Power transfer efficiency for obstructed wireless links using Bessel beams.

The power transfer efficiency of a partially obstructed wireless link operating in the Fresnel region is studied in this work. The wireless link consists of two equal apertures, axially aligned, radiating weakly-diffractive beams (truncated Bessel beams). A metallic obstacle is considered along the propagation path of the radiated beam to analyze its impact on the power transfer efficiency with respect to a clear line of sight link.

Instantaneous measurement of surface roughness spectra using white-light scattering projected on a spectrometer.

Following on from previous studies on motionless scatterometers based on the use of white light, we propose a new, to the best of our knowledge, experiment of white-light scattering that should overtake the previous ones in most situations. The setup is very simple as it requires only a broadband illumination source and a spectrometer to analyze light scattering at a unique direction. After introducing the principle of the instrument, roughness spectra are extracted for different samples, and the consistency of results is validated at the intersection of bandwidths.

Low coherence interferometric detection of the spectral dependence of the retro-reflection coefficient of an anti-reflective coated interface.

The measurement of very low reflection coefficients of anti-reflective coated interfaces has become a key issue for the realization of precision instruments such as the giant interferometers used for the detection of gravitational waves. We propose in this paper a method, based on low coherence interferometry and balanced detection, which not only allows to obtain the spectral dependence of this reflection coefficient in amplitude and phase, with a sensitivity of the order of 0.1 ppm and a spectral resolution of 0.

Analysis of the multilayer organization of a sunflower leaf during dehydration with terahertz time-domain spectroscopy.

We apply reverse engineering techniques (RET) to analyze the dehydration process of a sunflower leaf with terahertz time-domain spectroscopy. The multilayer structure of the leaf is extracted with accuracy during the entire process. Time variations of thickness and the complex index are emphasized for all leaf layers (2 cuticules, 2 epiderms, and 2 mesophylls).

Immediate and one-point roughness measurements using spectrally shaped light.

Capitalizing on a previous theoretical paper, we propose a novel approach, to our knowledge, that is different from the usual scattering measurements, one that is free of any mechanical movement or scanning. Scattering is measured along a single direction. Wide-band illumination with a properly chosen wavelength spectrum makes the signal proportional to the sample roughness, or to the higher-order roughness moments.

Micro-cavity optimization for ultra-sensitive all-dielectric optical sensors.

We present an analytical method for the optimization of luminescent micro-cavities to create a substrate that is extremely sensitive to contamination. Giant optical enhancement can thus be controlled arbitrarily and simultaneously at various frequencies within the substrate's evanescent field with the aim of obtaining ultra-sensitive optical sensors. This process provides an alternative to sensors based on illumination in free space.

Mechanisms driving self-organization phenomena in random plasmonic metasurfaces under multipulse femtosecond laser exposure: a multitime scale study.

Laser-induced transformations of plasmonic metasurfaces pave the way for controlling their anisotropic optical response with a micrometric resolution over large surfaces. Understanding the transient state of matter is crucial to optimize laser processing and reach specific optical properties. This article proposes an experimental and numerical study to follow and explain the diverse irreversible transformations encountered by a random plasmonic metasurface submitted to multiple femtosecond laser pulses at a high repetition rate.

Towards a metasurface adapted to hyperspectral imaging applications: from subwavelength design to definition of optical properties.

We numerically demonstrate the capability of a single metasurface to simultaneously separate and focus spectral features in accordance with the specifications of a pushbroom hyperspectral imager. This is achieved through the dispersion engineering of a library of two-level TiO nano-elements. Sommerfeld integrals are used to confirm our numerical simulations provided by our solver based on Fourier modal method.

Design of multilayer optical thin-films based on light scattering properties and using deep neural networks.

Despite limiting the performance of multilayer optical thin-films, light scattering properties are not as yet controllable by current design methods. These methods usually consider only specular properties: transmittance and reflectance. Among other techniques, design of thin-film components assisted by deep neural networks have seen growing interest over the last few years.

Trapped light scattering within optical coatings: a multilayer roughness-coupling process.

Despite numerous works devoted to light scattering in multilayer optics, trapped scattering has not been considered until now. This consists in a roughness-coupling process at each interface of the multilayer, giving rise to electromagnetic modes traveling within the stack. Such a modal scattering component is today necessary for completing the energy balance within high-precision optics including mirrors for gyro-lasers and detection of gravitational waves, where every ppm (part per million) must be accounted for.

Terahertz probing of sunflower leaf multilayer organization.

We analyze the multilayer structure of sunflower leaves from Terahertz data measured in the time-domain at a ps scale. Thin film reverse engineering techniques are applied to the Fourier amplitude of the reflected and transmitted signals in the frequency range f < 1.5 Terahertz (THz).

A New Optical Sensor Based on Laser Speckle and Chemometrics for Precision Agriculture: Application to Sunflower Plant-Breeding.

New instruments to characterize vegetation must meet cost constraints while providing accurate information. In this paper, we study the potential of a laser speckle system as a low-cost solution for non-destructive phenotyping. The objective is to assess an original approach combining laser speckle with chemometrics to describe scattering and absorption properties of sunflower leaves, related to their chemical composition or internal structure.

Wide-range wavelength and angle resolved light scattering measurement setup.

We present a new version of a light scattering measurement setup, using a high-power supercontinuum laser source, two volume hologram filters, and two low-noise scientific grade cameras. This configuration enables spectral and angle resolved characterization of the light scattered by complex thin-film filters from 400 to 1650 nm. Measurements carried out on specific filters illustrate the performances of the setup.

Combining light polarization and speckle measurements with multivariate analysis to predict bulk optical properties of turbid media.

This study aims to investigate the combination of speckle pattern analysis, polarization parameters, and chemometric tools to predict the optical absorption and scattering properties of materials. For this purpose, an optical setup based on light polarization and speckle measurements was developed, and turbid samples were measured at 405 and 660 nm. First, a backscattered polarized speckle acquisition was performed on a set of 41 samples with various scattering (${\mu}_s$μ) and absorbing (${{\mu}_a}$μ) coefficients.

Breakthrough spectrophotometric instrument for the ultra-fine characterization of the spectral transmittance of thin-film optical filters.

In this paper, we provide a detailed description of the main features of the upgraded version of a spectrophotometric apparatus developed by our team since 2014 [ Opt. Express23, 26863 (2015)]), and whose improved performance allows the characterization over the visible and near infrared part of the spectrum of the transmittance of complex interference filters with high spectral resolution (approximately one tenth of a nanometer) and an extremely wide dynamic range (thirteen decades).

Terahertz transmission imaging of inhomogeneous polymer multilayers: theory and experiment.

We extend an interferential multilayer model used in optics in the terahertz domain to be able to simulate the mapping of the transmissivity of a multilayer structure of polymers. In particular, we are interested in extracting the thickness gradient of a glue layer within an assembly of polymers. We developed an iterative procedure which we validated by terahertz imaging.

Broadband loss-less optical thin-film depolarizing devices.

For some space applications, sensors are sensitive to light polarization and can only be properly calibrated with non-polarized light. Here we propose new optical devices which allow to depolarize light in a spatial process. These devices are thin film multilayers which exhibit polarimetric phase variations in their plane.

Anomalous refraction of a low divergence monochromatic light beam in a transparent slab.

An exact formulation for the propagation of a monochromatic wave packet impinging on a transparent, homogeneous, isotropic, and parallel slab at oblique incidence is given. Approximate formulas are derived for low divergence light beams. These formulas show the presence of anomalous refraction phenomena at any slab thickness, including negative refraction and flat lensing effects, induced by reflection at the rear face.

Instantaneous one-angle white-light scatterometer.

We present a white light scatterometer operating at a unique scattering direction. Mechanical motions and wavelength scans are removed. The technique provides an immediate flexible characterization of roughness with no loss of resolution.

Transformational fluctuation electrodynamics: application to thermal radiation illusion.

Thermal radiation is a universal property for all objects with temperatures above 0K. Every object with a specific shape and emissivity has its own thermal radiation signature; such signature allows the object to be detected and recognized which can be an undesirable situation. In this paper, we apply transformation optics theory to a thermal radiation problem to develop an electromagnetic illusion by controlling the thermal radiation signature of a given object.

Backside absorbing layer microscopy: Watching graphene chemistry.

The rapid rise of two-dimensional nanomaterials implies the development of new versatile, high-resolution visualization and placement techniques. For example, a single graphene layer becomes observable on Si/SiO substrates by reflected light under optical microscopy because of interference effects when the thickness of silicon oxide is optimized. However, differentiating monolayers from bilayers remains challenging, and advanced techniques, such as Raman mapping, atomic force microscopy (AFM), or scanning electron microscopy (SEM) are more suitable to observe graphene monolayers.

Inverse heat mimicking of given objects.

We address a general inverse mimicking problem in heat conduction. The objects to cloak and mimic are chosen beforehand; these objects identify a specific set of space transformations. The shapes that can be mimicked are derived from the conductivity matrices.

Light scattered by optical coatings: numerical predictions and comparison to experiment for a global analysis.

Complex optical coatings may present highly disturbed scattering patterns, both spectrally and angularly. We show in this paper how the development of an accurate dedicated metrology allowed the optimization of numerical models. Our prediction is compared to our measurement.

Ultra-wide-range measurements of thin-film filter optical density over the visible and near-infrared spectrum.

We present the improved structure and operating principle of a spectrophotometric mean that allows us for the recording of the transmittance of a thin-film filter over an ultra-wide range of optical densities (from 0 to 11) between 400 and 1000 nm. The operation of this apparatus is based on the combined use of a high power supercontinuum laser source, a tunable volume hologram filter, a standard monochromator and a scientific grade CCD camera. The experimentally recorded noise floor is in good accordance with the optical density values given by the theoretical approach.

Phase retrieval of reflection and transmission coefficients from Kramers-Kronig relations.

Analytic and passivity properties of reflection and transmission coefficients of thin-film multilayered stacks are investigated. Using a rigorous formalism based on the inverse Helmholtz operator, properties associated with the causality principle and passivity are established when both the temporal frequency and spatial wave vector are continued in the complex plane. This result extends the range of situations where the Kramers-Kronig relations can be used to deduce the phase from the intensity.