Vincent Ginis

To what extent can we shift the frequency of signals using time-dependent surfaces?

Time-dependent metasurfaces are playing an increasingly important role as a platform to change the frequency of optical signals. This is a result of the fact that the interaction length through a single surface is so small that it is nearly impossible to generate new spectral components using nonlinear effects. However, although the idea to generate new spectral components using time-dependent metasurfaces has been around for quite some time, this emerging research field is still lacking a coherent, self-consistent framework that explains how frequencies of optical pulses can be manipulated.

In this manuscript, we introduce such a general framework that allows for the understanding and designing of time-dependent metasurfaces that shift the frequencies of electromagnetic pulses. Based on an analytical model and FDTD numerical simulations, we obtain physical insight into the complex process that occurs when broadband electromagnetic pulses interact with time-varying metasurfaces. In particular, we find that there is an intriguing relation between the bandwidth of the incident pulse, the targeted frequency shift, and the number of Lorentzian resonators that need to be implemented. We also demonstrate that in certain parameter regimes, pulse distortion and deviating pulse amplitudes cannot be avoided.

We thus expose a fundamental limitation of time-dependent metasurfaces. These results are independent of the mechanism that generates the time-dependence and they do not dependent on the operating frequencies, nor the geometry, size or material of the unit cell. They are a direct result of the subtle interplay of Lorentzian resonances. In addition, we provide a widely usable design method for anyone who wants to use time-dependent metasurfaces to shift frequencies of electromagnetic pulses.

The manuscript was published in Physical Review B as a Rapid Communication and can be found here. Full Pdf.

Open Science Conference 2018 – Back-to-Back Belgium-European Commission

Open Science goes beyond open access to publication, encompassing the sharing of open research data and citizen science. It also paves the way for the development of appropriate metrics, incentives and rewards. Since Open Science solutions have to be developed in a coherent way both at national and European level, this conference combines a discussion of the Belgian Open Science policies with debates relating to a H2020 Policy Support Facility Mutual Learning Exercise on Open Science with a prominent European outlook. Special emphasis is put on researchers working in an Open Science environment, skilling, incentivising and rewarding them.

I am honored to be one of the Belgian researchers in the panel debating open science and open access (Plan S).

Opportunities and challenges for implementing Plan S

Publicly funded research should not be hidden behind payment walls, but it should also not become a pay-to-publish game.

In a joint statement by several Young Academies, including the Belgian, a convinced “yes” to Open Access sounds. At the same time, the young scientists call for reflection on the best possible implementation of Plan S: stakeholder consultation with, among others, Young Academies is necessary.

The statement was authored by Koen Vermeir, Moritz Riede and Sabina Leonelli on behalf of the Global Young Academy (GYA), based on input from and discussion with the members of the respective Young Academies provided by Ericka Johnson and Jens Hjerling-Leffler (Young Academy of Sweden), Gergely Toldi (on behalf of the Hungarian National Young Academy Initiative), Ian Overton (Young Academy of Scotland), Olli Peltola (Young Academy Finland), Rens van de Schoot (Dutch Young Academy), Udi Sommer (Israel Young Acad- emy) and Vincent Ginis (Belgian Young Academy).

Mindblowers 2018

For the second year in a row, KVS and VUB unite forces to kick off the academic and cultural year. At MINDBLOWERS, don’t expect any puffed-up speeches or highfalutin lectures. No: short, powerful interventions at the crossroads of arts, culture, research and reflection. At the intersection of activism, ‘artivism’ and academia.

This second edition of MINDBLOWERS will be an ode to the imagination. Academics and artists both work from a place of curiosity, wonder, freedom, creativity and critical thinking. Only by imagining and envisioning the impossible can arts and science create new horizons.

Sophie Viaene successfully defended her PhD on the exploration of metamaterial horizons

Einstein’s theory of general relativity has dramatically changed our world view, by describing gravity as an intrinsic deformation of space and time. About fifteen years ago, John Pendry and Ulf Leonhard had the intriguing idea to emulate the behaviour of light in a deformed space by making use of carefully designed artificial metamaterials. Metamaterials consist of elements that are very small with respect to the characteristic length of light waves. By optimizing the shape, the density, and the size of these elements, metamaterials can control light rays in a very precise way.

A priori, it is very difficult, if not impossible, to determine what metamaterial properties impose a desired bend, split, or concentration of light rays. Transformation optics is a geometrical technique that allows determining what metamaterial properties reproduce the light path inside a deformed space. This has resulted in impressive and, at the same time, intuitive, material designs such as invisibility devices that hide an object by guiding light rays around it.

Thanks to rapidly advancing fabrication methods, metamaterial designs may result in a variety of novel properties, such as reconfigurable, two-dimensional, or quantum properties. In the first part of her thesis, Sophie introduces new concepts to describe reconfigurable metamaterials with properties that are controlled by an external signal. For example, she has developed a consistent description of materials that implement vector potentials for photons to enhance optical forces, and she has discovered that reconfigurable metamaterials are subject to a fundamental speed limit. These findings are important because they point out that active photonic switches can only be improved up to a certain point by making use of metamaterial structures. In the second part of her thesis, Sophie extends the geometrical tools of transformation optics to improve on the understanding of two-dimensional metamaterials and quantum metamaterials. This has allowed designing metamaterial layers that guide light along their surface and has provided new insights into the behaviour of light sources inside metamaterial black holes, which impose a gravitational wave vector shift.

By describing advanced electromagnetic phenomena inside reconfigurable, two-dimensional, and quantum metamaterials in an intuitive way, this thesis has contributed to advanced material designs that may be useful in future light-based applications.

Congratulations Sophie! We are proud supervisors.