Job Opportunity, PhD CALL OPENS, “Topological Metamaterials for Active Nanophotonic Devices”

This project intends to exploit concepts of topology to build photonic crystals with electromagnetic modes that propagate robustly along narrow paths, unperturbed by local defects or perturbations. These properties would enable the development of new materials for classical and quantum computation using photons instead of electrons.

The discovery of topological insulators in the mid 2000’s increased enormously the interest in topological phases of matter. Soon, it was demonstrated that topological concepts apply also to Photonics, paving the way to electromagnetic modes that can flow around large imperfections without back-reflection in topological metamaterials. This enables novel photonic devices, such as reflection-free sharply bent waveguides, robust delay lines, spin-polarized switches and non-reciprocal devices. Interestingly, the vectorial character of electromagnetic fields could lead to topological phases that couple the polarization and the wavevector, which may also open up interesting possibilities in quantum optics and computation.

Over the last years, we have investigated photonic and plasmonic crystals to demonstrate nonreciprocal propagation of light in the presence of magnetic fields, observable through angle-resolved reflectance spectroscopy. At the same time, the lab has shown enhanced gyrotropic properties and magneto-optic responses at frequencies around the stop-band edges of photonic crystals, as well as around plasmonic resonances and spin-dependent polaron transport [8]. More recently, we have investigated alternative approaches towards robust propagation of confined electromagnetic waves in topological crystals. One line of research has focused on graphene-like dielectric photonic crystals that can realize the analog of quantum spin Hall using photons instead of electrons. We have shown that helical edge modes are preserved for a certain range of the refractive index, opening up the possibility of using metamaterials with lower optical contrast than previously reported. This opens the way to the application of ferroelectrics in topological crystals, opening up a fascinating way to control edge propagation using electric fields, e.g., by incorporating ferroelectrics in the photonic crystals, which remains to be explored experimentally (Objective (a) below).

A second research line focuses on zero-index metamaterials with Dirac cones in the bandstructure, similar to the electronic band structure in graphene. At the Dirac point, these metamaterials exhibit zero effective permittivity and permeability, resulting in unique properties such as directional emission, cloaking and lensing, as well as robust unidirectional propagation, analogous to quantum Hall edge states. Our contribution to this field has been the theoretical investigation of gyromagnetic responses and polarization conversion close to the Dirac cone crystals that incorporate magnetic materials. Our study reveals new technological applications and paves way to other implementation of topological metamaterials, such as Weyl systems, where the required time-reversal symmetry breaking can be done by inducing artificial magnetism (Objective (b) below).

The objective of this project is to open/close edge propagation modes by electrostatic gating. Two sub-objectives will define the present project, as follows:

Objective a) Electrostatic modulation of edge modes in graphene-like photonic crystals. The objective is to control these modes by electrostatic fields though the incorporation of ferroelectric media in photonic crystals.

Objective b) Dirac/Weyl metamaterials with edge modes driven by artificial magnetism. The idea behind is to use synthetic gauge field for photons in photonic crystals, emulating the time-reversal symmetry breaking that happens in the presence of magnetic fields.



I can propose you  these projects:

Title: Hybrid Optical/Electronic Artificial Neural Networks Based on 2D Electron Gases.

The candidate will be trained in optical and transport characterization. Optical experiments will be carried out using high-numerical-aperture focusing in confocal microscopy with in-situ applied electric pulses on devices defined by means of optical and electron-beam lithography. The devices are defined on materials that host 2D electron gases (2DEGs) that respond to light with changes in conductance that mimic the synaptic plasticity of brains. Algorithms will be used to apply learning rules based on the synaptic-like photoconductive responses of artificial networks based on 2DEGs.

For more information on 2DEG properties: [1] G. Herranz et al., Nature Communications 6, 6028 (2015); [2] J. Gazquez et al., Phys. Rev. Lett. 119, 106102 (2017).

Title: Metasurfaces for Optical Telecommunications.

The successful candidate will be trained in magneto-optical spectroscopy with wavelengths in the near infrared and the visible. He/she will be also trained in a second kind of methodology, consisting of real- and reciprocal space mapping of optical responses with diffraction limitation [see our Reference 5]. These techniques allow the visualization down to just a few of hundreds of nanometers, enabling direct imaging of small devices. At the same time, it enables imaging of dispersion relations (frequency versus wavevector) of light propagating in photonic media (see, e.g., Figure 4 of Ref. [1]). The samples under analysis are obtained by structuring the matter (metals, dielectrics) using optical and electron-beam lithography to define small optical devices with length scales from around 100 nm up to around 100 microns.

For more information: [1] M. Rubio-Roy et al., Langmuir, 28, 9010 (2012); [2] J.M. Caicedo et al., ACS Nano, 5 2957 (2011); [3] J.M. Caicedo et al., Phys. Rev. B 89, 045121 (2014); [4] O. Vlasin et al., Phys. Rev. Applied. 2, 054003 (2014), [5] O. Vlasin et al., Scientific Reports 5 15800 (2015). [6] B. Casals et al., Physical Review Letters, 117, 026401 (2016)