Charles Loussert

Electrically controlled topological defects in liquid crystals as tunable spin-orbit encoders for photons.

We demonstrate experimentally that topological defects of vertically aligned nematic liquid crystal films induced by electric fields can be used as highly efficient natural optical spin-orbit encoders that do not need machining techniques. Moreover, we show that both the operating wavelength and operation mode of such natural quantum optical interfaces can be tuned in real time using low-voltage electric fields.

Efficient scalar and vectorial singular beam shaping using homogeneous anisotropic media

We propose and demonstrate a global and efficient approach for scalar and vectorial beam shaping based on the interaction of circularly polarized light with a single piece of homogeneous anisotropic medium. The main idea is to mimic the behavior of a two-dimensional inhomogeneous birefringent medium with a radial distribution of its optical axis. This is done by transforming an incident Gaussian beam into a conical nipple of light that further propagates along the optical axis of a c-cut uniaxial crystal.

Fluid flows driven by light scattering

We report on the direct experimental observation of laser-induced flows in isotropic liquids that scatter light. We use a droplet microemulsion in the two-phase regime, which behaves like a binary mixture. Close to its critical consolute line, the microemulsion undergoes large refractive index fluctuations that scatter light. The radiation pressure of a laser beam is focused onto the soft interface between the two phases of the microemulsion and induces a cylindrical liquid jet that continuously emits droplets. We demonstrate that this dripping phenomenon takes place as a consequence of a steady flow induced by the transfer of linear momentum from the optical field to the liquid due to light scattering. We first show that the cylindrical jet guides light as a step-index liquid optical fiber whose core diameter is self-adapted to the light itself. Then, by modelling the light-induced flow as a low-Reynoldsnumber, parallel flow, we predict the dependence of the dripping flow rate on the thermophysical properties of the microemulsion and the laser beam power. Satisfying agreement is found between the model and experiments.

Revolving supramolecular chiral structures powered by light in nanomotor-doped liquid crystals

Molecular machines operated by light have been recently shown to be able to produce oriented motion at the molecular scale1,2 as well as do macroscopic work when embedded in supramolecular structures3–5. However, any supramolecular movement irremediably ceases as soon as the concentration of the interconverting molecular motors or switches reaches a photo-stationary state6,7. To circumvent this limitation, researchers have typically relied on establishing oscillating illumination conditions—either by modulating the source intensity8,9 or by using bespoke illumination arrangements10–13. In contrast, here we report a supramolecular system in which the emergence of oscillating patterns is encoded at the molecular level. Our system comprises chiral liquid crystal structures that revolve continuously when illuminated, under the action of embedded light-driven molecular motors. The rotation at the supramolecular level is sustained by the diffusion of the motors away from a localized illumination area. Above a critical irradiation power, we observe a spontaneous symmetry breaking that dictates the directionality of the supramolecular rotation. The interplay between the twist of the supramolecular structure and the diffusion14 of the chiral molecular motors creates continuous, regular and unidirectional rotation of the liquid crystal structure under non-equilibrium conditions.When a helix-based liquid crystal that incorporates molecular motors is illuminated locally with intensity above a certain threshold, a continuous, regular and unidirectional rotation emerges.

Multiple chiral topological states in liquid crystals from unstructured light beams

It is shown experimentally that unstructured light beams can generate a wealth of distinct metastable defect structures in thin films of chiral liquid crystals. Various kinds of individual chiral topological states are obtained as well as dimers and trimers, which correspond to the entanglement of several topological unit cells. Self-assembled nested assemblies of several metastable particle-like topological states can also be formed. Finally, we propose and experimentally demonstrate an opto-electrical approach to generate tailor-made architectures.

Subnanowatt opto-molecular generation of localized defects in chiral liquid crystals

The controlled writing and deleting of topological states in soft matter systems is addressed. The reversible lightinduced topological structuring of chiral liquid crystals at the micrometer scale is reported. Various kinds of localized defect structures are generated at subnanowatt optical power levels, which is done by using chiroptical molecular switches that operate at the molecular scale.

Q-plates micro-arrays for parallel processing of the photon orbital angular momentum

We report on the realization of electrically tunable micro-arrays of space-variant optically anisotropic optical vortex generators. Each individual light orbital angular momentum processor consists of a microscopic self-engineered nematic liquid crystal q-plate made of a nonsingular topological defect spontaneously formed under electric field. Both structural and optical characterizations of the obtained spin-orbit optical interface are analyzed. An analytical model is derived and results of simulations are compared with experimental data. The application potential in terms of parallel processing of the optical orbital angular momentum is quantitatively discussed.

Topogical diversity of localized metastable states in chiral liquid crystals (presentation video)

Liquid crystalline materials are well-known to exhibit various kinds of structural defects, whose space-variant optical properties are actually useful for a number of applications, including recently developed integrated optical vortex generators. In addition, since liquid crystal defects display well-defined topological features, their very existence also appears attractive in terms of information storage at the microscopic scale. An illustrative example is provided by frustrated chiral liquid crystals films, in which different localized metastable states can be written by either structured or unstructured light beams. We will present recent results regarding the controlled generation of multiple chiral topological states in liquid crystals.

Singular photonics based on liquid crystals topological defects

Here we report on the use of liquid crystal topological defects for photonic applications that involve optical singularities. The well-dened molecular organization around a liquid crystal defect enables the coupling between the spin and the orbital angular momentum of light. Such an optical spin-orbit coupling is a general feature of light propagation through inhomogeneous or anisotropic media, which makes liquid crystal topological defects attractive micro-structures when orbital angular momentum of light is the key ingredient of an application.