Browsing by Author "Gökkavas, Mutlu"

Now showing 1 - 11 of 11

Open Access
AlGaN quadruple-band photodetectors
(IEEE, 2009) Gökkavas, Mutlu; Bütün, Serkan; Caban, P.; Strupinski, W.; Özbay, Ekmel
Quadruple back-illuminated ultraviolet metal-semiconductor-metal photodetectors with four different spectral responsivity bands were demonstrated. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 9.98 nm.
Open Access
AlxGa1-xN-based avalanche photodiodes with high reproducible avalanche gain
(2008) Tut, Turgut; Gökkavas, Mutlu; Özbay, Ekmel
We report high performance solar-blind photodetectors with reproducible avalanche gain as high as 1570 under ultraviolet illumination. The solar-blind photodetectors have a sharp cut-off around 276 nm. The dark currents of the 40 μm diameter devices are measured to be lower than 8 femto-amperes for bias voltages up to 20 V. The responsivity of the photodetectors is 0.13 A/W at 272 nm under 20 V reverse bias. The thermally limited detectivity is calculated as D* = 1.4 × 1014 cm Hz1/2 W-1 for a 40 μm diameter device. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
Open Access
Dark current reduction in ultraviolet metal-semiconductor-metal photodetectors based on wide band-gap semiconductors
(IEEE, 2009-10) Bütün, Serkan; Gökkavas, Mutlu; Yu, HongBo; Strupinski, Vlodek; Özbay, Ekmel
Photodetectors on semi-insulating GaN templates were demonstrated. They exhibit lower dark current compared to photodetectors fabricated on regular GaN templates. Similar behavior observed in photodetectors fabricated on epitaxially thick SiC templates. © 2009 IEEE.
Open Access
Design and characterization of resonant cavity enhanced Schottky photodiodes
(1996) Gökkavas, Mutlu
Recently, novel photodetectors which employ a multiple-pass detection scheme to increase the efficiency-bandwidth product have been developed. In this thesis. we present our work on .\iAs/GaAs resonant cavity enhanced (RCE) Schottky photodiodes w'ith an InGaAs absorber. Quantum efficiency enhancement is acconiplished by placing the InGaAs absorber inside a Fabry-Perot microcavity whose mirrors are formed b}' the Au Schottky layer cind an rVlAs/GaAs quarter wave stcick (QWS) reflector. In the design and analysis of the structures, scattering (S) matrices are used. Reflectivity, transmissivity, quantum efficiency, and the loss in the Schottky metal are calculated, a.nd it is shown that, it is ¡possible to diminish the front-surface reflectivity using a Si:iN.| dielectric coating to optimize the quantum efficiency. High speed and spectral efficiency/ measurements on fabricated photodiodes are also presented.
Open Access
Determining thermo-mechanical stress sources of an integrated optical device
(Elsevier, 2021-05-24) Dönertaş, Seval; Gökkavas, Mutlu; Özbay, Ekmel; Orhan, E. Ö.
In this study, we analyze the thermo-mechanical behavior of a multi-functional integrated optical chip (MIOC) via the finite element method (FEM). MIOC is a kind of fiber optic component which has a high thermal sensitivity. So; during temperature changes its mechanical resistance has a considerable critical significance. To investigate the thermo-mechanical effects, a three-dimensional device structure is modeled and time-dependent Von-Mises stress analyses are carried out via the multiphysics approach in the simulation environment. The simulation results are evaluated regarding the change of the crystal structure, epoxy layers, and waveguide core region. It is demonstrated that structural configuration has a crucial impact on the mechanical stability of the integrated optical device. Because of this reason, case D is accepted as a preferable geometrical structure for mass production. Also, the most dominant stress effect arises from the epoxy layer interface between the fiber pigtail carrier and lithium niobate (LiNbO3) chip called region 1. In this context, we investigate the influence of extreme thermal conditions on the induced stress on region 1. Apart from the mechanical strength of the devices, optical transmission is another subject to consider. Two claims arise on this point: power flow through the waveguide core is temperature-dependent and it is also related to the stress-optical effect. Finally, we compare the simulation results with the experimental results and there is certainly a consistency between the two separate analyses. Furthermore; it can be said that manufacturing more than 500 devices facilitates the quantitative analysis of device performance.
Open Access
Evidence of asymmetric beaming in a piecewise-linear propagation channel
(Optica, 2021-06) Gündoğdu, Tamara Funda; Gökkavas, Mutlu; Serebryannikov, Andriy E.; Özbay, Ekmel
Asymmetric beaming in a piecewise-linear propagation channel is demonstrated for a single photonic-crystal prism at Gaussian-beam illumination. The used hybrid refraction– diffraction mechanism exploits oblique incidence, the first-negative-order deflection at the longer interface, and asymmetry in coupling at the exit interfaces and does not need blocking of transmission by dispersion in the back ward illumination case. The Floquet–Bloch mode with left-handed behavior and nearly circular equifrequency dis persion contours is utilized. The outgoing waves may have significantly different spatial distributions for the forward and backward illumination cases, yielding asymmetry in the beaming regime.
Open Access
Few-layer bifunctional metasurfaces enabling asymmetric and symmetric polarization-plane rotation at the subwavelength scale
(Nature Publishing Group, 2024-06-13) Gökkavas, Mutlu; Gökkavas, Tamara Funda Gündoğdu; Özbay, Ekmel; Serebryannikov, Andriy E.
We introduce and numerically validate the concept of few-layer bifunctional metasurfaces comprising two arrays of quasiplanar subwavelength resonators and a middle grid (array of rectangular holes) that offer both symmetric and asymmetric transmissions connected, respectively, with symmetric and asymmetric polarization-plane rotation functionalities. The proposed structures are thinner than λ/7 and free of diffractions. Usually, the structure’s symmetry or asymmetry, i.e. unbroken or broken spatial inversion symmetries, are considered for metasurfaces as prerequisites of the capability of symmetric or asymmetric conversion of linearly polarized waves, respectively. Due to the achieved adjustment of the resonances enabling the rotation of the polarization plane simultaneously for both orthogonal polarizations of the incident wave, the symmetric polarization-plane rotation functionality can be obtained within one subwavelength band, whereas the asymmetric polarization-plane rotation functionality associated with the asymmetric transmission is obtained within another subwavelength band. This combination of the functionalities in one subdiffraction structure is possible due to the optimal choice of the grid parameters, since they may strongly affect the coupling between the two resonator arrays. Although normal incidence is required for the targeted bifunctionality, the variations of the incidence angle can also be exploited for the enrichment of the overall functional capability. Variations of the polarization angle give another important degree of freedom. The connection between the polarization-angle dependence of cross-polarized transmission and capability of symmetric and asymmetric polarization-plane rotation functionalities is highlighted. The feasible designs of the bifunctional metasurfaces are discussed.
Open Access
Optimization of the annealed proton exchange method with controlled annealing for multifunctional integrated optical chip production
(Optica, 2022-10-20) Asık, Fatma Yasemin; Gökkavas, Mutlu; Öztekin, Evren; Karagöz, Ercan; Ceylan, Abdullah; Özbay, Ekmel
The main objective of our studyis to develop a new approach to the annealed proton exchange (APE) method for the fabrication of the multifunctional integrated optical chip (MIOC) used in fiber-optic gyro systems and to eliminate the loss of time and material, especially in mass production applications. In this work, self-polarized waveguides, which are the basic components of a MIOC device, were produced by the APE method and studied. With the developed method, controlled annealing trials have been carried out from a certain region on the LiNbO3 substrate used in waveguide production, and the annealing time specific to the annealing process was determined. By utilizing a special setup for the hot acid process, the proton exchange process was accomplished without a sudden temperature change of the substrate. Using prism coupling measurements of the fabricated waveguides, annealing times were determined to obtain index change values suitable for 45%–50% optical throughput. Mode profiles of devices with high optical throughput that were produced by the proposed method were measured, and it was seen that devices from different proton exchange runs had similar profiles. As a result, many undamaged substrates were fabricated, and their optical quality was found to be within the expected values.
Open Access
Reduction in temperature-dependent fiber-optic gyroscope bias drift by using multifunctional integrated optical chip fabricated on pre-annealed LiNbO₃
(MDPI AG, 2024-12-11) Karagöz, Ercan; Aşık, Fatma Yasemin; Gökkavas, Mutlu; Akbaş, Erkut Emin; Yertutanol, Aylin; Özbay, Ekmel; Özcan, Sadan
The refractive index change obtained after annealed proton exchange (APE) in lithium niobate (LiNbO₃) crystals depends on both the proton exchange process carried out in hot acid and the structure of the crystals. In devices produced by the APE method, dislocations and lattice defects within the crystal structure are considered to be primary contributors to refractive index discontinuities and waveguide instability. In this study, the effects of pre-annealing LiNbO₃ crystals at 500 °C on multifunctional integrated optical chips (MIOCs) were investigated through interferometric fiber-optic gyroscope (IFOG) system-level tests. It was observed that the pre-annealing process resulted in an improvement in the optical throughput of MIOCs (from 34% to 51%) and the temperature-dependent bias drift stability of the IFOG (from 0.031–0.038°/h to 0.012–0.019°/h). The angle random walk (ARW) was measured as 0.0056 deg/√h.
Open Access
Ti-indiffused waveguide polarizers on lithium niobate for fiber optic gyroscope
(OSA, 2018) Kanlı, Yasemin; Öztekin, Evren; Dönertaş, Seval; Gökkavas, Mutlu; Özbay, Ekmel
We report our results on polarizing waveguides fabricated by Ti indiffusion technique on x-cut y-propagating LiNbO3. Polarizing Ti indiffused waveguides with polarization extinction coefficient (PER) higher than 47 dB at their outputs, operating at 1550 nm wavelength were demonstrated.
Open Access
Ultraminiature antennas combining subwavelength resonators and a very-high-ε uniform substrate: the case of lithium niobate
(IEEE, 2020) Serebryannikov, A. E.; Gökkavas, Mutlu; Gündoğdu, Tamara Funda; Volski, V.; Vandenbosch, G. A. E.; Vasylchenko, A.; Özbay, Ekmel
Combining the effects of subwavelength resonators and very-high-permittivity substrates enables a high extent of miniaturization, even for very simple, split-loop resonators. Here, we demonstrate how requirements to the substrate's permittivity are connected with the desired extent of miniaturization and why materials with a relative permittivity of 30 <; ε <; 100, like lithium niobate, may offer a real possibility to miniaturize. For demonstration purposes, we designed, in line with this approach, an ultraminiature dual-band antenna to operate at 2.8 and 4.2 GHz. The antenna is fabricated using microfabrication techniques and studied experimentally. There is good agreement between the measurement and simulation results. The realized gain is about -5 dB for the first resonance, at which the size of the substrate-resonator block is λ/24. The obtained results demonstrate the potential of the suggested approach, which is expected to be applicable to a very wide class of subwavelength resonators and a wide variety of substrates with high permittivity.