We have previously reported a new design for drift-free liquid-crystal polarization modulators (LCMs) based on liquid-crystal variable retarders (LCVRs). Here, we study their performance on Stokes and Mueller polarimeters. LCMs have polarimetric responses similar to LCVRs and can be used as temperature-stable alternatives to many LCVR-based polarimeters. We have built an LCM-based polarization state analyzer (PSA) and compared its performance to an equivalent LCVR-based PSA. Our system parameters remained stable over a wide range of temperature, precisely from 25°C to 50°C. Accurate Stokes and Mueller measurements have been conducted, paving the way to calibration-free polarimeters for demanding applications.

Polarization imaging has found many applications ranging from material sciences to biomedical applications to astronomy. A widely used class of polarimeters is based on Liquid Crystal Variable Retarders (LCVR). Indeed, LCVRs are ideal for imaging application: they are versatile polarization modulators with fast response times and a large aperture. However, the main drawback of such systems is their strong dependence on temperature. As a consequence, they require frequent and time-consuming calibration procedures. In this work, we propose a new design for a temperature-stable Variable Retarders cell compatible with LCVR-based polarimeter designs. We formalize a phenomenological model for the temperature dependency of LCVRs and derive theoretical expressions for the working points of the temperature stable cells. We used a heated enclosure to validate the proposed design experimentally. Stable operation of a single cell built from commercially available LCVRs is demonstrated on a wide range of temperatures (25-50°C). Two cells were then combined to obtain a Polarization State Analyzer (PSA), acting either as a standalone Stokes polarimeter or as part of a Muller polarimeter in combination with a Polarization State Generator (PSG). In both cases, excellent stability is demonstrated compared to similar LCVR based polarimeters.

We report a new design for temperature-stable polarization modulators. Each modulator is composed of two liquid crystal variable retarders (LCVRs) positioned in such a way that their temperature drifts mutually compensate. We propose a model for the temperature-dependent polarization response of LCVRs, which permits us to establish expressions for the operating point of the system and for its accessible retardance range. We have validated such a model experimentally by thorough analyses of LCVR temperature responses, and we have built a polarization modulator that is stable over a wide range of temperature with commercially available LCVRs.

A better understanding of tumor development is crucial for treating cancer. Polarimetric imaging is an interesting alternative for monitoring subcutaneous tumors as it is non-invasive. In this study, a Mueller spectro-polarimeter is used to monitor tumor development on mice injected with non-pigmented breast cancer cells or with pigmented murine melanoma cells. Three stages of non-pigmented tumor development are revealed with three polarimetric parameters. These stages also appear for pigmented tumors, although less clearly. A halo of high depolarization surrounding the non-pigmented tumor in the first stage allows the outlining of the tumor. Considering polarimetric parameters, a biological interpretation is proposed.

In this work, we propose using spectro-polarimetric imaging to monitor tumors growth of non-pigmented intra-dermal grafted tumors on murine models. We use for this purpose a full Mueller imaging polarimeter operating in the near infrared range. The injection site was imaged twice a week with our system from day 1 to the sacrifice of the mice while, in parallel, tumor volume was evaluated using standard caliper measurements. 40 nude mice were injected with a nonpigmented cancerous cell line (MDA) to produce intra-dermal tumors and separated into three different groups. The first group, called "early", received a Docetaxel treatment as soon as the polarization signal detected a change in the tissue matrix, the second one received the Docetaxel treatment as soon as the tumor reached an estimated volume of 20 mm(3) and the last group, was used as a control group. We demonstrate that early detection of tumor development based on depolarization metrics permits to control the tumor growth. We show also that while the tumor grows the depolarization contrast decreases while spectral contrasts appear. We observe also a change in fiber orientation around the tumor at the necrotic stage. This is highlighted by the azimuthal retardance orientation extracted from the polar decomposition of the Mueller matrix. This study confirms the usefulness of polarimetric contrast for tumor detection and monitoring. In the future, a precise characterization of different stages of tumor development could be useful for drug efficiency assessment in small-animal studies or in clinical applications.

Lensless inline digital holographic microscopy (LI-DHM) and Fourier ptychographic microscopy (FPM) are two widespread quantitative phase imaging (QPI) techniques. They have been employed in various fields, especially for biological slice imaging because of their simplicity in use, stability in structure, and also large field of view. Spherical phase response (for example from HeLa cells) is commonly observed in biological imagery. As a consequence, for calibration and validation purposes, small (several to tenth of microns in diameter) transparent microbeads have been used as standards. Phase imaging of their large counterparts (hundreds of microns in diameter) using either LI-DHM or FPM has not been reported so far. We are aiming to analyze the phase response of a 146-mu m soda-lime microsphere. It has been immersed in Canada balsam to reduce phase difference and to avoid overexposed diffraction rings. The phase estimation issue has been tackled using approaches that involve either Gerchberg-Saxton type algorithms or an inverse problem-based procedure. Confronting the results confirms the QPI capability for both imaging techniques to assess phase responses from such a large transparent object. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

Imaging spectropolarimetry is an informative technique that can be useful as a tool to detect and analyze cancerous tissues. However, to fit clinical standards, imaging spectropolarimeters must be fast, drift-free, and without any recurrent calibration, which is not the case for most imaging spectropolarimeters based on nematic liquid crystal phase modulators. Here, we present an instrument based on a novel architecture of differential liquid crystal variable retarders cells. A complete spectropolarimeter was built using this architecture and is now part of a clinical study at the dermatology department of the Strasbourg University Hospital.

The process of tumor growth is a phenomenon which, if understood better, could greatly improve the diagnostic and treatment of patients. In this context, the use of polarimetric imaging can offer more information than classical imaging methods. Here we use a new kind of calibration-free spectro-polarimeter which can be helpful for optical biopsy. Fifty different mice have been studied in full Mueller polarimetry with this device. Some were injected with very pigmented melanoma cells, others with non-pigmented breast cancer cells. Variations in depolarization were measured throughout this study: melanomas were accompanied with intense drops in depolarization, while mice injected with breast cancer cells showed a more diffuse decrease in depolarization. The study confirmed the potential of polarimetric imaging as an optical biopsy tool.

Surface plasmon modes propagating in metal nanowires are conveniently excited by focusing a laser beam on one extremity of the nanowire. We find that the precise positioning of the nanowire inside the focal region drastically influences the excitation efficiency of the different SPP modes sustained by the plasmonic waveguide. We demonstrate a spatially selective excitation of bound and leaky surface plasmon modes with excitation maps that strongly depend on the orientation of the incident linear polarization. We discuss this modal selection by considering the inhomogeneous distribution of the field components inside the focus. Our finding provides a way to discriminate the effective indices of the modes offering thus an increased coupling agility for future nanowire-based plasmonic architectures.

Nanoscale electronics and photonics are among the most promising research areas providing functional nanocomponents for data transfer and signal processing. By adopting metal-based optical antennas as a disruptive technological vehicle, we demonstrate that these two device-generating technologies can be interfaced to create an electronically driven self-emitting unit. This nanoscale plasmonic transmitter operates by injecting electrons in a contacted tunneling antenna feedgap. Under certain operating conditions, we show that the antenna enters a highly nonlinear regime in which the energy of the emitted photons exceeds the quantum limit imposed by the applied bias. We propose a model based upon the spontaneous emission of hot electrons that correctly reproduces the experimental findings. The electron-fed optical antennas described here are critical devices for interfacing electrons and photons, enabling thus the development of optical transceivers for on-chip wireless broadcasting of information at the nanoscale.

Artificial materials at optical frequencies have raised a strong interest in the last years, including photonic metamaterials, graded photonic crystals, and simple gradient index structures. The main common objective of these approaches is achieving a tight control of the electromagnetic guided-wave fields to play with light properties and propose versatile optical functions. In this general context, this work is focused on gradual photonic crystals (GPhCs) working in the diffraction regime, i.e. close to the photonic crystal (PhC) bandgap.

In this paper we describe a nonlinear imaging method employed to spatially map the occurrence of constrictions occurring on an electrically-stressed gold nanowire. The approach consists at measuring the influence of a tightly focused ultrafast pulsed laser on the electronic transport in the nanowire. We found that structural defects distributed along the nanowire are efficient nonlinear optical sources of radiation and that the differential conductance is significantly decreased when the laser is incident on such electrically-induced morphological changes. This imaging technique is applied to pre-determined the location of the electrical failure before it occurs.

By analogy to the three dimensional optical bottle beam, we introduce the plasmonic bottle beam: a two dimensional surface wave which features a lattice of plasmonic bottles, i.e. alternating regions of bright focii surrounded by low intensities. The two-dimensional bottle beam is created by the interference of a non-diffracting beam, a cosine-Gaussian beam, and a plane wave, thus giving rise to a non-diffracting complex intensity distribution. By controlling the propagation constant of the cosine-Gauss beam, the size and number of plasmonic bottles can be engineered. The two dimensional lattice of hot spots formed by this new plasmonic wave could have applications in plasmonic trapping.

The use of a birefringent graded photonic crystal (GPhC) is proposed for the realization of an efficient polarization beam splitter. This approach allows decoupling the two functions of efficient light injection for both polarizations and TE/TM beam splitting. A smooth light polarization splitting is naturally achieved due to the different curved trajectories followed within the graded medium by the TE and TM waves. A 160 nm operating bandwidth with insertion loss around 1 dB and interpolarization crosstalk below -15 dB is predicted by a finite difference time domain simulation. The unusually exploited electromagnetic phenomena are experimentally evidenced by scanning near-field optical measurements performed on samples fabricated using the silicon on insulator photonics technology. These experimental works open perspectives for the use of birefringent GPhCs to manage polarization diversity in silicon photonic circuits.

The transition between the long-wavelength and the short-wavelength regimes of light propagation in two-dimensional graded photonic crystal is investigated using a hyperspectral near-field scanning microscope. The experiments show an invariant quantity of only 1.78 times the lattice period as the criterion for the possible application of homogenization theories. These results are discussed in light of Fourier decomposition of the electromagnetic Bloch waves, and a physical interpretation of the observed transition between the two light propagation regimes is proposed. These results indicate the robustness of the homogenization approaches and suggest that the sharp transition between the two light propagation regimes could be profitably combined in graded optical artificial materials.

The transition between the long-wavelength and the short-wavelength regimes of light propagation in all-dielectric metamaterials is experimentally probed using a hyperspectral near-field scanning microscope technique. Our measurements lead to an invariant quantity “λ/n” of only 1.78 times the dielectric lattice period as the criterion for the possible application of homogenization theories.

La microscopie en champ proche optique permet d'analyser les phenomenes optiques avec une resolution spatiale sublongueur d'onde comme par exemple la localisation et la propagation de la lumiere dans des cristaux photoniques. D'une maniere generale, les methodes de microscopie en champ proche optique reposent sur le positionnement a l'echelle nanometrique d'une sonde locale a proximite de l'echantillon a analyser, puis sur la detection du signal diffuse et collecte lors du balayage de la sonde. En fonction du type de detection optique mise en oeuvre ou du type de sonde utilisee, les grandeurs physiques communement accessibles par ces methodes sont les distributions spatiales de l'amplitude et de la phase ou de l'intensite des composantes electriques ou magnetiques du champ sonde.Ce travail de these est consacre a la mise en place d'une detection hyperstectrale en champ proche optique dans le but de comprendre et de caracteriser, a des echelles sublongueurs d'onde, les proprietes spectrales et spatiales de systemes optiques miniaturises. L'imagerie hyperstectrale fournit en une seule acquisition, une serie d'image a chaque longueur d'onde dans les gammes spectrales visibles, infrarouges et aux longueurs d'onde des telecommunications optiques. Cette nouvelle technique d'imagerie a permis l'observation, sur une large bande spectrale, de phenomenes electromagnetiques dependant de la longueur d'onde tels que les effets superprisme et mirage dans les cristaux photoniques et la mise en forme de faisceaux de Bessel plasmoniques

Particles manipulation with optical forces is known as optical tweezing. While tweezing in free space with laser beams was established in the 1980s, integrating the optical tweezers on a chip is a challenging task. Recent experiments with plasmonic nanoantennas, microring resonators and photonic crystal nanocavities have demonstrated optical trapping. However, the optical field of a tweezer made of a single microscopic resonator cannot be shaped. So far, this prevents from optically driven micromanipulations. Here we propose an alternative approach where the shape of the optical trap can be tuned by the wavelength in coupled nanobeam cavities. Using these shapeable tweezers, we present micromanipulation of polystyrene microspheres trapped on a silicon chip. These results show that coupled nanobeam cavities are versatile building blocks for optical near-field engineering. They open the way to much complex integrated tweezers using networks of coupled nanobeam cavities for particles or bio-objects manipulation at a larger scale.