Characterization of liquid crystals impregnated micrometric diffraction gratings for an active near-IR light extraction
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TECHNICAL PAPER
Characterization of liquid crystals impregnated micrometric diffraction gratings for an active near-IR light extraction control A. He´liot1,2,3 • S. Pelloquin1 • O. Gauthier-Lafaye1 • A. Monmayrant1 • H. Camon1 • T. Gacoin2 K. Lahlil2 • L. Martinelli2 • C. Biver3 • S. Archambeau3
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Received: 20 September 2018 / Accepted: 18 August 2020 Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Due to their narrow reflection peak as well as their compact structure, guided mode resonance filters (GMRFs) are attractive for many applications. In this work, we will demonstrate the possibility to modulate the properties of a GMRF by associating it with liquid crystals (LCs). By impregnating the diffraction grating with LCs, it is possible to switch between an active and an inactive state depending on the polarization of the light or the applied voltage. In this paper we fabricated and characterized the first diffraction order of LC-impregnated gratings with different periods (0.8–5.0 lm) and depths (120 and 840 nm) to test the ability of liquid crystals to adjust the diffraction properties. Finally, without voltage, more than 99.8% of initial diffraction could be turned off with a 90 rotation polarization whereas, by applying a voltage of 30 V; 90–99% of the initial diffraction is turned off according to the grating dimensions. The effect of the grating dimension (period, depth) on the diffraction modulation capacity will be discussed.
1 Introduction Combining a waveguide and a diffractive structure, one can form a GMRF which is particularly effective to produce narrow-band spectral filters. These structures represent a very promising alternative to conventional Fabry–Perot multilayer filters (Wang and Magnusson 1993) and can also be used to directionally couple or extract light confined in the core of a waveguide (Taillaert et al. 2002). Modulating a GMRF by using an active material has been the subject of considerable interest as such filters are attractive for many applications from optical modulators (Sharon et al. 1996) to tunable lasers (Zheng et al. 2012) due to their simple and compact structure as well as their narrow and sharp reflection peak. Liquid crystals are of much interest in this purpose as they are the subject of many researches since the seventies and are used at an industrial scale in liquid crystal displays & A. He´liot [email protected] 1
LAAS-CNRS, Universite´ de Toulouse, CNRS, Toulouse, France
2
Laboratoire PMC, UMR 7643, Ecole Polytechnique, CNRS, Palaiseau, France
3
ESSILOR International, Labe`ge, France
(LCD) screens. More recently, the birefringence of nematic liquid crystals and their rotation capability under an electric field have enabled the realization of active optical structures. Many researches have already demonstrated the ability of liquid crystals to modulate the diffraction of a Bragg grating under an electric field. A first approach involves the use of a GMRF with liquid crystals in the cladding
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