Browsing by Author "Soydan, Mahmut Can"

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Open Access
All ceramic-based metal-free ultra-broadband perfect absorber
(Springer, 2019-06) Soydan, Mahmut Can; Ghobadi, Amir; Yıldırım, Deniz Umut; Ertürk, Vakur Behçet; Özbay, Ekmel
In this paper, we scrutinize unprecedented potential of transition metal carbides (TMCs) and nitrides (TMNs) for realization of light perfect absorption in an ultra-broad frequency range encompassing all of the visible (Vis) and near infrared (NIR) regions. For this purpose, two different configurations which are planar and trapezoidal array are employed. To gain insight on the condition for light perfect absorption, a systematic modeling approach based on transfer matrix method (TMM) is firstly utilized. Our modeling findings prove that the permittivity data of these TMCs and TMNs are closely matched with the ideal data. Thus, they can have stronger and broader absorption behavior compared to metals. Besides, these ceramic materials are preferred to metals due to the fact that they have better thermal properties and higher durability against erosion and oxidation than metals. This could provide the opportunity for design of highly efficient light harvesting systems with long-term stability. Numerical simulations are conducted to optimize the device optical performance for each of the proposed carbides and nitrides. Our findings reveal that these ceramic coatings have the broadest absorption response compared to all lossy and plasmonic metals. In planar configuration, titanium carbide (TiC) has the largest absorption bandwidth (BW) where an absorption above 0.9 is retained over a broad wavelength range of 405–1495 nm. In trapezoid architecture, vanadium nitride (VN) shows the widest BW covering a range from 300 to 2500 nm. The results of this study can serve as a beacon for the design of future high-performance energy conversion devices including solar vapor generation and thermal photovoltaics where both optical and thermal requirements can be satisfied.
Open Access
Colorimetric and near-absolute polarization-insensitive refractive-index sensing in all-dielectric guided-mode resonance based metasurface
(American Chemical Society, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Gökbayrak, Murat; Toprak, Ahmet; Bütün, Bayram; Özbay, Ekmel
Colorimetric detection of target molecules with insensitivity to incident-light polarization has attracted considerable attention in recent years. This resulted from the ability to provide rapid output and reduced assay times as a result of color changes upon altering the environment that are easily distinguishable by the naked eye. In this paper, we propose a highly sensitive refractive-index sensor, utilizing the excitation of guided modes of a novel two-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guided-mode resonances with an ultranarrow linewidth (full width at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high figure of merit of 190. The fabricated devices with 30 nm aluminum oxide and zinc oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highly sensitive, almost lossless, and compact molecular diagnostics in the field of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring.
Open Access
Correlation-based study of FEA and IR thermography to reveal the 2DEG temperature of a multi-fingered high-power GaN HEMT
(AIP Publishing LLC, 2022-02-23) Koçer, Hasan; Aras, Yunus Erdem; Soydan, Mahmut Can; Butun, Bayram; Özbay, Ekmel; Durna, Yılmaz
High electron mobility transistors (HEMTs) based on gallium nitride (GaN) with a wide range of application potentials need to be rigorously examined for reliability to take advantage of their intrinsically extraordinary properties. The most vital parameter of the reliability, the hotspot, or Tmax, resides in the two-dimensional electron gas (2DEG) temperature profile inside the device where optical access is often restricted. The device surface temperature can be measured by widespread IR thermography with the limitation of diffraction-based IR transmission losses. However, Tmax on the sub-surface cannot be reached thermographically. Although finite element analysis (FEA)-based thermal simulations can easily reveal the 2DEG temperature profile, accuracy is tightly dependent on the realistic modeling of material/structure parameters. Because these parameters are rather sensitive to fabrication and processing, it is quite difficult to specify them accurately. To overcome these drawbacks, a method integrating both IR thermography and FEA thermal analysis is demonstrated on a fabricated high-power 40 × 360 μm packaged GaN HEMT as a proof-of-concept. Utilizing the simulation and measurement temperature profiles, a correlation algorithm is developed so that accuracy of the FEA thermal simulation is improved by calibrating the parameters specific to fabrication/process conditions by thermographic measurement. Then, it is quantitatively shown that the proposed method is able to find the 2DEG temperature profile and Tmax with an accuracy that best suits the intrinsic and extrinsic characteristics of the device under test. The method sheds light on GaN reliability engineering by providing a feasible and reliable alternative to realistically reveal hotspot information for device lifetime assessments.
Open Access
Deep subwavelength light confinement in disordered bismuth nanorods as a linearly thermal‐tunable metamaterial
(Wiley-VCH Verlag, 2020) Soydan, Mahmut Can; Ghobadi, Amir; Yıldırım, Deniz Umut; Ertürk, Vakur Behçet; Özbay, Ekmel
Materials with a tunable optical response that can be controllably tailored using external stimuli excitation have undergone considerable research effort for the development of active optical devices, such as thermo‐optical modulators. Although bismuth (Bi) nanodots, embedded into glass matrices, have been proven to have a thermo‐optical response, the recyclability of the structure in solid–liquid phase transitions is a major challenge. Herein, a facile and lithography‐free fabrication method is proposed to realize densely packed stand‐alone Bi nanorods (NRs), with deep subwavelength gaps and a resonance at the midinfrared range (λ ≅ 4.462 μm). Owing to these ultrasmall gaps that support lossy Mie‐like resonances, strong field confinement is achieved, and the resonance wavelength exhibits great sensitivity to temperature, as the thermal sensitivity reaches as high as 1.0316 nm °C−1. This operation is conducted in the moderate temperature interval of 25–85 °C, which is far from the melting point of Bi. Overall, our simple, robust, and high‐performance device is highly promising for realizing optical switches, thermo‐optic modulators, and infrared camouflage.
Open Access
Design and analysis of metamaterial based perfect absorbers
(2019-08) Soydan, Mahmut Can
Subwavelength light absorbers have an enormous potential on applications such as photodetection, optoelectronics, solar cells and sensing. Scaling down the device dimensions provides artificial and advanced properties. That's why achieving higher performance devices with smaller sizes is the main trend in semiconductor technology. Design of an electromagnetic wave absorber has two dominant factors on the performance and spectral operation region: material selection and design configuration. Perfect light absorbers require an absorbing layer, such as a metal, semiconductor or any type of absorbing material, to achieve light confinement. While conventional metals have been mostly the primary choice in designs, there are various material types other than them which can have advantageous thermal properties in fabrication, integration or tunability besides having lossy nature. Although conventional metals are great absorbing materials due to lossy natures, they are not durable against erosion and oxidation. In the first work, we scrutinize unprecedented potential of transition metal carbides (TMCs) and nitrides (TMNs) as optional materials to conventional metals, for realization of light perfect absorption in an ultra-broad frequency range encompassing all of the visible (Vis) and near infrared (NIR) regions. To gain insight on the condition for light perfect absorption, a systematic modeling approach based on transfer matrix method (TMM) is firstly utilized. Our modeling findings prove that the permittivity data of these TMCs and TMNs are closely matched with the ideal data. Thus, they can have stronger and broader absorption behavior compared to metals. Besides, these ceramic materials are preferred to metals due to the fact that they have better thermal properties and higher durability against erosion and oxidation than metals. This could provide the opportunity for design of highly e cient light harvesting systems with long-term stability. Two different configurations which are planar and trapezoidal arrays are employed. Numerical simulations are conducted to optimize the device optical performance for each of the proposed carbides and nitrides. Our findings reveal that these ceramic coatings have the broadest absorption response compared to all lossy and plasmonic metals. In planar configuration, titanium carbide (TiC) has the largest absorption bandwidth (BW) where an absorption above 0.9 is retained over a broad wavelength range of 405 nm-1495 nm. In trapezoid architecture, vanadium nitride (VN) shows the widest BW covering a range from 300 nm to 2500 nm. The results of this study can serve as a beacon for the design of future high performance energy conversion devices including solar vapor generation and thermal photovoltaics where both optical and thermal requirements can be satisfied. Majority of existing designs necessitate a lithography-step during the fabrication, which hinders the repeatability, upscaling and large-scale compatibility of these designs. In the second work, we designed, fabricated and characterized a lithography free, double functional single Bismuth (Bi) metal nanostructure for ultra-broadband absorption in the visible and near-infrared, and narrowband response with ultra-high refractive-index sensitivity in mid-infrared (MIR) range. The superior permittivity data of Bi over conventional metals is comprehensively analyzed and explained using systematic modeling approaches based on TMM and Bruggeman's effective medium theory (EMT). To achieve a large scale fabrication of the design in a lithography-free route, oblique-angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It has been shown that this fabrication technique can provide a bottom-up approach to control the length and spacing of the design. Our characterization findings reveal a broadband absorption above 0.8 in Vis and NIR, and a narrowband absorption centered around 6.54 m. Due to densely packed architecture of the Bi nanostructures and its extraordinary permittivity response, they can provide strong field confinement in their ultra-small gaps and this could be utilized for sensing application. An ultrahigh sensitivity of 2.151 m/refractive-index-unit (RIU) is acquired for this Bi nanostructured absorber, which is, to the best of our knowledge, the experimentally attained highest sensitivity so far. The simple and large scale compatible fabrication route of the design together with extraordinary optical response of Bi coating, makes this design promising for many optoelectronic and sensing applications.
Open Access
Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface
(American Chemical Society, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, Ekmel
Precise control and stabilization of the operating temperature environment of spacecraft and satellites during their life cycle is of paramount importance to increase device reliabilities and reduce the thermomechanical constraints. Optical solar reflectors are the physical interface between the spacecraft and space, and they are broadband mirrors for the solar spectrum, while having strong thermal emission in the mid-infrared part of the electromagnetic spectrum. Strong light–matter interactions in metamaterials and metasurfaces offer significant advantages compared to the conventional methods in performance, weight, launch, and assembly costs. However, the fabrication complexity of these metastructures due to necessitating lithography hinders their upscaling, reproducibility, large-area compatibility, and mass production. In this regard, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design a metasurface based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal radiation to broad plasmonic resonances and, consequently, an experimental emissivity of 0.968, in a very wide range from 2.5 to 25 μm. In the solar spectrum, the low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. Our proposed design with high throughput, robustness, low cost, and high performance, therefore, shows great promise not only for space missions, but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling.
Open Access
Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface for the radiative cooling of spacecraft
(SPIE, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, Ekmel
Optical Solar Reflectors (OSRs) form the physical interface between the spacecraft and space and they are essential for the stabilization and uniform distribution of temperature throughout the spacecraft. OSRs need to possess a spectrally selective response of broadband and perfect electromagnetic wave absorption in the thermal-infrared spectral range, while strongly reflecting the solar energy input. In this work, we experimentally show that disordered and densely packed ITO nanorod forests can be used as an excellent top-layer metasurface in a metal-insulator-oxide cavity configuration, and a thermal-emissivity of 0.97 is experimentally realized in the spectral range from 2.5 to 25 μm. The low-loss dielectric response of ITO in the solar spectrum, from 300 nm to 2.5 μm range limited the solar absorptivity to an experimental value of 0.167. These make our proposed design highly promising for its application in space missions due to combining high throughput, robustness, low cost with ultra-high performance.
Open Access
Disordered plasmonic nanocavity enhanced quantum dot emission
(Institute of Physics, 2023-08-31) Kosger, Ali Cahit; Ghobadi, Amir; Omam, Zahra Rahimian; Soydan, Mahmut Can; Ulusoy Ghobadi, Türkan Gamze; Özbay, Ekmel
In this paper, a large-scale compatible plasmonic nanocavity design platform is utilized to achieve a nearly order of magnitude photoluminescence (PL) enhancement. The proposed design is made of multi-sized/multi-spacing gold (Au) nanounits that are uniformly wrapped with a thin aluminum oxide (Al2O3) layer, as a foreign host to form a metal-insulator-semiconductor cavity, as they are coated with semiconductor quantum dots (QDs). Our numerical and experimental data demonstrate that, in an optimal insulator layer thickness, the simultaneous formation of broadband Fabry-Perot resonances and plasmonic hot spots leads to enhanced light absorption within the QD unit. This improvement in absorption response leads to the PL enhancement of QDs. This work demonstrates the potential and effectiveness of a random plasmonic nanocavities host in the realization of lithography-free efficient emitters. © 2023 IOP Publishing Ltd
Open Access
Exceptional adaptable MWIR thermal emission for ordinary objects covered with thin VO2 film
(Elsevier Ltd, 2021-01-25) Durna, Yılmaz; Kocer, Hasan; Aydın, Koray; Cakir, Mehmet Cihan; Soydan, Mahmut Can; Odabasi, Oguz; Işık, Halil; Ozbay, Ekmel
Monotonous thermal radiation emitted from an ordinary object can be brought into a dynamic and versatile form that can be shaped according to the application area with the ingenious design of the surface coatings. Building the coatings with phase change materials provides exceptional and surprising properties in terms of tunability, adaptability and multifunctionality. In this paper, we investigate the thermal radiation properties in the MWIR band through comprehensive thermographic measurements and theoretical methods while a thin (similar to 90 nm thick) vanadium dioxide (VO2) layer on the sapphire substrate (VO2 thin film) is placed on different ordinary objects under heating/cooling conditions. It is indicated that the emission of the metal object (low emittance) can be boosted and the emission of the blackbody-like object (high emittance) can be suppressed at the relevant temperatures. The thermal emission of the objects covered with thin VO2 film at high temperatures (>75 degrees C) is determined by only the VO2 thin film, since the VO2 layer is completely metallized and the MWIR radiation of the underlying object is masked. When the actual temperature of the object behaving like a blackbody rises up to 95 degrees C, the temperature detected in the MWIR thermal camera is reduced by more than 20% to approx. 75 degrees C due to the VO2 thin film on this object, providing thermal camouflage. It is experimentally and theoretically revealed that the underlying physical mechanism on these strange results is associated with the drastic change in the infrared optical parameters of the VO2 as a result of the applied temperature. (C) 2020 Elsevier Ltd. All rights reserved.
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Geç kalmış bir göç hikayesi
(Bilkent University, 2014) Günendi, Sezen; Uysal, Ratipcan; Soydan, Mahmut Can; Aras, Yunus Erdem; Kostak, Gürol
Open Access
Lithography-free planar band-pass reflective color filter using a series connection of cavities
(Nature Publishing Group, 2019-01) Ghobadi, Amir; Hajian, Hodjat; Soydan, Mahmut Can; Bütün, Bayram; Özbay, Ekmel
In this article, a lithography-free multilayer based color flter is realized using a proper series connection of two cavities that shows relatively high efciency, high color purity, and a wide view angle. The proposed structure is a metal-insulator-metal-insulator-semiconductor (MIMIS) design. To optimize the device performance, at the frst step, transfer matrix method (TMM) modeling is utilized to fnd the right choices of materials for each layer. Simulations are carried out later on to optimize the geometries of the layers to obtain our desired colors. Finally, the optimized devices are fabricated and experimentally characterized to evaluate our modelling fndings. The characterization results of the fabricated samples prove the successful formation of efcient and wide view angle color flters. Unlike previously reported FP based designs that act as a band-stop flter in refection mode (absorbing a narrow frequency range and refecting the rest of the spectrum), this design generates a specifc color by refecting a narrow spectral range and absorbing the rest of the spectrum. The fndings of this work can be extended to other multilayer structures where an efcient connection of cavities in a tandem scheme can propose functionalities that cannot be realized with conventional FP resonators.
Open Access
Lithography-Free random bismuth nanostructures for full solar spectrum harvesting and mid-infrared sensing
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2020) Soydan, Mahmut Can; Ghobadi, Amir; Yıldırım, Deniz Umut; Duman, E.; Bek, A.; Ertürk, Vakur Behçet; Özbay, Ekmel
A lithography‐free, double‐functional single bismuth (Bi) metal nanostructure is designed, fabricated, and characterized for ultrabroadband absorption in the visible (vis) and near‐infrared (NIR) ranges, and for a narrowband response with ultrahigh refractive index sensitivity in the mid‐infrared (MIR) range. To achieve a large‐scale fabrication of the design in a lithography‐free route, the oblique‐angle deposition approach is used to obtain densely packed and randomly spaced/oriented Bi nanostructures. It is shown that this fabrication technique can provide a bottom‐up approach to controlling the length and spacing of the design. The characterization findings reveal a broadband absorbance above 0.8 in vis and NIR, and a narrowband absorbance centered around 6.54 µm. Dense architecture and extraordinary permittivity of Bi provide strong field confinement in ultrasmall gaps between nanostructures, and this can be utilized for a sensing application. An ultrahigh sensitivity of 2151 nm refractive‐index unit (RIU–1) is acquired, which is, as far as it is known, the experimentally highest sensitivity attained so far. The simple and large‐scale compatible fabrication route of the design together with the extraordinary optical response of Bi coating makes this design promising for many optoelectronic and sensing applications.
Open Access
One-way and near-absolute polarization insensitive near-perfect absorption by using an all-dielectric metasurface
(OSA - The Optical Society, 2020) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Serebryannikov, Andriy E.; Özbay, Ekmel
In this Letter, we numerically propose the one-way perfect absorption of near-infrared radiation in a tunable spectral range with high transmission in the neighboring spectral ranges. This functionality is obtained by using a two-dimensional, guided-mode resonance-based grating-waveguide metasurface that acts as a frequency-selective reflector, a spacer dielectric, and an absorbing oxide layer. Within the bandwidth of the excited guided-mode resonance excited at 1.82 µm (with a full-width at half-maximum of 19 nm), we confirmed perfect absorption when light was incident from one of the two opposite directions, whereas in the other direction, perfect reflection was observed. The forward-to-backward absorption ratio reached as high as 60, while the thickness of the entire structure was on the order of the operating wavelength. In addition to the spectral tunability of the excited resonances and their bandwidths, our proposed device supports transparency windows with 65% transmission in the adjacent frequency bands. Our 2D grating is also verified to enable near-absolute insensitivity to the polarization state of incident light. Geometrical parameter modification also gives our design great tunability, as we also designed a device with a 300 nm absorption/reflection linewidth.
Open Access
Strong light emission from a defective hexagonal boron nitride monolayer coupled to near-touching random plasmonic nanounits
(Optica, 2021-04) Eftekhari, Zeinab; Ghobadi, Amir; Soydan, Mahmut Can; Yıldırım, Deniz Umut; Cinel, Neval Ayşegül; Özbay, Ekmel
In this Letter, we demonstrate strong light emission from defective hexagonal boron nitride (hBN) defect centers upon their coupling with disorder near-touching plasmonic units. Based on numerical simulations and characterization results, the plasmonic design at thin layer thicknesses of 20 nm can provide above 2 orders of magnitude enhance ment in photoluminescence (PL) spectra. Moreover, this plasmonic platform shortens the luminescence lifetime of the emitters. The proposed design can be easily extended to other plasmonic-emitter combinations where strong light–matter interaction can be achieved using large-scale compatible routes.
Open Access
Strong light–matter interactions in Au plasmonic nanoantennas coupled with Prussian blue catalyst on BiVO4 for photoelectrochemical water splitting
(Wiley-VCH Verlag, 2020) Ulusoy-Ghobadi, Türkan Gamze; Ghobadi, Amir; Soydan, Mahmut Can; Vishlaghi, M. B.; Kaya, S.; Karadaş, Ferdi; Özbay, Ekmel
A facial and large‐scale compatible fabrication route is established, affording a high‐performance heterogeneous plasmonic‐based photoelectrode for water oxidation that incorporates a CoFe–Prussian blue analog (PBA) structure as the water oxidation catalytic center. For this purpose, an angled deposition of gold (Au) was used to selectively coat the tips of the bismuth vanadate (BiVO4) nanostructures, yielding Au‐capped BiVO4 (Au‐BiVO4). The formation of multiple size/dimension Au capping islands provides strong light–matter interactions at nanoscale dimensions. These plasmonic particles not only enhance light absorption in the bulk BiVO4 (through the excitation of Fabry–Perot (FP) modes) but also contribute to photocurrent generation through the injection of sub‐band‐gap hot electrons. To substantiate the activity of the photoanodes, the interfacial electron dynamics are significantly improved by using a PBA water oxidation catalyst (WOC) resulting in an Au‐BiVO4/PBA assembly. At 1.23 V (vs. RHE), the photocurrent value for a bare BiVO4 photoanode was obtained as 190 μA cm−2, whereas it was boosted to 295 μA cm−2 and 1800 μA cm−2 for Au‐BiVO4 and Au‐BiVO4/PBA, respectively. Our results suggest that this simple and facial synthetic approach paves the way for plasmonic‐based solar water splitting, in which a variety of common metals and semiconductors can be employed in conjunction with catalyst designs.
Open Access
Uncovering the non-radiative thermal characteristics of a passive radiative cooler under real operating conditions
(Institute of Physics Publishing Ltd., 2022-12-12) Koçer, Hasan; Durna, Yılmaz; Işık, Halil; Soydan, Mahmut Can; Khalichi, Bahram; Ghobadi, Amir; Kurt, H.; Özbay, Ekmel
Passive radiative cooling (PasRadCool), which emits thermal energy from objects to deep cold space through atmospheric transparency, offers complementary and alternative green energy solutions for passive cooling of buildings, clothing, and renewable energy harvesting. Depending on the spectral emissive/absorptive properties of the unit under test (UUT), radiative heat exchanges occur between the UUT, atmosphere, and sun, while at the same time non-radiative heat exchange occurs. The performance of the PasRadCool is determined by the combined thermal and thermodynamic effects of both exchange mechanisms. Although the non-radiative heat exchange, which consists of conductive and convective processes to the outer surfaces of the UUT and the surrounding air fluid, is very sensitive to environmental changes, the actual performance is not fully determined since this feature is considered statically in many studies. Herein, we propose a method that reveals the non-radiative thermal characteristics of the PasRadCool under real operating conditions. With a photonic radiative cooler structure, which we manufacture as a proof of concept, we perform nighttime field test measurements in varying non-radiative thermal conditions. The proposed method extracts the time-dependent non-radiative heat transfer coefficient of the UUT as accurately as possible. We also confirm that our experimental result shows good agreement with both numerical and analytical methods. The proposed approach, which highlights the realistic thermal management of PasRadCool, is not specific to the circumstances of our study and can be applied to all PasRadCool situations with different geometry, material, and environmental conditions.