The turbulent transport of flexible fibres is of paramount importance in a large range of industrial and environmental applications, from papermaking to sediment deposition in rivers and oceans. However, the dynamics of fibres in inhomogeneous and anisotropic turbulent flows, particularly relevant for these applications, has mainly been studied in numerical simulations and, only recently by a few experiments limited to rigid fibres. This project aims to go further in order to model the dynamics of long flexible fibres in wall-bounded turbulent flows. It relies on an experimental approach in a turbulent channel flow coupled with numerical simulations having similar control parameters. We will first study the rigid-flexible transition in the turbulent channel flow and compare it to the scaling law that has been obtained in homogeneous and isotropic turbulence. We will then study the effect of flexibility on the transport of the fibres in the turbulent channel flow, and particularly on their dynamics close to the walls. Finally, we will focus on the stretching of particles in wall-bounded turbulence, by investigating the coiled-stretched transition on home-made macroscopic polymers.

The STLight project focuses on the experimental study of photonic fluids in disordered environments. Superfluidity, the ability of a fluid to move without friction along a pipe or past an obstacle, is one of the most spectacular features of quantum fluids. In nonlinear optics, it manifests as light propagating without being altered by an inhomogeneous environment. The exact opposite happens in the linear case, where light undergoes spatial localisation in disordered media. The main ambition of the STLight project is to study the transition from spatial localisation to superfluidity in complex, but fully controlled, environments, positioning the project at the edge between quantum hydrodynamics and waves in nonlinear complex media. Strong turbulence in complex media will naturally arise in the system and will be investigated. It will, besides addressing transport in disordered optical systems, significantly benefit in both the nonlinear optics and quantum hydrodynamics communities. The project STLight relies on the main research hypothesis that the paraxial propagation of an optical field in a nonlinear transparent medium is formally analogous to the evolution of a two-dimensional quantum fluid. In this analogy, the spatial evolution of the optical field along the propagation direction is analogous to the temporal evolution of the wavefunction of a quantum gas. Simply put, each transverse plane in the nonlinear medium is equivalent to a “snapshot” of the temporal dynamics of a two-dimensional quantum fluid. To realise this analogy, photons need to acquire an effective mass and be in a fully controlled effective (repulsive) interaction – two features that are allowed in properly engineered photonic systems. For instance, a photorefractive crystal in which the optical index can be structured in an arbitrary and reconfigurable way, is a perfect candidate for studying fluids of light, notably in disordered environments, and is at the heart of our experimental apparatus. The project is outlined along 3 work packages organised following a progressive increase of the system’s complexity, from the simpler quasi-homogeneous case toward a completely disordered case; In particular, we will study: 1/ Turbulence in a nonlinear crystal with few obstacles --> Objective – Study the interplay between vortices and obstacles in simple configurations, through vortex generation in the wake of a 1D set of obstacles and the dynamics of vortices in the vicinity of obstacles. 2/ Fluids of light in complex media, through the robustness of superfluidity in a disordered system, as well as the robustness of localised states in a nonlinear medium. --> Objective – i) Design, implement and control the photo-induction of disordered patterns in the crystal in a 2D, z-independent configuration, ii) Study the extreme regimes of spatial localisation and superfluidity, iii) Study the intermediate turbulent regime. 3/ Towards fluids of light in “time-dependant” systems --> Objective – In this much more exploratory part, we will study time-dependant dynamics of the fluid of light with i) a time-dependant disorder and ii) a full 3D fluid of light. The main technical breakthrough to tackle is the control the creation of the disordered environment in a nonlinear system. This implementation is made possible in the experimental system that as the potential to overcome this technical barrier. The results are expected to strengthen the fundamental knowledge and experimental developments related to fluids of light and quantum turbulence and to open new perspectives on the spatial localisation of light in nonlinear environments.

Multimode optical fibers are the subject of very active research in view of the technological advances expected in the fields of telecommunications and imaging. However, mode dispersion and coupling pose practical implementation problems. RICOTTA is a fundamental project inspired by this issue, which aims to study light scattering in multimode optical fibers with controlled disorder. Scatterers will be photo-inscribed into the fibers using a local direct laser writing device, and the coherent wave transport properties will be measured through the disordered fiber transmission matrix. From the mesoscopic transport literature, it is known that the conductivity of these disordered quasi-1D wires is affected by weak localization, a reciprocity-induced phenomenon. In RICOTTA, we plan to break reciprocity by placing the samples in a magnetic field to induce Faraday effect. This will provide new insights into the transport properties, in addition to the results obtained by measuring other properties such as sample length, number of modes, or disorder intensity. The samples will be characterized by measuring the complete transmission matrix, paving the way for coherent wave control in such systems.

Lithium niobate (LN) is a material very exploited in photonics circuits (optical frequency generators, phase or intensity modulators, quantum states generators…) from labs use to satellites embedded devices. LN thin-films (LNOI) revolutionized integrated optics: by decreasing by several orders of magnitude the control voltage and increasing the modulation rate in intensity modulators, or enhancing frequency generators efficiency. However, the wafers substantial cost, the number and complexity of technological steps for manufacturing circuits, as well as their limited compatibility with standard fiber components (injection losses of several decibels) represent a bottleneck for further development of this technology. In the current project, we propose to combine processes well-known and mastered by the team members to fill the gap between standard photonics chip on bulk LN and LNOI based chips. The goal will be to fabricate a substrate, and components, approaching the outstanding results obtained with LNOI while reducing its fabrication complexity, using low-cost techniques: a layer of increased refractive index will be induced on the surface of a standard LN using a technique perfectioned in INPHYNI since 2013. Several methods of waveguides creation will be implemented: etching of the substrate or of a guiding deposited layer by mechanical or plasma. We will also combine our skills in numerical simulation and technology to keep a high compatibility of the waveguides with fiber-devices. As the originality of the project lies in the compatibility of the processes rather than in their technicality, the risks are low, and for each step we plan several backup solutions in case of failure.

Ultrafast nonlinear optic is currently employed in femtosecond lasers technology and gathers numerous powerfull techniques to gate or modulate the optical signal in the frequency, time or space domain. Current nonlinear media are crystals and gas. Liquid crystals intrinsically exhibit very interesting optical properties: a large birefringence, a wide spectral range of transparency and, above all, the possibility of modifying their optical properties through the molecular reorientation induced by an electric or magnetic field. Most common mesophases of liquid crystals have been widely studied for light manipulation. However, despite their exceptional optical properties, the applications of liquid crystal cells to the direct manipulation of ultrashort optical pulses trains have remained occasional so far. In the framework of the LABCOM SOFTLITE between the OCL group of Institut de Physique de Nice (INPHYNI) and FASTLITE company, we have recently started a novel research activity, devoted to the linear manipulation and spectro-temporal shaping of ultrashort pulses with devices based on liquid crystals. For the two last years, we have disclosed the use of thick nematic liquid crystal cells for ultrafast applications. These investigations have simultaneously opened a novel application field for ultrafast instrumentation as well as provided a new insight of liquid crystals dynamics, through the detection of unexpected collective molecular motions. A natural extension of this research consists in investigating the ultrafast nonlinear properties of liquid crystals. In the framework of the UNLOC project, we propose to extend our ultrafast facility with the implementation of a more energetic and shorter femtosecond source, in order to deeply investigate ultrafast nonlinear optics in liquid crystals. Although liquid crystals in two different phases (nematic and cholesteric) remain the targeted medium, we also plan to extend our expertise to photo-refractive crystals, whose properties for ultrafast optics hasn’t been studied so far. Using ultrashort pulses presents two main advantages. On one hand, it gives access to the ultrafast nonlinear temporal dynamics in the nonlinear medium. On the other hand, the facility will enable to reach high peak power and high average power, facilitating the excitation of third-order, or even higher, nonlinear processes. We expect to extent the fundamental knowledge about nonlinearity of soft matter through our performed ultrafast spectroscopy experiments. Moreover, potential applications include the nonlinear shaping of femtosecond pulses as well as the nonlinear tailoring of the considered nonlinear medium. At the end of the project, a novel experimental platform dedicated to ultrafast nonlinear optics will be fully operational at INPHYNI. Furthermore, we will have acquired the know-how to handle and quantify ultrafast nonlinear optics in liquid crystals. This know-how will lead to the development and fiabilization of the next generation of liquid-crystal based devices for ultrafast technology. The project will then strengthen the French position at the forefront of the international competition for this emerging research field. The UNLOC project will undoubtedly strengthen the coordinator’s research thematic at INPHYNI and will reinforce her scientific independence.