Browsing by Subject "Electromagnetic wave absorption"

Now showing 1 - 15 of 15

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
Absorption enhancement of molecules in the weak plasmon-exciton coupling regime
(Optical Society of American (OSA), 2014) Balci, S.; Karademir, E.; Kocabas, C.; Aydınlı, Atilla
We report on the experimental and theoretical investigations of enhancing the optical absorption of organic molecules in the weak plasmon-exciton coupling regime. A metal-organic hybrid structure consisting of dye molecules embedded in the polymer matrix is placed in close vicinity to thin metal films. We have observed a transition from a weak coupling regime to a strong coupling one as the thickness of the metal layer increases. The results indicate that absorption of the self-assembled J-aggregate nanostructures can be increased in the weak plasmon-exciton coupling regime and strongly quenched in the strong coupling regime. A theoretical model based on the transfer-matrix method qualitatively confirms the experimental results obtained from polarization-dependent spectroscopic reflection measurements.
Open Access
CdSe/ZnS core-shell nanocrystal based scintillators for enhanced detection in UV
(IEEE, 2006) Demir, Hilmi Volkan; Soğancı, İbrahim Murat; Mutlugün, Evren
In this work, we present a novel scintillator that incorporates CdSe/ZnS core-shell nanocrystals to utilize both their high quantum fluorescence efficiency in the visible and high optical absorption in the UV for the purpose of facilitating UV imaging on a Si platform. Here we demonstrate enhancement in UV detection up to 800% with respect to the host polymer in the UV.
Open Access
Comparative study of thin film n-i-p a-Si: H solar cells to investigate the effect of absorber layer thickness on the plasmonic enhancement using gold nanoparticles
(Elsevier Ltd, 2015) Islam, K.; Chowdhury F.I.; Okyay, Ali Kemal; Nayfeh, A.
In this paper, the effect of gold nanoparticles on n-i-p a-Si:H solar cells with different intrinsic layer (i-layer) thicknesses has been studied. 100nm and 500nm i-layer based n-i-p a-Si:H solar cells were fabricated and colloidal gold (Au) nanoparticles dispersed in water-based solution were spin-coated on the top surface of the solar cells. The Au nanoparticles are of spherical shape and have 100nm diameter. Electrical and quantum efficiency measurements were carried out and the results show an increase in short-circuit current density (Jsc), efficiency and external quantum efficiency (EQE) with the incorporation of the nanoparticles on both cells. Jsc increases from 5.91mA/cm2 to 6.5mA/cm2 (~10% relative increase) and efficiency increases from 3.38% to 3.97% (~17.5% relative increase) for the 100nm i-layer solar cell after plasmonic enhancement whereas Jsc increases from 9.34mA/cm2 to 10.1mA/cm2 (~7.5% relative increase) and efficiency increases from 4.27% to 4.99% (~16.9% relative increase) for the 500nm i-layer cell. The results show that plasmonic enhancement is more effective in 100nm than 500nm i-layer thickness for a-Si:H solar cells. Moreover, the results are discussed in terms of light absorption and electron hole pair generation. © 2015 Elsevier Ltd.
Open Access
Experimental determination of the asbsorption cross-section and molar extinction coefficient of colloidal CdSe nanoplatelets
(American Chemical Society, 2015) Yeltik A.; Delikanlı, S.; Olutas M.; Keleştemur, Y.; Güzeltürk, B.; Demir, Hilmi Volkan
There has been a strong interest in solution-processed two-dimensional nanomaterials because of their great potential in optoelectronics. Here, the absorption cross-section and molar extinction coefficient of four and five monolayer thick colloidal CdSe nanoplatelets (NPLs) having various lateral sizes are reported. The absorption cross-section of these NPLs and their corresponding molar extinction coefficients are found to strongly depend on the lateral area. An excellent agreement is observed between the experimental results and the calculated values based on the small-particle light absorption model. With these optical properties, NPLs hold great promise for optoelectronic applications. © 2015 American Chemical Society.
Open Access
Highly monodisperse low-magnetization magnetite nanocubes as simultaneous T1–T2 MRI contrast agents
(Royal Society of Chemistry, 2015) Sharma, V. K.; Alipour, A.; Soran-Erdem, Z.; Aykut, Z. G.; Demir, Hilmi Volkan
We report the first study of highly monodisperse and crystalline iron oxide nanocubes with sub-nm controlled size distribution (9.7 ± 0.5 nm in size) that achieve simultaneous contrast enhancement in both T1- and T2-weighted magnetic resonance imaging (MRI). Here, we confirmed the magnetite structure of iron oxide nanocubes by X-ray diffraction (XRD), selected area electron diffraction (SAED) pattern, optical absorption and Fourier transformed infrared (FT-IR) spectra. These magnetite nanocubes exhibit superparamagnetic and paramagnetic behavior simultaneously by virtue of their finely controlled shape and size. The magnetic measurements reveal that the magnetic moment values are favorably much lower because of the small size and cubic shape of the nanoparticles, which results in an enhanced spin canting effect. As a proof-of-concept demonstration, we showed their potential as dual contrast agents for both T1- and T2-weighted MRI via phantom studies, in vivo imaging and relaxivity measurements. Therefore, these low-magnetization magnetite nanocubes, while being non-toxic and bio-compatible, hold great promise as excellent dual-mode T1 and T2 contrast agents for MRI. © 2014 The Royal Society of Chemistry.
Open Access
Lateral overgrowth of germanium for monolithic integration of germanium-on-insulator on silicon
(Elsevier, 2015) Hyung Nam J.; Alkis, S.; Nam, D.; Afshinmanesh F.; Shim J.; Park, J.; Brongersma, M.; Okyay, Ali Kemal; Kamins, T.I.; Saraswat, K.
A technique to locally grow germanium-on-insulator (GOI) structure on silicon (Si) platform is studied. On (001) Si wafer, silicon dioxide (SiO2) is thermally grown and patterned to define growth window for germanium (Ge). Crystalline Ge is grown via selective hetero-epitaxy, using SiO2 as growth mask. Lateral overgrowth of Ge crystal covers SiO2 surface and neighboring Ge crystals coalesce with each other. Therefore, single crystalline Ge sitting on insulator for GOI applications is achieved. Chemical mechanical polishing (CMP) is performed to planarize the GOI surface. Transmission electron microscopy (TEM) analysis, Raman spectroscopy, and time-resolved photoluminescence (TRPL) show high quality crystalline Ge sitting on SiO2. Optical response from metal-semiconductor-metal (MSM) photodetector shows good optical absorption at 850 nm and 1550 nm wavelength. © 2015 Elsevier B.V. All rights reserved.
Open Access
Morphological control of mesoporosity and nanoparticles within Co3O4-CuO electrospun nanofibers: quantum confinement and visible light photocatalysis performance
(American Chemical Society, 2017-09) Pradhan, A. C.; Uyar, Tamer
The one-dimensional (1D) mesoporous and interconnected nanoparticles (NPs) enriched composite Co3O4-CuO nanofibers (NFs) in the ratio Co:Cu = 1/4 (Co3O4-CuO NFs) composite have been synthesized by electrospinning and calcination of mixed polymeric template. Not merely the mesoporous composite Co3O4-CuO NFs but also single mesoporous Co3O4 NFs and CuO NFs have been produced for comparison. The choice of mixed polymer templates such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) for electrospinning is responsible for the formation of 1D mesoporous NFs. The HR-TEM result showed evolution of interconnected nanoparticles (NPs) and creation of mesoporosity in all electrospun NFs. The quantum confinement is due to NPs within NFs and has been proved by the surface-enhanced Raman scattering (SERS) study and the UV-vis-NRI diffuse reflectance spectra (DRS). The high intense photoluminescence (PL) spectra showing blue shift of all NFs also confirmed the quantum confinement phenomena. The lowering of PL spectrum after mixing of CuO in Co3O4 nanofibers framework (Co3O4-CuO NFs) proved CuO as an efficient visible light response low cost cocatalyst/charge separator. The red shifting of the band gap in composite Co3O4-CuO NFs is due to the internal charge transfer between Co2+ to Co3+ and Cu2+, proved by UV-vis absorption spectroscopy. Creation of oxygen vacancies by mixing of CuO and Co3O4 also prevents the electron-hole recombination and enhances the photocatalytic activity in composite Co3O4-CuO NFs. The photocurrent density, Mott-Schottky (MS), and electrochemical impedance spectroscopy (EIS) studies of all NFs favor the high photocatalytic performance. The mesoporous composite Co3O4-CuO NFs exhibits high photocatalytic activity toward phenolic compounds degradation as compared to the other two NFs (Co3O4 NFs and CuO NFs). The kinetic study of phenolic compounds followed first order rate equation. The high photocatalytic activity of composite Co3O4-CuO NFs is attributed to the formation of mesoporosity and interconnected NPs within NFs framework, quantum confinement, extended light absorption property, internal charge transfer, and effective photogenerated charge separations.
Open Access
Nanocrystal emitters for enhanced photovoltaics in UV
(IEEE, 2008-11) Mutlugün, Evren; Soğancı, İbrahim Murat; Demir, Hilmi Volkan
[No abstract available]
Open Access
Observation of gate-tunable coherent perfect absorption of terahertz radiation in graphene
(American Chemical Society, 2016) Kakenov, N.; Balci, O.; Takan, T.; Ozkan, V. A.; Altan, H.; Kocabas, C.
We report experimental observation of electrically tunable coherent perfect absorption (CPA) of terahertz (THz) radiation in graphene. We develop a reflection-type tunable THz cavity formed by a large-area graphene layer, a metallic reflective electrode, and an electrolytic medium in between. Ionic gating in the THz cavity allows us to tune the Fermi energy of graphene up to 1 eV and to achieve a critical coupling condition at 2.8 THz with absorption of 99%. With the enhanced THz absorption, we were able to measure the Fermi energy dependence of the transport scattering time of highly doped graphene. Furthermore, we demonstrate flexible active THz surfaces that yield large modulation in the THz reflectivity with low insertion losses. We anticipate that the gate-tunable CPA will lead to efficient active THz optoelectronics applications.
Open Access
Oxygen partial pressure dependence of magnetic, optical and magneto-optical properties of epitaxial cobalt-substituted SrTiO3 films
(OSA - The Optical Society, 2015) Onbaşli, M.C.; Goto, T.; Tang, A.; Pan, A.; Battal, E.; Okyay, Ali Kemal; Dionne G.F.; Ross, C.A.
Cobalt-substituted SrTiO3 films (SrTi0.70Co0.30O3-δ) were grown on SrTiO3 substrates using pulsed laser deposition under oxygen pressures ranging from 1 μTorr to 20 mTorr. The effect of oxygen pressure on structural, magnetic, optical, and magneto-optical properties of the films was investigated. The film grown at 3 μTorr has the highest Faraday rotation (FR) and magnetic saturation moment (Ms). Increasing oxygen pressure during growth reduced Ms, FR and optical absorption in the nearinfrared. This trend is attributed to decreasing Co2+ ion concentration and oxygen vacancy concentration with higher oxygen partial pressure during growth. © 2015 Optical Society of America.
Open Access
Radar cross-section study of cylindrical cavity-backed apertures with outer or inner material coating: the case of E-polarization
(IEEE, 1993) Çolak, D.; Nosich, A. I.; Altintaş, A.
A dual-series-based solution is obtained for the scattering of an E-polarized plane wave from a cavity-backed aperture which is formed by a slitted infinite circular cylinder coated with absorbing material. The material coating can be done on the inner or outer surface of the cylinder. For both cases, numerical results are presented for the radar cross section and comparisons are given for two different realistic absorbing materials. The radar cross-section results are also given for the aspect angle of the screen. Finally, the dependence of radar cross section on the thickness of the absorbing layer is presented. © 1993 IEEE
Open Access
The role of the interface in germanium quantum dots: when not only size matters for quantum confinement effects
(Royal Society of Chemistry, 2015) Cosentino, S.; Mio, A. M.; Barbagiovanni, E. G.; Raciti, R.; Bahariqushchi, R.; Miritello, M.; Nicotra, G.; Aydınlı, Atilla; Spinella, C.; Terrasi, A.; Mirabella, S.
Quantum confinement (QC) typically assumes a sharp interface between a nanostructure and its environment, leading to an abrupt change in the potential for confined electrons and holes. When the interface is not ideally sharp and clean, significant deviations from the QC rule appear and other parameters beyond the nanostructure size play a considerable role. In this work we elucidate the role of the interface on QC in Ge quantum dots (QDs) synthesized by rf-magnetron sputtering or plasma enhanced chemical vapor deposition (PECVD). Through a detailed electron energy loss spectroscopy (EELS) analysis we investigated the structural and chemical properties of QD interfaces. PECVD QDs exhibit a sharper interface compared to sputter ones, which also evidences a larger contribution of mixed Ge-oxide states. Such a difference strongly modifies the QC strength, as experimentally verified by light absorption spectroscopy. A large size-tuning of the optical bandgap and an increase in the oscillator strength occur when the interface is sharp. A spatially dependent effective mass (SPDEM) model is employed to account for the interface difference between Ge QDs, pointing out a larger reduction in the exciton effective mass in the sharper interface case. These results add new insights into the role of interfaces on confined systems, and open the route for reliable exploitation of QC effects. © The Royal Society of Chemistry.
Open Access
Theoretical modeling of the doping process in polypyrrole by calculating UV/Vis absorption spectra of neutral and charged oligomers
(2008) Okur, S.; Salzner, U.
Changes in absorption spectra during doping of oligopyrroles were investigated with time-dependent density functional theory on optimized structures of neutral, singly, and doubly charged pyrrole oligomers with up to 24 rings. In the absence of counterions, defects are delocalized. Counterions induce localization. For dications two polarons on the same chain are preferred over a bipolaren. Intragap absorptions arise in charged species, no matter whether defects are localized or delocalized. Cations and dications give rise to two sub-band transitions. The cation peaks have lower energies than those of dications. The first excitations of cations have lower oscillator strengths than the second; for dications the second peak is weaker than the first. For very long oligomers, the second sub-band absorption vanishes and a third one appears at higher energy. The behavior of pyrrole oligomers is analogous to that of thiophene oligomers. Theoretical UV spectra for cations and dications of short oligomers (six to eight rings) match experimental spectra of polypyrrole at low and at high doping levels, respectively. The error in the theoretical calculations is about 0.4 eV, slightly larger than for thiophene oligomers at the same level of theory. © 2008 American Chemical Society.
Open Access
Ultrafast transient optical loss dynamics in exciton-plasmon nano-assemblies
(Royal Society of Chemistry, 2017) Elkabbash, M.; Rashed, A.R.; Kucukoz, B.; Nguyen, Q.; Karatay, A.; Yaglioglu, G.; Özbay, Ekmel; Caglayan, H.; Strangi, G.
We study the exciton-plasmon dynamics that lead to optical loss mitigation via ultrafast transient absorption spectroscopy (UTAS) on hybrid aggregates of core-shell quantum dots (QDs) and Au nanoparticles (NPs). We highlight that generating hot electrons in plasmonic NPs contributes to the transient differential absorption spectrum under optical excitation. The results suggest modifying the method of analyzing the transient absorption spectra of loss mitigated systems. Additionally, we investigate the effect of Electron Oscillation frequency-Phonon Resonance Detuning (EOPRD) on loss mitigation efficiency. Moreover, power dependent UTAS reveal a frequency pulling like effect in the transient bleach maximum towards the gain emission. We show that the appropriate choice of the pump wavelength and by changing the pump power we can conclusively prove the existence of loss mitigation using UTAS. Finally, we study the transient kinetics of hybrid gain-plasmon systems and report interesting hybrid transient kinetics.