Abstract
Photocurable monomer-doped liquid crystal (LC) cells were processed via holographic exposure using a low-power He–Ne laser to generate holographic polymer networks. The polymer network LC (PNLC) cells are used to fabricate infrared phase modulators at 1550 nm wavelength possessing favorable electro-optical performance. Compared with our previous work, the percentages of ingredients in the LC mixture filled in PNLC cells underwent a slight change. The 2 wt% concentration of anisotropic monomer RM257 were in place of isotropic monomer N–vinyl–2–pyrrolidinone (NVP). As a result, the fabricated phase modulators also maintained well homogeneous LC alignments and optical-scattering-free characteristics. Furthermore, NVP dopant successfully reduced the operating voltages from 95 Vrms to 79 Vrms to prevent polymer network deformation when electrically operating with higher voltages. The fabricated infrared phase modulators had a good average response time (i.e., rising time of 0.88 ms and falling time of 0.40 ms).
Highlights
Nematic liquid crystals (NLCs) are very popular and useful electro-optical materials due to their dielectric anisotropy and optical birefringence, which are sensitive to LC alignments, environmental temperature, and driving methods
The large thickness of the LC gap is unfavorable for optical response time in LC electro-optical devices except for several other reasons related to LC elastic constants, LC rotational viscosity [5,6], and electrically driving methods
During measurements of versus optical reflectance (V-R) curves, the anisotropic polymer networks generated by the RM257 dopant were continuously deformed due to anisotropic polymer networks generated by the RM257 dopant were continuously deformed due higher electrical voltages, which provided the phase variations in polymer network LC (PNLC) cells
Summary
Nematic liquid crystals (NLCs) are very popular and useful electro-optical materials due to their dielectric anisotropy and optical birefringence, which are sensitive to LC alignments, environmental temperature, and driving methods. Polymer network LC (PNLC) cells have been widely studied and demonstrated a fast response time [7,8,9,10,11] In this method, the molecules of an anisotropic monomer like RM257 possessing an LC building block structure are usually doped with LCs and processed with UV exposure, so that the generated polymer networks provide constraints to LCs to effectively speed up the falling time during the relaxation of LC molecular reorientations. LC cells to generate holographic polymer networks and to realize a LC phase modulator with low light scattering and ultra-fast response time at visible wavelengths [13]. PNLC cells processed via holographic exposure were used to fabricate optical-scattering-free infrared phase modulators with a fast response time. We found that increasing the percentage of NVP dopants successfully reduces operating voltages and the electrostriction effect
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