Vincent Ginis

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.

A question for Science

The campaign “A question for Science” is an initiative of the Flanders Scientific Research Fund (FWO), commissioned by Flemish Minister Muyters, responsible for science and innovation, in collaboration with a broad group of scientific organizations, such as the Flemish universities and colleges, the KVAB, the Young Academy, the strategic research centers …

As an ambassadors of this initiative, I have answered one of the questions that was submitted to the website. The answer was published in the Belgian newspaper Het Nieuwsblad.

The full article can be found here.

Ultrasensitive 3D force spectroscopy detects unambiguously exotic force on  a particle in an evanescent field

It was predicted in recent years that a non-chiral particle in an evanescent optical field (such as one generated by total internal reflection) is able to acquire linear momentum perpendicular to the plane of incidence from the spin component of the incoming light.  When the helicity of the light is flipped, this momentum changes direction.  This particular kind of spin-momentum coupling has attracted keen interest in the physics community, due to a fundamental connection with the BelinfanteRosenfeld energy-momentum tensor which emerged in quantum field theory in 1940. Many theoretical papers have been published exploring the interaction between this momentum with matter, yet only a qualitative confirmation of the existence of the resulting force has been thus far experimentally reported in the lab.
A true test of the theory is an experimental challenge as the spin-momentum force does not act in isolation.  Rather, it is merely a weak component of a 3D force vector whose magnitude is typically more than one hundred times larger in the other two directions.  More generally, all three components of this force vector are predicted to change as the incident light’s pure polarization state is altered.  Thus, measuring this spin-momentum force in context, that is, measuring both the magnitude and the direction of the composite vector force, using a probe whose geometry can be analytically modeled, is the only way to quantitatively determine the magnitude of the spin momentum force and unambiguously test the theory.
In our manuscript we report a series of such measurements.  We have built a floating-probe force microscope using a micron-sized polystyrene sphere held in an optical tweezer.  The instrument is capable of femtonewton force sensitivity and piconewton range with simultaneous, time-synchronized position readouts in the three orthogonal directions.  To test the theory we take the incident light along two different closed paths on the Poincare polarization sphere.  We record both the direction and magnitude of the 3D vector force acting on the trapped probe and compare against theoretical predictions.  Remarkable agreement was found between theory and experiment.
This work represents a significant advancement in the systematic study of exotic optical forces.  The volume-scanning capabilities of the force microscope can enable, among other applications, the 3D mapping of a vector force field with high resolution and dynamic range.  As such, our results can be of interest to researchers in other fields studying microscopic interactions.
The manuscript was published in Physical Review Letters and can be found here. Full pdf.
Image caption: The figure illustrates the interaction of the trapped
floating probe with the polarization- and amplitude-modulated evanescent field.

Taking it slow with reconfigurable metasurfaces

Ultrathin nanostructured resonators on surfaces, i.e., metasurfaces, have shown great potential for the miniaturization of advanced photonic technologies such as aberration-free lenses, beam shapers, and holograms. These metasurface designs focus strongly on obtaining desired optical properties through a careful optimization of the shape, the relative orientation, and the composition of structures. However, once manufactured the resulting optimized properties cannot change, preventing the use of metasurfaces in the implementation of displays or wearables, needed for applications involving augmented and virtual reality.

Recently, there has been a lot of interest in manipulating the properties of metasurfaces by mechanical deformation using an electrical current, an optical beam, or heat. A particularly successful design of optomechanical metasurfaces is based on elastic resonators with nontrivial mechanical and optical properties. The position, distance, or relative orientation of elastic resonators is changed by making use of an optical beam exerting an optical force. The optical properties of the surfaces change due to the reconfiguration of these resonators, depending on the the power of the pump. Therefore, changes in power may control the optical properties of metasurfaces and allow for their use in reconfigurable photonic devices.
Now, writing in Physical Review Letters, scientists from Chalmers University of Technology (Sweden), Vrije Universiteit Brussel (Belgium) and Harvard University (USA), point out that the reconfiguration process of these surfaces, i.e., the motion of elastic resonators towards their equilibrium configuration, crucially determines the refresh rate of optomechanical metasurfaces. For simple reconfiguration processes, the refresh rate is ultimately limited by the nonlinear interactions between elastic elements and the optical beam, rendering the process much too slow for practical applications. The results suggest that the current approach towards optomechanically reconfigurable metasurfaces needs to be reconsidered with a specific focus on the nonlinear dynamics of the system.
The manuscript was published in Physical Review Letters and can be found here. Full pdf.
Image caption: The figure illustrates the interaction of two beams with a reconfigurable metasurface: the red beam reconfigures the metasurface, the purple beam is modified by the surface. The unit cells are visualized as hourglasses, representing the unexpected long reconfiguration time of each unit cell, fundamentally limited by the nonlinear transient dynamics.