Diffraction and polarization properties of POLICRYPS permanent and switchable gratings

An experimental and theoretical study is presented on a novel class of electrically switchable diffraction gratings, formed by alternated stripes of polymer and liquid crystal (LC) called POLICRYPS (POlimer LIquid CRYstal Polymer Slides) [1]. The measurement of the diffraction efficiency versus the angle of incidence was performed for both p and s polarized probe beams at two wavelengths of 632.8 nm and 1550 nm. In particular infrared diffraction properties are interesting for possible applications of POLICRYPS as optical switches for fiber optic communication systems as demonstrated in a previous work [2]. The sample under test consisted of a 11.5 µm thick cell made of coupled ITO coated glass substrates filled with a mixture of 30%wt of nematic LC (NLC) 5CB and 70%wt of UV curable pre-polymer NOA61. A grating with a period of about 1.38 µm was written by laser curing at wavelength of 351 nm. A strong modulation of the refractive index across the sample is due to the NLC director which is oriented perpendicularly to the polymer stripes on the cell surface, and hence parallel to the wave vector of the impinging optical field. This geometry yields a high mismatch between the refractive index of NOA-61 (nP= 1.56) and the extraordinary refractive index of 5CB (ne= 1.7057) at 632.8 nm and T=25 °C. Diffraction efficiencies of about 95% at 632.8 nm and of about 40% at 1550 nm for a p-polarized beam probe were detected. Diffraction efficiencies below 8% were measured for an s-polarized beam probe. The grating can be switched off by applying an external electric field due to the matching between the refractive index of the liquid crystal and the polymer one. The presence of a threshold voltage for the decrease of the diffraction efficiency is due to the balancing between the dielectric torque and the elastic torque acting on the director of NLC molecules confined among the polymer stripes. At the Bragg angle the grating diffraction efficiency can be switched below 5% by applying an electric field of 5 V/µm at 632.8 nm and of 8 V/µm at 1550 nm. The coupled mode theory extended to thick anisotropic materials [3] has been used to fit the diffraction angular spectra at both wavelengths. Agreement between theoretical and experimental results for both polarizations has been achieved by including a statistical fluctuation of the LC molecule axis orientation with a Gaussianlike distribution respect to the grating vector [4]. References [1] R. Caputo, A. V. Sukhov, and C. Umeton, Mol. Cryst. Liq. Cryst., Vol. 372, pp. 263-274, 2001. [2] R. Asquini, A. d’Alessandro, C. Gizzi, P. Maltese, R. Caputo, A. V. Sukhov, C. Umeton, A. Veltri, Mol. Cryst. Liq. Cryst., 2003, in press. [3] G. Montemezzani and M. Zgonik, Phys. Rev. E, Vol. 55, pp. 1035-1047, 1997. [4] R. Sutherland, J. Opt. Soc. Am. B, Vol. 19, pp. 2995-3003, 2002.