Salah Obayya

Improved Complex-Envelope Alternating-Direction-Implicit Finite-Difference-Time-Domain Method for Photonic-Bandgap Cavities

In this paper, an improved complex-envelope alternating-direction-implicit finite-difference time-domain (CE-ADI-FDTD) method has been presented for the analysis of photonic-bandgap cavities. The improvement relies on a different approach of the perfectly matched-layer absorbing-boundary condition in order to avoid the formation of instability, as reported in the literature. The high numerical precision and efficiency obtained are clearly demonstrated through the agreement of the results obtained using CE-ADI-FDTD and their counterparts obtained using other rigorous approaches reported in the literature

Numerical analysis of bent waveguides: bending loss, transmission loss, mode coupling, and polarization coupling

A rigorous, full-vectorial and computationally efficient finite-element-based modal solution, together with junction analysis and beam propagation approaches have been used to study bending loss, transition loss, mode coupling, and polarization coupling in bent optical waveguides. The waveguide offset and their widths have been optimized to reduce the transition loss and the mode beating.

Accurate Perfectly Matched Layer Finite-Volume Time-Domain Method for Photonic Bandgap Devices

In this letter, a new explicit time-domain method for the analysis of light propagation in photonic bandgap (PBG) devices is suggested. In essence, this method is based on the finite volume that employs triangular elements in order to accurately represent curved boundaries encountered in PBG devices. Moreover, uniaxial perfectly matched layer absorbing boundary conditions has also been incorporated to rigorously truncate the computational domain. The high numerical precision of the suggested approach is demonstrated through numerical examples.

Implementation of Photonic Crystal Couplers

Finite Difference Time Domain (FDTD) analysis is effectively applied to investigate the transmission of the transverse electric polarized pulses in a 90deg bend photonic crystal waveguide. A comparison with a Time Domain Beam Propagation Method (BPM) based on finite clement scheme is made and excellent agreement is achieved. Moreover, a detailed study of photonic crystal couplers has been carried out. A series of simulations are performed to determine by how much the radius of holes would have to be tuned to result in realizing the functions of the truth tables of the proposed optical directional couplers.

Optimization of nanoantenna for solar energy harvesting based on particle swarm technique

This paper presents a new trend in the design of nanoantenna for solar energy harvesting. By creating a link between a particle swarm optimization (PSO) algorithm and an external finite element frequency domain (FEFD) solver, the algorithm refines all the design metrics to optimize harvesting efficiency. Simulation results show the effectiveness of applying PSO to nanoantenna design which offers a total solar energy harvesting efficiency of 98.7%. This is significantly higher than up to date solar cells.

Detection of DNA hybridization by hybrid alternative plasmonic biosensor

In order to detect DNA hybridization with label free and high sensitivity, a hybrid alternative plasmonic slot waveguide (HAPSW) biosensor based on silicon-on-insulator (SOI) is proposed and analyzed. The reported design increases the light interaction with the sensing region by using a slot-waveguide along with titanium nitride as an alternative plasmonic material. The suggested biosensor can detect the slightest change in the analyte refractive index with high sensitivity due to an ultra-high optical confinement in the low-index regions caused by the high index contrast and plasmonic enhancement. The effective index, normalized power confinement, and sensitivity are analyzed for the detection of the DNA hybridization. The simulation results are obtained using full vectorial finite element method (FVFEM). The suggested biosensor has high sensitivity of 1190 nm/RIU (refractive index unit) for DNA hybridization detection, which is very high relative to those reported in the literature to the best of our knowledge.

Modified conical silicon nanowires for highly efficient light trapping

A modified nanocone nanowire (NW) is proposed and analyzed for solar cell applications. The suggested NW consists of conical and truncated conical units. The geometrical parameters are studied by using 3D finite difference time domain (FDTD) method to achieve broadband absorption through the reported design and maximize its ultimate efficiency. The analyzed parameters are absorption spectra, ultimate efficiency and short circuit current density. The numerical results prove that the proposed structure is superior compared to cone, truncated cone and cylindrical nanowires (NWs). The reported design achieves an ultimate efficiency of 44.21% with an enhancement of 40.66% relative to the conventional conical NWs. Further, short circuit current density of 36.17 mA/cm2 is achieved by the suggested NW. The modified nanocone has advantages of broadband absorption enhancement, low cost and fabrication feasibility.

Recent Trends in Plasmonic Nanowire Solar Cells

Light trapping is crucial for low-cost and highly efficient nanowire (NW) solar cells (SCs). In order to increase the light absorption through the NWSCs, plasmonic materials can be incorporated inside or above the NW design. In this regard, two novel designs of plasmonic NWSCs are reported and analyzed using 3D finite difference time domain method. The geometrical parameters of the reported designs are studied to improve their electrical and optical efficiencies. The ultimate and power conversion efficiencies (PCE) are used to quantify the conversion efficiency of the light into electricity. The first design relies on funnel shaped SiNWs with plasmonic core while the cylindrical NWs of the second design are decorated by Ag diamond shaped. The calculated ultimate efficiency and PCE of the plasmonic funnel design are equal to 44% and 18.9%, respectively with an enhancement of 43.3 % over its cylindrical NWs counterpart. This enhancement can be explained by the coupling between the three optical modes, supported by the upper cylinder, lower cone and plasmonic material. Moreover, the cylindrical SiNWs decorated by Ag diamond offer an ultimate efficiency and short-circuit current density of 25.7%, and 21.03 mA∕cm2, respectively with an improvement of 63% over the conventional cylindrical SiNWs.

Why Do Field-Based Methods Fail to Model Plasmonics?

The paper studies plasmonics modeling issues and examines the reasons behind the failure of the field-based methods relying on Padé approximations widely used in the analysis of photonic devices based on dielectric materials. Through a study of evanescent, radiation, guided, and surface modes of a plasmonic structure where the failure appears clearly, we demonstrate the physical explanation of this failure and suggest some remedies. We developed a Bidirectional Beam Propagation Method (BiBPM) by adopting a Blocked Schur (BS) algorithm to introduce an unconditionally stable method for plasmonic structures with strong discontinuities. Central to BiBPMs is the accurate calculation of the square root operators that is very widely performed using Padé approximations. However, recent reports demonstrate convergence of Padé that is too slow to lend itself a stable solver in plasmonics. Moreover, Padé approximations completely fail in handling such a strong discontinuity between dielectric and plasmonic waveguides, where a very-wide spectrum of modes could be excited. Alternatively, we propose calculating these operators by the twice faster BS algorithm. Beyond the computational speed, our suggested approach overbears the Padé-based BiBPMs instability and accuracy problems, thanks to the proper physical treatment of surface and evanescent waves: the notorious sources of instability. Through the plasmonic discontinuity problems, the superiority of BS approach has been determined numerically and explained physically.

Bending loss, transition loss, mode coupling, and polarization coupling in bent waveguides

A rigorous, full-vectorial and computationally efficient finite element-based modal solution, together with junction analysis and beam propagation approaches have been used to study bending loss, transition loss, mode coupling and polarization coupling in bent optical waveguides with vertical or slanted side walls. The waveguide offset and their widths have been optimized to reduce the transition loss and the mode beating.