Holographic Polymer-dispersed Liquid Crystal Gratings Research Articles

Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings

Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities within the liquid-crystal-rich layers of the grating.

Tunable two-photon pumped lasing using a holographic polymer-dispersed liquid-crystal grating as a distributed feedback element

A holographic polymer-dispersed liquid-crystal (H-PDLC) grating film was employed as an angle-dependent and narrow spectral-band feedback control element for two-photon pumped lasing in a dye solution, 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4′-(6-hydroxyhexyl sulfonyl) stilbene (APSS) in dimethyl sulphoxide. The grating film contained about 80 layers of liquid-crystal domains periodically dispersed in an ∼15 μm thick polymer film, featuring a maximum reflectance of 75% at 561 nm position with an ∼9 nm spectral bandwidth. The output lasing wavelength could be tuned from 561.5 to 548.5 nm and the lasing bandwidth changed from 5 to 3 nm when the incidence angle on the grating film varied from 0° to 22°. The overall lasing efficiency was measured to be 10%.

Active U-turn electrooptic switch formed in patterned holographic polymer-dispersed liquid crystals

We have developed an active U-turn device by spatially patterning two identical holographic polymer dispersed liquid-crystal gratings. In the field-OFF state, the holographic planes of both gratings are slanted such that the angle of the diffraction beam is beyond the total internal reflection angle of the air-glass interface. The diffraction beam is trapped in the device until diffracted by the second slanted grating. In the field-ON state, the holographic grating is erased and the incident beam propagates directly through the sample. The index modulation is determined to be 0.059 /spl plusmn/ 0.006 at 1.55 /spl mu/m. Low switching voltage and sub-millisecond response times are reported.

Polarization and switching properties of holographic polymer-dispersed liquid-crystal gratings I Theoretical model

Polarization properties and electro-optical switching behavior of holographic polymer-dispersed liquid-crystal (HPDLC) reflection and transmission gratings are studied. A theoretical model is developed that combines anisotropic coupled-wave theory with an elongated liquid-crystal-droplet switching model and includes the effects of a statistical orientational distribution of droplet-symmetry axes. Angle- and polarization-dependent switching behaviors of HPDLC gratings are elucidated, and the effects on dynamic range are described. A new type of electro-optical switching not seen in ordinary polymer-dispersed liquid crystals, to the best of the author’s knowledge, is presented and given a physical interpretation. The model provides valuable insight to the physics of these gratings and can be applied to the design of HPDLC holographic optical elements.

Influence of partial matrix fluorination on morphology and performance of HPDLC transmission gratings

The morphology and the electro-optical performance characteristics were investigated in holographic polymer-dispersed liquid crystal (HPDLC) transmission gratings with partially fluorinated polymer matrices. HPDLC transmission gratings were prepared using standard UV curable monomer mixtures along with mono-functional fluorinated acrylate monomers and a nematic liquid crystal, TL203. Partial fluorination of the host polymer matrices by incorporating hexafluoroisopropyl acrylate (HFIPA) or trifluoroethyl acrylate (TFEA) in the standard formulation has been found to influence the morphological and the electro-optical properties of the resulting HPDLC transmission gratings. Significant decrease in switching voltages and higher relaxation times were observed in fluorinated HPDLCs. Conversely, an addition of methyl acrylate (MA), a non-fluorinated monomer with a similar structure in the standard formulation, resulted in an increase in the switching voltage and produced no significant change in the relaxation time in the HPDLC gratings. Presence of fluorine atoms at the polymer-liquid crystal (LC) interface not only decreased the surface anchoring strength but also influenced the orientation of LC droplet directors.