First manufactured diamond AGPM vector vortex for the L- and N-bands: metrology and expected performances

The AGPM (Annular Groove Phase Mask, Mawet et al. 2005) is an optical vectorial vortex coronagraph (or vector vortex) synthesized by a circular subwavelength grating, that is a grating with a period smaller than λ/n (λ being the observed wavelength and n the refractive index of the grating substrate). Since it is a phase mask, it allows to reach a high contrast with a small working angle. Moreover, its subwavelength structure provides a good achromatization over wide spectral bands. Recently, we have manufactured and measured our first N-band prototypes that allowed us to validate the reproducibility of the microfabrication process. Here, we present newly produced mid-IR diamond AGPMs in the N-band (~10 µm), and in the most wanted L-band (~3.5 µm). We first give an extrapolation of the expected coronagraph performances. We then present the manufacturing and measurement results, using diamond-optimized microfabrication techniques such as nano-imprint lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength grating profile metrology combines surface metrology (scanning electron microscopy, atomic force microscopy, white light interferometry) with diffractometry on an optical polarimetric bench and cross correlation with theoretical simulations using rigorous coupled wave analysis (RCWA).VORTE

First manufactured diamond AGPM vector vortex for the L- and N-bands: metrology and expected performances

The AGPM (Annular Groove Phase Mask, Mawet et al. 2005) is an optical vectorial vortex coronagraph (or vector vortex) synthesized by a circular subwavelength grating, that is a grating with a period smaller than λ/n (λ being the observed wavelength and n the refractive index of the grating substrate). Since it is a phase mask, it allows to reach a high contrast with a small working angle. Moreover, its subwavelength structure provides a good achromatization over wide spectral bands. Recently, we have manufactured and measured our first N-band prototypes that allowed us to validate the reproducibility of the microfabrication process. Here, we present newly produced mid-IR diamond AGPMs in the N-band (~10 µm), and in the most wanted L-band (~3.5 µm). We first give an extrapolation of the expected coronagraph performances. We then present the manufacturing and measurement results, using diamond-optimized microfabrication techniques such as nano-imprint lithography (NIL) and reactive ion etching (RIE). Finally, the subwavelength grating profile metrology combines surface metrology (scanning electron microscopy, atomic force microscopy, white light interferometry) with diffractometry on an optical polarimetric bench and cross correlation with theoretical simulations using rigorous coupled wave analysis (RCWA).VORTE

Achromatization of solar concentrator thanks to diffractive optics

Refractive solar concentrators suffer from important chromatic aberration. To enhance solar cell performance and increase the concentrating ratio, we propose an hybrid (diffractive/refractive) lens which allows a more uniform flux density and an achromatized (in the sense of achromatic doublet) illumination of the cell. To solve the problem of high diffraction efficiency, we turn to multilayer diffractive lenses to have broadband high diffraction efficiency optimized on solar spectrum. Rigorous coupled wave analysis simulations were performed in order to check the validity of scalar theory, they led to a new order of magnitude for the ratio period/wavelength which has been found to be around 100 in place of 10.WALI

Achromatization of solar concentrator using diffractive optics

Photovoltaic energy suffers from payback time. High solar concentration (above 500 suns) is a very interesting way to reduce production cost. The use of lenses allows more flexibility to make light flux more uniform. In addition lenses are less prone to errors of manufacture and can be easily duplicated. Unfortunately, optical materials are very dependent on the wavelength which causes important chromatic aberrations. One can think about achromatic doublets but doublets are thick, heavy and require exotic glasses which make them expensive. During the conference, I would like to present an innovative way to achromatize solar concentrators using hybrid (refractive/diffractive) lenses. Indeed, an hybrid lens is as thin as a Fresnel lens and is made in only one glass. But diffractive lenses suffer from limited broadband diffraction efficiency due to spurious orders. To overcome the lake of efficiency we are investigating the use of multi-layer diffractive optical element. This technique allows to achieve an efficiency greater than 97% over the full visible spectrum using only three components (e.g. two optical elements + air)

Antireflective subwavelength patterning of IR optics

peer reviewedThermal infrared (IR) lenses require efficient anti-reflection coating. Moth-eye (or egg-box) 2D subwavelength gratings have demonstrated their ability to reach a very high transmission for a wide wavelength and angular range. The use in thermal IR is simplified by the lower resolution for lithographic technology, compared to visible waveband. However, deeper structures must be engraved and lithography must be adapted to IR materials. In order to be cost-effective, the patterning must be produced by replication techniques, such as embossing. Our laboratory is now experimenting hot embossing of moth-eye patterns in chalcogenide substrates. In this paper, theoretical analysis, micro-lithographic technology and manufacturing processes are detailed

Interrupteur optique constitué de DOEs et d’une cellule à cristaux liquides ferroélectriques

peer reviewe

Non-Gaussian electrical fluctuations in a quasi-2d packing of metallic beads

The electrical properties of a two-dimensional packing of metallic beads are studied. Small mechanical perturbations of the packing lead to giant voltage fluctuations. Fluctuations are found to be non-Gaussian and seem to belong to Levy stable distributions. Anticorrelations have been also found for the sign of these fluctuations

Design optimization of large-size format edge-lit light guide units

In this paper, we present an original method of dot-pattern generation dedicated to design optimization of large-size format ELBUs (Edge-Lit Light Guide Unit), where the number of dots greatly exceeds the maximum allowable number of optical objects supported by most common ray-tracing software. In the proposed method, in order to simplify the computational problem, an original optical system is replaced by an equivalent one. Accordingly, the original dot pattern is splitted into multiple small sections. Then, these sections are replaced by equivalent cells with continuous diffusing film. After that, we adjust the TIS (Total Integrated Scatter) two-dimensional distribution over the grid of equivalent cells, using an iterative optimization procedure. Finally, the obtained optimal TIS distribution is converted into the dot size distribution by applying an appropriate conversion rule. An original semi-empirical equation dedicated to rectangular large-size LGPs (Light Guide Plate) is proposed for the initial guess of the TIS distribution. It allows significantly reducing the time needed to dot pattern optimization