Tanya Z. Kosc

Electric-field-induced motion of polymer cholesteric liquid-crystal flakes in a moderately conductive fluid.

Polymer cholesteric liquid-crystal flakes suspended in a fluid with nonnegligible conductivity can exhibit motion in the presence of an ac electric field. The plateletlike particles with a Grandjean texture initially lie parallel to the cell substrates and exhibit a strong selective reflection that is diminished or extinguished as the flakes move. Flake motion was seen within a specific frequency bandwidth in an electric field as low as 5 mV(rms)/microm. Flakes reoriented about their longest axis to align parallel to theapplied field, and a rise time of 560 ms was seen in a 40-mV(rms)/microm field at 100 Hz.

Polymer cholesteric liquid-crystal flake reorientation in an alternating-current electric field

The motion of highly dielectric polymer cholesteric liquid crystal (PCLC) flakes suspended in a host fluid can be controlled with an ac electric field. The electric field acts to induce a dipole moment on the flake due to interfacial, or Maxwell–Wagner, polarization. Theoretical modeling of PCLC flakes as oblate spheroids shows that the flakes will reorient to align one of the two major axes parallel to the electric field. The theory also supports the observed dependence of the particle reorientation time on the electric-field magnitude, frequency, and particle shape. A PCLC flake’s orientation determines its ability to reflect light of a specific wavelength and circular polarization. The ability to switch the position of PCLC flakes with an electric field has implications for electro-optic devices and display applications.

62.3: Doped Multilayer Polymer Cholesteric-Liquid-Crystal (PCLC) Flakes: A Novel Electro-Optical Medium for Highly Reflective Color Flexible Displays

Polymer cholesteric-liquid-crystal (PCLC) flake/fluid-host suspensions are a novel particle display technology for full-color reflective display applications on rigid or flexible substrates. These “polarizing pigments” require no polarizers or color filters, switch rapidly at very low voltages, and produce highly saturated colors with a reflection efficiency approaching 80%.

Electro-optical behavior of polymer cholesteric liquid crystal flake/fluid suspensions in a microencapsulation matrix

When flakes of polymer cholesteric liquid crystals (PCLCs) are dispersed in a fluid host and subjected to an applied electric field, their bright, polarization-selective reflection color is extinguished as they undergo field-induced rotation. Maxwell-Wagner (interfacial) polarization is the underlying physical mechanism for flake motion and results from the large difference in dielectric properties of the flake and fluid hosts. Flake reorientation times can be as short as 300 ms to 400 ms at exceedingly low driving fields (10 to 100 mVrms/μm) and are dependent on flake size and shape, fluid host dielectric constant and viscosity, and drive-filed frequency and magnitude. These attributes make this new materials system of special interest in electro-optical and photonics applications, where reflective-mode operation, polarization selectivity, and low power consumption are of critical importance (e.g., reflective displays). Until very recently, the electro-optical reorientation of PCLC flakes has been studied only in sandwich-type cells using glass substrates. In this work, we report on the dc field-induced reorientation behavior of PCLC flakes contained in confined spherical or near-spherical fluid-filled cavities formed by microencapsulation of the flake/fluid host dispersion in a water-borne flexible binder. This PCLC flake-fluid host/binder emulsion is coated onto either rigid or flexible condutive-coated substrates and then overcaoted (uniformly or patterned) using a conductive emulsion or paint that is either absorbing (black) or reflecting (silver). In addition to providing a unique environment to study flake motion, this device geometry also extends the application scope of the technology to conformal, electrically switchable coatings for large planar areas and flexible media for information display applications (e.g., electronic paper).

The multiple-pulse driver line on the OMEGA laser

The multiple-pulse driver line (MPD) provides on-shot co-propagation of two separate pulse shapes in all 60 OMEGA beams at the Laboratory for Laser Energetics (LLE). The two co-propagating pulse shapes would typically be (1) a series of 100-ps “picket” pulses followed by (2) a longer square or shaped “drive” pulse. Smoothing by spectral dispersion (SSD), which increases the laser bandwidth, can be applied to either one of the two pulse shapes. Therefore, MPD allows for dynamic bandwidth reduction, where the bandwidth is applied only to the picket portion of a pulse shape. Since the use of SSD decreases the efficiency of frequency conversion from the IR to the UV, dynamic bandwidth reduction provides an increase in the drive-pulse energy. The design of the MPD required careful consideration of beam combination as well as the minimum pulse separation for two pulses generated by two separate sources. A new combined-pulse-shape diagnostic needed to be designed and installed after the last grating used for SSD. This new driver-line flexibility is built into the OMEGA front end as one component of the initiative to mitigate cross-beam energy transfer on target and to demonstrate hydro-equivalent ignition on the OMEGA laser at LLE.

Electric Field Induced Rotation of Polymer Cholesteric Liquid Crystal Flakes: Mechanisms and Applications

Electric fields can induce motion of polymer cholesteric liquid crystal (pCLC) flakes suspended in a fluid medium. The platelet-shaped pCLC flakes with a Grandjean texture show strong selective reflection when lying flat in the plane of a conventional cell. As their orientation with respect to normally incident light changes, their selective reflection color shifts toward the blue and diminishes until the flakes are no longer easily visible beyond 7-12° of rotation. Reproducibility and control of motion has been observed in moderately conductive host fluid. Flakes in such hosts do not respond to a DC electric field, but they rotate 90° in an AC field within a given frequency band. The response times and frequency regions for motion depend partially on the field magnitude, the dielectric properties of the host fluid and the flake geometry. We observe flakes reorienting in less than 500 ms in an electric field of 0.17 Vrms/μm, while sub-second reorientation is seen in fields as low as 5x10-2 Vrms/μm. This response time is comparable with typical electronic-paper applications, but with a significantly lower electric field. Displays using pCLC flakes would not require backlighting, sheet polarizers, color filters or alignment layers. Numerous additional applications for pCLC flakes are envisioned, including filters, polarizers, and spatial light modulators.

Optically robust photoalignment materials for liquid crystal device applications in the near-UV region (Conference Presentation)

Photo-alignment technology is important for liquid crystal (LC) device applications where both high resistance to incident optical energy and spatially distributed alignment states over the device clear aperture are required. Coumarin-based photo-alignment materials developed at the Laboratory for Laser Energetics (LLE) possess near-IR laser damage resistance approaching that of fused silica and have been employed in the development and fabrication of a wide variety of LC high-peak-power laser optics. One example is a photo-patterned LC beam shaper, developed for use in LLE’s four-beam, petawatt-peak-power OMEGA EP laser, that has demonstrated 1054-nm, 1 ns laser-damage thresholds approaching those of dielectric thin-film Brewster’s angle polarizers (30 to 40 J/cm2). Achieving similar performance levels in LC devices for near-UV applications is challenging due to a scarcity of both UV-transparent LC materials and polymer alignment layers that can withstand repeated exposure to intense pulsed- or CW UV irradiation without degradation. Previously-employed alignment materials for UV-LC devices such as buffed polyvinyl alcohol (PVA) or Nylon 6/6 have limited usefulness; buffing embeds particulates and scratches into the alignment layer that reduce its UV damage thresholds to only a few J/cm2 and is incapable of producing highly resolved and spatially-distributed LC alignment states. In recent experiments, we have found that coumarin photoalignment materials are remarkably more resistant to damage from both incident 351 nm, 1 ns high-energy laser pulses [~11.42 J/cm2 (1-on-1) and ~15.70 J/cm2.(N-on-1)] and broad-band, continuous wave (CW) UV-visible light than would be expected due to their highly conjugated aromatic electronic structures. This finding opens a new chapter in the development of LC devices for UV applications in high-peak-power lasers (e.g. wave plates, polarization rotators, radial polarization converters, photo-patterned beam shapers) and other areas of optics and photonics where UV stability is important (e.g., space-based applications).

Damage testing of nematic liquid crystalline materials for femtosecond to nanosecond pulse lengths at 1053 nm

Damage-test data are scarce for liquid crystalline (LC) materials at 1-ns pulse lengths and nonexistent at shorter pulselengths. Here we describe the methodology to develop a comprehensive database of damage performance for typical nematic LC’s for a wide range of pulse lengths at 1053 nm. This series of nematic LC materials investigates the effect of a varying degree of π-electron delocalization. Obtaining damage-threshold measurements is of fundamental interest for the consideration of LC materials for applications in short-pulse laser systems.

Enhancements to the timing of the OMEGA laser system to improve illumination uniformity

Two diagnostics have been developed to improve the uniformity on the OMEGA Laser System, which is used for inertial confinement fusion (ICF) research. The first diagnostic measures the phase of an optical modulator (used for the spectral dispersion technique employed on OMEGA to enhance spatial smoothing), which adds bandwidth to the optical pulse. Setting this phase precisely is required to reduce pointing errors. The second diagnostic ensures that the arrival times of all the beams are synchronized. The arrival of each of the 60 OMEGA beams is measured by placing a 1-mm diffusing sphere at target chamber center. By comparing the arrival time of each beam with respect to a reference pulse, the measured timing spread of the OMEGA Laser System is now 3.8 ps.

P-94: Polymer Cholesteric Liquid Crystal Flakes for Particle Displays

Polymer cholesteric liquid crystal (PCLC) flakes suspended in a moderately conductive host fluid reorient in the presence of an AC electric field within a specific frequency range, causing a dramatic change in flake reflectivity. PCLC flakes reflect light of a specific color and (circular) polarization, thereby eliminating the need for backlighting, color filters, and polarizers to attain unique optical effects. Reorientation can be seen in fields as low as several milli-volts per micron, and reorientation times are on the order of several hundred milliseconds, thereby approaching response times for both nematic liquid crystal displays and other particle displays. Applications for electro-optic devices, decorative commercial and consumer products, and displays, particularly electronic paper, are envisioned.