KAUST-UCSB-NSF Workshop on Solid-State Lighting

February 13-14, 2012 King Abdullah University of Science & Technology

Thuwal, Saudi Arabia

Poster Abstracts

1

P-01

Optical attractors and light control on photorefractive structures

Damian San Roman Alerigi, Yaping Zhang, and Tien Khee Ng (Photonics Lab, KAUST)

Controlling and reshaping light propagation is capital to future photonic applications, from beam reshaping for optical telecommunications and OCT scanners, to light trapping for optical memories and time delays. In our work we study the possible applications of time dependent and static photo-refractive phenomena to implement optical attractors and light trapping. Specifically we investigate an optical trap, based on a feasible refractive index profile in photo-refractive materials, which mimes the dynamics of photons in the vicinity of black hole horizons. Moreover the theoretical scheme developed to study this phenomena can be expanded to model light propagation in moving refractive index perturbations, which could enable ultra-fast light coupling and package confinement.

2

P-02

Porous Silicon Nanowires Prepared by Metal-Assisted Electroless Etching Method

Adel Najar, Ahmed Ben Slimane, Mohamed Nejib Hedhili, Dalaver Anjum, and Tien Khee Ng (Photonics Lab., ANIC Core Lab., KAUST)

We report on the structural and optical properties of porous silicon nanowires (PSiNWs) fabricated using silver (Ag) ions assisted electroless etching method. The scanning electron microscopy (SEM) observations showed that the PSiNWs, at the optimum HF concentration, can have an average pore diameter of 10-40 nm with Si nanocrystallites sizes of < 5 nm embedded in amorphous silica. The strongest photoluminescence (PL) signal has been measured from PSiNWs etched with 4.8 Molar (M) of HF, and beyond which the PL emission intensity reduced significantly. The enhancement in PL signals is associated with the formation of PSiNWs, and to quantum confinement of electron in the Si nanocrystallites, observed in transmission electron microscopy (TEM). Furthermore, the resultant PSiNWs exhibited red emissions originated from SiOx/Si interface defects and/or defect states in surface oxide, as deduced from X-ray photoelectron spectroscopy (XPS) and PL measurements. Ultra-low reflectance less than 0.1% was achieved over a wavelength from 250 to 900 nm for a length of 6 μm. PSiNWs fabricated using the electroless etching method can be used as optical sensors, and as an anti-reflection layer in silicon-based solar cells.

3

P-03

Device characterization of high performance tunnel injection Quantum Dot laser

Ashique Ahmed Rafi (Photonics Lab., KAUST)

Recently the cost effective comb based laser sources are considered to be one of the prominent emitters for optical communication (OC) and photonic integrated circuits (PIC). Also, the data computer network is moving to higher bit rate and enormous data traffic is involved, we are lacking of economical lasers to support on chip optical signaling. Therefore, these high performance multi-wavelength 1.3-μm quantum dot (Q.Dot) comb lasers need to satisfy several challenges before real system implementations like high uniform broadband gain spectrum from the active layer, small relative intensity noise with lower bit error rate (BER) and better temperature stability. Thus, such short wavelength comb lasers offering higher bandwidth can be a feasible solution to address these issues and require thorough characterization before system level implementation. In this project, we will briefly characterize the novel quantum dot comb laser by varying duty cycle based electrical injection and temperature variations. Hence, different system level performance trends are demonstrated with pulse and continuous (CW) electrical pumping for the several promising applications to support next generation computer-communications.

4

P-04

ZnO Nanorods for Simultaneous Light Trapping and Transparent Electrode Application in Solar Cells

Yasser Khan, Yaping Zhang, Muhammad Amin, Adrian Bayraktaroglu, Tien Khee Ng, Hakan Bagci, Jamie Phillips (Photonics Lab., KAUST)

While researchers strive to increase internal quantum efficiency of solar cells, there exists a huge scope to enhance overall quantum yield of these energy harvesting devices by researching into light trapping. Transparent conductive oxides have been studied extensively in display and photovoltaic devices. But, incorporating light trapping capability and transparent electrode characteristics in the same layer has not been explored that well, which may increase overall quantum efficiency significantly. In this work, we investigate the efficacy of using vertically grown ZnO nanorod arrays for enhancing electromagnetic field intensity, and at the same time serve as the top contact layer as transparent electrodes. FDTD simulation, optical measurements, sheet resistance measurements, and SEM micrographs shows boost in adequate characteristics that can enhance overall quantum yield of solar cells.

5

P-05

Feasibility of Fabricating Passive Optical Components on As2S3 Chalcogenide Thin Film

Yaping Zhang, Damian San Roman Alerigi, and Tien Khee Ng (Photonics Lab., KAUST)

Arsenic trisulfide (As2S3) glass is an interesting material for photonic integrated circuits (PICs) as infrared (IR) or nonlinear optical components. In this work, material characterization methods such as X-ray diffraction (XRD) measurement, X-ray photoelectron spectroscopy (XPS), UV-Vis-NIR spectrophotometer have been implemented to examine the micro-structure, chemical composition and optical properties of As2S3 thin films deposited by pulsed laser deposition (PLD), thermal evaporation (TE) and e-beam evaporation (EBE). Direct laser writing (DLW) was applied to engineer the refractive index of thermal evaporated As2S3 thin film. Film samples were exposed to focused light with above band-gap energy (405nm) using different fluence adjusted by laser power and exposure time. The refractive index before and after laser irradiation was calculated by fitting the experimental data obtained from Spectroscopic Ellipsometer (SE) measurement to Tauc-Lorenz dispersion formula. A positive change in refractive index (Δn=0.19 at 1.55µm) was achieved in As2S3 film by using 10mW, 0.3µs laser irradiation. Due to the rapid and large photo-induced modification of refractive index obtainable with high spatial resolution, this process is promising for integrated optic device fabrication. In addition, material parameters obtained from the experiments have been adopted in the design of ring resonator.

6

P-06

Charge Carrier Density in Li-Intercalated Graphene

T. P. Kaloni, Y. C. Cheng, M. Upadhyay Kahaly, and U. Schwingenschlögl (PSE, KAUST)

The electronic structures of bulk C6Li, Li-intercalated free-standing bilayer (BL) graphene, and Li-intercalated BL and trilayer (TL) graphene on SiC(0001) are studied using density functional theory. Our estimate of Young’s modulus suggests that Li intercalation increases the intrinsic stiffness. For decreasing Li-C interaction, the Dirac point shifts to the Fermi level and the associated band splitting vanishes. For Li-intercalated BL graphene on SiC(0001) the splitting at the Dirac point is tiny. It is also very small at the two Dirac points of Li-intercalated TL graphene on SiC(0001). For all the systems under study, a large enhancement of the charge carrier density is achieved by Li intercalation.

7

P-07

Oxidation of Monovacancies in Graphene by Oxygen Molecules

T. P. Kaloni, and U. Schwingenschlögl (PSE, KAUST)

We study the oxidation of monovacancies in graphene by oxygen molecules using first principles calculations. In particular, we address the local magnetic moments which develop at monovacancies and show that they remain intact when a molecule is adsorbed such that the dangling carbon bonds are not fully saturated. However, the lowest energy configuration does not maintain dangling bonds and is found to be semiconducting. Our data can explain the experimentally observed behavior of graphene under exposure to an oxygen plasma.

8

P-08

Bio-Inspired 3D Nano-Photodiode Devices for Smart CMOS Image Sensing and High Efficiency Solar Energy Harvesting

Fayçal Saffih (PSE/EE & N-Imaging Inc., KAUST)

Previous attempts have been devoted to mimic biological vision intelligence at the architectural system level. In this presentation, a novel imitation of biological visual system intelligence is suggested, at the device level with the introduction of novel photodiode morphology. The proposed bio-inspired nanorod photodiode puts the depletion region length on the path of the incident photon instead of on its width, as the case is with the planar photodiodes. The depletion region has a revolving volume to increase the photodiode responsivity, and thus its photosensitivity. In addition, it can virtually boost the pixel fill factor (FF) above the 100% classical limit due to the decoupling of its vertical sensing area from its limited planar circuitry area. Furthermore, the suggested nanorod photodiode photosensitivity is analytically proven to be higher than that of the planar photodiode. In addition, we show, semi-empirically, that the responsivity of the suggested device varies linearly with its height. This important feature has been confirmed "electro-optically" using Sentaurus device simulation and "biologically" in the cone photocells of Bullfrogs and a kind of fish called Haplochromis burtoni. The proposed nano-photorod is believed to meet the increasingly stringent High-Resolution-Low-Light (HRLL) detection requirements of the camera-phone and biomedical imaging markets as well as the increasing demand for high-efficiency solar cells (photovoltaic).

9

P-09

A Simple Algorithm for Simulation of 3D Anisotropic Material in FDTD

Ahmad Ali Al-Jabr, Mohammad Al-Suniadi (Photonics Lab., KAUST)

Many semiconductor material have anisotropic behavior in the optical range. The existing FDTD algorithms are complicated and require a sort of approximation because the fields are coupled in time. In this work a new and simple algorithm is presented. This algorithm can handle coupling between the fields efficiently. It can simulate light propagation in material with dispersion relations including the lorentze, the Drude, the Debye and also the gyrotropic dispersion found in the magnetized plasma, it works fine for 2D and 3D problems. The well-known problem of the-9-mm slab of magnetized plasma is simulated and excellent agreement with analytical reflection and transmission coefficient is shown.

10

P-10

Thermal Treatment Induced Relaxation of Tensile Strain In Porous GaN Structures

Ahmed Ben Slimane, Adel Najar, Tien Khee Ng (Photonics Lab, KAUST)

We report on the effect of annealing on porous GaN synthesized using UV-assisted electroless etching in a HF:CH3OH:H2O2 solution. The optical properties of: (i) annealed bulk GaN, (ii) as-etched porous GaN, and (iii) annealed porous GaN were characterized using photoluminescence (PL) spectroscopy. A redshift of 26 meV in peak PL wavelength has been observed in the annealed porous GaN, which was attributed to the significant reduction in the bi-axial tensile strain. This process technology is most suitable for the preparation of a porous-GaN-on-sapphire template substrate for potential low dislocation density epitaxy growth.

11

P-11

InGaN Nanowires and Nanomushrooms for Solid-State Lighting

Anwar Gasim (Photonics Lab, KAUST)

InGaN is a promising large bandgap material for solid-state lighting and high-power electronics applications. InGaN nanowire structures have recently attracted great interest due to their property of eliminating the detrimental effects of the lattice mismatch. In this study, InGaN nanowires were grown on Si(111) via molecular beam epitaxy. We report on the first observation of unique nanostructures, taking the form of mushrooms, which were found to have grown alongside the nanowires in certain regions on the sample. Photoluminescence characterization suggests that the nanomushrooms possess a smaller bandgap and thus emit at a longer wavelength. The simultaneous emission from nanowires (420nm) and nanomushrooms (570nm) forms white light. Controlling the growth of both nanostructures may potentially pave the way towards high-efficiency white LEDs.

12

P-12

Slow Thermal Quenching in Indium-Rich InGaN/GaN Self Assembled Quantum Dots

Rami Afandy (Photonics Lab., KAUST)

Differences in optical and structural properties of indium rich (27 %), indium gallium nitride (InGaN) self-organized quantum dot (QD), with red wavelength emission, and the two dimensional underlying wetting layer are investigated. Temperature dependent micro-photoluminescence, reveals a slower thermal quenching of the QDs integrated intensity compared to that of WL. This difference in behaviour is due to the 3-D localization of carriers within the QDs preventing them from thermalization to nearby traps causing an increase in the internal quantum efficiency of the device. Such a slow thermal quenching will enable the development of red InGaN light emitting diodes (LEDs) with high efficiencies at room temperature which when incorporated with blue and green InGaN LEDS can be used to produce white solid state lighting.

13

P-13

Photoluminescence in Amorphous Porous Silicon Suspensions

Asad Mughal and Sahraoui Chaieb (PSE, KAUST)

Transforming bulk Si into the nano-scale results in the emission of visible light from Si at room temperature. Studying the mechanism behind this emission is important for its use in optoelectronics, sensors, and other applications. In this work, we synthesized and characterized amorphous porous Si (ap-Si) suspensions through a chemical etching method. ap-Si was characterized through BET, TEM, XPS, Raman, spectrofluorometry, and FT-IR. As synthesized, it was found to have a high surface area porous structure (493m^2/g) composed of amorphous Si. Photoluminescent emission wavelengths ranged between 523nm (2.37eV) to 621nm (2eV). FT-IR measurements indicated that the PL emission was effected by surface oxidation.

14

P-14

Modeling the Optical Properties of Thermochromic and Electrochromic Devices

Goumri-Said S (PSE division, KAUST)

Recently electrochromic materials have received an increase interest because of their applications in a wide variety of optical modulation devices including smart windows for solar control and automotive sectors. Thermochromic coatings are known to reduce the transmission of solar energy as the temperature rises and can prevent overheating. They found application in the thermal control of buildings, satellites and spatial equipments. In this work, I am interested in the calculation of the theoretical reflectance spectra from the continued fraction method, which relies on exactly solving Maxwell's equations for an arbitrarily stratified medium. In this model, the reflection coefficients for transverse-electric or transverse-magnetic polarized light are expressed in terms of the surface impedances, which take the form of continued fractions. These continued fractions terminate for a finite number of layers deposited onto an infinitely thick substrate (of known refractive index). Their values are determined as soon as the thickness and the refractive index of all the layers are known and the incidence conditions are given. The reflectance is calculated from the value of the surface impedance at z=0, i.e. at the interface between the incidence medium and the first layer of the stack. Complex refractive index data which were obtained from ellipsometric spectral measurements were used as inputs in the developed code. We will report optical measurements and theoretical spectra of DC sputtered thin films of WO3 and VO2 for electrochromic and thermochromic devices.

15

P-15

Large Band Gap Graphene Nanoribbons Synthesize on Patterned Substrates

Guy Olivier Ngongang Ndjawa1, John Anthony2, Aram Amassian1 (1: OEPV, KAUST, 2: University of Kentucky)

Graphene Nanoribbons (GNRS), unlike graphene sheets, display semiconductors properties and while maintaining unique carriers features. Application of GNRs to transistors requires that they have a large band gap and low defects on the edges. A Bottom up synthesis approach from carefully design monomers offers an attractive route towards ultrafast transistors based on large band gap GNRs. As silicon approaches its fundamental limits, which translates to its inability to follow Moore’s law, there is an urgent need for a new kind of material that meets this demand. Graphene emerges as a good candidate with unique electrical properties but presents significant drawbacks because of its metallic behaviour. Graphene Nanoribbons with controlled edges and precise dimensions offer a genuine promise towards the so desired new generations of transistors. The aim of the study is to design of functionalized monomers for the bottom-up Synthesis of large band gap GNRs by Molecular Beam Epitaxy MBE as well as their characterization by In-Situ Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). Application to Fabrication of transistors on patterned substrates will also be investigated.

16

P-16

Resonance Splitting in Ring Resonators

V.A. Melnikov, I.S. Roqan (PSE Division, KAUST)

All-pass and add-drop ring resonator filters with embedded reflective element are analyzed by space-domain coupled-mode theory. This analysis is applied to demonstrate various possibilities for resonance lineshape engineering in ring resonators. For illustration, fitting of resonance lineshapes with different degree of splitting in experimental spectra of add-drop ring resonator fabricated on silicon-on-insulator platform is presented.

17

P-17

Laser Drilling in Disordered Materials Exploiting Disorder for Efficient Drilling for Oil and Gas

Klemens Katterbauer, Andrea Fratalocchi (PRIMALIGHT, KAUST)

The oil and gas industry is the most important industry in world accounting for over half of export revenues of many countries on earth and provides the vital energy supply that keeps most economies afloat. However, with the growing demand for oil and gas in Asia, the need to commercialize previously non-financially viable reservoirs and to extract oil in increasingly more difficult terrains has become a major issue as the existing bit drilling technology has approached its limit. Starting in 1994 with the phase-out of the Star Wars project by the Clinton Administration the Argonne National Lab, the Gas technology Institute and the Colorado School of Mines have investigated revolutionizing the industry by replacing conventional bits by lasers. The results showed that new lasers are easily able to spall and melt rock and that previous calculations (in the 70's) of the energy required for achieving this were highly overestimated. A further result was that of the three components usually found by drilling for oil and gas, the major component sandstone (disordered material) required the highest specific energy. Therefore, we will present the economic significance and an outline of current laser drilling technology, present an ab-initio model via molecular dynamics and FDTD for investigating the light propagation and spalling properties of disordered materials and present a setup for experimentally investigating laser beam propagation within these materials whose beam is emitted by high power fiber lasers.

18

P-18

Nano-Tomography of Metallic Materials

T. Boll, T. Al-Kassab, C. Wille (Tomography of Nano Structured Materials, KAUST)

Atom Probe Tomography (APT) is an emerging characterization technique for anomaterials. At KAUST we have obtained two state of the art instruments: The Local Electrode Atom Probe (LEAP 4000 HR) and the Laser Assisted Wide Angle Atom Probe (LAWATAP). Our group is involved in utilizing these instruments for materials characterization and development as well as in introducing new methods for analysis. The strength of APT is its ability to chemically characterize non-organic materials in 3 dimensions with a resolution of up to 0.1 nm. The samples are typically needle shaped with a tip diameter of about 100 nm, which can be produced by electrochemical methods or FIB. In this poster we will present experimental results of two metallic materials, which were recently obtained at the KAUST facilities:

1) Ni-11.3at.%Ti was subjected to different heat treatments and the formation of L12-Ti rich precipitates is analyzed. In APT we observe a superstructured sequence of atomic planes within the precipitates, even for early stages of the precipitation. This leads to the conclusion that the precipitates exhibit a high degree of ordering.

2) For ball-milled powder samples of Fe-10at.%Cu, cluster formation upon heat treatment was analyzed by means of APT and TEM. The nanostructure and chemical composition with emphasis on the spatial distribution of the minority component Cu is presented.

19

P-19

Ferromagnetism in (Zn, Gd)O: A Potential p-Type Ferromagnetic Semiconductor for Spin-LEDs

I. Bantounas, S. Venkatesh, I. S. Roqan and U. Schwingenschlogl (KAUST)

In the current study we investigate the magnetic properties and carrier type (p or n) of Gd-doped ZnO as a potential magnetic semiconductor and try to elucidate the magnetic behaviour using ab initio calculations. Magnetic semiconductors could pave the path toward electronic devices which exploit both the charge and spin of an electron, with spin-LEDs being amongst the list of proposed spin dependent devices. Interest in ZnO follows as it has been put forward as a promising candidate material for the realisation of a room temperature ferromagnetic semiconductor. In addition, if the difficulties in obtaining p-type ZnO can be overcome, ZnO may be able to compete with GaN in the current optoelectronic device industry offering the potential for lower cost and simpler fabrication techniques. Our experimental findings show room temperature ferromagnetism and samples displaying p-type carriers under certain deposition conditions, though, such findings suffer from difficulties with reproducibility. This lack of consistency is supported by ab initio total energy calculations, which show there to be no coupling between the magnetic ions in the (Zn, Gd)O system. A finding that points toward extrinsic sources of ferromagnetism, such as foreign elements or unwanted secondary phases formed during deposition. Work is under way to examine the possibility of extrinsic ferromagnetic ordering.

20

P-20

Removal of Aniline from Aqueous Solutions by Multi-walled Carbon Nanotubes

Hind Al-johani and Mohamed Abdel Salam (KCC, KAUST)

Aromatic amines are widely used in agriculture, drugs, resins, inks, marking, perfume, shoe polish, dyes, polymers and many other industries. Aromatic amines has great harmful effect for public health and environmental quality, aromatic amines contained wastewater has brought a series of serious environmental problem due to their high toxicity and accumulation in the environment. Thus, there is a need to treat the wastewater contain high levels of aromatic amines before being discharged into the environment. Traditional methods of removal or destruction of aromatic amines from wastewaters include solvent extraction, biodegradation, oxidation catalyst, and separation of the membrane, ultrasonic degradation, supercritical water oxidation, and electrochemical oxidation. However, they are in some cases restricted because of economic constraints or technology. Many research studies have showed the ability of carbon nanotubes to adsorb different pollutants from various aqueous samples due to their exceptional properties. In this research work, the effect of different factors, such as adsorbent mass, solution temperature, and pH on the removal of Aniline; as an example of the aromatic amines, from aqueous solution was studied using different multi-walled carbon nanotubes. The adsorption of aniline on pristine MWCNTs at different temperatures was studied and the thermodynamic parameters showed that the adsorption process is product favoured, and becomes more so at lower temperature, since the adsorption is exothermic.

21

P-21

Biofunctionalized Magnetic Nanowires as Promising Materials for Cancer Cell Destruction

María Fernanda Contreras†, §, Timothy Ravasi† and Jürgen Kosel§ (†Integrative Systems Biology Lab, CLSE, KAUST, §Sensing, Magnetism and Microsystems Group, PSE, KAUST)

Magnetic micro and nanomaterials are increasingly interesting for biomedical applications since they posses many advantageous properties: they can become biocompatible, they can be functionalized to target specific cells and they can be remotely manipulated by magnetic fields. The main goal of this project is to investigate magnetic nanowires as an alternative method overcoming the limitations of current cancer treatments that lack specificity and are highly cytotoxic. Nanowires are developed so that they selectively attach to cancer cells, potentially destroying them when a magnetic field induces their vibration. This will transmit a mechanical force to the targeted cells, which is expected to induce apoptosis on them.

22

P-22

Omnidirectional Photonic Band Gap Using Low Refractive Index Contrast Materials in a Disordered One-Dimensional Photonic Crystal

Angelo Vidal Faez, Andrea Fratalocchi (PRIMALIGHT, KAUST)

Photonic crystals researchers have argued for many years that one of the conditions for omnidirectional reflection in a one-dimensional photonic crystal is a strong refractive index contrast between the two constituent dielectric materials. Using numerical simulations and the theory of Anderson localization of light, in this work we demonstrate that an omnidirectional band gap can indeed be created utilizing low refractive index contrast materials when they are arranged in a disordered manner. Moreover, the size of the omnidirectional band gap becomes a controllable parameter, which now depends on the number of layers and not only on the refractive index contrast of the system, as it is widely accepted.

23

P-23

Detecting Electromagnetic Cloaks Using Backward Propagating Waves

Mohamed A. Salem, and Hakan Bağcı (Computational Electromagnetics Lab., KAUST)

A novel approach for detecting transformation-optics invisibility cloaks is proposed. The detection method takes advantage of the unusual backward-propagation characteristics and spectral structure of recently reported beams and pulses to induce electromagnetic scattering from the cloak. Even though waves with backward-propagating energy flux cannot penetrate the cloaking shell and interact with the cloaked objects (i.e., they do not make the cloaked object visible), they provide a mechanism for detecting the presence of cloaks.

24

P-24

Reflection and Transmission of Full-Vector X-Waves Through a Dielectric Slab

Mohamed A. Salem, and Hakan Bağcı (Computational Electromagnetics Lab., KAUST)

The reflection and transmission of full-vector X-Waves incident normally on a lossless dielectric slab are investigated. Full-vector X-Waves are obtained by superimposing transverse electric and magnetic polarization components, which are derived from the scalar X-Wave solution. The analysis of transmission and reflection is carried out via a straightforward but yet effective method: First, the X-Wave is decomposed into vector Bessel beams via the Bessel-Fourier transform. Then, the reflection and transmission coefficients of the beams are obtained in the spectral domain. Finally, the transmitted and reflected X-Waves are obtained via the inverse Bessel-Fourier transform carried out on the X-wave spectrum weighted with the corresponding coefficient.

25

P-25

Generation of X-Wave Pulse Trains Using Gradient-Index (GRIN) Components

Mohamed A. Salem, and Hakan Bağcı (Computational Electromagnetics Lab., KAUST)

The appropriate choice of phase functions on a linear positive group-velocity dispersive medium coupled to a plane-wave light source could yield a class of propagation invariant localized wave solutions. In this work, we demonstrate the possibility of achieving a localized wave pulse train of the X-Wave type propagating along the optical axis gradient-index (GRIN) axicons. We demonstrate the effect of varying the GRIN material dispersion characteristics, the phase function and the physical dimension of the GRIN component on the generated X-Wave pulse train. This approach is particularly suitable for applications in free-space optic communication systems to overcome diffraction and possible atmospheric attenuation, to increase the link distance or to diminish the power emitted.

26

P-26

On the Behavior of Vector Bessel Beams Obliquely Incident onto a Dielectric Half-Space

Mohamed A. Salem, Bağcı H. (Computational Electromagnetics Lab., KAUST)

Full-vector Bessel beams are derived by superimposing transverse electric and transverse magnetic polarized vector Bessel beam solutions of Maxwell equations. This superposition was shown to give the full-vector Bessel beams unique energy flow characteristics, which can be tuned by varying the relative phase shift between the two polarization components. In this work, the reflection and transmission of full-vector Bessel beams obliquely incident at a planar (refracting) half-space from free-space are studied using a semi-numerical approach. The incident full-vector Bessel beams are expanded in terms of vector spherical waves. The reflection and transmission coefficients of each component of a given vector spherical wave are obtained analytically. The expressions of the reflected and transmitted wave fields are then computed numerically by superimposing all reflected and scattered spherical wave components, respectively. The study is carried out for various material types (double-positive, double-negative, epsilon-negative and mu-negative) and novel shifts are observed.

27

P-27

Simulation of Transient Electromagnetic Fields on Photonic Devices via Explicit Marching-on-in-time Solution of the Time Domain Volume Integral Equation

Ahmed Al-Jarro and Hakan Bagci (Computational Electromagnetics Lab., PSE, KAUST)

Simulation methods capable of accurately and efficiently characterizing transient electromagnetic wave interactions with arbitrarily-shaped and inhomogeneous penetrable (dielectric) bodies are indispensable for many practical applications in the field of photonics. For this purpose, marching-on-in-time (MOT)-based solutions of the time domain volume integral equation (TDVIE) solvers are now becoming an attractive alternative to finite-element time domain (FETD) and finite-difference time domain (FDTD) methods. Unlike FETD and FDTD methods, MOT-TDVIE solvers i) require only the discretization of the scatterers domain, ii) implicitly satisfy radiation condition (i.e., they do not require absorbing or perfectly matched boundary conditions to truncate the computation domains), and iii) do not suffer from numerical dispersion. Nevertheless, MOT-TDVIE solvers require the computation of retarded fields/potentials, which involve temporal and spatial convolutions of the time history of currents/fields induced on scatterers with the Green’s function. This renders them susceptible to late-time instabilities and increases their computational complexity. In this work we report on the efficacy of the recent developments for stabilizing and accelerating (via parallelization) the MOT-TDVIE solvers when applied to the characterization of transient electromagnetic wave interactions on photonic structures. These include development of stable predictor-corrector schemes, memory efficient explicit MOT algorithms and highly scalable parallelization schemes. Numerical examples include simulations of the backscattered diffraction patterns of biological cells - a practical problem in life sciences microscopy - as well as narrow high intensity light beams (photonic nanojets) - a carefully engineered excitation mechanism for the detection of dielectric sub-wavelength nano features.

28

P-28

Explicit Solution of Second-kind Time Domain Integral Equations Using a Predictor-Corrector Algorithm

Ülkü H. A., Bağcı H. (Computational Electromagnetics Lab., KAUST), Michielssen E. (University of Michigan, Ann Arbor, MI, USA)

A marching-on-in-time based second kind integral equation (MOT-SKIE) solver, which derives from a recently developed novel predictor-corrector scheme, i.e., PE(CE)m, is presented. Contrary to the polynomial based traditional PE(CE)m schemes, the new scheme approximates the solution in terms of exponentials but still assumes the same form as traditional predictor-corrector methods. To apply this new PE(CE)m scheme in MOT-SKIE solvers, we construct a second kind time domain integral equation that imposes a boundary condition on the time derivatives of the magnetic and electric fields. Current density values are updated via a straightforward application of the PE(CE)m recipe; the evaluation of all retarded boundary integrals is carried out semi-analytically, which continues to require the notion of a temporal basis function. In contrast to previous explicit MOT schemes, the proposed scheme yields controllable errors that can be matched to those of the spatial discretization used. Numerical results that demonstrate the effectiveness of the proposed scheme and compare its accuracy and stability characteristics to those of existing MOT schemes, will be presented.

29

P-29

On the Low-Frequency Behavior of Time Domain Magnetic Field Integral Equation

Ülkü H. A., Bağcı H. (Computational Electromagnetics Lab., KAUST), Bogaert I. (Ghent University, Ghent, Belgium), Cools K. (University of Nottingham, Nottingham, UK), Andriulli F. P. (TELECOM Bretagne, Brest, France)

Transient electromagnetic scattering from closed perfect electrically conducting surfaces can be analyzed by solving the time-domain magnetic field integral equation (TD-MFIE) using marching-on-in-time (MOT) techniques. The classical MOT discretization schemes use Rao-Wilton-Glisson (RWG) and polynomial basis functions to expand the current in space and time respectively; RWG and Dirac delta functions are then used to test the equations in space and time. In this work, it will be shown that the solution of the MOT-TD-MFIE matrix system constructed using RWG basis and testing functions loses its accuracy under the large time-step condition even though MOT-TD-MFIE matrix system does not suffer from ill-conditioning. It will be shown that the mixed discretization of MFIE, which uses the RWG basis functions and the (curl conforming) Buffa-Christiansen (BC) testing functions, eliminates the inaccuracy of the solution at the low frequencies by implicitly enforcing the correct frequency-scaling of the solution’s Helmholtz components (without the need for a loop-tree/star decomposition).

30

P-30

Highly-Accurate Modeling of Diffraction Gratings Using Time Domain Techniques

Kostyantyn Sirenko1, Anna Krivchikova2, Yuriy Sirenko2, and Hakan Bağcı1

1 Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia 2 Institute of Radiophysics and Electronics of National Academy of Sciences of Ukraine (IRE NASU), Kharkiv, Ukraine

Almost all well-established methods that are used for mathematically modeling diffraction gratings are frequency-domain methods, namely, Fourier modal method, C-method, method of analytical regularization. Time-domain methods posses some advantages over frequency-domain ones, especially when a grating is studied as an open resonator. Numerical implementation of time domain differential equation based methods inevitably faces the problem of computational domain truncation. Due to their limited error controllability, use of perfectly matched layers (PMLs) for this purpose might reduce the accuracy of the overall solution especially for simulations with long observation times used in analysis of resonant wave behavior. This work focuses on development of exact absorbing boundary conditions (EACs) for error-free computation domain truncation for 2D and 3D diffraction gratings. Implementation details of the proposed EACs and numerical results demonstrating their effectiveness will be presented.

31

P-31

Plasmon Induced Transparency via Higher-Order Modes on a Gold Disk with an Elongated Cavity

Muhammad Amin, Bağcı H. (Computational Electromagnetics Lab., KAUST)

We propose a planar meta-material medium of sub-wavelength thickness to realize Electromagnetic Induced Transparency (EIT) capable of strongly dispersive and low loss transmission behavior in the optical spectrum. A Fano resonance is observed in the transmission spectrum of a meta-atom composed of a gold nano-disk with an embedded non-concentric elongated cavity. It has been shown previously that a gold disk with an embedded non-concentric circular cavity and a thin wire supports hybridization-induced localized surface plasmon resonances over a band of frequency, which includes visible and near infrared part of spectrum. In this work, we extend this to allow for the generation of a Fano resonance in the same region of the spectrum under normal incidence. In this new configuration, the symmetry breaking non-concentric elongated cavity forces the otherwise dark higher order modes to brighten. By tuning the geometric parameters of the design, these narrow sub-radiant higher-order modes are moved closer to wide super-radiant dipole modes in the spectrum to cause interference between modes. This destructive interference between the hybridized dipole-dipole mode and the brightened quadruple-quadruple mode generates a Fano resonance in the visible range of the spectrum. The slow light propagation within the Fano resonance band is demonstrated via group index calculation making use of meta-atom’s S-parameters. A periodic arrangement of such meta-atom can be used to construct an ultra-compact and tunable dispersive medium, which might be useful in various applications of “slow” light.

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P-32

Development of Sparsity-based Optimization Algorithms for Inverse Electromagnetic Scattering Problems

Abdulla Desmal, Bağcı H. (Computational Electromagnetics Lab., KAUST)

The difficult of solving electromagnetic inverse problems stems from (i) the non-linearity of the scattering equations and (ii) the ill-conditioning of the problem. The former can be avoided via the use of Born iterations, which convert the non-linear inverse problem into a sequence of linear inverse problems. To alleviate the ill-conditioning of the problem, a regularization scheme has to be applied at each step of the Born iterations. For this purpose, 2nd - norm regularizers have been used extensively, and the resulting optimization problem has been solved by using Tikhonov- or conjugate gradient-based schemes. However, 2nd - norm regularizers promote the smoothness in the solution; and produce high-levels of error when applied in domains with sharp variations and sparse content. Such domains exist in many practical applications, such as see-through-the-wall imaging and molecular detection. In this type of applications, priori knowledge of the domain’s sparsness could be used to alleviate the ill-conditioning, i.e., for regularization. Four methods have been tested and compared for sparsity regularization of Born iterations, namely ISTA (Iterative Shrinkage Thresholding Algorithm), FIST (Fast ISTA), TwIS (Two Steps ISTA) and Salsa (Split Augmented Lagrangian Shrinkage Algorithm).

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P-33

Studying the Structural and Optical Characteristics of Thin Film InGaN

Y. Basma, I. Roqan (Optical Spectroscopy Lab., KAUST)

We present Optical and Structural studies of state of the art InxGa1-xN.the structural and optical properties of InxGa1-xN/GaN were investigated by using X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy dispersive X-ray (EDX), Raman spectroscopy, Photoluminescence (PL) and PLE. The III-nitride material system with band gap ranging from 0.7ev to 6.2ev has high potential to develop high efficiency solar cells. The III-nitride materials are grown by MOCVD on a lattice mismatched sapphire substrate (0001). Four InGaN bulk materials grown by MOCVD at different temperature (720<T<920) degrees with Indium concentrations (0.3%<x<24%) on a GaN buffer.

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P-34

Plasmonic Thin Film Solar Cell

Bilal Janjua (Solar and Alternative Energy Center)

Amorphous Silicon (a-Si) thin film solar cells have the potential to provide cheap and clean energy to the rapidly growing energy demand. Up till now, thin film solar cells trivial role in the solar cell market has been limited by their small efficiencies. The main limitation in these devices is short carrier diffusions length in the material and large bandgap (1.7eV) of a-Si. Numerous ideas have been proposed to improve the absorption in these devices. In conventional solar cells, to improve absorption, front and back surface texturing is used to trap light. With this technique enhancement of up of 4n2, where n is the refractive index of the absorbing material, can be achieved. In thin film solar cells, sub-wavelength texturing is required to achieve the desired results. Current surface texturing designs does not provide efficient scattering of near IR region of the solar radiation. Plasmonics is an emerging field which allows scattering, guiding and localization of light at sub wavelength scales using nanostructures. Merging plasmonics with thin film solar cells has opened new frontiers into improving the efficiencies of thin film solar cells. Enhanced absorption in plasmonic solar cells, allow thinner absorbing layer considerably reducing the material cost.

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P-35

The Influence of Metal Interlayers on the Structural and Optical Properties of Nano-Crystalline TiO Films

Yang Yang, Qiang Zhang, Bei Zhang, Long Chen, Liang Li, Chao Zhao, Elhadj Marwan Diallo, and Xixiang Zhang (Advanced Nanofabrication and Imaging Core Lab., KAUST)

Anatase TiO2 films have been mostly investigated due to the photocatalysis property that enables wide applications including water splitting, anti-fogging and self-cleaning glass, and solar cells. Since the structural and electronic properties of TiO2 film hold the key to its potential applications, pure anatase TiO2 film with a narrower band gap has attracted extensive attention in recent years. TiO2-M-TiO2 (M=W, Co and Ag) multilayer films deposited on glass substrates have been systematically investigated using micro-Raman spectroscopy combined with various characterization methods.

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P-36

Monolithic Si-based Gaussian to Non-linear beam converter

Damian San Roman Alerigi, Tien Khee Ng, Yaping Zhang, Ahmed Ben Slimane (Photonics Lab., KAUST)

We present the theoretical studies of a refractive index mapping to implement a Gauss to J0 Bessel-Gauss convertor. The proposed device consists of a silicon oxide slab, 200um length and 25um width; where the refractive index varies in controllable steps across the light propagation and transversal directions. The conversion efficiency and loss are 90%, a loss 0.457dB respectively. The results obtained from the beam conversion-efficiency, self-regeneration, and propagation through an opaque obstruction; demonstrate that a 2D graded index mapping can transform a Gauss beam into a J0 -Bessel-Gauss beam. To the best of our knowledge, this is the first demonstration of such beam transformation by means of a monolithic 2D index-map fully integrable with silicon photonics based planar lightwave circuits (PLC). The concept device is significant for the eventual development of a new array of technologies such as micro optical tweezers, optical traps, beam reshaping and non-linear beam diode lasers.

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P-37

Chalcogenide Glass for a Monolithic Mid-Infrared Laser Design

Ajymurat Orozaliev, Karim R. Mahmoud Gad Elrab, Faleh AlTal, Matteo Chiesa, Clara Dimas (Masdar Institute of Science and Technology, Abu Dhabi, UAE)

Erbium-doped Gallium Lanthanum Sulfide (GLS) is studied as a possible gain medium to fabricate micro-disk lasers emitting in the MIR regime. A mathematical model was built to simulate lasing at 4.5 µm and 2 µm wavelengths, given doping level, film quality and device designs parameters which are achievable through low-cost process development. Initial thin films of evaporated GLS are characterized to begin the experimental validation of our model and microfabrication plan. These simulation and preliminary experimental results are a first step to the fabrication of a CMOS or detector array compatible laser sources that can enable absorption spectroscopy on-a-chip.

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P-38

Optical properties of CdO Ni dopped nano structured thin films

L.E. Al-Otibi, W.A. Farooq, A.S. Al-Dwayyan, F. Yakuphanoglu

We have investigated the effect of laser irradiation on optical properties of CdO and Ni doped CdO thin films prepared with Sol Gel technique on glass substrate. In this study we have prepared CdO and Ni doped CdO thin films using Sol-gel technique and irradiate the thin film samples with Nd:YAG laser (Barilliant,A02BR) at wavelength of 1064 nm for different energy densities. Then absorption of the irradiated samples is carried out in the range of 200-700 nm using spectrophotometer lamda-40 (UV-VISBILE SPECTROSCOPY, SHIMADZU CORPORATION, UV-1650PC). Emission spectra of these samples are also investigated using fluoro-spectrophotometer (SpectroFlouroPhotometer, RF-3501(PC)S). The obtained results indicate that the optical band gap of the CdO films can be controlled by laser irradiation energy. We have also observed shift in emission spectra with changing power density.