Tunable Polarization Gratings Based on Nematic Liquid Crystal Mixtures Photoaligned with Azo Polymer-Coated Substrates

Abstract

Liquid crystal polarization gratings are of great interest for optical communications as elements performing beam steering, splitting, multiplexing or beam combining. Material birefringence, cell thickness or a period of the liquid crystal director pattern influence, among other features, spectroscopic and electro-optical characteristics of fabricated devices, determining thus their functionality and applicability. Here, we report on liquid crystal polarization gratings that allow for complete maximization of the first-order diffraction efficiency (resulting in total elimination of the zeroth-order diffraction) for any wavelength of an incident beam from green to the near-infrared spectral region by applying a low electric voltage. The gratings with periods as small as 10 μm were obtained by holographic exposure of the cell substrates coated with light-sensitive azo polymer alignment layers, and then filled with three different liquid crystal mixtures. The influence of gold nanoparticle dopants in the liquid crystalline mixtures on spectroscopic and electro-optical properties of the devices is presented. Moreover, on the basis of the measured transmittance spectra of the fabricated gratings, the unknown birefringence of liquid crystal mixtures as well as their effective birefringence due to molecular reorientation in the electric field in the visible and near IR region were determined.

Dual-Period Polarization-Dependent Diffraction Gratings Based on a Polymer-Stabilized Liquid Crystal

“…Additionally, LC-based diffraction gratings with no polymer added to the material have been demonstrated. In those cases, the gratings were either created by irradiation of photosensitive alignment layers [32][33][34][35][36][37][38] or by introducing the LC into LC cells with periodic electrodes [39]. Moreover, the diffraction efficiencies of the presented LC-based diffraction gratings were dependent on polarization of incident light and the value of applied voltage in cases of the ones with periodic electrodes.…”

Influence of period and surface anchoring strength in liquid crystal optical axis gratings

“…In practice, it is difficult to realize grating periods smaller than the thickness of the layer. Gratings of 10 μm, 21 5 μm 22 and 3.4 μm 4 have been reported. Kun et al reported optical axis gratings with a period down to 2.2 μm.…”

Electrically Tuneable Optical Diffraction Gratings Based on a Polymer Scaffold Filled with a Nematic Liquid Crystal

“…Tunability is one of the important properties of diffraction gratings. Various tuning methods have been proposed; diffractive properties can be modulated by an applied electrical voltage [ 25 , 26 , 27 , 28 ], magnetic field [ 29 , 30 , 31 ], pressure, temperature, or by another electromagnetic wave [ 32 , 33 , 34 , 35 , 36 , 37 ]. Liquid crystals (LCs) are very suitable for the construction of tuneable optical diffraction gratings [ 38 , 39 ] since they possess a unique combination of mechanical softness and strong optical anisotropy that provides relatively easy control of LC molecules’ orientation.…”