Photopatterned azo poly(amide imide) layers as aligning substrates of holographic liquid crystal diffraction gratings for beam steering applications

We demonstrate that custom synthesized azobenzene-functionalized poly(amide imide) enables fabrication of tunable liquid crystal photonics devices. Firstly, it is shown that the studied azo poly(amide imide) exhibits a very large and stable photoinduced optical anisotropy arising from directional orientation of its azo chromophores due to irradiation with linearly polarized blue light. Secondly, it is proved that the polymer layers with light-induced anisotropy possess an excellent capability to align nematic liquid crystals. The azimuthal anchoring energy characterizing this capability is significantly higher than the highest anchoring energy reported so far for the class of amorphous azo polymers, reaching simultaneously the level of anchoring energies found for photo-crosslinking materials. Furthermore, we demonstrate that it is possible to inscribe a specific polarization hologram in the azo poly(amide imide) layers, and then to transfer the generated azo chromophore pattern to liquid crystal molecules brought into contact with the polymer substrates. As a consequence, a diffraction grating allowing for complete redirecting of the passing light from its initial propagation direction into the first diffraction order(s) is obtained. A specific form of the Euler-Lagrange equations regarding reorientation of liquid crystal molecules has been derived and then solved numerically in the case of biasing the structure with a low external electric voltage. The obtained results make it possible to predict and explain the device performance observed under experimental conditions.