TiO 2 Nanoparticle±Photopolymer Composites for Volume Holographic Recording** By Carlos Sµnchez, Michael J . Escuti, Chris van Heesch, Cees W . M. Bastiaansen,* Dirk J . Broer, Joachim Loos ,and RenØ Nussbaumer 1. Introduction Photopolymerizable materials with a high refractive-index modulation have been intensively investigated in the past, sincetheyaregoodcandidatesforwriteonce,readmanytimes (WORM)volumeholographicstorage,owingtotheirlargedy- namicrange,highsensitivity,anddimensionalstability. [1] Apart from storage applications, these materials are highly suitable fortheproductionofauthentication/securityfeaturesandholo- graphic optical elements (HOEs) for different optical applica- tions. [2] For example, commercial photopolymers are often used in holographic components such as reflectors, diffusers, andcolorfilterstomakeliquid-crystaldisplays(LCDs)bright- erandmoreefficient. Usually,holographicphotopolymersconsistoflight-sensitive monomersand/ormonomer±polymermixturesthatareableto capture microscopic light patterns, such as those generated by the interference of two or more coherent laser beams. Inter- ference patterns are captured as a refractive-index change throughpreferentialphotoreactionintheregionsexposedtoa higherfluxofenergy.Asignificantamountofbasicandapplied research has been performed over the last few decades to un- ravel the fundamental details of the holographic recording mechanism and to improve the performance of materials in terms of dynamic response, sensitivity, and recording fidelity. Severalphotopolymermaterialfamilies,whichrelyondifferent philosophies in order to achieve high refractive-index modula- tion, have been proposed. For example, in the classic Polaroid material DMP-128, [3] exposure to a holographic pattern is fol- lowed by solvent processing and thermal treatments to obtain a refractive-index profile based on nanoscopic voids that map to the amplitude front of the hologram. In holographic poly- mer-dispersed liquid crystals (HPDLCs), [4] a homogeneous mixture of reactive monomers and liquid crystals (LCs) is ex- posed to patterned irradiation, leading to LC-rich regions and polymer-rich regions that manifest the refractive-index con- trast.Despitethehighrefractive-indexmodulationspossiblein thesematerials,theHPDLChologramsarenotfullypolymeric bydesign,apropertythatisnotalwaysdesiredincertainappli- cations. DuPont has also developed holographic materials in which the difference in refractive index between the reactive monomers and a polymeric binder is responsible for the holo- gram formation. [5] In these systems, the binder acts as a sup- porting matrix for the monomer mixture, which preferentially diffuses and reacts in the volumes of highest irradiance. This photopolymerizationprocessleadstoaheterogeneouspolymer network exhibiting a refractive-index profile that mimics the exposing irradiance pattern. These materials have the advan- tagesofexcellenthumidityandtemperaturestability,andtheir solvent-free processing generates completely polymeric films that are easy to laminate. Materials beyond acrylate-only sys- tems have also been pursued, most notably those containing epoxymonomersthatpolymerizethroughacationicring-open- Adv. Funct. Mater. 2005, 15, 1623±1629 DOI: 10.1002/adfm.200500095  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1623 ± [*] Dr. C. W. M. Bastiaansen,Dr. C. Sµnchez,Dr. M. J. Escuti, C. van Heesch, Dr. J. Loos Eindhoven University of Technology Den Dolech 2, NL-5600 MB Eindhoven (The Netherlands) E-mail:c.w.m.bastiaansen@tue.nl Dr. C. W. M. Bastiaansen,Dr. C. Sµnchez,Dr. M. J. Escuti, C. van Heesch, Dr. J. Loos Dutch Polymer Institute (DPI) P.O. Box 902, NL-5600 AX Eindhoven (The Netherlands) Prof. D. J. Broer Eindhoven Polymer Laboratories Eindhoven University of Technology P.O. Box 513, NL-5600 MB Eindhoven (The Netherlands) Prof. D. J. Broer Philips Research Laboratories Prof. Holstlaan 4, NL-5656 AA Eindhoven (The Netherlands) Dr. R. Nussbaumer Department of Materials ETH Zentrum UNO-C15, CH-8092 Zürich (Switzerland) [**] The research of Carlos Sµnchez, Michael Escuti, Chris van Heesch, and Cees Bastiaansen forms part of the research program of the Dutch Polymer Institute (DPI), projects DPI#298, 304, and 426. A new and efficient photopolymer for the recording of volume holograms is presented. The material comprises a mixture of UV-sensitive acrylates and grafted titanium dioxide nanoparticles with an average size of 4 nm. We report the formation of holographicgratingswithrefractive-indexmodulationamplitudesofupto15.510 ±3 Ðanimprovementofmorethanafactor offouroverthebasematerialwithoutnanoparticlesÐwhilemaintainingalowlevelofscatteringandahightransparencyinthe visible-wavelength range. The influence of the composition of the acrylate system on the final properties of the holographic materialisalsoinvestigatedanddiscussed.Thepresenceofmultifunctionalmonomersfavorsthecompositionalsegregationof thedifferentcomponents,whiletheadditionofmonofunctionalacrylate,highlycompatiblewiththegraftingofthenanoparti- cles,favorsthedilutionofthesenanoparticles. FULL PAPER