Pierre Wahl

Multiple-wavelength focusing of surface plasmons with a nonperiodic nanoslit coupler.

A novel type of multiple-wavelength focusing plasmonic coupler based on a nonperiodic nanoslit array is designed and experimentally demonstrated. An array of nanoslits patterned on a thin metal film is used to couple free-space light into surface plasmon polaritons (SPPs) and simultaneously focus different-wavelength SPPs into arbitrary predefined locations in the two-dimensional plane. We design and fabricate a compact triplexer on a glass substrate with an integrated silicon photodetector. The photocurrent spectra demonstrate that the incident light is effectively coupled to SPPs and routed into three different focal spots depending on the wavelength. The proposed scheme provides a simple method of building wavelength-division multiplexing and spectral filtering elements, integrated with other plasmonic and optoelectronic devices.

Routing and photodetection in subwavelength plasmonic slot waveguides

Abstract The ability to manipulate light at deeply sub-wavelength scales opens a broad range of research possibilities and practical applications. In this paper, we go beyond recent demonstrations of active photonic devices coupled to planar plasmonic waveguides and demonstrate a photodetector linked to a two conductor metallic slot waveguide that supports a mode with a minute cross-sectional area of ∼λ2/100. We demonstrate propagation lengths of ∼10λ (at 850 nm), routing around 90° bends and integrated detection with a metal-semiconductor-metal (MSM) photodetector. We show polarization selective excitation of the slot mode and measure its propagation characteristics by studying the Fabry-Perot oscillations in the photocurrent spectra from the waveguide-coupled detector. Our results demonstrate the practicality of transferring one of the most successful microwave and RF waveguide technologies to the optical domain, opening up many opportunities in areas such as biosensing, information storage and communication.

Simple Electroabsorption Calculator for Designing 1310 nm and 1550 nm Modulators Using Germanium Quantum Wells

With germanium showing significant promise in the design of electroabsorption modulators for full complementary metal oxide semiconductor integration, we present a simple electroabsorption calculator for Ge/SiGe quantum wells. To simulate the quantum-confined Stark effect electroabsorption profile, this simple quantum well electroabsorption calculator (SQWEAC) uses the tunneling resonance method, 2-D Sommerfeld enhancement, the variational method and an indirect absorption model. SQWEAC simulations are compared with experimental data to validate the model before presenting optoelectronic modulator designs for the important communication bands of 1310 nm and 1550 nm. These designs predict operation with very low energy per bit ( <; 30×fJ/bit).

Investigation of Limits to the Optical Performance of Asymmetric Fabry-Perot Electroabsorption Modulators

We have investigated the suitability of surface-normal asymmetric Fabry-Perot electroabsorption modulators for short-distance optical interconnections between silicon chips. These modulators should be made as small as possible to minimize device capacitance; however, size-dependent optical properties impose constraints on the dimensions. We have thus performed simulations that demonstrate how the optical performance of the modulators depends on both the spot size of the incident beam and the dimensions of the device. We also discuss the tolerance to nonidealities such as surface roughness and beam misalignment. The particular modulators considered here are structures based upon the quantum-confined Stark effect in Ge/GeSi quantum wells. We present device designs that have predicted extinction ratios greater than 7 dB and switching energies as low as 10 fF/bit, which suggests that these silicon-compatible devices can enable high interconnect bandwidths without the need for wavelength division multiplexing.

B-CALM: AN OPEN-SOURCE MULTI-GPU-BASED 3D-FDTD WITH MULTI-POLE DISPERSION FOR PLASMONICS

Numerical calculations based on finite-difference timedomain (FDTD) simulations for metallic nanostructures in a broad optical spectrum require an accurate modeling of the permittivity of dispersive materials. In this paper, we present the algorithms behind B-CALM (Belgium-CAlifornia Light Machine), an open-source 3D-FDTD solver simultaneously operating on multiple Graphical Processing Units (GPUs) and efficiently utilizing multi-pole dispersion models while hiding latency in inter-GPU memory transfers. Our architecture shows a reduction in computing times for multi-pole dispersion models and an almost linear speed-up with respect to the amount of used GPUs. We benchmark B-CALM by computing the absorption efficiency of a metallic nanosphere in a broad spectral range with a six-pole Lorentz model and compare it with Mie theory and with a widely used Central Processing Unit (CPU)-based FDTD simulator.

Energy-per-Bit Limits in Plasmonic Integrated Photodetectors

The energy consumption per transmitted bit is becoming a crucial figure of merit for communication channels. In this paper, we study the design tradeoffs in photodetectors, utilizing the energy per bit as a benchmark. We propose a generic model for a photodetector that takes optical and electrical properties into account. Using our formalism, we show how the parasitic capacitance of photodetectors can drastically alter the parameter values that lead to the optimal design. Finally, we apply our theory to a practical case study for an integrated plasmonic photodetector, showing that energies per bit below 100 attojoules are feasible despite metallic losses and within noise limitations without the introduction of an optical cavity or voltage amplifying receiver circuits.

A Performance Model for GPU-Accelerated FDTD Applications

In this work we develop, validate and use a performance model for a Finite-Difference Time-Domain (FDTD) application which is parallelized on multiple GPUs. FDTD is a method for simulating electrodynamic interaction and is applied in a number of research and engineering areas. In this work we focus on a particular implementation called B-CALM (Belgium-California Light Machine). We adopt a simple, semi-empirical modelling approach to design a model which we validate for different hardware architectures. Using the model allows making implementation decisions and exploring the architectural design space with the goal of optimizing HPC systems for this application.

Modal Source Radiator Model for Arbitrary Two-Dimensional Arrays of Subwavelength Apertures on Metal Films

We present a theoretical and numerical framework to analyze optical properties of subwavelength apertures that are distributed arbitrarily on the 2-D plane of a metal film. Using the radiation patterns linked to individual eigenmodes inside the aperture, the coupling between multiple apertures is described efficiently by a small-rank linear system of equations. The complicated contributions from both the surface plasmon polariton (SPP) and creeping wave are inherently included in the analysis. Three-dimensional fully vectorial finite-difference time-domain calculations are used to verify the model in several test cases. The model accurately predicts all the salient features in extraordinary optical transmission (EOT) spectra of 2-D nanoslit arrays, including the surface plasmon resonances and Rayleigh-Wood anomalies. We also explore the effects of finite array size on EOT and discover a novel regime where the EOT efficiency decreases with an increasing number of apertures. Finally, we apply the model in calculating SPP excitation by an arbitrarily patterned nanoslit array. The presented method not only provides deeper physical insights into EOT and related phenomena, but should also be useful in designing a variety of novel aperiodic plasmonic devices with drastically less computational cost.

B-CALM: An open-source GPU-based 3D-FDTD with multi-pole dispersion for plasmonics

Numerical calculations with finite-difference time-domain (FDTD) on metallic nanostructures in a broad optical spectrum require an accurate approximation of the permittivity of dispersive materials. Here, we present the algorithms behind B-CALM (Belgium-California Light Machine), an open-source 3D-FDTD solver operating on Graphical Processing Units (GPUs) with multi-pole dispersion models. Our modified architecture shows a reduction in computational times for multi-pole dispersion models for a broad spectral range. We benchmark B-CALM by computing the absorption efficiency of a metallic nanosphere with a one-pole and a three-poles Drude-Lorentz model and compare it with Mie theory.

Design of large scale plasmonic nanoslit arrays for arbitrary mode conversion and demultiplexing

We present an iterative design method for the coupling and the mode conversion of arbitrary modes to focused surface plasmons using a large array of aperiodically randomly located slits in a thin metal lm. As the distance between the slits is small and the number of slits is large, significant mutual coupling occurs between the slits which makes an accurate computation of the field scattered by the slits difficult. We use an accurate modal source radiator model to efficiently compute the fields in a significantly shorter time compared with three-dimensional (3D) full-field rigorous simulations, so that iterative optimization is efficiently achieved. Since our model accounts for mutual coupling between the slits, the scattering by the slits of both the source wave and the focused surface plasmon can be incorporated in the optimization scheme. We apply this method to the design of various types of couplers for arbitrary fiber modes and a mode demultiplexer that focuses three orthogonal fiber modes to three different foci. Finally, we validate our design results using fully vectorial 3D nite-difference time-domain (FDTD) simulations.