Characterization of PDLC Holographic Gratings

We have studied the optical properties of electrically switched holographic gratings fabricated using polymer-dispersed liquid crystal (PDLC) materials. We have found that the PDLC mixture can be used to control the diffractive properties of the composite gratings, including the anisotropy of the grating. 2000 Optical Society of America OCIS codes: (050.0050) Diffraction and gratings, (050.7330) Volume holographic gratings, (160.3710) Liquid crystals In this paper we present the results of an experimental study of the diffractive properties of electrically switched holographic gratings fabricated using polymer-dispersed liquid crystal mixtures. These composite gratings can be used to fabricate high contrast, electrically switchable, diffractive elements for the visible and near infrared region of the spectrum for optical switching and beam steering applications [1]. In this study we have found that the diffractive properties can be controlled using different PDLC mixtures. In fact by changing the PDLC mixture the diffractive properties of the gratings can be changed from highly anisotropic to highly isotropic. The liquid crystal composite gratings that we have characterized were fabricated using a prepolymer mixture similar to the mixtures used by Sutherland et. al. [2]. The prepolymer mixture consists of a multifunctional acrylate mixture, the nematic liquid crystal E7 or TL205, a photoinitiator dye and a coinitiator. The gratings studied in this work were fabricated using 30 % by weight the liquid crystal TL205 or E7. To characterize the gratings we measure diffraction efficiency as a function of angle for many different wavelengths. By measuring the diffraction efficiency at many different wavelengths, we are effectively probing the composite medium at many different Bragg angles. This is a sensitive measure of the relative permittivity modulation tensor and therefore of the liquid crystal alignment. 0 0.2 0.4 0.6 0.8 1.0 10 20 30 40 50 60 70 Bragg angle within prism (degrees) di ffr ac tio n ef fic ie nc y Fig. 1 Diffraction efficiency of a PDLC grating fabricated using E7 liquid crystal. Theoretical predictions are obtained by modeling the grating as a highly anisotropic composite grating. Figure 1 shows the experimentally measured values for the normalized diffraction efficiency for ppolarized light at the Bragg angle for each wavelength for the case of no field applied to the sample (circles) and the case when a switching field was applied to the sample (squares). The diffraction efficiency for s-polarized light is near zero for all wavelengths and is not shown. The grating in this case was fabricated using E7 liquid crystal. A striking feature of the data is that the diffraction efficiency at a Bragg angle of 45 is not equal to zero for either the field-on or field-off case whereas if the grating were isotropic the diffraction efficiency at 45 would be zero independent of all other grating parameters. We used the coupled wave theory developed by Montemezzani and Zgonik [3] to model the grating as a highly birefringent grating. The solid and dashed lines in Fig. 1 represent the calculations obtain from this model and show good agreement with the experimentally measured values.