HAL Portal IOGS (nstitut d'Optique Graduate School)
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On the applicability of Kirchhoff’s law to the lasing regime
No full textInternational audienceIn 1860, Kirchhoff showed that the radiance emitted by a hot body is the product of its absorptivity by a universal function of temperature and frequency, which he defined as blackbody radiance. With the advent of semiconductors, it has been shown that Kirchhoff's law can be extended to account for electroluminescence and photoluminescence in a cavity. Hence, the question of the applicability of Kirchhoff's law in the lasing regime arises naturally. In this work, we show that Kirchhoff's law accounts for the lasing transition. It also enables the modeling of important features of the lasing regime, such as the frequency, directivity, and polarization of both the first lasing mode and the non-lasing background. Finally, we show that it enables to recover the Schawlow-Townes spectral linewidth. In summary, Kirchhoff's law appears to be a versatile tool to model light emission from the thermal to the lasing regime
Sensing forces in cells using fluorescence FRET microscopy and optical tweezers
No full textInternational audienceForces inside cells play a fundamental role in cell behavior, for example in cancer cell migration. We focus on the vinculin protein which is involved in the stabilization of cell adhesion. Through fluorescence transfer (FRET), forces within vinculin can be measured with picoNewton sensitivity. We measure these internal forces while applying a calibrated external force with a laser-based optical tweezer via a microbead attached to the cell. Our most recent results using fibroblast cells show that the force applied with the optical tweezer induces the recruitment of vinculin and the formation of focal adhesions on the bead within a few minutes. Once the bead is attached to the cell, we record its trajectory and infer the force exerted by the cell. We correlate this force with the FRET efficiency of the force sensor
Correlated phonon pairs in a time modulated Bose-Einstein Condensate
No full textInternational audienceIn standard cosmological models, inflation is driven by a quantum field known as the inflaton, whose constant energy density causes the universe to expand at superluminal speeds. When inflation ends, the universe has an extremely low density, but the inflaton field begins to oscillate around its energy minimum and decays into entangled pairs of particles. This phase is known as pre-heating. The particles then start to interact, leading to decoherence and thermalization, marking the re-heating stage. Although in situ observation of the inflaton particle creation process is impossible, this pair production through parametric amplification is analogous to the creation of phonon pairs in a Bose-Einstein condensate (BEC) with temporally modulated interaction strength. Modulating the stiffness of a dipole trap in a cigar-shaped BEC is equivalent to modulating the effective one-dimensional interaction strength in a BEC. Our work is both theoretical, introducing new criteria to probe non-separability, and experimental. We report the observation of entangled phonons with opposite momenta. The entanglement of the phonon pairs is observed to decrease as the excitation duration increases
Driven-dissipative phase separation in free-space atomic ensembles
No full textInternational audienceThe driven Dicke model, wherein an ensemble of atoms is driven by an external field and undergoes collective spontaneous emission due to coupling to a leaky cavity mode, is a paradigmatic example of a system exhibiting a driven-dissipative phase transition as a function of driving strength. Recently, a similar phenomenon was experimentally observed, not in a cavity setting, but rather in a free-space atomic ensemble. The reason why similar behavior should emerge in free space is not obvious, as the system interacts with a continuum of optical modes, which encodes light-propagation effects. Here, we present and solve a simple model to explain the behavior of the free-space system, based on the one-dimensional Maxwell-Bloch equations. On one hand, we show that a free-space ensemble at a low optical depth can exhibit similar behavior as the cavity system, as spatial propagation effects are negligible. On the other hand, in the thermodynamic limit of large atom number, we show that certain observables such as the transmittance or the atomic excited population exhibit nonanalytic behavior as a function of the driving intensity, reminiscent of a phase transition. However, a closer analysis reveals that the atomic properties are highly inhomogeneous in space, and based on this we argue that the free-space system does not undergo a phase transition but rather a “phase separation,” roughly speaking, between saturated and unsaturated regions
The quest for resolution; a dynamic view on laser volume nanostructuring
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Systematic design of a robust half-W1 photonic crystal waveguide for interfacing slow light and trapped cold atoms
No full textInternational audienceNovel platforms interfacing trapped cold atoms and guided light in nanoscale waveguides are a promising route to achieve a regime of strong coupling between light and atoms in single pass, with applications to quantum non-linear optics and quantum simulation. A strong challenge for the experimental development of this emerging waveguide-QED field of research is to combine facilitated optical access for atom transport, atom trapping via guided modes and robustness to inherent nanofabrication imperfections. In this endeavor, here we propose to interface Rubidium atoms with a photonic crystal waveguide based on a large-index GaInP slab. With a specifically tailored half-W1 design, we show that a large coupling to the waveguide can be obtained and guided modes can be used to form two-color dipole traps for atoms at about 100 nm from the edge of the structure. This optimized device should greatly improve the level of experimental control and facilitate the atom integration
Approximation Error of Sobolev Regular Functions with tanh Neural Networks: Theoretical Impact on PINNs
No full textInternational audienceConsidering the key role played by derivatives in Partial Differential Equations (PDEs), using the tanh activation function in Physics-Informed Neural Networks(PINNs) yields useful smoothness properties to derive theoretical guarantees in Sobolev norm. In this paper, we conduct an extensive functional analysis, unveiling tighter approximation bounds compared to prior works, especially for higher order PDEs. These better guarantees translate into smaller PINN architectures and improved generalization error with arbitrarily small Sobolev norms of the PDE residuals
Fluorescence-Guided Surgical Techniques in Adult Diffuse Low-Grade Gliomas: State-of-the-Art and Emerging Techniques: A Systematic Review
No full textInternational audienceDiffuse low-grade gliomas are infiltrative tumors whose margins are not distinguishable from the adjacent healthy brain parenchyma. The aim was to precisely examine the results provided by the intraoperative use of macroscopic fluorescence in diffuse low-grade gliomas and to describe the new fluorescence-based techniques capable of guiding the resection of low-grade gliomas. Only about 20% and 50% of low-grade gliomas are macroscopically fluorescent after 5-amino-levulinic acid (5-ALA) or fluorescein sodium intake, respectively. However, 5-ALA is helpful for detecting anaplastic foci, and thus choosing the best biopsy targets in diffuse gliomas. Spectroscopic detection of 5-ALA-induced fluorescence can detect very low and non-macroscopically visible concentrations of protoporphyrin IX, a 5-ALA metabolite, and, consequently, has excellent performances for the detection of low-grade gliomas. Moreover, these tumors have a specific spectroscopic signature with two fluorescence emission peaks, which is useful for distinguishing them not only from healthy brain but also from high-grade gliomas. Confocal laser endomicroscopy can generate intraoperative optic biopsies, but its sensitivity remains limited. In the future, the coupled measurement of autofluorescence and induced fluorescence, and the introduction of fluorescence detection technologies providing a wider field of view could result in the development of operator-friendly tools implementable in the operative routine
Femtosecond Laser Cutting of Human Crystalline Lens Capsule and Decellularization for Corneal Endothelial Bioengineering
No full textInternational audienceThe bioengineering of corneal endothelial grafts consists of seeding in vitro cultured corneal endothelial cells onto a thin, transparent, biocompatible, and sufficiently robust carrier which can withstand surgical manipulations. This is one of the most realistic alternatives to donor corneas, which are in chronic global shortage. The anterior capsule of the crystalline lens has already been identified as one of the best possible carriers, but its challenging manual preparation has limited its use. In this study, we describe a femtosecond laser cutting process of the anterior capsule of whole lenses in order to obtain capsule discs of 8 mm diameter, similar to conventional endothelial grafts. Circular marks made on the periphery of the disc indicate its orientation. Immersion in water for 3 days is sufficient to completely remove the lens epithelial cells and to enable the seeding of corneal endothelial cells, which remain viable after 27 days of culture. Therefore, this method provides a transparent, decellularized disc ready to form viable tissue engineered endothelial grafts
Optical properties and applications of laser-induced disordered plasmonics metasurfaces
No full textInternational audienceThe lecture explores laser-induced plasmonic metasurfaces, focusing on their structural characteristics and optical properties. Despite their disordered nature, these metasurfaces excel in applications such as color printing, sensing, light manipulation, and energy harvesting, offering advantages over traditional ordered structures through detailed case studies