Imanol Andonegui

Topological Optical Waveguiding in Silicon and the Transition between Topological and Trivial Defect States

One-dimensional models with topological band structures represent a simple and versatile platform to demonstrate novel topological concepts. Here we experimentally study topologically protected states in silicon at the interface between two dimer chains with different Zak phases. Furthermore, we propose and demonstrate that, in a system where topological and trivial defect modes coexist, we can probe them independently. Tuning the configuration of the interface, we observe the transition between a single topological defect and a compound trivial defect state. These results provide a new paradigm for topologically protected waveguiding in a complementary metal-oxide-semiconductor compatible platform and highlight the novel concept of isolating topological and trivial defect modes in the same system that can have important implications in topological physics.

The finite element method applied to the study of two-dimensional photonic crystals and resonant cavities

A critical assessment of the finite element (FE) method for studying two-dimensional dielectric photonic crystals is made. Photonic band structures, transmission coefficients, and quality factors of various two-dimensional, periodic and aperiodic, dielectric photonic crystals are calculated by using the FE (real-space) method and the plane wave expansion or the finite difference time domain (FDTD) methods and a comparison is established between those results. It is found that, contrarily to popular belief, the FE method (FEM) not only reproduces extremely well the results obtained with the standard plane wave method with regards to the eigenvalue analysis (photonic band structure and density of states calculations) but it also allows to study very easily the time-harmonic propagation of electromagnetic fields in finite clusters of arbitrary complexity and, thus, to calculate their transmission coefficients in a simple way. Moreover, the advantages of using this real space method in the context of point defect cluster quality factor calculations are also stressed by comparing the results obtained with this method with those obtained with the FDTD one. As a result of this study, FEM comes out as an stable, robust, rigorous, and reliable tool to study light propagation and confinement in both periodic and aperiodic dielectric photonic crystals and clusters.

Designing integrated circuitry in nanoscale photonic crystals

Current data networks rely on a mix of optical carriers and electronic circuitry. Signals are transported in the optical domain by fibers that support high bandwidths and low losses. Meanwhile, electronic devices distributed along the backbone provide the rest of the essential functions needed for a modern packetswitching network, such as routing, storing, and queuing the data traffic. The performance of such networks, however, is limited by the electronics, which have a maximum speed of only a few gigabits per second. To satisfy the increasing bandwidth requirements of emerging applications and to eliminate this bottleneck, we wish to replace the electronic circuitry with analogous optical systems that provide transparency and high bandwidth while consuming significantly less energy. Furthermore, we imagine a broad range of applications for such optical circuits beyond their use in networks, such as for optical computing or lightweight, highly integrated automotive and aeronautical monitoring. Conventional optical circuits, however, must be large to reduce losses when guiding light around curves. Photonic crystals offer a handy alternative technology that allows us to design nanoscale circuits that can control the flow of light with innovative silicon patterns (see Figure 1). A photonic crystal is an engineered inhomogeneous periodic structure made of two or more materials with very different dielectric constants.1 It supports a complete photonic band gap: a frequency range within which no photons can propagate through the material. Irregularities (‘defects’) introduced in the structure form localized defect states, which allow light to be guided through the structure on a subwavelength scale with minimal losses. By introducing different forms of disorder in a photonic crystal, one can achieve more complex functions. Tailoring this disorder for a particular application, however, is extremely difficult. The traditional design approach starts with a proposed structure, then allows only a few degrees of freedom to vary Figure 1. Schematic view of basic functions that an optical circuit must be able to perform to manage the flow of light in a photonic crystal structure.

An Intelligent Decision Support System for Assessing the Default Risk in Small and Medium-Sized Enterprises

In the last years, default prediction systems have become an important tool for a wide variety of financial institutions, such as banking systems or credit business, for which being able of detecting credit and default risks, translates to a better financial status. Nevertheless, small and medium-sized enterprises did not focus its attention on customer default prediction but in maximizing the sales rate. Consequently, many companies could not cope with the customers’ debt and ended up closing the business. In order to overcome this issue, this paper presents a novel decision support system for default prediction specially tailored for small and medium-sized enterprises that retrieves the information related to the customers in an Enterprise Resource Planning (ERP) system and obtain the default risk probability of a new order or client. The resulting approach has been tested in a Graphic Arts printing company of The Basque Country allowing taking prioritized and preventive actions with regard to the default risk probability and the customer’s characteristics. Simulation results verify that the proposed scheme achieves a better performance than a naive Random Forest (RF) classification technique in real scenarios with unbalanced datasets.

Novel Light Coupling Systems Devised Using a Harmony Search Algorithm Approach

We report a critical assessment of the use of an Inverse Design (ID) approach steamed by an improved Harmony Search (IHS) algorithm for enhancing light coupling to densely integrated photonic integratic circuits (PICs) using novel grating structures. Grating couplers, performing as a very attractive vertical coupling scheme for standard silicon nano waveguides are nowadays a custom component in almost every PIC. Nevertheless, their efficiency can be highly enhanced by using our ID methodology that can deal simultaneously with many physical and geometrical parameters. Moreover, this method paves the way for designing more sophisticated non-uniform gratings, which not only match the coupling efficiency of conventional periodic corrugated waveguides, but also allow to devise more complex components such as wavelength or polarization splitters, just to cite some.

Topological optical waveguiding in SOI structures

We experimentally demonstrate topologically protected optical waveguiding in silicon at the interface between two topologically distinct dimer chains. Further, we propose and demonstrate beating between topological and trivial defect modes.

Topological optical waveguiding in silicon and beating between trivial and topological defect modes

We experimentally demonstrate topologically protected optical waveguiding in silicon at the interface between two topologically distinct dimer chains. Further, we propose and demonstrate beating between topological and trivial defect modes.

Coupling light into photonic integrated circuits using non-periodic surfaces

We report a critical assessment of the use of non-uniform gratings for enhancing light coupling to photonic integrated circuits (PICs). Grating couplers, performing as a very attractive vertical coupling scheme for the silicon waveguide, have been widely demonstrated. It is shown that the coupling efficiency of grating couplers can be enhanced by using an Inverse Design (ID) methodology that can deal simultaneously with many physical and geometrical parameters. Moreover, this method paves the way for designing non-uniform gratings in an efficient way, which not only match the coupling efficiency of conventional periodic corrugated waveguides but also allow to devise more sophisticated components such as wavelength or polarization splitters.

Towards silicon all-optical nanophotonic circuitry

We report on the design of novel, highly efficient, photonic crystal (PC) devices capable of fulfilling various functionalities for a given topology. These novel devices have been obtained by means of an inverse design (ID) method. This new procedure implements both the limitations of a given PC manufacturing technique and the typical errors due to random imperfections in the PC manufacturing process. Therefore, the resulting designs operation is very robust against imperfections and close to optimality. A more advanced version of this technique also allows us to design highly efficient photonic integrated circuits (PIC) of practical relevance that look very promising in terms of packaging density, improved functionality, and cost effectiveness.