dipole moment and dipole polarizabilities could be obtained using low-level quantum chemistry at a satisfactory level, the functionalization of more intensive methods was required to accurately obtain the nonlinear optical properties. © 2021 Elsevier B.V.
  • No Thumbnail Available
    Item
    Three-dimensional modelling and terahertz imaging of malignant cells with convolutional time-reversed FDTD method
    (Taylor and Francis Ltd., 2022) Bardak C.
    A novel image reconstruction approach to terahertz pulsed imaging for detecting malignant and fibrous cells within healthy tissue is presented. The non-ionizing effect of the terahertz radiation on healthy tissue makes it a future diagnosis method in medical imaging. The implemented method is based on the convolutional time-reversed FDTD algorithm in the terahertz range. It is computationally efficient and accurately reconstructs images of malignancies from reflected terahertz signals. Besides, the technique is capable of differentiating malignant, fibrous, and fatty cells through the variations of their electrical properties at the terahertz frequency range and well reconstructing their images in 3D. The images created in this method are in high resolution, even for cell-sized malignancies. Furthermore, the proposed technique is compared with conventional terahertz imaging methods and found to have more sophisticated outcomes. © 2022 Taylor & Francis.
  • No Thumbnail Available
    Item
    External field intensity and wavelength dependency from IR to deep UV of linear and nonlinear optical properties of 1-butyl-3-methylimidazolium dicyanamide ionic liquid
    (Elsevier B.V., 2023) Bardak F.; Bardak C.
    The electronic structure, inter-ionic interactions and charge transfer, polarizability, and first-order and second-order hyperpolarizabilities of 1-butyl-3-methylimidazolium dicyanamide ionic liquid have been explored via modeling at M06-2X/6-311++G(d,p) level of theory. External field intensity dependence of electronic properties was investigated by utilization of field intensity from 0.0001 to 0.002 au applied in the direction of anion-cation. Interionic interactions have been characterized through the reduced density gradient method to reveal and distinguish hydrogen bonding and other non-covalent interactions. Extraordinary bonding interactions were observed between the anion and ring system of the cation. Electronic excitation characteristics were analyzed based on the inter-fragment charge transfer method to distinguish charge transfer contributions from local excitations. Static and dynamic nonlinear optical parameters namely the total dipole moment μ, the mean polarizability α(0,0) and α(−ω; ω), the anisotropy of polarizability Δα(0,0) and Δα(−ω; ω) the first hyperpolarizability in the direction of dipole moment β(0,0,0), β(−ω; ω,0), and β (−2ω; ω,ω), the average second hyperpolarizability γ(0;0,0,0), γ(−ω; ω,0,0) and γ(−2ω; ω,ω,0) have been calculated for specific laser wavelengths corresponding to the UV, visible, and infrared region. Thus, a novel way of mapping the field intensity and wavelength dependence of NLO properties was achieved. The hyperpolarizability calculations indicate that 1-butyl-3-methylimidazolium dicyanamide has a significant potential to be used in electro-optics Pockels effect, second-harmonic generation, and electro-optical Kerr effect applications. NLO property maps indicate that the wavelength dependence in the IR and visible region is almost ignorable when compared with that of the UV region where the first hyperpolarizabilities can increase by ten thousand and the second hyperpolarizabilities can reach up to ten billion times. © 2023 Elsevier B.V.
  • No Thumbnail Available
    Item
    Electric field modulation of electronic structure, charge transfer, and nonlinear optical properties of crocetin for dye-sensitized solar cells
    (Elsevier B.V., 2025) Bardak F.; Bardak C.
    Natural sensitizing dyes in solar energy applications have gained attention due to their sustainability, cost-effectiveness, and environmental friendliness. However, enhancing their performance, light-harvesting efficiency, and durability requires a deeper understanding of their structural and electronic properties under solar irradiance. Accordingly, this study explores the electronic structure and nonlinear optical properties of crocetin dye (cis and trans conformers) under varying external electric fields using M06-2X/6-311+G(d,p) model chemistry. The absorption characteristics strongly depend on field intensity, with electron injection efficiency expected to decrease as the field increases. Key nonlinear optical (NLO) parameters, such as dipole moment, polarizability, and hyperpolarizabilities, were calculated at solar irradiance peak frequencies. The NLO properties are wavelength-independent in the infrared region but show significant wavelength dependency in the visible region, increasing substantially near the absorption energy. © 2024 Elsevier B.V.