Gain dynamics in liquid crystal photorefractive hybrids

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ABSTRACT Photorefractive (PR) hybrid liquid crystal (LC) cells have combined th e space-charge field generated in either a polymer (using e.g. PVK;C 60 ) with the large birefringence from a LC layer to generate PR grating for beam coupling applications. The efficiency of PR beam coupling in hybrid devices is dependent on the amplitude of the space-charge field, as well as the ability of the LC mo lecules to align with the corresponding field. In this paper the time dynamics of the formation of the PR gratings are measured in LC hybrid systems and are used to explain the large variation of gain coefficients found in the literature. 1. INTRODUCTION There has been a considerable interest in using light activated materials to control the orientation of a liquid crystal (LC) layer within optical devices [1-5]. In par ticular hybrid optical devices composed of photorefractive (PR) substrates with an organic LC layer are capable of overcoming several of the disadvantages present in pure inorganic or pure organic PR materials[2, 6]. PR hybrid devices take advantage of the high effective trap density present in inorganic PR materials to generate a large space-charge field capable of controlling an organic a liquid crystal layer [2, 3]. The low birefringence present in PR materials is compensated by the large birefringence possible in LCs. The combination these two fundamentally different materials into a single PR hybrid device result in gain coefficients two orders of magnitude larger than those in pure inorganic or organic PR materials[5, 7]. There have been two types of hybrid devices that have su ccessfully demonstrated the ability of a PR material to control a LC layer; inorganic/organic hybrids and pure organic hybrids. Inorganic/organic hybrids are composed of crystalline inorganic PR windows, such as SBN:Ce, and an organic LC gain media. The space-charge field generated in the PR material is capable of modulating the LC layer and a measurable increase to the PR gain coefficient has been observed [6]. Inorganic/organic hybrids take advantage of the high effective trap density that is present in PR crystals and can operate at grating sp acings that are in the Bragg regime [6]. An even larger gain coefficient has been achieved with the incorporation of ferroelectric nanoparticles that possess a large spontaneous polarization [7-9]. Furthermore, inorganic/ organic hybrid devices have been constructed using non-traditional PR materials such as semiconductors, which have expanded their wavelength sensitivity into the infrared [10]. The second type of hybrid is a pure organic LC hybrid, which will be the focus of this work. Pure organic hybrids are LC cells where the standa rd Elvamide alignment layer has been replaced with a space-charge field generating alignment layer closely related to a PR polymer. PR polymers have been extensively studied over the last several decades are considered a cheaper, more flexible materials possessing PR properties [11, 12]. PR polymer materials have large diffraction efficiencies and the ability to write and store holographic data [13, 14].