Georgiy Tkachenko

Le piégeage et la manipulation optique de microsphères chiraux contrôlées par l'hélicité du photon: Exploiter le degré de liberté angulaire de la lumière pour contrôler les forces optiques ouvre une nouvelle voie pour la manipulation optique de systèmes matériels. Dans ce contexte, notre travail porte sur l’interaction lumière-matière en présence de chiralité, qu’elle soit matérielle ou ondulatoire. Expérimentalement, nous avons utilisé des gouttes de cristaux liquides cholestériques interagissant avec un ou plusieurs champs lumineux polarisés circulairement et nous avons apporté une description quantitative de nos observations. Notre principal résultat correspond à la démonstration que la pression de radiation optique peut être contrôlée par l’hélicité du photon. Ce phénomène est ensuite utilisé, d’une part pour faire une démonstration de principe du tri de la chiralité matérielle via une approche optofluidique et d’autre part pour réaliser un piège optique tridimensionnel sensible à la chiralité de l’objet piégé.

Small composite objects, known as Janus particles, drive sustained scientific interest primarily targeted at biomedical applications, where such objects act as micro- or nanoscale actuators, carriers, or imaging agents. A major practical challenge is to develop effective methods for the manipulation of Janus particles. The available long-range methods mostly rely on chemical reactions or thermal gradients, therefore having limited precision and strong dependency on the content and properties of the carrier fluid. To tackle these limitations, we propose the manipulation of Janus particles (here, silica microspheres half-coated with gold) by optical forces in the evanescent field of an optical nanofiber. We find that Janus particles exhibit strong transverse localization on the nanofiber and much faster propulsion compared to all-dielectric particles of the same size. These results establish the effectiveness of near-field geometries for optical manipulation of composite particles, where new waveguide-based or plasmonic solutions could be envisaged.

This publisher’s note reports a correction to the funding section of Optica 7, 59 (2020)OPTIC8 2334-2536 10.1364/OPTICA.374441 .

Evanescent electromagnetic fields near a waveguide can exert a transverse radiation force on scattering objects. To prove this experimentally, we demonstrate light-induced orbiting of isotropic, dielectric microparticles around an optical nanofiber that guides elliptically polarized, fundamental modes. The orbit frequency is proportional to the helicity of the coupled light. Interestingly, the observed motion is opposite to the energy flow circulation around the fiber. This result verifies the theoretically predicted negative optical torque on a sufficiently large particle in the vicinity of a nanofiber.

Directional coupling of light in nanophotonic circuits has recently attracted increasing interest, with numerous experimental realizations based on broken rotational or mirror symmetries of the light-matter system. The most prominent underlying effect is the spin-orbit interaction of light in subwavelength structures. Unfortunately, coupling of light to such structures is, in general, very inefficient. In this work, we experimentally demonstrate an order of magnitude enhancement of the directional coupling between two nanowaveguides by means of a whispering gallery microcavity. We also show that both transverse magnetic and transverse electric modes can be used for the enhancement.

We present a simple method for controlling the polarisation state of light at the waist of a single-mode optical nanofibre. The method consists of complete polarisation compensation based on imaging scattered light from inherent inhomogeneities both on the fibre surface and in the glass material itself. In contrast to the recently reported protocol exploiting two imaging systems oriented at 45° to each other, our method requires only one lens and a video camera. It is particularly useful for nanofibre-based applications with severe geometric constraints, such as inside vacuum chambers for experiments with cold atoms. The measured fidelity of the achieved control is about 98% using lenses with moderate numerical apertures.

Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback—their inability to maintain light’s polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two nonorthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.

Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback - their inability to maintain light's polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin-locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two non-orthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.

Contactless manipulation of micron-scale objects in a microfluidic environment is a key ingredient for a range of applications in the biosciences, including sorting, guiding, and analysis of cells and bacteria. Optical forces are powerful for this purpose but, typically, require bulky focusing elements to achieve the appropriate optical field gradients. To this end, realizing the focusing optics in a planar format would be very attractive and conducive to the integration of such microscale devices, either individually or as arrays. Here we report on, to the best of our knowledge, the first experimental demonstration of optical trapping using planar silicon metalenses illuminated with a collimated laser beam. The structures consist of high-contrast gratings with a locally varying period and duty cycle. They are designed to mimic parabolic reflectors with a numerical aperture of 0.56 at a vacuum wavelength of 1064 nm. We achieve both two- and three-dimensional trapping in water, with the latter realized by omitting the central Fresnel zones. This Letter highlights the versatility of such lithographically defined metastructures for exerting optical forces without the need for traditional optical elements.