Switchable volume hologram materials and devices
View Patent ↗A new photopolymerizable material allows single-step, fast recording of volume holograms with properties that can be electrically controlled. Polymer-dispersed liquid crystals (PDLCs) in accordance with the invention preferably comprise a homogeneous mixture of a nematic liquid crystal and a multifunctional pentaacrylate monomer, in combination with photoinitiator, coinitiator and cross-linking agent. Optionally, a surfactant such as octanoic acid may also be added. The PDLC material is exposed to coherent light to produce an interference pattern inside the material. Photopolymerization of the new PDLC material produces a hologram of clearly separated liquid crystal domains and cured polymer domains. Volume transmission gratings made with the new PDLC material can be electrically switched between nearly 100% diffraction efficiency and nearly 0% diffraction efficiency. By increasing the frequency of the switching voltage, switching voltages in the range of 50 Vrms can be achieved.
1. A hologram made by exposing an interference pattern inside a polymer-dispersed liquid crystal material, the polymer-dispersed liquid crystal material, before exposure, compnsing:
(a) a polymerizable semifluorinated acrylate monomer;
(b) a liquid crystal;
(c) a cross-linking monomer;
(d) a coinitiator comprising an organic borate; and
(e) a photoinitiator dye.
2. The hologram according to claim 1 , wherein
the polymerizable monomer comprises, in combination, di-acrylates, tri-acrylates and penta-acrylates.
3. The hologram according to claim 1 , wherein
the liquid crystal comprises 4-methoxybenzylidene-4′-butylaniline.
4. The hologram according to claim 1 , wherein the coinitiator comprises trialkylborate.
5. The hologram according to claim 1 , wherein the coinitiator comprises triphenyl borate.
6. The hologram according to claim 1 , wherein the photoinitiator dye comprises indolinocarbocyanine.
7. The hologram according to claim 1 , wherein
the coinitiator comprises triethylamine.
8. The hologram according to claim 1 , wherein
the photoinitiator dye comprises a cationic cyanine dye.
9. The hologram according to claim 1 , wherein the polymer-dispersed liquid crystal material comprises, before exposure, a surfactant.
10. The hologram according to claim 1 , wherein
wherein the hologram comprises opposing surfaces and a plurality of polymer regions having a first refractive index and polymer-dispersed liquid crystal regions having a second refractive index disposed at an angle to the opposing surfaces of the hologram whereby the symmetry axis of the liquid crystal is disposed at an angle to the opposing surfaces of the hologram.
11. The hologram according to claim 1 , wherein
wherein the hologram comprises opposing surfaces and a plurality of alternating planes of polymer channels having a first refractive index and polymer-dispersed liquid crystal channels having a second refractive index disposed at an angle to the opposing surfaces of the hologram whereby the symmetry axis of the liquid crystal is disposed at an angle to the opposing surfaces of the hologram.
12. The hologram according to claim 1 , wherein the photoinitiator dye comprising iodobenzospiropyran.