196,047 research outputs found
Differential Evolution Algorithm Based Photonic Structure Design: Numerical and Experimental Verification of Subwavelength ? /5 Focusing of Light
No full textPhotonic structure designs based on optimization algorithms provide superior properties compared to those using intuition-basedapproaches. In the present study, we numerically and experimentally demonstrate subwavelength focusing of light using wavelength scale absorption-free dielectric scattering objects embedded in an air background. An optimization algorithm based on differential evolution integrated into the finite-difference time-domain method was applied to determine the locations of each circular dielectric object with a constant radius and refractive index. The multiobjective cost function defined inside the algorithm ensures strong focusing of light with low intensity side lobes. The temporal and spectral responses of the designed compact photonic structureprovided a beam spot size in air with a full width at half maximum value of 0.19?, where ? is the wavelength of light. The experiments were carried out in the microwave region to verify numerical findings, and very good agreement between the two approaches was found. The subwavelength light focusing is associated with a strong interference effect due to nonuniformly arranged scatterers and an irregular index gradient. Improving the focusing capability of optical elements by surpassing the diffraction limit of light is of paramount importance in optical imaging, lithography, data storage, and strong light-matter interaction. © The Author(s) 2016
Design and analysis of all-dielectric subwavelength focusing flat lens
No full textIn this letter, we numerically designed and experimentally demonstrated a compact photonic structure for the subwavelength focusing of light using all-dielectric absorption-free and nonmagnetic scattering objects distributed in an air medium. In order to design the subwavelength focusing flat lens, an evolutionary algorithm is combined with the finite-difference time-domain method for determining the locations of cylindrical scatterers. During the multi-objective optimization process, a specific objective function is defined to reduce the full width at half maximum (FWHM) and diminish side lobe level (SLL) values of light at the focal point. The time-domain response of the optimized flat lens exhibits subwavelength light focusing with an FWHM value of 0.19 lambda and an SLL value of 0.23, where. denotes the operating wavelength of light. Experimental analysis of the proposed flat lens is conducted in a microwave regime and findings exactly verify the numerical results with an FWHM of 0.192 lambda and an SLL value of 0.311 at the operating frequency of 5.42 GHz. Moreover, the designed flat lens provides a broadband subwavelength focusing effect with a 9% bandwidth covering frequency range of 5.10 GHz-5.58 GHz, where corresponding FWHM values remain under 0.21 lambda. Also, it is important to note that the designed flat lens structure performs a line focusing effect. Possible applications of the designed structure in telecom wavelengths are speculated upon for future perspectives. Namely, the designed structure can perform well in photonic integrated circuits for different fields of applications such as high efficiency light coupling, imaging and optical microscopy, with its compact size and ability for strong focusing.
Generation of a Two-Dimensional Limited-Diffraction Beam With Self-Healing Ability by Annular-Type Photonic Crystals
No full textIn this work, we present the design of a photonic structure for the generation of in-plane two-dimensional (2D) limited-diffraction beam. We have numerically investigated the characteristics of the light propagation passing through a 2D square-lattice annular-type photonic crystal shaped in an axicon configuration. Careful selection of the operating frequency as well as the optimization of the apex rod position creates a less diffracted beam whose transverse intensity profile closely resembles a zero-order Bessel function. The created beam dramatically resists against the spatial spreading over a propagation distance of 50 mu m, after focusing with a spot size of similar to 0.23 mu m. The self-healing capability of the generated limited-diffraction beam is demonstrated by placing obstacles with different sizes and shapes along the optical axis. The two features that accompany with such beams, i.e., diffraction-limited propagation and reconstruction ability after encountering obstructions, may strengthen its usage in manipulation of light propagation in various environments. (C) 2012 Optical Society of AmericaThe authors gratefully acknowledge the financial support of The Scientific and Technological Research Council of Turkey (TUBITAK), Project No: 110T306. H. Kurt also acknowledges support from the Turkish Academy of Sciences Distinguished Young Scientist Award (TUBA GEBIP).Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [110T306]; Turkish Academy of Sciences (TUBA GEBIP)Turkish Academy of Science
Photonic crystal based polarization insensitive flat lens
No full textThe paper proposes a new design of an inhomogeneous artificially created photonic crystal lens structure consisting of annular dielectric rods to efficiently focus both transverse electric and transverse magnetic polarizations of light into the same focal point. The locations of each individual cell that contains the annular dielectric rods are determined according to a nonlinear distribution function. The inner and outer radii of the annular photonic dielectric rods are optimized with respect to the polarization insensitive frequency response of the transmission spectrum of the lens structure. The physical background of the polarization insensitive focusing mechanism is investigated in both spatial and frequency domains. Moreover, polarization independent wavefront transformation/focusing has been explored in detail by investigating the dispersion relation of the structure. Corresponding phase index distribution of the lens is attained for polarization insensitive normalized frequency range of a/lambda = 0.280 and a/lambda = 0.300, where a denotes the lattice constant of the designed structure and lambda denotes the wavelength of the incident light. We show the wave transformation performance and focal point movement dynamics for both polarizations of the lens structure by specially adjusting the length of the structure. The 3D finite-difference time domain numerical analysis is also performed to verifiy that the proposed design is able to focus the wave regardless of polarization into approximately the same focal point (difference between focal distances of both polarizations stays below 0.25 lambda) with an operating bandwidth of 4.30% between 1476 nm and 1541 nm at telecom wavelengths. The main superiorities of the proposed lens structure are being all dielectric and compact, and having flat front and back surfaces, rendering the proposed lens design more practical in the photonic integration process in various applications such as optical switch, attenuators and couplers, where the polarization insensitive focusing without any additional polarization control components plays an important role.
Modified Maxwell Fish-Eye Approach for Efficient Coupler Design by Graded Photonic Crystals
No full textWe present a novel design of two dimensional graded index medium that provides coupling of light with high coupling efficiency between two planar dielectric waveguides of different widths (15.46 mu m vs. 2.21 mu m). Poor light coupling performance of butt-coupler can be mitigated by implementing tapered coupler at the expense of long coupler section. In order to reduce coupling losses, a new coupling device approach based on graded index (GRIN) concept is proposed. The refractive index distribution is in the form of modified version of the Maxwell fish-eye lens. The inhomogeneous refractive index distribution is approximated by photonic crystals (PCs) such that the positions of each PC rods are appropriately arranged. Strong electric field focusing ability of the designed GRIN PC medium provides relatively high coupling efficiency that is around 90%. Spectral region corresponding to coupling efficiency over 75% has a bandwidth of Delta omega = 18.56% (284 nm). Finally, we discuss the durability of the proposed coupler against the lateral displacement and angular misalignment of output waveguides. (C) 2012 Optical Society of AmericaThe authors gratefully acknowledge the financial support of the Scientific and Technological Research Council of Turkey (TUBITAK), project 110T306. H. K. also acknowledges partial support from the Turkish Academy of Sciences Distinguished Young Scientist Award (TUBA GEBIP).Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [110T306]; Turkish Academy of Sciences (TUBA GEBIP)Turkish Academy of Science
Design of Flat Lens-Like Graded Index Medium by Photonic Crystals: Exploring Both Low and High Frequency Regimes
No full textIn this manuscript, we propose the design of an inhomogeneous artificially created graded index (GRIN) medium to enrich the optical device functionalities of light by using periodic all-dielectric materials. Continuous GRIN profile with hyperbolic secant indexdistribution is approximated using two-dimensional photonic crystal (2D PC) dielectric rods with a fixed refractive index. The locations of each individual cell that contain dielectric rods of certain radii are determined based on the results of the frequency domain analysis. The desired index distribution is attained at long wavelengths using dispersion engineering approach. The frequency response of the transmission spectrum exhibits high transmission windows appearing at both larger and smaller wavelengths regions. Two regions are separated by a local band gap that blocks the incident light for a certain frequency interval. Light manipulation characteristics such as focusing, de-focusing and collimation are systematically and quantitatively compared for artificially designed GRIN medium within lowand high frequency regimes. We show different field manipulation capabilities and focal point movement dynamics of the GRIN medium by special adjustment of the length of the structure. In addition, an analytical formulation based on ray theory is derived to investigate the focusing, de-focusing and collimation properties of proposed GRIN medium. The analytical approach utilizes Ray theory and computational tools are based on plane wave expansion and finite-difference time-domain methods. Implementing the GRIN medium by periodic optical materials provides frequency selectivity and strong focusing effects at higher frequency region. The designed structure can be used in integrated nanophotonics as a compact optical element with flat surfaces. © 2014 Elsevier B.V. All rights reserved
Mode Transformation Using Graded Photonic Crystals With Axial Asymmetry
No full textWe propose a mode conversion method that enables transformation of the propagating mode from fundamental to higher-order modes by utilizing asymmetric graded index (A-GRIN) structures. Refractive index variations of two different asymmetric gradient profiles, i.e., exponential and Luneburg lens profiles, have been approximated by two-dimensional photonic crystals (PCs). The basic structure is composed of constant radii with different lattice sizes. The designed GRIN mode converters provide relatively high transmission efficiency in the spectral region of interest and achieve the transformation in compact configuration. Numerical approaches utilizing the finite-difference time-domain and plane wave expansion methods are used to analyze the mode conversion phenomenon of proposed GRIN PC media. Analytical formulation based on ray theory is outlined to explore both ray trajectories and the physical concept of a wavefront retardation mechanism. (C) 2013 Optical Society of AmericaThe authors gratefully acknowledge the financial support of the Scientific and Technological Research Council of Turkey (TUBITAK), project 110T306. H. K. also acknowledges the partial support of the Turkish Academy of Science.Scientific and Technological Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [110T306]; Turkish Academy of ScienceTurkish Academy of Science
High Extinction Ratio Polarization Beam Splitter Design by Low-Symmetric Photonic Crystals
No full textA novel concept of polarization beam splitting with high polarization extinction ratio is proposed based on the polarization sensitive self-collimation feature of symmetry reduced photonic crystals. The idea utilizes self-collimation mechanism and rotated isofrequency contours arising due to the deliberately implemented symmetry reduction in the photonic structure. Square lattice of low-symmetric rectangular air holes in dielectric background are numerically analyzed in both frequency and time domains. The operating bandwidth of the device is Delta lambda = 53 nm. At lambda = 1.55 mu m, polarization extinction ratios are similar to 23 and similar to 18 dB for transverse electric and transverse magnetic ports, respectively. The investigated device is 46.4 mu m x 12.4 mu m in size and based on a uniform, planar, and homogenous structure. Therefore, there is no need for an additional splitting assistance to separate the two orthogonal polarizations. Possible fabrication imperfections are also analyzed and it is observed that polarization extinction ratios stay above 17 dB when error percentage is around 5.5%. With having these advantages, symmetry reduced photonic crystal-based polarization beam splitters can be a good candidate for optical communication applications.
Polarization splitting phenomenon of photonic crystals constructed by two-fold rotationally symmetric unit-cells
No full textWe present an intrinsic polarization splitting characteristic of low-symmetric photonic crystals (PCs) formed by unit-cells with C 2rotational symmetry. This behavior emerges from the polarization sensitive self-collimation effect for both transverse-magnetic (TM) and transverse-electric (TE) modes depending on the rotational orientations of the unit-cell elements. Numerical analyzes are performed in both frequency and time domains for different types of square lattice two-fold rotational symmetric PC structures. At incident wavelength of ? = 1550 nm, high polarization extinction ratios with ?26 dB (for TE polarization) and ?22 dB (for TM polarization) are obtained with an operating bandwidth of 59 nm. Moreover, fabrication feasibilities of the designed structure are analyzed to evaluate their robustness in terms of the unit-cell orientation: for the selected PC unit-cell composition, corresponding extinction ratios for both polarizations still remain to be over 18 dB for the unit-cell rotation interval of ? = [40°-55°]. Taking all these advantages, two-fold rotationally symmetric PCs could be considered as an essential component in photonic integrated circuits for polarization control of light. © 2017 IOP Publishing Ltd
Inverse design of ultra-compact photonic gates for all-optical logic operations
No full textLogic gates have great importance in realization of rapid data transmission as well as low loss transfers. In this paper, a multi-objective inverse-design approach is implemented by using objective-first algorithm to design optical AND, OR, NAND and NOT logic gates on Si-platform at the design wavelength of 1.30 mu m. For all gates, the design area is fixed to 2.24 mu m x 2.24 mu m. The optical logic '1' output is accepted to be optical power values greater than 0.8 times of the input optical power. By implementing a Bias waveguide as well as two input ports, we made it possible to achieve logic '1' output for logic operations having no inputs such as '0 NAND 0 = 1' and '0 NOT = 1'. We binarized the proposed logic gates, and then numerically analyzed them by using finite-difference time-domain method. Proposed AND gate yields 1.20 times of input power for '1 AND 1 = 1' logic operation and highest logic '0' is obtained for logic operation of '1 AND 0 = 0' as 0.40 times of the input power at the operating wavelength. It is also observed that proposed logic gates can operate not only at the design wavelength of 1.30 mu m but also at broad wavelength regions as well. Finally, we demonstrate that it is possible to carry out complex logic operations by combining the proposed logic AND, OR and NAND gates to construct an XOR gate in the same platform.
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