Institut Fresnel

We formulate an optimization problem for the dependence of the eigenvalues of Maxwell's equations in a cavity with perfect electric conducting boundary upon variation of the electric permittivity, and we prove a corresponding Maximum Principle.

An electromagnetic model is developed to predict the thermal radiation which is trapped in a multilayer structure and transferred to its guided modes. The theory is based on the electromagnetic power supplied by the thermal currents given by the fluctuation-dissipation theorem. The source of the radiation is the ambient temperature or that caused by the optical absorption of the component subjected to spatio-temporal illumination. A numerical example is given for a multi-dielectric mirror at thermodynamic equilibrium. It is shown that the thermal radiation transferred to the guided modes of the multilayer can be much larger or lower than the radiation emerging in free space outside the component.

This paper presents a new approach to nanoparticle detection using an active micro-resonator operating in the transparency regime. Simulations demonstrate that when nanoparticles interact with the micro-resonator surface, they induce optical losses. To compensate for these losses, the optical gain is amplified to restore the transparency regime. Simulation results show a linear relationship between nanoparticle concentration and the pump power required to compensate for optical losses. By the use of micro-resonator with a very high quality factor, this approach offers an accurate and sensitive method for detecting nanoparticles, without the need for complex equipment.

Deriving analytical expressions of dielectric permittivities is required for numerical and physical modeling of optical systems and the soar of non‐Hermitian photonics motivates their prolongation in the complex plane. Analytical models are based on the association of microscopic models to describe macroscopic effects. However, the question is to know whether the resulting Debye–Drude–Lorentz models are not too restrictive. Here, it is shown that the permittivity must be treated as a meromorphic transfer function that complies with the requirements of complex analysis. This function can be naturally expanded on a set of complex singularities. This singularity expansion of the dielectric permittivity allows to derive a generalized expression of the Debye–Drude–Lorentz model that complies with the requirements of complex analysis and the constraints of physical systems. It is shown that the complex singularities and other parameters of this generalized expression can be retrieved from experimental data acquired along the real frequency axis. The accuracy of this expression is assessed for a wide range of materials including metals, 2D materials and dielectrics, and it is shown how the distribution of the retrieved poles helps in characterizing the materials.

Nanoantennas capable of large fluorescence enhancement with minimal absorption are crucial for future optical technologies from single-photon sources to biosensing. Efficient dielectric nanoantennas have been designed, however, evaluating their performance at the individual emitter level is challenging due to the complexity of combining high-resolution nanofabrication, spectroscopy and nanoscale positioning of the emitter. Here, we study the fluorescence enhancement in infinity-shaped gallium phosphide (GaP) nanoantennas based on a topologically optimized design. Using fluorescence correlation spectroscopy (FCS), we probe the nanoantennas enhancement factor and observe an average of 63-fold fluorescence brightness enhancement with a maximum of 93-fold for dye molecules in nanogaps between 20 and 50 nm. The experimentally determined fluorescence enhancement of the nanoantennas is confirmed by numerical simulations of the local density of optical states (LDOS). Furthermore, we show that beyond design optimization of dielectric nanoantennas, increased performances can be achieved via tailoring of nanoantenna fabrication.

The article introduces an optical microscopy technique capable of simultaneously acquiring quantitative fluorescence and phase (or equivalently wavefront) images with a single camera sensor, avoiding any delay between both images, or registration of images acquired separately. The method is based on the use of a 2-dimensional diffraction grating (aka cross-grating) positioned at a millimeter distance from a 2-color camera. Fluorescence and wavefront images are extracted from the two color channels of the camera, and retrieved by image demodulation. The applicability of the method is illustrated on various samples, namely fluorescent micro-beads, bacteria and mammalian cells.

The rise of metasurfaces to manipulate the polarization states of light motivates the development of versatile numerical methods able to model and analyze their polarimetric properties. Here we make use of a scattered-field formulation well suited to the Finite Element Method (FEM) to compute the Stokes-Mueller matrix of metasurfaces. The major advantage of the FEM lies in its versatility and its ability to compute the optical properties of structures with arbitrary and realistic shapes, and rounded edges and corners. We benefit from this method to design achiral, pseudo-chiral, and chiral metasurfaces with specific polarimetric properties. We compute and analyze their Mueller matrices. The accuracy of this method is assessed for both dielectric and metallic scatterers hosting Mie and plasmonic resonances.

Optical contrasts in microscopy are sensitive to light polarization, whose interaction with molecular dipoles provides an important lever for probing molecular orientation. Polarization microscopy has evolved considerably during the last decade, integrating strategies ranging from traditional linear dichroism to single-molecule orientation and localization imaging. This review aims to provide a summary of concepts and techniques behind orientation and structural imaging at the molecular level, from ensemble microscopy in 2D to single-molecule super-resolution microscopy in 3D.

Topological insulators, such as the Bi2Se3 material, exhibit significant optical nonlinearities. This work investigates the impact of the pulse duration on the nonlinear optical responses of Bi2Se3 layers. Scanning electron microscopy studies have been performed to reveal the crystalline structure of the samples. The nonlinear optical performance has been investigated for a wide range of pulse durations, from 400 fs to 10 ps, using 1030 nm laser excitation. The nonlinear absorption coefficients recorded in this study range from -1.45 x10⁻⁷ m/W to -4.86 x10⁻⁷ m/W. The influence of two different mechanisms on optical nonlinearities was observed and discussed. Identical experimental conditions have been employed throughout the studies allowing a direct comparison of the results.

COVID-19 has spread rapidly worldwide, despite the availability of vaccines, the fear of the World Health Organization continues due to the mutation of the Coronavirus. This is what prompted us to propose this work of social distance and wearing a face mask to fight against this pandemic to save lives. In this work, we propose a real-time four-stage model with monocular camera and deep learning based framework for automating the task of monitoring social distancing and face mask detection using video sequences. This work based on Scaled-You Only Look Once (Scaled-YOLOv4) object detection model, Simple Online and Real-time Tracking with a deep association metric approach to tracking people. The perspective transformation is used to approximate the three-dimensional coordinates with Euclidean metric to compute distance between boxes. The Dual Shot Face Detector (DSFD) and MobileNetv2 face mask model used to detect faces of people who violate or cross the social distance. Accuracy of 56.2% and real-time performance of 32 frames per second are achieved by the Social-Scaled-YOLOv4 (Social-YOLOv4-P6) model trained on the MS COCO dataset and Google-Open-Image dataset. The results are compared with other popular state-of-the-art models in terms of Mean-Average-Precision, frame rate and loss of values. The DSFD&MobileNetv2 facemask detectors trained on Wider Face and Real Face mask dataset achieves an accuracy of 99.3%. The proposed approach is validated on indoor/outdoor public images and video sequences such as wider face dataset, Oxford Town Center dataset and open access sequences.

Mechanical metamaterials, also known as architected materials, are rationally designed composites, aiming at elastic behaviors and effective mechanical properties beyond (“meta”) those of their individual ingredients – qualitatively and/or quantitatively. Due to advances in computational science and manufacturing, this field has progressed considerably throughout the last decade. Here, we review its mathematical basis in the spirit of a tutorial, and summarize the conceptual as well as experimental state-of-the-art. This summary comprises disordered, periodic, quasi-periodic, and graded anisotropic functional architectures, in one, two, and three dimensions, covering length scales ranging from below one micrometer to tens of meters. Examples include extreme ordinary linear elastic behavior from artificial crystals, e.g., auxetics and pentamodes, “negative” effective properties, behavior beyond classical linear elasticity, e.g., arising from local resonances, chirality, beyond-nearest-neighbor interactions, quasi-crystalline mechanical metamaterials, topological band gaps, cloaking based on coordinate transformations and on scattering cancellation, seismic protection, nonlinear and programmable metamaterials, as well as space-time-periodic architectures.

This paper demonstrates whispering gallery mode (WGM) resonance with the help of an encaved optical nano-probe developed inside an optical fiber tip cavity. The nano-probe generates a tightly focused beam with a spot-size of ∼3 µm. A barium titanate microsphere is placed besides the optical axis inside the cavity. The focused beam remains off-axis of the microresonator and excites the WGM. The off-axis excitation shows unique resonating properties depending on the location of the resonator. A resonant peak with quality factor as high as Q ∼7 × 10⁴ is achieved experimentally. Another design with a shorter cavity length for a bigger resonator is also demonstrated by embedding a bigger microsphere on the cleaved fiber tip surface. The optical probe holds great potential for photonic devices and is ideal for studying morphology-based scattering problems.

Quantitative phase microscopy (QPM) represents a non-invasive alternative to fluorescence microscopy for cell observation with high contrast and for the quantitative measurement of dry mass (DM) and growth rate at the single cell level. While DM measurements using QPM have been widely conducted on mammalian cells, bacteria have been less investigated, presumably due to the high-resolution and high-sensitivity required by their smaller size. This article demonstrates the use of cross-grating wavefront microscopy (CGM), a high-resolution and high-sensitivity QPM, for accurate DM measurement and monitoring of single micro-organisms (bacteria and archaea). The article covers strategies for overcoming light diffraction and sample focusing, and introduces the concepts of normalized optical volume (OV) and optical polarizability (OP) to gain additional information beyond DM. The algorithms for DM, OV, and OP measurements are illustrated through two case studies: monitoring dry mass evolution in a microscale colony forming unit as a function of temperature, and using OP as a potential species-specific signature.

We report a bending-insensitive multi-core fiber (MCF) for lensless endoscopy imaging with modified fiber geometry that enables optimal light coupling in and out of the individual cores. In a previously reported bending insensitive MCF (twisted MCF), the cores are twisted along the length of the MCF allowing for the development of flexible thin imaging endoscopes with potential applications in dynamic and freely moving experiments. However, for such twisted MCFs the cores are seen to have an optimum coupling angle which is proportional to their radial distance from the center of the MCF. This brings coupling complexity and potentially degrades the endoscope imaging capabilities. In this study, we demonstrate that by introducing a small section (1 cm) at two ends of the MCF, where all the cores are straight and parallel to the optical axis one can rectify the above coupling and output light issues of the twisted MCF, enabling the development of bend-insensitive lensless endoscopes.

Optical detection of ultrasound for photoacoustic imaging provides a large bandwidth and high sensitivity at high acoustic frequencies. Therefore, higher spatial resolutions can be achieved using Fabry-Pérot cavity sensors than conventional piezoelectric detection. However, fabrication constraints during the deposition of the sensing polymer layer require precise control of the interrogation beam wavelength to provide optimal sensitivity. This is commonly achieved by employing slowly tunable narrowband lasers as interrogation sources, hence limiting the acquisition speed. We propose instead to use a broadband source and a fast-tunable acousto-optic filter to adjust the interrogation wavelength at each pixel within a few microseconds. We demonstrate the validity of this approach by performing photoacoustic imaging with a highly inhomogeneous Fabry-Pérot sensor.

Coherence quantifies the statistical fluctuations in an optical field and has been extensively studied in the space, time, and polarization degrees of freedom. In the context of space, coherence theory has been formulated between two transverse positions as well as between two azimuthal positions, referred to as transverse spatial coherence and angular coherence, respectively. In this paper, we formulate the theory of coherence for optical fields in the radial degree of freedom and discuss the associated concepts of coherence radial width, radial quasi-homogeneity, and radial stationarity with some physically realizable examples of radially partially coherent fields. Furthermore, we propose an interferometric scheme for measuring radial coherence.