Material response of photopolymer containing four different photosensitizers Yue Qi a , Haoyu Li a , Jinxin Guo a,b , Michael R. Gleeson a,c , John T. Sheridan a,n a School of Electrical, Electronic and Communications Engineering, Communications and Optoelectronic Research Centre, The SFI-Strategic Research Cluster in Solar Energy Conversion, College of Engineering and Architecture, University College Dublin, Belfield, Dublin 4, Ireland b Department of Engineering Science, University of Electro-Communications, 1-5-1 Chofu-gaoka, Chofu, Tokyo 182-8585, Japan c III–V Materials and Devices, Photonics Centre, Tyndall National Institute, Lee Maltings, Dyke Parade, Cork, Ireland article info Article history: Received 12 December 2013 Received in revised form 9 January 2014 Accepted 12 January 2014 Available online 28 January 2014 Keywords: Photosensitizer Dyes Photopolymer Holography Refractive index modulation abstract Building on a previous studies, the behaviour of four different photosensitizers in an acrylamide/ polyvinyl alcohol (AA/PVA) photopolymer material are examined using a 1-D Nonlocal Photo- polymerisation Driven Diffusion (NPDD) model. In order to characterise the effects of using different photosensitizers, holographic illuminations with different spatial frequencies and intensities are applied. Material parameters, i.e., the nonlocal response parameter, s, the diffusion rate of monomer, D m , the chain initiation kinetic constant, k i , and the termination rate, k t , are extracted by numerically fitting the predictions of the NPDD to experimentally measured refractive index modulation growth curves. Four photosensitizers, [Erythrosin B (EB); Eosin Y (EY); Phloxine B (PB); and Rose Bengal (RB)], are examined. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The first step in any photo-polymerisation process involves the absorption of light by the photosensitiser in the material. Depend- ing on the material and dye type, this leads to an initiation process. Clearly this first step is critically important in determining the material response characteristics. In earlier models it was gener- ally assumed that the exposing intensity [1–5] directly determined the rate of initiation or the rate of polymerisation (and thus the process of grating formation) in the layer. To concisely characterise the material response Zhao and Mouroulis [1], whose model well describes the behaviour of photopolymers for larger period and lower exposure intensities, introduced the parameter R ¼ K 2 D m =F 0 : ð1Þ In this equation, K ¼ 2π=Λ is the magnitude of the grating vector, D m is the diffusion constant of the monomer, and F 0 ¼ κI τ 0 , is the polymerisation rate [1,4]. The κ parameter is a material constant, while τ describes the nonlinear response of the material to the exposing radiation and I 0 is the average exposing irradiance [1,4]. When the material layer is illuminated by a cosinusoidal interference pattern a holographic grating of fundamental period Λ is formed. The R parameter is useful because it can be shown that the larger the value of R the higher the fidelity and strength of the recorded grating [1]. For low spatial frequency (SF) cases, the recorded cosinusoidal grating period Λ is large, i.e., SF ¼ 1/Λ. Larger grating periods mean that the monomer must diffuse a longer distance from the dark interference fringe regions into the bright regions, in order to equalise the monomer concentration in the layer. Returning to the R parameter we note that as Λ increases K decreases and thus R decreases, and therefore one might expect a weaker grating to be recorded. For low exposing intensity (or low absorptivity dyes) the values of I 0 (or κ) are smaller and therefore R is bigger. In these cases the monomer is used up (polymerised) more slowly. Therefore the monomer has time to diffuse into the brightly illuminated regions and be polymerised there, and thus a stronger grating will be formed. High exposing intensity will cause fast growth of the grating but will also lead to poor fidelity recording, because of the formation of higher harmonic of the refractive index modulation. Such higher harmonic can be formed even if τ ¼ 1 (i.e., linear material response), e.g. if monomer, diffusing from the dark regions does not penetrate into the centre of the bright regions, but is rapidly polymerised at the edges of these regions. For this case the spatial distribution of the photo- polymer becomes less sinusoidal, thus higher harmonics of the photopolymer concentration becomes more significant leading to the generation of the higher harmonics of the refractive index Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2014.01.026 n Corresponding author. Tel.: þ353 1716 1 927; fax: þ353 1 283 092. E-mail address: john.sheridan@ucd.ie (J.T. Sheridan). Optics Communications 320 (2014) 114–124