Browsing by Subject "Photonic crystal"

Now showing 1 - 20 of 24

Open Access
Absorption enhancement in InGaN-based photonic crystal-implemented solar cells
(SPIE, 2012-07-26) Gundogdu, T. F.; Gokkavas, M.; Özbay, Ekmel
We 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]
Open Access
AIBIIICVI 2 (A = Cu, Ag; B = Ga, In; C = S, Se, Te) based photonic crystal superlattices: optical properties
(Wiley-VCH Verlag, 2017) Simsek S.; Palaz S.; Akhundov, C.; Mamedov, A. M.; Özbay, Ekmel
In this study, we present an investigation of the optical properties and band structures for the photonic structures based on AIBIIICVI 2 with a Fibonacci sequence that can act as a multi-wavelength birefringent filter. The filtering wavelengths are analyzed by the indices concerning the quasi-periodicity of a Fibonacci sequence and the average lattice parameter. The transmittances of filtering wavelengths can be tuned by varying structure parameters such as the lengths of poled domains, filling factor, and dispersion relation. In our simulation, we employed the finite-difference time domain (FDTD) technique, which implies a solution from Maxwell equation.
Open Access
Complete photonic band gaps in Sn2P2X6 (X = S, Se) supercell photonic crystals
(Taylor & Francis, 2020-04) Şimşek, Ş.; Palaz, S.; Koç, H.; Mamedov, Amirullah M.; Özbay, Ekmel
In this work, we present an investigation of the optical properties and band structures for the photonic crystal structures (PCs) based on Sn2P2X6: X = S, Se) with Fibonacci superlattices. The optical properties of PCs can be tuned by varying structure parameters such as the lengths of poled domains, filling factor, and dispersion relation. In our simulation, we employed the finite-difference time domain technique and the plane wave expansion method, which implies the solution of Maxwell equations with centered finite-difference expressions for the space and time derivatives.
Open Access
Development of left-handed composite materials and negative refracting photonic crystals with subwavelength focusing
(SPIE, 2005) Özbay, Ekmel
We review the studies conducted in our group concerning electromagnetic properties of metamaterials and photonic crystals with negative effective index of refraction. In particular, we demonstate the true left handed behavior of a 2D composite metamaterial, by analyzing the electric and magnetic response of the material components systematically. The negative refraction, subwavelength focusing, and flat lens phenomena using 2D dielectric photonic crystals are also presented.
Open Access
Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides
(Elsevier, 2012-02-28) Akosman, Ahmet E.; Mutlu, Mehmet; Kurt, H.; Özbay, Ekmel
A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.
Open Access
Enhanced electro-optic modulation of LiNbO3-based photonic crystal cavities with dual mode and polarization operation
(TUBITAK, 2017) Baǧcı, F.; Kurt G.; Akaoǧlu, B.; Özbay, Ekmel
A high output transmission and high quality factor, compact LiNbO3 -based Fabry-Perot-type photonic crystal cavity is reported that can be used in electro-optical modulators at optical communication wavelengths. The electro-optic effect is 123 times enhanced as compared to the bulk material as a result of the slow light effect. The transmission wavelength is found to depend linearly on the applied voltage with 1.2 nm/V modulation sensitivity. Moreover, an externally switchable dual mode regime with high quality factors and transmissions is realized. High transmissions and high quality factors are also obtained for both transverse-electric and transverse-magnetic polarizations simultaneously. © 2017 TÜBITAK.
Open Access
Enhancing light extraction efficiency of InGaN/GaN multi quantum well light emitting diodes with embedded two dimensional photonic crystal structures
(2010-08) Kaya, Ali Güneş
Advance in the growth methods of III-Nitrides and researches in order to eliminate doping problems of gallium nitride (GaN) resulted in high band gap materials with increased crystal quality which have led to tremendous improvement in opto-electronic devices. Their durability under harsh environmental and operational conditions such as high pressure and high temperature, and large spectral coverage range including 200 nm deep ultra-violet (deepUV) through 1500 nm infra-red (IR) make them excellent candidates for opto-electronic applications such as full color LCD panels, biomedical sensor devices, high resolution printers, high density storage devices, defense systems. Among III-Nitrides, GaN has attracted the most interest with its high electronic band gap of 3.4 eV at room temperature and ability to form compounds with other group III elements aluminum and indium which have band gaps of 6.2 eV and 0.7 eV respectively which results in a large optical spectrum. However, potential performance of III-Nitride based devices is hindered mainly because of an optical phenomenon called total internal reflection (TIR). High dielectric constant of these materials prevents light to escape from the structure. Light is totally reflected back from the air-nitride interface for incident angles larger than critical angle. With a refractive index of 2.7, GaN material has 22°-24° critical angle which means less than 12% of light can just escape the structure. While 66% of the light generated in quantum well region of a GaN based light emitting diode (LED) is trapped in the GaN layer, 22% of the light is guided in the sapphire substrate. Because of TIR, although internal quantum efficiency of GaN based LED with emission wavelength of around 400 nm is almost unity (higher than 90%), external quantum efficiency is very low. To enhance extraction, a lot of geometrical methods including surface roughening and facet shaping have been tried to reduce the effects of dielectric contrast between the device and medium. In order to increase the extraction efficiency of GaN based LEDs, two dimensional photonic crystals were used in this thesis. LED wafers used were fabricated in collaboration with University of Santa Barbara, California (UCSB) which are InGaN/GaN multi quantum well (MQW) structures that emit light at 390 nm and 410 nm respectively. These LED wafers were processed in the scope of the thesis and photonic crystal (PC) structures were patterned on the p layer of the device. This thesis work is concentrated on two parts; first part is characterization and fabrication, and second part is simulation. In characterization and fabrication part, firstly GaN material etching characterizations were completed using dry etching method by reactive ion etcher (RIE) since there had been no optimized recipe for GaN processing in the clean room know-how. In this characterization, main parameter was the etching anisotropy since vertical side-wall is crucial for LED processing and PC formation. After characterization step was completed, p doping activation was done in Middle East Technical University, Ankara (METU) and then LED wafers were processed in clean room class-100 environment at Bilkent University, Ankara by using the know-how and recipe obtained from characterization stage. As a final step, two dimensional square and triangular photonic crystal lattice structures were patterned by using electron beam writer and these structures were transferred on to p layer of GaN LED by using RIE. Measurements and imaging regarding material and optical properties, fabrication quality and extraction enhancement were done by using scanning electron microscope (SEM), atomic force microscopy (AFM), spectrophotometer (UV-Vis), ellipsometer and probe station. I-V characteristics, optical power measurements and intensity plots on black&white CCD camera images were taken. In simulations part, two and three dimensional simulations using plane wave expansion method, integral method and finite difference time domain method were completed. More than 40000 simulations were run in total during this thesis work. As a result, PCs with 520 nm lattice period and 260 nm hole diameters in square and triangular geometries were modeled and fabricated. Final depth of lattices was around 100-120 nm. Results of 2D integral simulations suggested around 15-20% of error between modeling and experiments because of imperfectness regarding fabrication of PC structures. Furthermore, damaging effects of RIE and focused electron beams were not considered. In measurements, extraction efficiency enhancement factors of about 2.2 and about 2.6 were found using square and triangular PC lattices with respect to LED devices without PC structure patterning. In simulations, while square PC lattice models showed 2.8 times enhancement in extraction efficiency, triangular lattice resulted in 3.1 times with respect to no PC models. In comparison of measurements and simulations, difference in the range of 15- 25% was found which were expected as stated above with also considering the effect of processing damages on top p and p++ layer and quantum well region.
Open Access
Experimental demonstration of sub-wavelength imaging by left handed metamaterials
(SPIE, 2007) Özbay, Ekmel
We review the studies conducted in our group concerning electromagnetic properties of metamaterials and photonic crystals with negative effective index of refraction. In particular, we demonstate the true left handed behavior of a 2D composite metamaterial, by analyzing the electric and magnetic response of the material components systematically. The negative refraction, subwavelength focusing, and flat lens phenomena using left handed metamaterials and photonic crystals are also presented.
Open Access
Highly directional enhanced radiation from sources embedded inside two and three-dimensional photonic crystals
(SPIE, 2005) Çağlayan, Hümeyra; Bulu, İrfan; Özbay, Ekmel
In this work, we have experimentally and theoretically studied the emission of radiation from a monopole source embedded in a two and three dimensional photonic crystal. We have demonstrated the enhancement of radiation at the band edges and at the cavity modes including coupled cavity modes. We have shown that the emission of radiation from a source depends on the group velocities of the modes and on the electric field intensities of the modes at the source location. Moreover, we have studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. Our results show that it is possible to obtain highly directive radiation sources operating at the band edge of the photonic crystal.
Open Access
Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures
(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.
We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.
Open Access
Multifrequency spatial filtering: a general property of two-dimensional photonic crystals
(Elsevier, 2016) Serebryannikov, A. E.; Colak, E.; Petrov, A.; Usik, P. V.; Özbay, Ekmel
Spatial filtering, an analog of frequency-domain filtering that can be obtained in the incidence angle domain at a fixed frequency is studied in the transmission mode for slabs of two-dimensional rod-type photonic crystals. In the present paper, the emphasis is put on the demonstration of the possibility to obtain various regimes of spatial filtering, i.e., band-stop, band-pass, and low-pass filtering in different frequency ranges in one simple configuration. The operation is based on the use of several Floquet-Bloch modes with appropriate dispersion properties, so that such one or two co-existing mode(s) contribute to the forming of a proper filter characteristic within each specific frequency range. It is shown that high-efficiency transmission and steep switching between pass and stop bands can be obtained in the angle domain for wide ranges of variation of the problem parameters. In particular, by varying the rod-diameter-to-lattice-constant ratio, one attains lots of freedom in the engineering of spatial filters with desired transmission characteristics.
Open Access
Negative refraction and subwavelength focusing in two-dimensional photonic crystals
(Springer-Verlag, 2007) Özbay, Ekmel; Özkan, Gonca; Krowne, C. M.; Zhang, Y.
We experimentally and theoretically demonstrate negative refraction and focusing on electromagnetic (EM) waves by using two-dimensional photonic crystal slabs at microwave frequencies. Negative refraction is observed both for transverse magnetic (TM) and transverse electric (TE) polarized incident EM waves. Gaussian beam shifting method is used to verify negative refractive index. Subwavelength imaging and flat lens behavior of photonic crystals are successfully demonstrated. We have been able to overcome the diffraction limit and focus the EM waves to a spot size of 0.21θ. Metallodielectric photonic crystals are employed to increase the range of angles of incidence that results in negative refraction. Experimental results and theoretical calculations are in good agreement throughout the work.
Open Access
Negative refraction and subwavelength focusing using photonic crystals
(SPIE, 2005-01) Özbay, Ekmel; Aydın, Koray; Alıcı, Kamil Boratay; Güven, Kaan
We review certain novel optical properties of two-dimensional dielectric photonic crystals (PCs) which exhibit negative refraction behavior. We investigate two mechanisms which utilize the band structure of the PC and lead to a negative effective index of refraction (neff < 0). The negative refraction phenomenon is demonstrated experimentally and by simulations when the incident beam couples to a photonic band with neff < 0. Further, the PC slab acts like a focusing lens to an omnidirectional source where the properties of focusing depends on the details of the band structure. In one case, by utilizing the TM polarized first band, an image of the source can be formed in the vicinity of the interface with subwavelength resolution. In another case, a TE polarized upper band is used which is able to focus the omnidirectional field far away from the interface with a resolution on par with the wavelength. In the latter case, we explicitly show the flat lens behavior of the structure. These examples indicate that PC based lenses can surpass limitations of conventional lenses and greatly enhance and extend optics applications.
Open Access
One-way Rayleigh-Wood anomalies and tunable narrowband transmission in photonic crystal gratings with broken structural symmetry
(American Physical Society, 2013-05-03) Serebryannikov, A. E.; Özbay, Ekmel
In photonic crystal (PC) gratings whose structural symmetry is broken owing to one-side corrugations, the Rayleigh-Wood anomalies can appear in a one-way manner. In the combination with the vanishing coupling to the lowest Floquet-Bloch mode at the upper band edge, these anomalies lead to the appearance of the defect-mode-like, i.e., very narrow one-way transmission, peak, which cannot appear in the corresponding slab of PC without corrugations. Such a sole peak can be transformed into a one-way transmission band with a desired width by varying angle of incidence. The nonoverlapping and thus switchable one-way (higher-order) and two-way (zero-order) transmission bands can be obtained in both the frequency and incidence-angle domains. Adjustment of PC lattice and corrugation parameters allows one to obtain two subsequent one-way bands, every being connected with a certain higher diffraction order. Conditions required for the appearance of a narrow one-way transmission band in the vicinity of a Rayleigh-Wood anomaly are well consistent with typical dispersion features of a very large class of PCs and do not need unusual performances.
Open Access
Photonic band gap of multiferroic-dielectric materials in the IR region: FDTD method
(Taylor & Francis, 2019) Palaz, S.; Şimşek, Ş.; Mamedov, Amirullah M.; Özbay, Ekmel
In this report, we present an investigation of the optical properties and band structure calculations for the photonic structures based on the multiferroic materials- BaMnF4. We calculate the photonic bands and optical properties of BaMnF4/LiNbO3 based photonic crystal. We study the photonic band gap and optical properties of the photonic structures, numerically analyzed in the IR frequency region by using the FDTD method for various incidence angles, number of periods in the PC and the nature/geometry of the materials.
Open Access
Photonic band gap structures for WDM applications
(IEEE, 2002) Bayındır, Mehmet; Akarca, S. S.; Özbay, Ekmel
A new structure by combining a single cavity and coupled-cavity waveguides (CCWs) for wavelength division multiplexing (WDM) applications was proposed. As such, a structure in which the coupling between the cavity mode and the guiding model allows to drop a selective wavelength λi was designed. The selectivity of dropping wavelength was determined by local properties of the cavity modes. Such results were said to be important for designing future ultrasmall optical circuits.
Open Access
Photonic crystal-based resonant antenna with a very high directivity
(American Institute of Physics, 2000-09-24) Temelkuran, B.; Bayındır, Mehmet; Özbay, Ekmel; Biswas, R.; Sigalas, M. M.; Tuttle, G.; Ho, K. M.
We investigate the radiation properties of an antenna that was formed by a hybrid combination of a monopole radiation source and a cavity built around a dielectric layer-by-layer three-dimensional photonic crystal. We measured a maximum directivity of 310, and a power enhancement of 180 at the resonant frequency of the cavity. We observed that the antenna has a narrow bandwidth determined by the cavity, where the resonant frequency can be tuned within the band gap of the photonic crystal. The measured radiation patterns agree well with our theoretical results. (C) 2000 American Institute of Physics
Open Access
Physics and applications of defect structures in photonic crystals
(Springer, 2003) Özbay, Ekmel; Bayındır, Mehmet; Shumovsky, Alexander S.; Rupasov, V. I.
We propose and demonstrate a new type of propagation mechanism for electromagnetic waves in photonic band gap materials. Photons propagate through coupled cavities due to interaction between the highly localized neighboring cavity modes. We report a novel waveguide, which we called coupled-cavity waveguide (CCW), in three-dimensional photonic structures. By using CCWs, we demonstrate lossless and reflectionless waveguide bends, efficient power splitters, and photonic switches. We also experimentally observe the splitting of eigenmodes in coupled-cavities and formation of defect band due to interaction between the cavity modes. The tight-binding (TB) approach, which is originally develop for the electronic structures, is applied to the photonic structures, and compared to the experimental results. Our achievements open a new research area, namely physics and applications of coupled-cavities, in photonic structures. We think that our results are very important for constructing future all-optical components on a single chip.
Open Access
Physics and applications of photonic crystals
(Nanotechnology Research Center NANOTAM, 2004) Özbay, Ekmel; Bulu I.; Aydin, K.; Caglayan H.; Guven, K.
In this article, we investigate how the photonic band gaps and the variety of band dispersions of photonic crystals can be utilized for various applications and how they further give rise to completely novel optical phenomena. The enhancement of spontaneous emission through coupled cavity waveguides in a one-dimensional silicon nitride photonic microcrystal is investigated. We then present the highly directive radiation from sources embedded in two-dimensional photonic crystals. The manifestation of novel and intriguing optical properties of photonic crystals are exemplified experimentally by the negative refraction and the focusing of electromagnetic waves through a photonic crystal slab with subwavelength resolution. © 2004 Elsevier B.V. All rights reserved.
Open Access
Physics and applications of photonic nanocrystals
(Inderscience Publishers, 2004) Özbay, Ekmel; Güven, Kaan; Aydın, Koray; Bayındır, Mehmet
Photonic nanocrystals are periodic dielectric or metallic structures having photonic bands in analogy to electronic bands of semiconductors. The presence of photonic band-gaps, where the propagation of photons of certain frequencies is prohibited, and the variety of photon dispersions give rise to novel and unusual optical phenomena. Consequently, photonic crystals are now envisaged as an essential building block of future photonic devices. This paper aims to provide a review of contemporary developments on the physics and applications of photonic crystals with an emphasis on optical properties of coupled microcavity waveguides and on the negative refraction phenomenon. The enhancement of spontaneous emission in a silicon nitride photonic nanocrystal is investigated in detail. Both the negative refraction of a Gaussian beam and the focusing of a microwave point source through a photonic crystal slab with subwavelength resolution are studied experimentally.