Browsing by Subject "Light sources"
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Item Open Access Circularly configured multi-SLM holographic display system(IEEE, 2011) Yaraş, Fahri; Kang, Hoonjong; Onural, LeventThe designed circular holographic display system produces ghost-like 3D optical reconstructions of a computer generated 3D model. System uses six phase-only reflective-type spatial light modulators (SLMs) that are configured circularly. Alignment of the SLMs are successful and gap problem is solved by using half-mirrors. The total number of pixels of the resultant display is 11520 1080. Reconstructions show that increase in the viewing angle is significant compared to the single SLM case. With the help of the proposed system, observer can see the reconstructions binocularly. As a result, comfortable 3D perception is achieved. In order to avoid eye-hazard, LED illumination is also used as an alternative light source. Experimental results are satisfactory. Proposed system can be used as a holographic display system.Item Open Access Energy-saving quality road lighting with colloidal quantum dot nanophosphors(Walter de Gruyter GmbH, 2014) Erdem, T.; Kelestemur, Y.; Soran-Erdem, Z.; Ji, Y.; Demir, Hilmi VolkanHere the first photometric study of road-lighting white light-emitting diodes (WLEDs) integrated with semiconductor colloidal quantum dots (QDs) is reported enabling higher luminance than conventional light sources, specifically in mesopic vision regimes essential to street lighting. Investigating over 100 million designs uncovers that quality road-lighting QD-WLEDs, with a color quality scale and color rendering index ≥85, enables 13-35% higher mesopic luminance than the sources commonly used in street lighting. Furthermore, these QD-WLEDs were shown to be electrically more efficient than conventional sources with power conversion efficiencies ≥16-29%. Considering this fact, an experimental proof-of-concept QD-WLED was demonstrated, which is the first account of QD based color conversion custom designed for street lighting applications. The obtained white LED achieved the targeted mesopic luminance levels in accordance with the road lighting standards of the USA and the UK. These results indicate that road-lighting QD-WLEDs are strongly promising for energy-saving quality road lighting. © 2014 Science Wise Publishing & De Gruyter 2014.Item Open Access High-precision laser master oscillators for optical timing distribution systems in future light sources(European Physical Society Accelerator Group (EPS-AG), 2006) Winter, A.; Schmüser, P.; Ludwig, F.; Schlarb, H.; Chen, J.; Kärtner, F. X.; ilday, F. ÖMERAn ultra-stable timing and synchronization system for linac-driven FELs has been designed providing 10 fs precision over distances of several kilometers. Mode-locked fiber lasers serve as master oscillators. The optical pulse train is distributed through length-stabilized fiber links. The layout of the optical synchronization system and its phase noise properties are described. A prototype system has been tested in an accelerator environment and has achieved the required stability.Item Open Access High-speed resonant-cavity-enhanced Schottky photodiodes(IEEE, 1998) Ata, Erhan P.; Bıyıklı, Necmi; Demirel, Ekrem; Özbay, Ekmel; Gökkavas, M.; Onat, B.; Ünlü, M. S.; Tuttle, G.The top-illuminated Schottky photodiodes were fabricated by a microwave-compatible monolithic microfabrication process. Fabrication started with formation of ohmic contacts to n+ layers. Mesa isolation was followed by a Ti-Au interconnect metallization. Following this, a semitransparent Au Schottky metal and a silicon nitride layer was deposited. Finally, a thick Ti-Au layer was deposited to form an air bridge connection between the interconnect and the Schottky metal. The optical properties of the photodiodes were simulated using a transfer matrix method.Item Open Access Highly flexible, electrically driven, top-emitting, quantum dot light-emitting stickers(American Chemical Society, 2014) Yang X.; Mutlugun, E.; Dang, C.; Dev, K.; Gao, Y.; Tan, S.T.; Sun X.W.; Demir, Hilmi VolkanFlexible information displays are key elements in future optoelectronic devices. Quantum dot light-emitting diodes (QLEDs) with advantages in color quality, stability, and cost-effectiveness are emerging as a candidate for single-material, full color light sources. Despite the recent advances in QLED technology, making high-performance flexible QLEDs still remains a big challenge due to limited choices of proper materials and device architectures as well as poor mechanical stability. Here, we show highly efficient, large-area QLED tapes emitting in red, green, and blue (RGB) colors with top-emitting design and polyimide tapes as flexible substrates. The brightness and quantum efficiency are 20 000 cd/m2 and 4.03%, respectively, the highest values reported for flexible QLEDs. Besides the excellent electroluminescence performance, these QLED films are highly flexible and mechanically robust to use as electrically driven light-emitting stickers by placing on or removing from any curved surface, facilitating versatile LED applications. Our QLED tapes present a step toward practical quantum dot based platforms for high-performance flexible displays and solid-state lighting. © 2014 American Chemical Society.Item Open Access Indium rich InGaN solar cells grown by MOCVD(Springer New York LLC, 2014) Çakmak, H.; Arslan, E.; Rudziński, M.; Demirel, P.; Unalan, H. E.; Strupiński, W.; Turan, R.; Öztürk, M.; Özbay, EkmelThis study focuses on both epitaxial growths of InxGa 1-xN epilayers with graded In content, and the performance of solar cells structures grown on sapphire substrate by using metal organic chemical vapor deposition. The high resolution X-ray and Hall Effect characterization were carried out after epitaxial InGaN solar cell structures growth. The In content of the graded InGaN layer was calculated from the X-ray reciprocal space mapping measurements. Indium contents of the graded InGaN epilayers change from 8.8 to 7.1 % in Sample A, 15.7-7.1 % in Sample B, and 26.6-15.1 % in Sample C. The current voltage measurements of the solar cell devices were carried out after a standard micro fabrication procedure. Sample B exhibits better performance with a short-circuit current density of 6 mA/cm2, open-circuit voltage of 0.25 V, fill factor of 39.13 %, and the best efficiency measured under a standard solar simulator with one-sun air mass 1.5 global light sources (100 mW/cm2) at room temperature for finished devices was 0.66 %.Item Open Access Lighting monuments: reflections on outdoor lighting and environmental appraisal(Elsevier, 2006-06) Tural, M.; Yener, C.For more than 10 years, highlighting urban pattern, revealing the artifacts in our surroundings, and providing relationships among the elements of cities, outdoor lighting practice gained more significance. The following study aims at suggesting the needs for monument lighting, taking it as an essential architectural and outdoor lighting issue, and focuses on aspects to be considered both in the approach and application phases by discussing some examples from Ankara, the capital of Turkey. In order to determine approaches in attaining an effective lighting scheme, a case study was conducted. The participants were shown a series of photographs of the Bilkent University Atatürk Monument and its model taken under daylight and artificial lighting conditions, and the differences and tendencies in their perceptional preferences were examined. The results implied that lighting the monument with down-lighting technique is more preferable compared to up-lighting. The findings also suggested that there is no significant perceptional difference on the figure when daylight condition (direction) is imitated using artificial lighting sources. In the analyses, it was also observed that the difference in the incident angle (45° and 60°) of light did not considerably change the perception of participants. © 2005 Elsevier Ltd. All rights reserved.Item Open Access Nanocrystal hybridized white light sources integrated on near UV leds(World Scientific Publishing, 2007) Nizamoğlu, Sedat; Demir, Hilmi VolkanWe report on CdSe/ZnS core-shell nanocrystal (NC) based white light sources integrated on near-UV InGaN/GaN light emitting diodes (LEDs). We present the design, epitaxial growth, fabrication, integration and characterization of such hybrid NC-LEDs that incorporate the dual combinations of cyan and yellow nanocrystals (λPL=500-580 nm) and the trio combinations of cyan, green, and red nanocrystals (λPL=500-540-620 nm).Item Open Access Nanosecond pulsed laser ablated sub-10 nm silicon nanoparticles for improving photovoltaic conversion efficiency of commercial solar cells(Institute of Physics Publishing Ltd., 2017) Rasouli, H. R.; Ghobadi, A.; Ghobadi, T. G. U.; Ates, H.; Topalli, K.; Okyay, Ali KemalIn this paper, we demonstrate the enhancement of photovoltaic (PV) solar cell efficiency using luminescent silicon nanoparticles (Si-NPs). Sub-10 nm Si-NPs are synthesized via pulsed laser ablation technique. These ultra-small Si nanoparticles exhibit photoluminescence (PL) character tics at 425 and 517 nm upon excitation by ultra-violet (UV) light. Therefore, they can act as secondary light sources that convert high energetic photons to ones at visible range. This down-shifting property can be a promising approach to enhance PV performance of the solar cell, regardless of its type. As proof-of-concept, polycrystalline commercial solar cells with an efficiency of ca 10% are coated with these luminescent Si-NPs. The nanoparticle-decorated solar cells exhibit up to 1.64% increase in the external quantum efficiency with respect to the uncoated reference cells. According to spectral photo-responsivity characterizations, the efficiency enhancement is stronger in wavelengths below 550 nm. As expected, this is attributed to down-shifting via Si-NPs, which is verified by their PL characteristics. The results presented here can serve as a beacon for future performance enhanced devices in a wide range of applications based on Si-NPs including PVs and LED applications.Item Open Access Performance assessment of a diffraction field computation method based on source model(IEEE, 2008-05) Esmer, G. Bora; Onural, Levent; Özaktaş, Haldun M.; Uzunov, V.; Gotchev, A.Efficient computation of scalar optical diffraction field due to an object is an essential issue in holographic 3D television systems. The first step in the computation process is to construct an object. As a solution for this step, we assume that an object can be represented by a set of distributed data points over a space. The second step is to determine which algorithm provides better performance. The source model whose performance is investigated is based on superposition of the diffraction fields emanated from the hypothetical light sources located at the given sample points. Its performance is evaluated according to visual quality of the reconstructed field and its algorithmic complexity. Source model provides acceptable reconstructed patterns when the region in which the samples are given has a narrow depth along the longitudinal direction and a wide extent along the transversal directions. Also, the source model gives good results when the cumulative field at the location of each point due to all other sources tends to be independent of that location. ©2008 IEEE.Item Open Access Simulating a wavelength-size 2-D lens with an accurate numerical method(IEEE, 2001) Boriskin, A. V.; Nosich, A. I.; Altıntaş, AyhanThe effect of a localized light source directivity improvement due to an arbitrarily shaped dielectric cylinder taken as a 2-D model of a dielectric lens is studied. The source is simulated by the field of a complex source-point (CSP). An efficient algorithm for the solution of 2-D problem of wave scattering by a smooth dielectric cylinder is developed, based on the concept of analytical regularization. The basic properties of the algorithm are studied. Numerical results for the accuracy of the algorithm and sample far-field characteristics such as the total radiated power, directivity and radiation patterns for various lens parameters are presented.Item Open Access Sun position estimation and tracking for virtual object placement in time-lapse videos(Springer, 2017) Balcı, H.; Güdükbay, UğurRealistic illumination of virtual objects placed in real videos is important in terms of achieving visual coherence. We propose a novel approach for illumination estimation on time-lapse videos and seamlessly insert virtual objects in these videos in a visually consistent way. The proposed approach works for both outdoor and indoor environments where the main light source is the Sun. We first modify an existing illumination estimation method that aims to obtain sparse radiance map of the environment in order to estimate the initial Sun position. We then track the hard ground shadows on the time-lapse video by using an energy-based pixel-wise method. The proposed method aims to track the shadows by utilizing the energy values of the pixels that forms them. We tested the method on various time-lapse videos recorded in outdoor and indoor environments and obtained successful results. © 2016, Springer-Verlag London.Item Open Access Time resolved photoluminescence study of magnetic CdSe/CdMnS/CdS core/multi-shell nanoplatelets(SPIE, 2017) Murphy, J. R.; Delikanlı, Savaş; Zhang, T.; Scrace, T. A.; Zhang, P.; Norden, T.; Thomay, T.; Cartwright, A. N.; Demir, Himli Volkan; Petrou, A.Colloidal semiconductor nanoplatelets (NPLs) are quasi 2D-nanostructures that are grown and processed inexpensively using a solution based method and thus have recently attracted considerable attention. We observe two features in the photoluminescence spectrum, suggesting two possible recombination channels. Their intensity ratio varies with temperature and two distinct temperature regions are identified; a low temperature region (10K < T < 90K) and a high temperature region (90K < T < 200K). This ratio increases with increasing temperature, suggesting that one recombination channel involves holes that are weakly localized with a localization energy of 0.043meV. A possible origin of these localized states are energy-variations in the xy-plane of the nanoplatelet. The presence of positive photoluminescence circular polarization in the magnetically-doped core/multi-shell NPLs indicates a hole-dopant exchange interaction and therefore the incorporated magnetic Manganese ions act as a marker that determines the location of the localized hole states.1 Time-resolved measurements show two distinct timescales (τfast and τslow) that can be modeled using a rate equation model. We identify these timescales as closely related to the corresponding recombination times for the channels. The stronger hole localization of one of these channels leads to a decreased electron-hole wave function overlap and thus a decreased oscillator strength and an increased lifetime. We show that we can model and understand the magnetic interaction of doped 2D-colloidal nanoplatelets which opens a pathway to solution processable spin controllable light sources. Copyright © 2017 SPIE.Item Open Access Ultra-low timing-jitter passively mode-locked fiber lasers for long-distance timing synchronization(SPIE, 2006) İlday, F. Ömer; Winter, A.; Kim J.-W.; Chen, J.; Schmüser, P.; Schlarb, H.; Kärtner, F. X.One of the key challenges for the next-generation light sources such as X-FELs is to implement a timing stabilization and distribution system to enable ∼ 10 fs synchronization of the different RF and laser sources distributed in such facilities with distances up to a few kilometers. These requirements appear to be beyond the capability of traditional RF distribution systems based on temperature-stabilized coaxial cables. A promising alternative is to use an optical transmission system: A train of pulses generated from a laser with low timing jitter is distributed over length-stabilized fiber links to remote locations. The repetition frequency of the pulse train and its higher harmonics contain the synchronization information. At the remote locations, RF signals are extracted simply by using a photodiode and a suitable bandpass filter to pick the desired harmonic of the laser repetition rate. Passively mode-locked Er-doped fiber lasers provide excellent long-term stability. The laser must have extremely low timing jitter, particularly at high frequencies (>1 kHz). Ultimately, the timing jitter is limited by quantum fluctuations in the number of photons making up the pulse and the incoherent photons added in the cavity due to spontaneous emission. The amplitude and phase noise of a home-built laser, generating 100-fs, 1-nJ pulses, was characterized. The measured phase noise (timing jitter) is sub-10 fs. from 1 kHz to Nyquist frequency. In addition to synchronization of accelerators, the ultra-low timing jitter pulse source can find applications in next-generation telecommunication systems.Item Open Access Warm white light generating nanocrystal hybridized LEDs with high color rendering index(IEEE, 2008-11) Demir, Hilmi Volkan; Nizamoğlu, Sedat; Zengin, GülişBy hybridizing custom-design CdSe/ZnS core-shell NC emitters on InGaN/GaN based blue LEDs, we demonstrated three warm-white light sources with desirably low CCT ranging from 3227 K to 1982 K as is required for SSL indoor applications. In these proof-of-concept demonstrations, high color rendering indices (82.4) and high luminous efficacies of emitted spectra (327 lm/W) were achieved, while the color temperature was simultaneously kept low as desired. Our proof-of-concept demonstrations indicated that such nanocrystal luminophor based warm-white LEDs with high-quality photometric properties hold great promise especially for future indoor lighting applications.Item Open Access White light generation with CdSe/ZnS core-shell nanocrystals and InGaN/GaN light emitting diodes(IEEE, 2006) Nizamoğlu, Sedat; Özel, Sedat; Sarı, Emre; Demir, Hilmi VolkanWe present hybrid white light sources that integrate CdSe/ZnS core-shell nanocrystals on blue InGaN/GaN light emitting diodes (LED). We report on the demonstrations of white light generation using yellow nanocrystals (λPL=580 nm) hybridized on a blue LED (λEL= 440 nm) with tristimulus coordinates of x=0.37 and y=0.25, correlated color temperature of Tc=2692 K, and color rendering index of R a=14.6; cyan and red nanocrystals (λPL=500 nm and 620 nm) on a blue LED (λEL=440 nm) with x=0.37, y=0.28, T c=3246 K, and Ra=19.6; and green, yellow, and red nanocrystals (λPL=540 nm, 580 nm, and 620 nm) on a blue LED (λEL=452 nm) with x=0.30, y=0.28, Tc =7521 K, and Ra=40.9. © 2006 IEEE.