Engineered Near and Far Field Optical Response of Dielectric Nanostuctures using Focused Cylindrical Vector Beams

Near- and far-field optical properties of silicon nanostructures under linear polarization (Gaussian beam), and azimuthally or radially focused cylindrical vector beams are investigated by finite-difference time-domain method (FDTD) in Meep open-source software. A python toolkit allowing FDTD simulations in Meep for using those excitation sources is provided. In addition to the preferential excitation of specific electric or magnetic resonance modes as function of the excitation beam polarization, it is shown in the case of spheroids that shape anisotropy affects the resonance wavelength and the dipole orientation of the magnetic or electric dipole mode. For radial or linear polarization, the electric dipole resonance is split by an anapole mode depending on the spheroid symmetry axis with respect to the electric field orientation. Finally, the optical properties in both far-field (scattering pattern) and near-field (electric and magnetic field hot spots) can be tuned by changing the excitation polarization at a fixed wavelength and selecting properly the spheroid shape and dimensions. These numerical simulations can be extended to more complex shapes, or fabrication-friendly nanostructures such as nanocylinders with circular or elliptic sections

Universal shape law of stochastic supercritical bifurcations: Theory and experiments

A universal law for the supercritical bifurcation shape of transverse one-dimensional (1D) systems in presence of additive noise is given. The stochastic Langevin equation of such systems is solved by using a Fokker-Planck equation leading to the expression for the most probable amplitude of the critical mode. From this universal expression, the shape of the bifurcation, its location and its evolution with the noise level are completely defined. Experimental results obtained for a 1D transverse Kerr-like slice subjected to optical feedback are in excellent agreement.Comment: 5 pages, 5 figure

Directional silicon nano-antennas for quantum emitter control designed by evolutionary optimization

We optimize silicon nano-antennas to enhance and steer the emission of local quantum sources. We combine global evolutionary optimization (EO) with frequency domain electrodynamical simulations, and compare design strategies based on resonant and non-resonant building blocks. Specifically, we investigate the performance of models with different degrees of freedom but comparable amount of available material. We find that simpler geometric models allow significantly faster convergence of the optimizer, which, expectedly, comes at the cost of a reduced optical performance. We finally analyze the physical mechanisms underlying the directional emission that also comes with an emission rate enhancement, and find a surprising robustness against perturbations of the source emitter location. This makes the structures highly interesting for actual nano-fabrication. We believe that optimized, all-dielectric silicon nano-antennas have high potential for genuine breakthroughs in a multitude of applications in nanophotonics and quantum technologies.Comment: 8 pages, 6 figure

Effets du bruit et d'un flot transverse sur les instabilités spatio-temporelles dans un système optique à cristaux liquides

Président du Jury: Jorge Tredicce Institut non lineaire de Nice Valobonne (France) tél: +39 055 23081 Rapporteurs: William J Firth Department of Physics, University of Strathclyde, John Anderson Building 107 Rottenrow, GLASGOW G4 ONG (Scotland) tél : +44 141 548 3269 Pier Luigi Ramazza Istituto Nazionale di Ottica Applicata, Largo E. Fermi 6 I-50125 Firenze (Italy) tél: +39 055 23081 Examinateurs: Thorsten Ackemann Department of Physics, University of Strathclyde, John Anderson Building 107 Rottenrow, GLASGOW G4 ONG (Scotland) tél : +44 141 548 3269 Jean Pierre Huignard Thalès Research and Technology, Orsay (France) tél: +39 055 23081Spontaneous pattern formation has attracted interest in many different scientific domains, as it is a widespread phenomenon manifested, for instance, in sand ripples, cloud stripes and leopard's skin. This work is a contribution to the study of these phenomena in the field of optics. It combines theoretical, numerical and experimental study, and focuses onto a liquid crystal system with optical feedback. First, non-trivial effects of noise on the pattern formation are studied. For instance, we show the presence of noise-induced precursors that anticipate some properties of the patterns appearing above the threshold. Second, we evidence that a transversal drift leads to the existence of a new instability (convective) leading to new noise-sustained structures. Their properties, their domains of existence as well as their combinations are studied.Les instabilités spatio-temporelles sont le nom scientifique pour nommer la formation spontannée de structures spatiales plus ou moins régulières. Nous avons tous eu l'occasion de voir des ridules se former sur le sable ou encore les nuages s'organiser en "troupeaux de moutons". Ce sont deux exemples parmi une très grande variété de structures et de systèmes capables de développer des instabilités spatio-temporelles. En optique, elles s'observent dans la section transverse des faisceaux laser. La diversité des organisations et la richesse de leur dynamique ont entraîné un intérêt croissant pour leur étude. Ce travail est une contribution à cet effort scientifique et s'appuie sur un système composé d'un faisceau laser faisant un aller-retour dans à travers un échantillon de cristal liquide. Une étude à la fois expérimentale, numérique et théorique est réalisée autour de deux axes principaux: l'étude des effets (i) du bruit sur la formation des structures et (ii) d'un flot transverse sur leur dynamique. Dans une première partie, nous montrons en particulier que la présence de bruit d'origine microscopique (fluctuation thermique des molécules de cristaux liquides) induit des effets macroscopiques non triviaux. C'est à dire qu'ils n'ont pas d'équivalent dans le système sans bruit généralement étudié lors de sa modélisation. Plus précisément, nous caractérisons un effet précurseur induit par le bruit qui se manifeste sous le seuil, là où aucun effet n'est attendu par le modèle classique. Il est appelé précurseur car il anticipe certaines propriétés du réseau hexagonal qui apparaît au seuil. Dans une seconde partie, nous montrons que lorsque la symétrie du système est brisée par un courant transverse, un nouveau régime dynamique -- l'instabilité convective -- peut apparaître. Nous mettons alors en évidence expérimentalement que celui-ci se traduit par la formation de structures entretenues par le bruit, i.e. qui n'existent qu'en présence de bruit. Enfin, l'étude de leurs propriétés et de leur dynamique nous permet de définir des conditions de formation de nouvelles structures complexes (superlattice) purement entretenues par le bruit. Bienvenu dans un monde où l'ordre naît du désordre..

Spin–orbit photonic diode from biomimetic 3D chiral liquid crystal architectures

International audienceSpin–orbit photonic devices usually rely on 2D (transverse) material structuring and are designed for optimal coupling between the polarization state and the spatial degrees of freedom at a given wavelength. Exploiting the third dimension (longitudinal) provides ways to bypass monochromatic limitations. Within a singular optics framework, here we show that chiral liquid crystals endowed with non-singular 3D helix axis orientational distribution exhibit transmissive broadband spin–orbit optical vortex generation as well as an optical diode effect. These results are in stark contrast to the properties of spin–orbit optical elements fabricated from chiral liquid crystals with a uniform orientation of the helix axis, which are reflective devices that process forward and backward propagating waves equally. Moreover, the similarities between the proposed 3D chiral structure and that of the cuticle of some insects invites considering spin–orbit photonics from a biological perspective

Cholesteric liquid crystalline materials with a dual circularly polarized light reflection band fixed at room temperature

International audienceAn unpolarized normal-incidence light beam reflected by a cholesteric liquid crystal is left- or right-circularly polarized, in the cholesteric temperature range. In this article, we present a novel approach for fabricating a cholesteric liquid crystalline material that exhibits reflection bands with both senses of polarization at room temperature. A cholesteric liquid crystal that presents a twist inversion at a critical temperature Tc is blended with a small quantity of photopolymerizable monomers. Upon ultraviolet irradiation above Tc, the liquid crystal becomes a polymer-stabilized liquid crystal. Below Tc, the material reflects a dual circularly polarized band in the infrared. By quenching the experimental cell at a temperature below the blend's melting point, the optical properties of the material in an undercooled state are conserved for months at room temperature, which is critical to potential applications such as heat-repelling windows and polarization-independent photonic devices

Color selectivity lent to a cholesteric liquid crystal by monitoring interface-induced deformations

International audienceThe cholesteric liquid crystalline structure is omnipresent in living matter and concerns many applications in optics because of its property of selective light reflection. The color reflected by this structure depends on material parameters such as the molecular chirality or the concentration of chiral dopant, the helical pitch of the twisted structure and the optical indices. In the present publication, we show that the color may be selected simply by varying the annealing time of a cholesteric oligomer film with hybrid anchoring. Experimental and simulated transmittance spectra reveal the color shift. The three-dimensional representation of the structure is provided from the macroscopic scale to the mesoscale by combining complementary imaging techniques (optical microscopy, transmission and scanning electron microscopies, atomic force microscopy) to image the texture in different directions. We show that the color selectivity is due to controlled changes of the orientation of the helical axis with respect to the air–material interface. The optical behavior is described with a single parameter, which is the angle between the film surface normal and the orientation within the film of the helical axis. Potential applications in the field of chiro-optical devices concern self-assembled chiral microreflectors fabricated via a low-cost controllable thermal process. Additionally, the texture formation provides insight to aid the understanding of the exoskeleton morphogenesis of beetles like Chrysina Gloriosa

Cholesteric liquid crystal gels with a graded mechanical stress

International audienceIn cholesteric liquid-crystalline gels, the mechanical role of the polymer network over the structure of the whole gel has been ignored. We show that it is the stress gradient exerted by the network over the helical structure that drives the broadening of the optical band gap, as evidenced by the absence of a gradient in chiral species. Model calculations and finite-difference time-domain simulations show that the network acts as a spring with a stiffness gradient. The present results indicate a revision to the common understanding of the physical properties of liquid-crystalline gels is necessary when a concentration gradient in a polymer network is present

Cholesteric liquid crystals doped with gold nanoparticles

International audienceThe reflection color of a cholesteric liquid crystal depends on material parameters such as the molecular chirality or the concentration of chiral dopant, the helical pitch of the twisted structure and the optical indices. We show that the color may be selected simply by varying the annealing time of an open cholesteric oligomer film with hybrid anchoring. The 3D representation of the structure is provided by combining complementary imaging techniques. The color selectivity is due to controlled changes of the orientation of the helix axis with respect to the air-material interface. Potential applications are chiral microreflectors and microlenses. Then, we demonstrate the symbiotic association of gold nanoparticles within such cholesteric textures and their long-range self-organized arrangements. We show that the nanoparticles can be patterned on demand only by playing with the film thickness and the interfacial properties of the CLC film. We investigate how the selective reflection is affected by the in situ organization of gold nanoparticles and what is the plasmon response of nanoparticle chains. Potential applications are envisioned in the field of soft nanotechnology and optical materials

Wavelength-tunable light shaping with cholesteric liquid crystal microlenses

International audienceThe ability to guide light on the mesoscopic scale is important both scientifically and technologically. Especially relevant is the development of wavelength-tunable light-shaping microdevices. Here we demonstrate the use of cholesteric liquid crystal polygonal textures organized as an array of microlenses for this purpose. The beam shaping is controlled by tuning the wavelength of the incident light in the visible spectrum. By taking advantage of the self-organization property of liquid crystals, the structure of the lens and its optical response are tailored by changing the annealing time of the single layer material during a completely integrated one-step process. The intrinsic helical organization of the layer is the cause of the light shaping and not the shape of the surface as for conventional lenses. A new concept of light manipulation using the structure chirality of liquid crystals is demonstrated, which concerns soft matter photonic circuits to mould the light