Browsing by Author "Gokkavas, M."
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Item Open Access Absorption enhancement in InGaN-based photonic crystal-implemented solar cells(SPIE, 2012-07-26) Gundogdu, T. F.; Gokkavas, M.; Özbay, EkmelWe investigate the absorption characteristics of InGaN solar cells with high indium (0.8) content and a one-dimensional periodic nano-scale pattern (implemented) in the InGaN layer theoretically. The short-circuit current of our InGaN-based solar cell structure is calculated for different lattice constant, etch depth, and fill factor values. A substantial increase in the absorption (17.5% increase in short-circuit current) is achieved when the photonic crystal pattern is thoroughly optimized. (c) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.JNP.6.061603]Item Open Access Broadband quarter-wave plates at near-infrared using high-contrast gratings(2013) Mutlu, M.; Akosman, A.E.; Kurt G.; Gokkavas, M.; Özbay, EkmelIn this paper, we report the theoretical and experimental possibility of achieving a quarter-wave plate regime by using high-contrast gratings, which are binary, vertical, periodic, near-wavelength, and two-dimensional high refractive index gratings. Here, we investigate the characteristics of two distinct designs, the first one being composed of silicon-dioxide and silicon, and the second one being composed of silicon and sapphire. The suggested quarter-wave plate regime is achieved by the simultaneous optimization of the transverse electric and transverse magnetic transmission coefficients, TTE and TTM, respectively, and the phase difference between these transmission coefficients, such that |TTM| ≅ |TTE| and \TTM - \TTE ≅ -/2. As a result, a unity circular polarization conversion efficiency is achieved atλ0 = 1.55 μm for both designs. For the first design, we show the obtaining of unity conversion efficiency by using a theoretical approach, which is inspired by the periodic waveguide interpretation, and rigorous coupled-wave analysis (RCWA). For the second design, we demonstrate the unity conversion efficiency by using the results of finite-difference time-domain (FDTD) simulations. Furthermore, the FDTD simulations, where material dispersion is taken into account, suggest that an operation percent bandwidth of 51% can be achieved for the first design, where the experimental results for the second design yield a bandwidth of 33%. In this context, we define the operation regime as the wavelength band for which the circular conversion efficiency is larger than 0.9. © 2013 SPIE.Item Open Access Chiral Structures: Manipulation of Asymmetric Transmission in Planar Chiral Nanostructures by Anisotropic Loss (Advanced Optical Materials 7/2013)(2013) Li, Z.; Gokkavas, M.; Özbay, EkmelAsymmetric transmissions of circularly polarized optical waves can be achieved when the waves are incident normal to planar chiral structures, provided that the structures are anisotropic and lossy. In order to clarify how the factor of loss affects the asymmetric transmission, Z. Li, M. Gokkavas, and E. Ozbay studied a typical planar chiral structure by using an optical lumped element model. On page 482, they found that the anisotropy of loss, instead of the whole loss, plays a crucial role for achieving asymmetric transmission. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.Item Open Access Experimental realization of a high-contrast grating based broadband quarter-wave plate(Optical Society of America, 2012-12-03) Mutlu, M.; Akosman, A. E.; Kurt, G.; Gokkavas, M.; Özbay, EkmelFabrication and experimental characterization of a broadband quarter-wave plate, which is based on two-dimensional and binary silicon high-contrast gratings, are reported. The quarter-wave plate feature is achieved by the utilization of a regime, in which the proposed grating structure exhibits nearly total and approximately equal transmission of transverse electric and transverse magnetic waves with a phase difference of approximately pi/2. The numerical and experimental results suggest a percent bandwidth of 42% and 33%, respectively, if the operation regime is defined as the range for which the conversion efficiency is higher than 0.9. A compact circular polarizer can be implemented by combining the grating with a linear polarizer. (C) 2012 Optical Society of AmericaItem Open Access High performance AlxGa1-xN-based avalanche photodiodes(Elsevier BV, 2007-10) Tut, T.; Butun, B.; Gokkavas, M.; Özbay, EkmelWe report high performance solar-blind photodetectors with reproducible avalanche gain as high as 820 under ultraviolet illumination. The solar-blind photodetectors have a sharp cut-off around 276 nm. We improved the device performance by designing different epitaxial wafer structure with thinner active multiplication layer. We compare the resulting fabricated devices from these wafers in terms of dark current, photoresponse, avalanche gain performances.Item Open Access High-speed transparent indium-tin-oxide based resonant cavity Schottky photodiode with Si/sub 3/N/sub 4//SiO/sub 2/ top Bragg mirror(IEEE, Piscataway, NJ, United States, 2000) Bıyıklı, Necmi; Kimukin, I.; Aytur, O.; Özbay, Ekmel; Gokkavas, M.; Unlu, S.Photodetectors demonstrating high bandwidth-efficiency (BWE) products are required for high-performance optical communication and measurement systems. For conventional photodiodes the BWE product is limited due to the bandwidth-efficiency trade-off. A resonant cavity enhanced (RCE) photodetection scheme offers the possibility to overcome this limitation. Very high BWE products are achieved using Schottky and p-i-n type RCE photodiodes, which could not be reached with conventional detector structures. Even better performances should be possible for RCE Schottky photodiodes if one can get rid of the optical losses and scattering caused by the Schottky metal, Au, which also serves as the top mirror of the resonant cavity. The transparent, low resistivity material indium-tin-oxide (ITO) is a potential alternative to thin semi-transparent Au as a Schottky-barrier contact material. We report our work on high-performance ITO-based RCE Schottky photodiodes.Item Open Access Manipulation of Asymmetric Transmission in Planar Chiral Nanostructures by Anisotropic Loss(Wiley, 2013) Li, Z.; Gokkavas, M.; Özbay, EkmelPlanar chiral structures may provide asymmetric transmission for circularly polarized optical waves at normal incidence if the structures are anisotropic and lossy, but the role of loss has not yet been clarified. Here, a typical planar chiral structure is studied, and the mechanism of asymmetric transmission is analyzed. It is demonstrated for the first time that asymmetric transmission can be manipulated by changing the anisotropy of loss.Item Open Access Optically implemented broadband blueshift switch in the terahertz regime(American Physical Society, 2011-01-18) Shen, N. H.; Massaouti, M.; Gokkavas, M.; Manceau J. M.; Özbay, Ekmel; Kafesaki, M.; Koschny, T.; Tzortzakis, S.; Soukoulis, C. M.We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.Item Open Access A planar metamaterial with dual-band double-negative response at EHF(IEEE, 2009-10-02) Gundogdu, T. F.; Guven, K.; Gokkavas, M.; Soukoulis, C. M.; Özbay, EkmelWe report the fabrication and electromagnetic characterization of a planar composite metamaterial (CMM) that is designed to achieve dual-frequency double-negative response at the lower end of the extremely high-frequency (EHF) band. The CMM is based on cut wire pairs and continuous wire elements. Dual-frequency operation is obtained by employing cut wire pairs of two different lengths within the unit cell of the CMM. The magnetic response of the cut wire pairs and the left-handed transmission band of the CMM are demonstrated by experiment and numerical simulations. It is found that the combined electric response of the dual-band CMM is complicated and imposes certain restrictions to the structure design in achieving true left-handed response at both designated frequencies.Item Open Access Solar-blind AlxGa1-xN-based avalanche photodiodes(American Institute of Physics, 2005) Tut, T.; Butun, S.; Butun, B.; Gokkavas, M.; Yu, H. B.; Özbay, EkmelWe report the Metalorganic Chemical Vapor Deposition (MOCVD) growth, fabrication, and characterization of solar blind AlxGa1-xN/GaN-based avalanche photodiodes. The photocurrent voltage characteristics indicate a reproducible avalanche gain higher than 25 at a 72 V applied reverse bias. Under a 25 V reverse bias voltage, the 100 mu m diameter devices had a maximum quantum efficiency of 55% and a peak responsivity of 0.11 A/W at 254 nm, and a NEP of 1.89x10(-16) W/Hz(1/2).Item Open Access Ultra-miniature dual-band antenna based on subwavelength resonators on LiNbO3 substrate(IET, 2018) Serebryannikov, A. E.; Gokkavas, M.; Gundogdu, F. T.; Vandenbosch, G. A. E.; Vasylchenko, A.; Özbay, EkmelThe common effect of subwavelength resonators and a high-permittivity lithium niobate substrate is used for deep-subwavelength miniaturization of a dual-band S/C-band monopole antenna. The resulting size is just about 1/18th of a wavelength for the lower band.