Etienne Brasselet

Photopolymerized microscopic vortex beam generators: Precise delivery of optical orbital angular momentum

Direct femtosecond laser photopolymerization is used to fabricate high resolution microscopic spiral phase plates. The total phase change all around their center is prepared to be a integer multiple of 2π for the operating wavelength in the visible domain. The optical performances of the spiral plates are measured and we propose a simple single beam interferometric technique to characterize the phase singularity of the generated vortex beams. The experimental results are compared to simulations and a satisfying agreement is obtained. Potential of large scale fabrication, templating, and smart spiral plate architectures are also illustrated.

Spatially engineered polarization states and optical vortices in uniaxial crystals

We describe how the propagation of light through uniaxial crystals can be used as a versatile tool towards the spatial engineering of polarization and phase, thereby providing an all-optical technique for vectorial and scalar singular beam shaping in optics. Besides the prominent role played by the linear birefringence, the influence of circular birefringence (the optical activity) is discussed as well and both the monochromatic and polychromatic singular beam shaping strategies are addressed. Under cylindrically symmetric light-matter interaction, the radially, azimuthally, and spirally polarized eigen-modes for the light field are revealed to be of a fundamental interest to describe the physical mechanisms at work when dealing with scalar and vectorial optical singularities. In addition, we also report on nontrivial effects arising from cylindrical symmetry breaking, e.g. tilting the incident beam with respect to the crystal optical axis.

Optofluidic sorting of material chirality by chiral light

The lack of mirror symmetry, chirality, plays a fundamental role in physics, chemistry and life sciences. The passive separation of entities that only differ by their handedness without need of a chiral material environment remains a challenging task with attractive scientific and industrial benefits. To date, only a few experimental attempts have been reported and remained limited down to the micron scale, most of them relying on hydrodynamical forces associated with the chiral shape of the micro-objects to be sorted. Here we experimentally demonstrate that material chirality can be passively sorted in a fluidic environment by chiral light owing to spin-dependent optical forces without chiral morphology prerequisite. This brings a new twist to the state-of-the-art optofluidic toolbox and the development of a novel kind of passive integrated optofluidic sorters able to deal with molecular scale entities is envisioned.

Dynamics of optical spin-orbit coupling in uniaxial crystals

We study theoretically and verify experimentally the detailed dynamics of spin-to-orbital angular momentum conversion for a circularly polarized Gaussian beam propagating along the optical axis of a uniaxial crystal. We extend the results to the case of white-light beams when each of the spectral components undergoes its own wavelength-dependent angular momentum conversion process.

Reversible optical nonreciprocity in periodic structures with liquid crystals

We suggest a novel approach to achieve reversible nonreciprocal optical response in a periodic photonic structure with a pair of defects, one of them being a nonlinear liquid crystal. We show that nonreciprocal effects can be reversed by changing the wavelength due to change of localization properties of defect modes.

Creation and manipulation of topological states in chiral nematic microspheres

Topology is a universal concept that is encountered in daily life and is known to determine many static and dynamical properties of matter. Taming and controlling the topology of materials therefore constitutes a contemporary interdisciplinary challenge. Building on the controllable spatial properties of soft matter appears as a relevant strategy to address the challenge, in particular, because it may lead to paradigmatic model systems that allow checking theories experimentally. Here we report experimentally on a wealth of complex free-standing metastable topological architectures at the micron scale, in frustrated chiral nematic droplets. These results support recent works predicting the formation of free-standing knotted and linked disclination structures in confined chiral nematic fluids. We also demonstrate that various kinds of external fields (thermal, electrical and optical) can be used to achieve topological remote control. All this may foster the development of new devices based on topologically structured soft media.

Helicity-dependent three-dimensional optical trapping of chiral microparticles

The rule of thumb of tailored optical forces consists in the control of linear momentum exchange between light and matter. This may be done by appropriate selection of the interaction geometry, optical modes or environmental characteristics. Here we reveal that the interplay of the helicity of light and the chirality of matter turns the photon spin angular momentum into an efficient tool for selective trapping of chiral particles. This is demonstrated, both experimentally and theoretically, by exploring the three-dimensional optical trapping of chiral liquid crystal microspheres with circularly polarized Gaussian or Laguerre-Gaussian beams. These results suggest the development of novel optomechanical strategies that rely on the photon helicity towards selective trapping and manipulation of chiral objects by chiral light.

Monolithic generators of pseudo-nondiffracting optical vortex beams at the microscale

We report on the fabrication and characterization of micro-optical elements with typical size of 100 μm, which enable the production of pseudo-nondiffracting optical vortex beams of arbitrary order. This is made possible from the monolithic integration of spiral phase plates and axicons into helical axicons by direct laser writing using femtosecond laser nanopolymerization. The optical performances of the fabricated three-dimensional singular microstructures are experimentally measured and compared with their expected theoretical behavior, both in intensity and phase. The proposed approach thus represents an attempt to merge the field of singular integrated optics with that of nondiffracting light fields.

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.