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.
Highlights
Diffraction gratings are optical elements with periodic structures spatially modulating the amplitude or/and phase of an incident light
We have reported on a successful fabrication of tunable liquid crystals (LCs) polarization gratings based on azo poly layers irradiated with two beams of opposite circular polarizations [29]
We focus our attention on the influence of gold nanoparticle concentration in the nematic LC mixtures on diffraction and electro-optical properties of the obtained gratings
Summary
Diffraction gratings are optical elements with periodic structures spatially modulating the amplitude or/and phase of an incident light. Numerous experiments have shown that irradiation of azo polymer layers with two beams of opposite circular polarizations results in efficient diffraction of the probe beam by volume birefringence (polarization) grating and surface relief grating (SRG) almost simultaneously formed in the material [10,11,12,13]. The latter, being sinusoidal variations of a sample thickness, modulates the phase of the incident wave, increasing the intensity of the first-order diffraction signal. From the measured transmittance spectra at 0 V and at higher voltages, we calculate the unknown values of LC mixture material birefringence as well as effective birefringence, practically in the whole visible/near-IR spectral range
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