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. The power transfer efficiency in the obstructed case is derived by resorting to a scattered field formulation. In the proposed approach, the distance between the apertures is considered larger than their radius, which is also bigger than the operating wavelength. A paraxial approximation is then applied to the formulation. Numerical results validate the proposed approach. It appears that the transverse propagation constant of the Bessel Beam and resulting non-diffractive range strongly affects the distance of operation of the wireless link in both the clear and obstructed cases. In addition, we observe how the self-healing property of Bessel beams preserves the efficiency of the partially obstructed link by establishing a resilient link under defined conditions for the propagating beam and size of the obstruction.

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. The technique will be of great use for samples that cannot be moved.

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 0.2 nm, but also to eliminate any spurious influence related to the possible presence of uncoated interfaces. This method also implements a data processing similar to that used in Fourier transform spectrometry. After establishing the formulas that control the accuracy and the signal-to-noise ratio of this method, we present the results that provide a complete demonstration of its successful operation in various experimental conditions.

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). The global thickness of the sunflower leaf is reduced by up to 40% of its initial value.

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. Spectral shaping is carried out with gratings and a spatial light modulator. We validate the technique by cross-checking with a classical angle-resolved scattering set-up. Though the bandwidth is reduced, this white light technique may be of key interest for on-line measurements, large components that cannot be displaced, or other parts that do not allow mechanical movement around them.

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.

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 TiO2 nano-elements. Sommerfeld integrals are used to confirm our numerical simulations provided by our solver based on Fourier modal method. As a proof of concept, a metasurface with a 175 µm diameter is designed to be compatible with hyperspectral imaging over a spectral range of ± 50 nm around 650 nm with a spectral resolution of 8.5 nm and a field of view of 8° around the normal incidence (angular resolution of 0.2°).

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. We show how to calculate this trapped light and compare its order of magnitude with the free space scattering component emerging outside the multilayer.

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. This paper presents an implementation of a deep neural network model for light scattering design and proposes an optimization process for complex multilayer thin-film components to comply with expectations on both specular and scattering spectral responses.

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). Validation is first performed with success on etalon samples. The optimal structure of the leaf is found to be a 8-layer stack, in good agreement with microscopy investigations. Results may open the door to a complementary classification of leaves.

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. A laser diode system at two wavelengths 660 nm and 785 nm combined with polarization has been set up to differentiate four sunflower genotypes. REP-ASCA was used as a method to analyze parameters extracted from speckle patterns by reducing sources of measurement error. First findings have shown that measurement errors are mostly due to unwilling residual specular reflections. Moreover, results outlined that the genotype significantly impacts measurements. The variables involved in genotype dissociation are mainly related to scattering properties within the leaf. Moreover, an example of genotype classification using REP-ASCA outcomes is given and classify genotypes with an average error of about 20%. These encouraging results indicate that a laser speckle system is a promising tool to compare sunflower genotypes. Furthermore, an autonomous low-cost sensor based on this approach could be used directly in the field.

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 (${\mu}_s$µs) and absorbing (${{\mu}_a}$µa) coefficients. Then, several parameters were computed from the polarized speckle images, and prediction models were built using stepwise multiple linear regression. For scattering media, ${{\mu}_s}$µs was predicted with ${R^{2} = 0.9}$R2=0.9 using two parameters. In the case of scattering and absorbing media, prediction results using two parameters were ${R^{2} = 0.62}$R2=0.62 for ${{\mu}_s}$µs and ${R^{2} = 0.8}$R2=0.8 for ${{\mu}_a}$µa. The overall results obtained in this research showed that the combination of speckle pattern analysis, polarization parameters, and chemometric tools to predict the optical bulk properties of materials show interesting promise.

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. A zero spatial polarization degree can be reached with high accuracy in a controlled bandwidth.

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.

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.

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. Starting from the fluctuation dissipation theorem where thermally fluctuating sources are related to the radiative losses, we demonstrate that it is possible for objects residing in two spaces, virtual and physical, to have the same thermal radiation signature if the complex permittivities and permeabilities satisfy the standard space transformations. We emphasize the invariance of the fluctuation electrodynamics physics under transformation, and show how this result allows the mimicking in thermal radiation. We illustrate the concept using the illusion paradigm in the two-dimensional space and a numerical calculation validates all predictions. Finally, we discuss limitations and extensions of the proposed technique.

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/SiO2 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. The first two techniques are slow, and the third is operated in vacuum; hence, in all cases, real-time experiments including notably chemical modifications are not accessible. The development of optical microscopy techniques that combine the speed, large area, and high contrast of SEM with the topological information of AFM is therefore highly desirable. We introduce a new widefield optical microscopy technique based on the use of previously unknown antireflection and absorbing (ARA) layers that yield ultrahigh contrast reflection imaging of monolayers. The BALM (backside absorbing layer microscopy) technique can achieve the subnanometer-scale vertical resolution, large area, and real-time imaging. Moreover, the inverted optical microscope geometry allows its easy implementation and combination with other techniques. We notably demonstrate the potentiality of BALM by in operando imaging chemical modifications of graphene oxide. The technique can be applied to the deposition, observation, and modification of any nanometer-thick materials.