Multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals: Static and dynamic studies

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France
Applied Optics (Impact Factor: 1.78). 10/2005; 44(25):5273-80. DOI: 10.1364/AO.44.005273
Source: PubMed


The optimization of the experimental parameters of two multiplexed holographic transmission gratings recorded in holographic polymer-dispersed liquid crystals is investigated. Two methods are used to record the holograms: simultaneous and sequential multiplexing. These two processes are optimized to produce two multiplexed Bragg gratings that have the same and the highest possible diffraction efficiencies in the first order. The two methods show similar results when suitable recording parameters are used. The parameters of the recorded gratings (mainly the refractive-index modulation) are retrieved by use of an extension of the rigorous coupled-wave theory to multiplexed gratings. Finally, the response of the holograms to an electric field is studied. We demonstrate few coupling effects between the behavior of both gratings, and we expect a possibility of switching from one grating to the other.

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Available from: Raymond Chevallier, Jul 04, 2014
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    • "Therefore, the grating develops a dynamic behavior that may be modified by electronic means. In this manner, it is possible to make dynamic devices such as tunable-focus lenses, sensors, phase modulators, or prism gratings [10] [11] [12] [13] [14] [15] [16]. There are many parameters involved in hologram formation . "
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    • "In this manner, it is possible to make dynamic devices such as tunable-focus lenses, sensors, phase modulators, or prism gratings [11] [12] [13] [14] [15] [16] [17]. "
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    ABSTRACT: The response of a H-PDLC device is improved by means of a two-step method. First, component optimization-initiator system, crosslinker, and cosolvent-enables the diffraction efficiency of the hologram to be maximized. Second, the use of N-methyl-2-pyrrolidone in combination with N-vinyl-2-pyrrolidone prevents the overmodulation in photopolymers containing ethyl eosin.
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    ABSTRACT: Introduction In optical holography, among photopolymerizing materials, which are used as registering me-dia, the holographic polymer-dispersed liquid crystals (H-PDLC) controlled by the external electric field, form a special direction [1]. These composite materials are developed to produce displays and photonic crystals, 3D holographic optical memory [2 – 5]. The recent technologies for H-PDLC ap-plications suggest the recording of several transmission or reflection gratings within one material to increase the functional abilities of the material. The technology of superimposed grating formation (multiplexing) in the H-PDLC was initially developed for the reflective displays. One method was the angular multiplexing, which is based on the configuring two or more pairs of laser beams inter-fering at various angles onto the same volume of the photopolymerizing composition confined into a special cell, wherein the H-PDLC structure is formed. Later, the angular multiplexing, both simul-taneous and sequential (temporal) has been used to form superimposed transmission Bragg gratings in a thin layer of the photopolymerizing composition containing nematic liquid crystal [4]. The preparatory stage of the recording of the multiplexed gratings involves an independent re-cording of the various-period gratings, purposed to study the dynamics of their formations. The aim of the work at this stage is to adjust such recording conditions to get the gratings of different period with similar and maximally high diffraction efficiency (DE) in the first order. The results of these investigations will enable to optimize the recording conditions (exposure time and power density) for each grating and to reduce their possible mutual influence during the multiplexing. Experimental results and discussion The dependence of the formed gratings DE on the exposure time during the recording has been studied. The gratings of the periods of 1.3 – 3 µm were recorded with the р-polarized interfering beams of a semiconductor diode (λ=658 nm). The power density in the recording plane was 3 – 6 mW/cm 2 . The initial pre-polymer composition includes a multifunctional monomer – dipentaeritrol penta/hexa acrylate, a nematic liquid crystal (NLC) – the commercial mixture BL038 (∆n=0.27, n 0 =1.527, ∆ε=16.4) and a photo-initiating system (methylene blue, triethanolamine, N-vinylpyrrollidone). The preparation of experimental cells with the pre-polymer composition and ba-sic recording scheme are described in [6 – 7]. The measurements of angular dependences of light transmission have been performed using la-boratory experimental setup [6]. The light source was the semiconductor diode with the wavelength λ=658 nm. A half-wave plate and polarizer were placed before the cell with the recorded grating to set the polarization of the reading beam. The measurements were made at strictly fulfilled Bragg conditions. From the angular dependences we could estimate the grating period, the first-order DE (the ratio between the diffracted beam intensity and the incident s-or p-polarized beam intensity η 1s or η 1р). The polarization contrast was determined as the diffraction efficiencies ratio η 1s /η 1р .
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