The GRADFLEX experiment of ESA has shown that under microgravity conditions a stationary thermodiffusion process is accompanied by giant non-equilibrium fluctuations with size as large as the size of the sample. In the presence of small concentration gradients, the features of the non-equilibrium fluctuations can be described by means of linearized hydrodynamics. However, the linear models are not suitable to describe most cases of applicative interest, such as fluctuations induced by large gradients and under non-stationary conditions. Moreover, presently the investigation of non-equilibrium fluctuations has mainly involved single component fluids and binary mixtures, but recently transport processes in ternary mixtures have attracted increasing interest due to the experiments performed on the International Space Station in the framework of the DCMIX project of ESA. The Giant Fluctuations (NEUF-DIX) project of ESA will investigate non-equilibrium fluctuations during diffusive processes occurring in complex multi-component mixtures, where one of the components is a macromolecule, such as a polymer, a colloid or a protein. Important objectives will be the exploration of the features of the fluctuations under non-ideal conditions, such as large gradients, transient processes, and concentrated samples, and the understanding of how the fluctuations affect the interactions between macromolecules. The project involves the development of a dedicated facility, consisting of an array of shadowgraph optical instruments working in parallel, each one equipped with a thermal gradient cell. Here we outline the design concept of the facility and the results of performance tests performed on a breadboard to evaluate the suitability of the designed instrument to carry out scientific measurements of non-equilibrium concentration fluctuations in space.

ABSTRACTInterchain interactions can play a positive role in reaching amplified spontaneous emission in an interesting core–polymer system where the donor (side chains) and the acceptor (core) are chemically linked together. Different degree of interchain interactions modifies the photophysical characteristics of the polymer. By means of transient absorption spectroscopy we show that the stimulated emission from the core decreases passing from solid state to concentrated solution and it is almost absent in the diluted solution. The conformational rearrangements of the core–polymer chain in solution limits the efficiency of the intrachain Förster energy transfer mechanism. The free chain rotations decrease the exciton hopping along the conjugated chains, the ratio between donor and acceptor moieties in the polymer, and change the relative orientation of the transition dipoles of the donor and acceptor causing a strong decrease of energy transfer efficiency and subsequently of the gain. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 965–969

In this letter, the authors demonstrate Drop‐on‐Demand printing of variable focus, polarization‐independent, liquid crystal (LC) microlenses. By carefully selecting the surface treatment applied to a glass substrate, the authors are able to deposit droplets with a well‐defined curvature and contact angle, which result in micron‐sized lenses with focal lengths on the order of 300–900 µm. Observations with an optical polarizing microscope confirm the homeotopic alignment of the LC director in the droplets, which is in accordance with the polarization independent focal length. Results show that microlenses of different focal lengths can be fabricated by depositing successive droplets onto the same location on the substrate, which can then be used to build up programmable and arbitrary arrays of microlenses of various lens sizes and focal lengths. Finally, the authors utilize the thermal dependency of the order parameter of the LC to demonstrate facile tuning of the focal length. This technique has the potential to offer a low‐cost solution to the production of variable focus, arbitrary, microlens arrays.

We discuss the role played by time-dependent scattering on light propagation in liquid crystals. In the linear regime, the effects of the molecular disorder accumulate in propagation, yielding a monotonic decrease in the beam spatial coherence. In the nonlinear case, despite the disorder-imposed Brownian-like motion to the self-guided waves, self-focusing increases the spatial coherence of the beam by inducing spatial localization. Eventually, a strong enhancement in the beam oscillations occurs when power is strong enough to induce self-steering, i.e., in the non-perturbative regime.

We study the interplay between multiple scattering and self-focusing in Nematic Liquid Crystals (NLCs). At low powers self-focusing increases the coherence, but for large powers a continuous temporal oscillation is observed.

A uniform lying helix (ULH) alignment of cholesteric liquid crystals (LCs) is obtained using a solvent evaporation technique. The solvent evaporation method allows for the spontaneous formation of a virtually defect-free alignment, even in the absence of an external electric field. A small amount of solvent diffuses into the LC and changes its phase into isotropic state where the individual LC molecules are more mobile. As the solvent diffuses out of the LC and consequently evaporates, additional mobility provided by the solvent allows the molecules to reach the lowest energy configuration, dictated by the boundary conditions, the solvent evaporation direction and the elastic forces among the molecules. Compared to a shear-flow-induced alignment, the solvent-induced ULH exhibits a contrast ratio between the bright and dark states that is a factor of 4 times larger, due to the low number of defects in the structure. From measurements of the flexoelectro-optic effect, the difference between the splay and bend flexoelectric coefficients, , for the nematic LC E7 is found to be in agreement with the measured values reported in the literature (12.11.0 pC/m), demonstrating that the solvent self-aligning does not change the electric response of the medium, while improving its optical properties. [GRAPHICS] .

We present modeling of the spatial instabilities in nematicon propagation where the long range interaction in liquid crystals is taken into account with a correlated noise. The experimental measurements support the numerical results.

Starting from the de Gennes theory of director fluctuations in nematics, we report on a model of the spatial fluctuations in nematicon propagation. We demonstrate that, when the long-range correlation that characterizes nematic liquid crystals is taken into account in the thermal noise, it is possible to account for the spatial oscillations and propagation losses experienced by nematicons. Increasing the power of the nematicon, the oscillation amplitudes increase and the propagation losses decrease. The nematicon is then more strongly confined and deviates more, but is less scattered by the thermally induced perturbations of the refractive index. All the results are in good agreement with the experimental observations.

ABSTRACTPolyfluorene (PFO) embedded in a nematic liquid crystal (LC) matrix is investigated. For low PFO weight contents, a homogeneous dispersion is obtained which displays a strong fluorescence anisotropy along the LC director, indicating a significant alignment of the polymeric chains along this direction. Besides, for relatively high PFO weight contents, phase separation takes place. Under these conditions, the sample is composed of micrometer‐sized domains, where the two species are in solution, enclosed by segregated polymeric boundaries. By polarized‐photoluminescence imaging and spectroscopy, it is found that most of the light emission originates from these boundaries and gets strongly pinned along their orientation. Since boundaries are mainly oriented orthogonal to the LC chains, this morphological alignment results in a system in which the orientation of the polarization emission can be predicted and possibly controlled. Conversely, in the homogeneous sample one can obtain a homogeneous emission polarization by controlling the alignment of the LC. These features are potentially relevant for the development of flexible polarization‐sensitive optoelectronic devices. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1558–1563

We investigate experimentally the interaction between amplified spontaneous emission (ASE) and a soliton, which are both generated in a dye-doped nematic liquid crystal (LC) cell. A light beam is injected through an optical fiber slid into the cell to form a soliton beam. ASE is then automatically collected by this self-induced waveguide and efficiently coupled into the same optical fiber, in the backward direction. We demonstrate that the presence of the soliton improves the ASE collection by one order of magnitude. We also show that the ASE is highly polarized in the plane of the LC cell and that the ASE spectrum depends on the pump stripe orientation with respect to the LC director. The origin of the spectral anisotropy of the gain curves is determined with the help of femtosecond pump-probe spectroscopy.

Amplified spontaneous emission (ASE) generated in nematic liquid crystals is efficiently collected by a soliton generated in the same medium. The soliton guides and injects the weakly coherent polarized ASE into an optical fiber.

By means of confocal photoluminescence measurements and fs pump-probe spectroscopy, we observe the optical properties of phase separation in a mixture of polyfluorene and Liquid Crystals (LCs). The boundaries of LC-rich micro-domains display a large polarized gain region from keto defects stemming from the single-chain nature of this defect.

We demonstrate a way of light harvesting in integrated microfluidic chips fabricated by femtosecond laser micromachining. The architecture consists of waveguide arrays fabricated in the vicinity of the microchannel filled with a fluorescent organic solution (e. g., polyfluorene solution). Amplified spontaneous emission from the microchannel is efficiently coupled by the waveguides to the outside of the chip. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

Liquid crystals can induce β-phase of PFO with anisotropic absorption and emission spectra. Pump-probe spectra also show the presence of a stimulated emission band polarized parallel to the PFO chains.

Les sujets de Mémoires de fin d'études suivants sont proposés, à titre principal, aux étudiants de 2e année du grade de Master en Ingénieur civil physicien, Ingénieur civil électricien (spécialisation télécommunications) ou aux étudiants de 2e année du Master en Sciences physiques. Les thèmes proposés s'intègrent dans la palette des activités de recherche développées par le groupe Optique et Photonique du Service OPERA. MOTIVATION Modulation instability (MI) is a universal effect existing in many nonlinear systems and has been studied in a wide range of fields including fluid dynamics, plasma physics and nonlinear optics. It means that perturbations of the plane wave solution will get amplified along the propagation direction. The explanation of this phenomenon is the energy transfer between spectral modes, namely between the zero-order mode (the plane wave) and higher-order Fourier components in the signal. When adding a sinusoidal perturbation, this perturbation gets amplified and under certain circumstances, after some propagation distance, the perturbation decreases again and the system returns to its initial state. This phenomenon was first reported by Fermi, Pasta and Ulam and is nowadays commonly called FPU recurrence. The figure below shows simulations of such a nonlinear system. The initial sinusoidal perturbation is initially amplified. After reaching the maximum amplification, the system can go back to to the initial state or alternatively, it can lead to quite complex behavior. DESCRIPTION Modeling and experimental demonstration of modulation instability, induced modulation instability and recurrence has been done for materials with optical nonlinearities. In this thesis, the purpose is to extend the model and/or provide experimental evidence that modulation instability also occurs in materials without optical nonlinearity but with optical gain. Optical gain (e.g. due to stimulated emission) is also a form of nonlinearity. Depending on the interest of the student, the focus of the thesis can be devoted more to theoretical work or more to experimental work. This means that the thesis may consist of the following tasks:  Theory :  Calculation of modulation instability for optical gain nonlinearity. Determination of the gain coefficient and frequency with maximum gain.  Investigation of the effect of deviations from pure gain: effect of saturability, etc  Experiment :  Devices need to be prepared with strong optical gain. These devices will be fabricated in collaboration with UGent.  Building an experimental setup including a pump laser to excite the laser dyes, a signal beam which will be amplified …

We report a new approach to improve the gain characteristics of a red-emitting based molecule. The insertion of the active dye between two polymeric arms prevents the inter-molecular interactions deleterious for the gain. By means of the ultra-fast pump-probe technique we found an efficient energy transfer between the polymer and dye. High gain in the nanosecond timescale regime has been proved.

Soluble and processable polymeric conjugated materials are of particular interest since they provide a simple technology which allows for low cost large area and flexible devices. We report on the photophysical characterization of a new copolymer (R1A) obtained by a recently developed “core-type” design approach [1] (the chemical structure is shown in Fig. 1b). By means of ultra-fast pump-probe technique we are able to resolve the energy transfer from the side chains towards the core. Fig. 1a shows that the highest stimulated emission signal (660 nm) is reached at around 10 ps probe delay and after 400 ps the signal is still positive.

A uniform lying helix (ULH) alignment of cholesteric liquid crystals (CLCs) is of particular interest for flexoelectro-optic switching, which is a fast in-plane rotation of the optic axis when an electric field is applied perpendicular to the helix axis [1]. As the response times can be sub-millisecond, this opens up the possibility of realizing frame sequential color in next generation flat panel displays. However, achieving a high quality ULH still remains a challenge [2-4]. Here we propose a new method using solvent evaporation to spontaneously induce a defect-free ULH in a CLC, even in the absence of an electric field. Compared to the shear flow induced ULH (Fig.1a-c), the solvent induced ULH (Fig.1d-f) exhibits a contrast ratio between the bright and dark states that is a factor of 4 greater, due to the low number of defects in the structure (Fig.1f). The measured e1-e3 for E7 is found to be the same for both techniques (12.2±1.0 pC/m), direct observation of flexoelectrically-driven instabilities in the presence of an electric field.References: [1] J. S. Patel and R. B. Meyer, Phys. Rev. Lett. 58, 1538 (1987).[2] Y. Inoue and H. Moritake, Appl. Phys. Expr. 8, 071701 (2015).[3] P.S. Salter et al., Liq. Cryst. 36, 1355 (2009). [4] H.J. Coles et al., J. Appl. Phys. 99, 034104 (2006).