Browsing by Author "Butun, B."

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Open Access
97 percent light absorption in an ultrabroadband frequency range utilizing an ultrathin metal layer: randomly oriented, densely packed dielectric nanowires as an excellent light trapping scaffold
(Royal Society of Chemistry, 2017) Ghobadi, A.; Dereshgi, S. A.; Hajian, H.; Birant, G.; Butun, B.; Bek, A.; Özbay, Ekmel
In this paper, we propose a facile and large scale compatible design to obtain perfect ultrabroadband light absorption using metal-dielectric core-shell nanowires. The design consists of atomic layer deposited (ALD) Pt metal uniformly wrapped around hydrothermally grown titanium dioxide (TiO2) nanowires. It is found that the randomly oriented dense TiO2 nanowires can impose excellent light trapping properties where the existence of an ultrathin Pt layer (with a thickness of 10 nm) can absorb the light in an ultrabroadband frequency range with an amount near unity. Throughout this study, we first investigate the formation of resonant modes in the metallic nanowires. Our findings prove that a nanowire structure can support multiple longitudinal localized surface plasmons (LSPs) along its axis together with transverse resonance modes. Our investigations showed that the spectral position of these resonance peaks can be tuned with the length, radius, and orientation of the nanowire. Therefore, TiO2 random nanowires can contain all of these features simultaneously in which the superposition of responses for these different geometries leads to a flat perfect light absorption. The obtained results demonstrate that taking unique advantages of the ALD method, together with excellent light trapping of chemically synthesized nanowires, a perfect, bifacial, wide angle, and large scale compatible absorber can be made where an excellent performance is achieved while using less materials.
Open Access
Disordered nanohole patterns in metal-insulator multilayer for ultra-broadband light absorption: atomic layer deposition for lithography free highly repeatable large scale multilayer growth
(Nature Publishing Group, 2017) Ghobadi, A.; Hajian, H.; Dereshgi, S. A.; Bozok, B.; Butun, B.; Özbay, Ekmel
In this paper, we demonstrate a facile, lithography free, and large scale compatible fabrication route to synthesize an ultra-broadband wide angle perfect absorber based on metal-insulator-metal-insulator (MIMI) stack design. We first conduct a simulation and theoretical modeling approach to study the impact of different geometries in overall stack absorption. Then, a Pt-Al2O3 multilayer is fabricated using a single atomic layer deposition (ALD) step that offers high repeatability and simplicity in the fabrication step. In the best case, we get an absorption bandwidth (BW) of 600 nm covering a range of 400 nm-1000 nm. A substantial improvement in the absorption BW is attained by incorporating a plasmonic design into the middle Pt layer. Our characterization results demonstrate that the best configuration can have absorption over 0.9 covering a wavelength span of 400 nm-1490 nm with a BW that is 1.8 times broader compared to that of planar design. On the other side, the proposed structure retains its absorption high at angles as wide as 70°. The results presented here can serve as a beacon for future performance enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity. © 2017 The Author(s).
Open Access
Fano resonances in THz metamaterials composed of continuous metallic wires and split ring resonators
(Optical Society of America, 2014) Li, Z.; Cakmakyapan, S.; Butun, B.; Daskalaki, C.; Tzortzakis, S.; Yang, X.; Özbay, Ekmel
We demonstrate theoretically and experimentally that Fano resonances can be obtained in terahertz metamaterials that are composed of periodic continuous metallic wires dressed with periodic split ring resonators. An asymmetric Fano lineshape has been found in a narrow frequency range of the transmission curve. By using a transmission line combined with lumped element model, we are able to not only fit the transmission spectra of Fano resonance which is attributed to the coupling and interference between the transmission continuum of continuous metallic wires and the bright resonant mode of split ring resonators, but also reveal the capacitance change of the split ring resonators induced frequency shift of the Fano resonance. Therefore, the proposed theoretical model shows more capabilities than conventional coupled oscillator model in the design of Fano structures. The effective parameters of group refractive index of the Fano structure are retrieved, and a large group index more than 800 is obtained at the Fano resonance, which could be used for slow light devices. (C) 2014 Optical Society of America
Open Access
High performance AlxGa1-xN-based avalanche photodiodes
(Elsevier BV, 2007-10) Tut, T.; Butun, B.; Gokkavas, M.; Özbay, Ekmel
We 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.
Open Access
High-performance visible-blind GaN-based p-i-n photodetectors
(AIP Publishing LLC, 2008) Butun, B.; Tut, T.; Ulker, E.; Yelboga, T.; Özbay, Ekmel
We report high performance visible-blind GaN-based p-i-n photodetectors grown by metal-organic chemical vapor deposition on c -plane sapphire substrates. The dark current of the 200 μm diameter devices was measured to be lower than 20 pA for bias voltages up to 5 V. The breakdown voltages were higher than 120 V. The responsivity of the photodetectors was ∼0.23 AW at 356 nm under 5 V bias. The ultraviolet-visible rejection ratio was 6.7× 103 for wavelengths longer than 400 nm.
Open Access
High-speed 1.55 μm operation of low-temperature-grown GaAs-based resonant-cavity-enhanced p-i-n photodiodes
(American Institute of Physics, 2004) Butun, B.; Bıyıklı, Necmi; Kimukin, I.; Aytur, O.; Özbay, Ekmel; Postigo, P. A.; Silveira, J. P.; Alija, A. R.
The 1.55 μm high-speed operation of GaAs-based p-i-n photodiodes was demonstrated and their design, growth and fabrication were discussed. A resonant-cavity-detector structure was used to selectively enhance the photoresponse at 1.55 μm. The bottom mirror of the resonant cavity was formed by a highly reflecting 15-pair GaAs/AlAs Bragg mirror and molecular-beam epitaxy was used for wafer growth. It was found that the fabricated devices exhibited a resonance of around 1548 nm and an enhancement factor of 7.5 was achieved when compared to the efficiency of a single-pass detector.
Open Access
A hybrid light source with integrated inorganic light-emitting diode and organic polymer distributed feedback grating
(Institute of Physics Publishing Ltd., 2008) Butun, B.; Aydin, K.; Ulker, E.; Cheylan, S.; Badenes, G.; Forster, M.; Scherf, U.; Özbay, Ekmel
We report a compact light source that incorporates a semiconductor light-emitting diode, nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugated polymer film. With this hybrid structure, we transferred electrically generated 390 nm ultraviolet light to an organic polymer via optical pumping and out-couple green luminescence to air through a second-order DFB grating. We demonstrate the feasibility of electrically driven, hybrid, compact light-emitting devices and lasers in the visible range.
Open Access
Hybrid plasmon-phonon polariton bands in graphene-hexagonal boron nitride metamaterials [Invited]
(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Dereshgi, S. A.; Butun, B.; Özbay, Ekmel
We theoretically investigate mid-infrared electromagnetic wave propagation in multilayered graphene-hexagonal boron nitride (hBN) metamaterials. Hexagonal boron nitride is a natural hyperbolic material that supports highly dispersive phonon polariton modes in two Reststrahlen bands with different types of hyperbolicity. Due to the hybridization of surface plasmon polaritons of graphene and hyperbolic phonon polaritons of hBN, each isolated unit cell of the graphene-hBN metamaterial supports hybrid plasmon-phonon polaritons (HPPs). Through the investigation of band structure of the metamaterial we find that, due to the coupling between the HPPs supported by each unit cell, the graphene-hBN metamaterial can support HPP bands. The dispersion of these bands can be noticeably modified for different thicknesses of hBN layers, leading to the appearance of bands with considerably flat dispersions. Moreover, analysis of light transmission through the metamaterial reveals that this system is capable of supporting high-k propagating HPPs. This characteristic makes graphene-hBN metamaterials very promising candidates for the modification of the spontaneous emission of a quantum emitter, hyperlensing, negative refraction, and waveguiding. © 2017 Optical Society of America.
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InGaAs-based high-performance p-i-n photodiodes
(IEEE, 2002-03) Kimukin, I.; Bıyıklı, Necmi; Butun, B.; Aytur, O.; Ünlü, S. M.; Özbay, Ekmel
In this letter, we have designed, fabricated, and characterized high-speed and high efficiency InGaAs-based p-i-n photodetectors with a resonant cavity enhanced structure. The devices were fabricated by a microwave-compatible process. By using a postprocess recess etch, we tuned the resonance wavelength from 1605 to 1558 nm while keeping the peak efficiencies above 60%. The maximum quantum efficiency was 66% at 1572 nm which was in good agreement with our theoretical calculations. The photodiode had a linear response up to 6-mW optical power, where we obtained 5-mA photocurrent at 3-V reverse bias. The photodetector had a temporal response of 16 ps at 7-V bias. After system response deconvolution, the 3-dB bandwidth of the device was 31 GHz, which corresponds to a bandwidth-efficiency product of 20 GHz.
Open Access
InGaN green light emitting diodes with deposited nanoparticles
(Elsevier BV, 2007) Butun, B.; Cesario J.; Enoch, S.; Quidant, R.; Özbay, Ekmel
We grew an InGaN/GaN-based light-emitting diode (LED) wafer by metal-organic chemical vapor deposition (MOCVD), fabricated devices by optical lithography, and successfully deposited ellipsoidal Ag nano-particles by way of e-beam lithography on top. The diodes exhibited good device performance, in which we expected an enhancement of the radiated intensity by the simulations and emission measurements. The obtained results showed the feasibility of plasmon-assisted LED emission enhancement.
Open Access
ITO-schottky photodiodes for high-performance detection in the UV-IR spectrum
(IEEE, 2004) Bıyıklı, Necmi; Kimukin, I.; Butun, B.; Aytür, O.; Özbay, Ekmel
High-performance vertically illuminated Schottky photodiodes with indium-tin-oxide (ITO) Schottky layers were designed, fabricated, and tested. Ternary and quarternary III-V material systems (AlGaN-GaN, AlGaAs-GaAs, InAlGaAs-InP, and InGaAsP-InP) were utilized for detection in the ultraviolet (UV) (λ < 400 nm), near-IR (λ ∼ 850 nm), and IR (λ ∼ 1550 nm) spectrum. The material properties of thin ITO films were characterized. Using resonant-cavity-enhanced (RCE) detector structures, improved efficiency performance was achieved. Current-voltage, spectral responsivity, and high-speed measurements were carried out on the fabricated ITO-Schottky devices. The device performances obtained with different material systems are compared.
Open Access
Large-area, cost-efective, ultrabroadband perfect absorber utilizing manganese in metal-insulator-metal structure
(Nature Publishing Group, 2018) Aalizadeh, M.; Khavasi, A.; Butun, B.; Özbay, Ekmel
Achieving broadband absorption has been a topic of intensive research over the last decade. However, the costly and time consuming stage of lithography has always been a barrier for the large-area and mass production of absorbers. In this work, we designed, fabricated, and characterized a lithography-free, large-area compatible, omni-directional, ultra-broadband absorber that consists of the simplest geometrical configuration for absorbers: Metal-Insulator-Metal (MIM). We introduced and utilized Manganese (Mn) for the first time as a very promising metal for broadband absorption applications. We optimized the structure step-by-step and compared Mn against the other best candidates introduced so far in broadband absorption structures and showed the better performance of Mn compared to them. It also has the advantage of being cheaper compared to metals like gold that has been utilized in many patterned broadband absorbers. We also presented the circuit model of the structure. We experimentally achieved over 94 percent average absorption in the range of 400-900 nm (visible and above) and we obtained absorption as high as 99.6 percent at the wavelength of 626.4 nm. We also experimentally demonstrated that this structure retains broadband absorption for large angles up to 70 degrees.
Open Access
Metal-semiconductor-metal photodetector on as-deposited TiO2 thin films on sapphire substrate
(AIP Publishing LLC, 2013-03-06) Caliskan, D.; Butun, B.; Ozcan, S.; Özbay, Ekmel
TiO2 thin films are prepared on c-plane sapphire substrates by the RF magnetron sputtering method. The performance of the Pt contact metal–semiconductor–metal (MSM) photodetector fabricated on as-deposited films is studied. The dark current density and the responsivity obtained were 1.57 × 10−9 A/cm2 at 5 V bias and 1.73 A/W at 50 V bias, respectively. Breakdown is not observed up to 50 V bias. Rise and fall times for the photocurrent were 7 and 3 s, respectively. Our results show that high quality MSM photodetectors can be fabricated without high temperature and complicated fabrication steps.
Open Access
Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals
(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Butun, B.; Özbay, Ekmel
In this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design. © 2017 Optical Society of America.
Open Access
Silicon nanowire network metal-semiconductor-metal photodetectors
(AIP Publishing LLC., 2013-08-23) Mulazimoglu, E.; Coskun, S.; Gunoven, M.; Butun, B.; Özbay, Ekmel; Turan, R.; Unalan, H. E.
We report on the fabrication and characterization of solution-processed, highly flexible, silicon nanowire network based metal-semiconductor-metal photodetectors. Both the active part of the device and the electrodes are made of nanowire networks that provide both flexibility and transparency. Fabricated photodetectors showed a fast dynamic response, 0.43 ms for the rise and 0.58 ms for the fall-time, with a decent on/off ratio of 20. The effect of nanowire-density on transmittance and light on/off behavior were both investigated. Flexible photodetectors, on the other hand, were fabricated on polyethyleneterephthalate substrates and showed similar photodetector characteristics upon bending down to a radius of 1 cm.
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, Ekmel
We 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).
Open Access
Spectral response modification of TiO2 MSM photodetector with an LSPR filter
(Optical Society of America, 2014) Caliskan, D.; Butun, B.; Ozcan, S.; Özbay, Ekmel
We fabricated UVB filtered TiO2 MSM photodetectors by the localized surface plasmon resonance effect. A plasmonic filter structure was designed using FDTD simulations. Final filter structure was fabricated with Al nano-cylinders with a 70 nm radius 180 nm period on 360 nm SiO2 film. The spectral response of the TiO2 MSM photodetector was modified and the UVB response was reduced by approx. 60% with an LSPR structure, resulting in a peak responsivity shift of more than 40 nm. To our knowledge, this is the first published result for the spectral response modification of TiO2 photodetectors with LSPR technique. © 2014 Optical Society of America.
Open Access
Ultra-broadband asymmetric light transmission and absorption through the use of metal free multilayer capped dielectric microsphere resonator
(Nature Publishing Group, 2017) Ghobadi, A.; Dereshgi, S. A.; Butun, B.; Özbay, Ekmel
In this paper, we propose a simple design with an excellent performance to obtain high contrast in transmission asymmetry based on dielectric microspheres. Initially, we scrutinize the impact of the sphere radius on forward and backward transmissions. Afterward, by introducing a capping layer on top of the sphere, transmission response for the backward illuminated light will be blocked. In the next step, in order to replace the reflecting metal cap with a metal free absorbing design, we adopt a modeling approach based on the transfer matrix method (TMM) to explore an ideal material to achieve metal free perfect absorption in a multilayer configuration of material-insulator-material-insulator (MIMI). As a result of our investigations, it is found that Titanium Nitride (TiN) is an excellent alternative to replace metal in a MIMI multilayer stack. Setting this stack as the top capping coating, we obtain a high contrast between forward and backward light transmission where in an ultra-broadband range of 400 nm-1000 nm, forward transmission is above 0.85 while its backward response stays below 0.2. Moreover, due to the existence of multilayer stack, a high asymmetry is also observed for absorption profiles. This design has a relatively simple and large scale compatible fabrication route. © 2017 The Author(s).
Open Access
Ultra-broadband, lithography-free, and large-scale compatible perfect absorbers: the optimum choice of metal layers in metal-insulator multilayer stacks
(Nature Publishing Group, 2017) Dereshgi, S. A.; Ghobadi, A.; Hajian, H.; Butun, B.; Özbay, Ekmel
We report ultra-broadband perfect absorbers for visible and near-infrared applications that are based on multilayers of metal-insulator (MI) stacks fabricated employing straightforward layer deposition techniques and are, therefore, lithography-free and large-scale compatible. We scrutinize the impact of different physical parameters of an MIMI absorber structure with analysis of each contributing metal layer. After obtaining the optimal design parameters (i.e. material selection and their thicknesses) with both simulation and numerical analysis (Transfer Matrix Method) methods, an experimental sample is fabricated and characterized. Our fabricated MIMI absorber consists of an optically thick tungsten (W) back reflector layer followed by 80 nm aluminum oxide (Al2O3), 10 nm titanium (Ti), and finally another 80 nm Al2O3. The experimental results demonstrate over 90 percent absorption between 400 nm and 1640 nm wavelengths that is optimized for ultra-broadband absorption in MIMI structures. Moreover, the impedance matching method with free-space is used to shed light on the metallic layer selection process. © 2017 The Author(s).
Open Access
Ultra-broadband, wide angle absorber utilizing metal insulator multilayers stack with a multi-thickness metal surface texture
(Nature Publishing Group, 2017) Ghobadi, A.; Dereshgi, S. A.; Hajian, H.; Bozok, B.; Butun, B.; Özbay, Ekmel
In this paper, we propose a facile route to fabricate a metal insulator multilayer stack to obtain ultra-broadband, wide angle behavior from the structure. The absorber, which covers near infrared (NIR) and visible (Vis) ranges, consists of a metal-insulator-metal-insulator (MIMI) multilayer where the middle metal layer has a variant thickness. It is found that this non-uniform thickness of the metal provides us with an absorption that is much broader compared to planar architecture. In the non-uniform case, each thickness is responsible for a specific wavelength range where the overall absorption is the superposition of these resonant responses and consequently a broad, perfect light absorption is attained. We first numerically examine the impact of different geometries on the overall light absorption property of the multilayer design. Afterward, we fabricate the designs and characterize them to experimentally verify our numerical findings. Characterizations show a good agreement with numerical results where the optimum absorption bandwidth for planar design is found to be 620 nm (380 nm-1000 nm) and it is significantly boosted to an amount of 1060 nm (350 nm-1410 nm) for multi-thickness case. © 2017 The Author(s).