Kinetics of the grating formation in holographic polymer-dispersed liquid crystals: NMR measurement of diffusion coefficients

Polymer films with embedded liquid crystal inclusions (polymer-dispersed liquid crystals) are superb composites for addressable windows, flexible displays and optical storage. Their scattering behavior and electro-optic properties depend essentially on the shape and size of the liquid crystal inclusions, which are typically formed by phase separation from a multicomponent homogeneous mixture. Here, pulsed field gradient NMR is used to measure the self-diffusion coefficients of the liquid crystal and a photo-reactive monomer, which compose such a precursor mixture. The kinetics of holographic grating formation in this mixture can be predicted by inserting the NMR diffusion coefficient of the monomer and the polymerization rate in a reaction diffusion model. The ratio of diffusion rate over reaction rate is found to be in the limiting regime in which the kinetics of the grating formation is not sensitive to this parameter

Nanocomposites of a nematic liquid crystal doped with magic-sized CdSe quantum dots

We here report on the optical, alignment and electro-optic properties of a nematic liquid crystal affected by the presence of semiconductor CdSe magic-sized nanocrystals (MSNCs). Three single-sized CdSe samples were tested, exhibiting bright bandgap photoluminescence (PL) with \u3bbmax 48 463 nm and 10 nm full width at half-maximum (fwhm). The three quantum dot (QD) samples were passivated with a monolayer of myristic acid. Two of them (QD1 and QD2) only vary in the amount of defects as indicated by different bandgap and deep trap PL. The third MSNC sample (QD3) is compositionally different, doped with Zn. These MSNCs with almost identical sizes were doped at different concentrations (1\u20135 wt%) into the nematic phase of 5-n-heptyl-2-(4-n-octyloxyphenyl)-pyrimidine (LC1). Only QD3 showed the formation of birefringent stripes surrounded by areas of homeotropic alignment between plain glass slides at all concentrations as observed for many other nanoparticle-doped nematic liquid crystals reported earlier by our group. In polyimide-coated glass slides favouring planar orientation of the nematic director, planar alignment was observed. Surprisingly, only the Zn-doped magic-sized QD3 quantum dots (CdSe@Zn) significantly lower the dielectric anisotropy as well as the splay elastic constant of the nematic host, despite identical size and surface functionality, which highlights the tremendous effect of the nanocrystal core composition on the electro-optic properties of the nematic host. In addition, fluorescence confocal (polarizing) microscopy studies show the director field within and around the birefringent stripes and confirm locally elevated concentrations or aggregates of the MCNC that are otherwise randomly distributed in the nematic host.Peer reviewed: YesNRC publication: Ye

Enhanced organic light-emitting diode based on a columnar liquid crystal by integration in a microresonator

A study of resonant cavity-enhanced light-emitting diodes based on a columnar liquid crystal was presented in this article. An organic layer made of the electroluminescent liquid crystal was sandwiched between a Bragg mirror deposited on a silicon substrate and a semitransparent top electrode. The fabrication of the substrates was complementary metal oxide semiconductor compatible, the maximum electroluminescence intensity was enhanced by a factor of 3-4, and the spectral width of the emission could be reduced down to 18 nm, thereby improving the chromaticity coordinates, considerably

Tuning of the Optical Properties in Photonic Crystals Made of Macroporous Silicon

It is well known that robust and reliable photonic crystal structures can be manufactured with very high precision by electrochemical etching of silicon wafers, which results in two- and three-dimensional photonic crystals made of macroporous silicon. However, tuning of the photonic properties is necessary in order to apply these promising structures in integrated optical devices. For this purpose, different effects have been studied, such as the infiltration with addressable dielectric liquids (liquid crystals), the utilization of Kerr-like nonlinearities of the silicon, or free-charge carrier injection by means of linear (one-photon) and nonlinear (two-photon) absorptions. The present article provides a review, critical discussion, and perspectives about state-of-the-art tuning capabilities.Peer Reviewe