Browsing by Subject "Composite structures"

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    Microwave transmission through metamaterials in free space
    (2002) Aydın, Koray; Bayındır, Mehmet; Özbay, Ekmel
    To investigate properties of metamaterials, composite structures which consists of periodical arrangement of thin copper wires and SRRs on a circuit board was constructed. Through this, the transmission properties of composite metamaterials at microwave frequencies were investigated. It was observed that a pass-band was formed within the forbidden transmission bands of thin wire and SRR structures.
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    ItemOpen Access
    Observation of negative refraction and negative phase velocity in true left-handed metamaterials
    (SPIE, 2005) Özbay, Ekmel
    We report a true left-handed (LH) behavior in a composite metamaterial consisting of periodically arranged split ring resonator (SRR) and wire structures. The magnetic resonance of the SRR structure is demonstrated by comparing the transmission spectra of SRRs with that of closed SRRs. We confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined left-handed transmission band with a peak value of -1.2 dB (-0.3 dB/cm) is obtained. We also report the transmission characteristics of a 2D composite metamaterial (CMM) structure in free space. At the frequencies where left-handed transmission takes place, we experimentally confirmed that the CMM structure has effective negative refractive index. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments and the phase measurements are in good agreement. The experimental results agree extremely well with the theoretical calculations.
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    Substrate temperature influence on the properties of GaN thin films grown by hollow-cathode plasma-assisted atomic layer deposition
    (AIP Publishing LLC, 2016-02) Alevli, M.; Gungor, N.; Haider A.; Kizir S.; Leghari, S. A.; Bıyıklı, Necmi
    Gallium nitride films were grown by hollow cathode plasma-assisted atomic layer deposition using triethylgallium and N2/H2 plasma. An optimized recipe for GaN film was developed, and the effect of substrate temperature was studied in both self-limiting growth window and thermal decomposition-limited growth region. With increased substrate temperature, film crystallinity improved, and the optical band edge decreased from 3.60 to 3.52 eV. The refractive index and reflectivity in Reststrahlen band increased with the substrate temperature. Compressive strain is observed for both samples, and the surface roughness is observed to increase with the substrate temperature. Despite these temperature dependent material properties, the chemical composition, E1(TO), phonon position, and crystalline phases present in the GaN film were relatively independent from growth temperature.