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

Abstract

Custom synthesized “T-type” azobenzene-functionalized poly(amide imide) allows for effective fabrication of a tunable liquid crystal photonic device for light beam steering.

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

“…Nevertheless, the above results confirmed the ability of irradiated PAI layers to align nematic LC mixtures periodically, with a period twice as dense as that presented recently [29]. It is important to note that successful fabrication of the gratings with a 10 µm period allowed for increasing the diffraction angle of 690 nm light to 4 • compared to only 1.7 • for the mentioned LC polarization grating with the 23 µm period [29]. Larger separation between the zeroth and the first orders increases an application potential of the fabricated optical elements and encourages future research efforts on further minimizing the LC structure period.…”

“…The presence of additional diffracted beams behind the LC_2 grating indicated deviations in the LC director alignment pattern from that presented in Figure 2b, introduced by gold nanoparticles. Nevertheless, the above results confirmed the ability of irradiated PAI layers to align nematic LC mixtures periodically, with a period twice as dense as that presented recently [29]. It is important to note that successful fabrication of the gratings with a 10 µm period allowed for increasing the diffraction angle of 690 nm light to 4 • compared to only 1.7 • for the mentioned LC polarization grating with the 23 µm period [29].…”

“…The latter shows results of certain calculations and numerical simulations performed for the grating, illustrating influence of voltage-driven molecular reorientation on diffraction efficiency (and thus on the zeroth-order transmittance). Details of the performed numerical analysis with the specific form of the Euler-Lagrange differential equations, which were solved with the use of the relaxation scheme, are provided in our previous work [29]. Specifically, the inset in Figure 5b represents numerically calculated spatial distribution of the molecular director within one period of the grating for the voltage above the threshold value.…”

“…Three LC cells with the measured gap (d) in the range 4.6/4.8 µm were assembled from the PAI-coated substrates using 5 µm glass spacers. Then, the empty cells were exposed to a light interference pattern formed by two coherent 442 nm 40 mW/cm 2 beams (from a He-Cd laser) with orthogonal circular polarizations (a detailed scheme of the experimental set-up was presented in our recent paper [29]). In the arranged geometry, the beams crossed at the angle (θ) of ca.…”

“…Moreover, reversible photochemistry in azo polymers allows for recording of erasable and rewritable LC alignment patterns, which is not possible with photocrosslinkable polymers. Very recently, we have reported on a successful fabrication of tunable LC polarization gratings based on azo poly (amide imide) layers irradiated with two beams of opposite circular polarizations [29]. Regrettably, a period of the obtained LC structure was ca.…”