RESUMO
We have performed a detailed study of the order and dynamics of the commercially available BL038 liquid crystal (LC) inside nanosized (50-300 nm) droplets of a reflection-mode holographic-polymer dispersed liquid crystal (H-PDLC) device where LC nanodroplet layers and polymer layers are alternately arranged, forming a diffraction grating. We have determined the configuration of the LC local director and derived a model of the nanodroplet organization inside the layers. To achieve this, we have taken advantage of the high sensitivity of the ESR spin probe technique to study a series of temperatures ranging from the nematic to the isotropic phase of the LC. Using also additional information on the nanodroplet size and shape distribution provided by SEM images of the H-PDLC cross section, the observed director configuration has been modeled as a bidimensional distribution of elongated nanodroplets whose long axis is, on the average, parallel to the layers and whose internal director configuration is a uniaxial quasi-monodomain aligned along the nanodroplet long axis. Interestingly, at room temperature the molecules tend to keep their average orientation even when the layers are perpendicular to the magnetic field, suggesting that the molecular organization is dictated mainly by the confinement. This result might explain, at least in part, (i) the need for switching voltages significantly higher and (ii) the observed faster turn-off times in H-PDLCs compared to standard PDLC devices.
RESUMO
We show proof of concept of a transmission-mode wavelength filtering device consisting of layered holographically formed polymer dispersed liquid crystal (H-PDLC) cells. H-PDLC cells were fabricated from a thiolene based polymer composite to have transmission notches in the near-IR wavelength range. Wavelength filtering was achieved by stacking four H-PDLC cells with transmission notches spaced at 10 nm intervals. Results show a broad transmission notch spanning the spectral width of the constituent cells. With bias applied to an individual cell within the stack, the transmission notch of the cell inverts and the overall transmission envelope changes shape. Using a transmitted energy sensing device and a lineshape mapping algorithm, spectral content can be determined to a resolution of 0.1 nm for narrow banded signals. Applications for this switchable wavelength filtering device include serial detection of spectral content for telecom data signals or chemical and biological sample identification through absorption or emission spectroscopy.
