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This Letter presents, to our knowledge, the first direct measurement of the three-dimensional distribution of photonic jets (PJs) generated by shaped-tip multimode optical fibers. A PJ at the distal end of optical fibers makes it easier to scan a sample, for lithography or optical analysis, for example, with a spot smaller than the diffraction limit. The backscattered light can also be easily collected. In this study, the volume of the PJ has been reconstructed using a stack of image planes and compared to numerical simulations. For the first time, the power distribution of the non-fundamental mode around the PJ has been observed, giving a better understanding of PJ-based laser etching using multimode optical fibers. An original 50/125 fiber with a microlens fitting just on its core has made it possible to strongly reduce the power spread compared to the thermoformed 100/140 fibers used in our previous works.

In this study, multicrystalline mc-Si wafers were subjected to a rapid thermal processing (RTP) cycle at a peak temperature of 620°C. Two wafers were passivated by silicon nitride (SiNx:H) layers, deposited by a PECVD system. A first wafer was protected (PW) from the direct radiative heating of the RTP furnace by placing the wafer between two as-cut mc-Si shield wafers during the heat treatment. A second one was not protected (NPW) and followed the same RTP cycle. Moreover, charges carrier lifetime was measured by the quasi-steady-state photoconductance (QSSPC) method before and after RTP process, an enhancement in the electrical proprieties was detected for the PW wafer. Further, wafers surface profiling by interferometer microscope show a morphological degradation of the NPW wafer compared to the protected one. Furthermore, solar cells with aluminum oxide (AlO x ) passivated rear side (Al-BSF) were elaborated using as substrates the treated mc-Si wafers. The electrical characteristics of the solar cells were determined by a sun simulator, before and after an illumination step by exposing the simples to 500W halogen lamps for 5h duration in work condition temperature of 60°C ±5°C. It was observed that the solar cell based on the PW protected wafer has indicated a good electrical stabilization and a weak efficiency degradation after the illumination step. Finally, the proposed method can lead to improving solar cell efficiency by an adequate second RTP processing.

Conference Presentation; International audience; In classical microscopy, the diffraction of the light limits the resolving power of the system. Observation of nanoscale elements through an optical microscope appears often to be restricted. From the cut-off frequency of the optical transfer function, the lateral resolution of an optical microscope can thus be quantified as 0.5 Lambda/NA, where Lambda and NA are the wavelength of the light source and the numerical aperture of the microscope objective, respectively. A white-light microscope thus allows the visualisation of objects having a minimum size that is just greater than half of the wavelength of the illumination in air. Recently, several far-field methods have been developed in order to overcome this limitation. Microsphere-assisted microscopy is a modern technique which allows the diffraction limit to be broken. In 2011, Wang et al. developed two-dimensional super-resolution imaging through glass microspheres. They showed that microsphere-assisted microscopy distinguishes itself from others by being able to perform label-free and full-field acquisitions. In addition, with only slight modifications of classical white-light microscopy, microsphere-assisted microscopy makes it possible to reach a lateral resolution of a few hundred nanometres (~λ/7 in immersion). Placing a microsphere on a sample, directly or at a few hundreds of nanometres distance allows the generation of a super-resolved virtual image of the object which is then collected by a microscope objective. Currently several studies have been aimed at providing a better understanding of the super-resolution phenomenon in microsphere imaging. Now we know that the performance in microsphere-assisted microscopy depends on the optical and geometrical parameters. According to many studies on the PJ prediction, the imaging process in microsphere-assisted microscopy can be addressed by considering the sphere as a photonic jet lens. However, recently, we demonstrated that the photonic jet (PJ) generation by a microsphere, and considered here as the point spread function, is not small enough to justify this resolution improvement in the imaging process. Although the size of the focus spot overcomes the diffraction limit, the full width at half maximum of the PJ waist is around a third of the wavelength, which is lower than the super-resolving power which is around λ/7 in immersion. However, the PJ phenomenon can explain the imaging through the microsphere, the nature of the image, the position of the image plane and the lateral magnification dependence provided by the microsphere. Moreover, we have investigated the contrast in the virtual image according to the relative phase difference between close point sources. When the point sources are initially in-phase, the two virtual images cannot be distinguished. However, when the point sources are initially out-of-phase, the two virtual images can be clearly distinguished. Our work considers a third hypothesis by contributing to the investigation of the role of whispering gallery modes (a radially evanescent wave) in the microsphere assisted microscopy mechanism through numerical simulations using a finite element method.

The photonic nanojet (PNJ) was first reported by Chen et al.in 2004 through finite-difference time domain modeling of cylindrical structures under plan wave illumination. PNJ is a propagative beam with a full width at half maximum (FWHM) slightly smaller than a half wavelength. The power density can be more than 200 times higher than the one of the incident wave. This concentration can be achieved using a transparent dielectric microcylinder with a wavelength-scale radius. Gustav Mie obtained the exact solution of Maxwell’s equations for a dielectric microsphere in 1908. Applying this method, PNJ in the 3D case of a dielectric sphere was first reported by Li and Lecler. The photonic nanojet appears as a narrow and elongated spot with a high intensity electromagnetic field. The key parameters of PNJs, which are their FWHM, focal distance, decay length, and light intensity maximum, have been the subject of extensive theoretical and experimental studies. An in-depth understanding of photonic nanojet is nevertheless needed to fully exploit the potential of microspheres as optical super-lens. The original properties of PNJs make them ideal candidate in a wide range of applications, including nonlinear absorption enhancement, single-molecule fluorescence measurement, laser surgery, super-resolved microscopy, manipulation and detection of single sub-100 nm nanoparticles and biomolecules and laser sub-micro etching. Recently, the possibility to obtain a PNJ using an optical fiber with a shaped tip has opened new interests in the challenging field of direct laser subwavelength micromachining. To start with, the fiber is easier to move than the microsphere. Furthermore, the sphere is corrupted after the first irradiation due to their contact with the sample. On the opposite, the fiber tips have no contact with the processed surface and are not altered by the removed material. PNJs have already demonstrated the ability to reduce the laser etching size using shaped optical fiber tips. With a shaped optical fiber tips, the PNJ phenomenon is only due to the fundamental mode of the fiber. However, a standard single-mode fiber has three drawbacks for PNJ generation. (1) The small core diameter (

This article presents a fibre-optic sensor that measures temperature and pressure. Its operating principle is based on the amplitude modulation caused by the variation in the radius of a concave micro-mirror crafted into the end of an SMF optical fibre. In fact, a micro-cavity engraved into the end of the fibre by selective chemical etching is filled with a PDMS (Polydimethylsiloxane)-type polymer. Due to surface tension, the polymer micro-drop takes on a hemispheric shape characterised by a certain radius. After polymerisation in an oven at 100 °C for one hour, the hemispheric micro-drop is coated with a thin layer of gold using the vacuum evaporation technique. Typically, concave micro-mirrors can be obtained with bend radii of between 10μm and 30μm. Under the action of a temperature gradient or a variation in pressure, the thickness of the PDMS changes and causes a variation in the bend radius of the micro-mirror. As a result, the light intensity guided by the optical fibre and reflected by the micro-mirror is modulated by the variation in its bend radius. In this configuration, the sensor has a thermo-sensitivity of – 0.08dB/°C with a resolution of 0.13 °C in a range of between 20 °C and 100 °C. It also has a pressure sensitivity of 0.11dB/bar between 10 and 20 bars. The measurements are taken by a reflectometer (OTDR). In addition, the experimental results have been validated by theoretical modelling. This sensor is relatively simple to make and can be used in a wide range of applications, in particular biomedical and industrial ones.

In this paper, we present a technique for producing fibers optics micro-collimators composed of polydimethylsiloxane PDMS microlenses of different radii of curvature. The waist and working distance values obtained enable the optimization of optical coupling between optical fibers, fibers and optical sources, and fibers and detectors. The principal is based on the injection of polydimethylsiloxane (PDMS) into a conical micro-cavity chemically etched at the end of optical fibers. A spherical microlens is then formed that is self-centered with respect to the axis of the fiber. Typically, an optimal radius of curvature of 10.08 μm is obtained. This optimized micro-collimator is characterized by a working distance of 19.27 μm and a waist equal to 2.28 μm for an SMF 9/125 μm fiber. The simulation and experimental results reveal an optical coupling efficiency that can reach a value of 99.75%.

The sensor optimization is devoted to the study of the improvement of the sensor performance (sensitivity, operating interval) as a function of the radius of curvature of the micro-mirrors and the core diameter of the optical fibres. When the mode field diameter of the single mode fibre is smaller, the size of the waist image should have the same value (at a temperature or pressure value) to have a maximum coupling efficiency. During the fluctuation of T or P, the waist size image, which depends on the bend radius of the micro-mirror, scans the mode field diameter of the SMF beyond the Rayleigth range. As a result, the variation in the intensity of the reflected light is relatively faster. So, the slope of the coupling efficiency curve is quite steep. The sensitivity is improved but the range of temperature or pressure variation is shorter. Conversely, if the sizes are relatively large (SMF core as well as waist image), the scanning area is longer, so the sensitivity decreases, but the dynamic range is enlarged. This imposes a choice of detection. The main results and the structure of the sensor will be presented.

Concave cone fabricated by a chemical etching process similar to the technique used to make fibre tips (convex cone) is presented. Typically, optical fibres are etched in a hydrofluoric (HF) solution with a thin layer of oil floating on top of the HF solution. The present investigation shows that concave cone etched fibre (CCEF) can be obtained by following the beginning of the etching process. Comprehensive measurements of the dependence of the width and height of the taper angle of the concave cone are plotted. The results of this investigation have led to an optimum approach for core fibre alignment that is suitable for use in connecting fibres for efficient coupling between single-mode fibres.

Concave cone fabricated by chemical etching process similar to previous technique used to make fiber tips (convex cone) is presented. Typically, optical fibers are etched in a fluorhydric (HF) solution with a thin layer of oil floating on top of the HF. The present investigation shows that concave cone etched fiber (CCEF) can be obtained by following the beginning of etching process. Comprehensive measurements of the dependence of the width and height of taper angle of the concave cone are plotted. The results of this investigation have led to an optimum approach for fiber cores alignment that is suitable for use in connecting fibers for efficient coupling between single-mode fibers.

International audience; Several techniques of centring a microlens onto the fibre optic end face are studied. In most of them, microsphere lenses are centred with the aid of high-accuracy micro-positioners. This process is complicated with regard to the difficulty in manipulating microsphere lenses. In this paper, a simple and accurate self-centring method for mounting microsphere lenses using a concave cone etched fibre (Demagh et al 2006 Meas. Sci. Technol. 17 119-22) is described. This technique allows the centring of a wide variety of microlens radii, typically 7 mu m to over 24 mu m. The proposed process, however, is not affected by any spatial positioning control of microspheres. In over 85% of the attempts, the microsphere lenses were centred on the fibre axis to within 0.12 mu m.