M. Gnan

Efficient coupling into slow-light photonic crystal channel guides using photonic crystal tapers.

Photonic crystal tapers have been designed for coupling of light from ridge waveguides into low group velocity photonic crystal channel guides. The coupling efficiency is increased from 3 % (case of butt-coupling) to 97 % for frequencies in the band-edge region, corresponding to a group index close to 100, as predicted using 2D finite-difference time-domain simulations.

Tapered Photonic Crystal Microcavities Embedded in Photonic Wire Waveguides With Large Resonance Quality-Factor and High Transmission

We present the design, fabrication, and characterization of a microcavity that exhibits simultaneously high transmission and large resonance quality-factor (Q-factor). This microcavity is formed by a single-row photonic crystal (PhC) embedded in a 500-nm-wide photonic wire waveguide - and is based on silicon-on-insulator. A normalized transmission of 85%, together with a Q-factor of 18 500, have been achieved experimentally through the use of carefully designed tapering on both sides of each of the hole-type PhC mirrors that form the microcavity. We have also demonstrated reasonably accurate control of the cavity resonance frequency. Simulation of the device using a three-dimensional finite-difference time-domain approach shows good agreement with the experimental results.

Photonic crystal and photonic wire nano-photonics based on silicon-on-insulator

Silicon-on-insulator (SOI) is a strong candidate for application in future planar waveguide integration technology, whether or not luminescence is extracted from the silicon. We review recent research on photonic devices based on silicon-on-insulator. These devices exploit either photonic crystal or photonic wire concepts—or combinations of both. Aspects of the technologies used that are particularly critical for successful implementation of SOI-based photonics are addressed.

Enhanced stitching for the fabrication of photonic structures by electron beam lithography

Large-area electron beam lithography tools pattern substrates as a series of writing fields that are stitched together. Pattern defects, termed stitching errors, can arise at field boundaries and these can have detrimental effects on device performance. These problems are exaggerated by substrate tilt. In this article, the authors demonstrate the application of a substrate tilt correction procedure to minimize stitching errors in the fabrication of photonic structures by electron beam lithography. The authors show that the magnitude of stitching errors is dependent on the position within the field boundary and is influenced by substrate tilt. Application of tilt correction procedures is shown to reduce stitching errors and give rise to a corresponding reduction in propagation losses in photonic wire waveguides. The authors show that the results of measured propagation losses arising from stitching errors are in good agreement with numerical results.

Closure of the stop-band in photonic wire Bragg gratings.

Photonic Wire Bragg Gratings, made by periodic insertion of lateral rectangular recesses into photonic wires in silicon-on-insulator, can provide large reflectivity with short device lengths because of their large index contrast. This type of design shows a counter-intuitive behaviour, as we demonstrate - using experimental and numerical data - that it can have low or null reflectance, even for large indentation values. We provide physical insight into this phenomenon by developing a model based on Bloch mode theory, and are able to find an analytical expression for the frequency at which the grating does not sustain the stop-band. Finally we demonstrate that the stop-band closing effect is a general phenomenon that may occur in various types of periodic device that can be modeled as transmission line structures.

Retrieval of Bragg grating transmission spectra by post-process removal of spurious Fabry-Pérot oscillations.

Efficient post-process suppression is demonstrated of spurious Fabry-Pérot oscillations, introduced by multiple cavity effects in transmission spectra measurements of various Bragg grating devices. These devices were fabricated within access waveguides and terminated with cleaved facets. The tool, based on a curve-fitting to an equivalent scattering matrix model, is shown to extract transmission spectra of devices, without a-priori knowledge of their properties. Simple and complex grating structure spectra are successfully extracted and compare well with simulated results. The technique exhibits robust behaviour for varying facet conditions and device geometries, outperforming classical averaging techniques.

Systematic investigation of misalignment effects at junctions between feeder waveguide and photonic crystal channel waveguide

Feature Issue on Nanoscale Integrated Photonics for Optical Networks The coupling between two different guiding elements, a ridge waveguide and a photonic crystal channel waveguide (obtained by a single line defect in the crystal), is investigated both computationally and experimentally. The study concentrates on the effects that different widths for the ridge waveguide, as well as positions with respect to the channel waveguide, have on the coupling efficiency--thus allowing the assessment of the sensitivity to drift effects that can occur in fabrication--and the optimal design parameters. Characterization of devices fabricated in GaAs/AlGaAs epitaxial waveguide material shows good overall agreement with the simulated trends for all the configurations of the junction considered. On the other hand, the trends from experiments also demonstrate reduced dependency of transmission on mismatch, by comparison with simulations. Finally, the configuration, which allows optimum coupling and transmission (98%), is found to be verified by both simulation and characterization.

Effect of lithography stitching errors on Silicon-on-Insulator photonic wires

Electron beam lithography (EBL) is a widely used tool in the patterning of photonic integrated circuits, either by direct writing or by UV lithography with e-beam defined masks. It provides the benefit of pattern flexibility, high accuracy and the required nanometre-scale resolution.

Photonic crystal and photonic wire device structures

Photonic devices that exploit photonic crystal (PhC) principles in a planar environment continue to provide a fertile field of research. 2D PhC based channel waveguides can provide both strong confinement and controlled dispersion behaviour. In conjunction with, for instance, various electro-optic, thermo-optic and other effects, a range of device functionality is accessible in very compact PhC channel-guide devices that offer the potential for high-density integration. Low enough propagation losses are now being obtained with photonic crystal channel-guide structures that their use in real applications has become plausible. Photonic wires (PhWs) can also provide strong confinement and low propagation losses. Bragg-gratings imposed on photonic wires can provide dispersion and frequency selection in device structures that are intrinsically simpler than 2D PhC channel guides--and can compete with them under realistic conditions.

Post-Process Removal of Spurious Fabry-Pérot Oscillations in Measurements of Transmission Spectra

A post-processing tool for suppression of unwanted Fabry-Perot fringes in integrated optic device transmission measurements is presented. The method is shown to perform better than a shifting window averaging technique on various Bragg grating spectra.