L. V. Natarajan

Electrically switchable volume gratings in polymer‐dispersed liquid crystals

We report electrical switching of the diffraction efficiency in volume Bragg gratings written holographically in polymer‐dispersed liquid crystals (PDLCs). Scanning electron microscopy confirms the volume nature of the gratings and shows that they consist of periodic PDLC planes. The diffraction efficiency can be switched from a high value (∼50%) to a value near zero at fields ∼11 V/μm.

The morphology and performance of holographic transmission gratings recorded in polymer dispersed liquid crystals

Abstract Holographic transmission gratings are formed by the anisotropic visible laser radiation curing of a multifunctional acrylate monomer blended with the liquid crystal (LC) mixture E7. This results in an anisotropic spatial distribution of phase-separated LC droplets within the photochemically cured polymer matrix. The morphology of thin films (5–20 μm) containing the gratings is examined by low-voltage, high-resolution scanning electron microscopy and transmission electron microscopy. Low concentrations of E7 (16% LC) coupled with rapid curing kinetics result in the formation of narrow LC-rich Bragg lamellae without well defined boundaries. These LC-rich lamellae, with approximate widths of 100 nm, are composed of small droplets measuring 20–50 nm in diameter. Increasing the concentration of LC in the prepolymer mixture results in larger lamellae (canals) of LC-rich material. Films formed from a mixture containing the highest LC concentration (34% LC) exhibited lamellae approximately 200–250 nm wide that are separated by polymer lamellae that also possess a small fraction of phase-separated LC droplets. The other variable examined in detail, laser writing intensity, has little effect on the morphologies exhibited in these films. The morphology is related to the performance (diffraction efficiency, transmission, switching times and fields) through a simple model.

Volume holographic image storage and electro-optical readout in a polymer-dispersed liquid-crystal film.

We report storage and electrical switching of holographic image data in an economical polymer-dispersed liquidcrystal material. The hologram is recorded in a fast, single-step process and can be reversibly erased and restored repeatedly by the application of fields of approximately 10-15 V/ microm, with a response time of 22 micros and a relaxation time of 42 micros. Simple (quasi-sinusoidal) holographic transmission gratings also are studied with switching fields of <5 V/ microm and with response and relaxation times of 25 and 44 micros, respectively.

Monomer functionality effects in the anisotropic phase separation of liquid crystals

Abstract Holographic gratings formed through the anisotropic phase separation of liquid crystals show promise as switchable optical elements. In order to form useful elements, however, it is necessary to control the nanoscale morphologies within the grating films. In this manuscript, we evaluate the role of monomer functionality on the morphology and the electro-optical properties of both gratings and conventional scattering polymer-dispersed liquid crystal (PDLC) films. Both of these structures are formed using polymerization-induced phase separation (PIPS) of liquid crystals from a cross-linked polymer formed through free-radical photo-polymerization. Floodlit (uniform illumination) films and holographic gratings (from non-uniform illumination caused by the interference of two laser beams) were made using monomers with 2–5 acrylate groups, while keeping the LC concentration constant in the syrups. The morphologies of these films were examined using low voltage scanning electron microscopy (LVSEM). In all cases, very small LC domains were formed with little indication of growth or coalescence. Lowering monomer functionality reduced the volume fraction of phase-separated domains in the floodlit samples. For the grating samples, the local volume fraction and the LC domain sizes decreased substantially as the monomer functionality was decreased. Using detailed image analysis, differences in the anisotropy of the domains was also probed. A much stronger tendency to form anisotropically-shaped domains was observed for the higher functional syrups. These domain anisotropy differences are correlated with the number of reactive double bonds per monomer and are suggestive of local environmental differences exerted at the time of the domain formation.

Evolution of anisotropic reflection gratings formed in holographic polymer-dispersed liquid crystals

The temporal evolution of an anisotropic reflection grating produced in a holographic polymer-dispersed liquid crystal film is investigated. We find that this type of grating is preceded in time by an isotropic concentration grating, and that the development of the anisotropic grating can be delayed until several seconds after laser exposure. The formation of an anisotropic grating is nearly coincident with the onset of phase separation of liquid crystal and implies a macroscopic ordering of liquid crystal droplet directors. Detailed knowledge of grating evolution may allow in situ control over the polarization sensitivity of the hologram.

Morphology of anisotropic polymer‐dispersed liquid crystals and the effect of monomer functionality

The morphology of transmission gratings with varying Bragg spacings formed using polymer-dispersed liquid crystals were examined using high-resolution scanning electron microscopy and image analysis techniques. The effect on the morphology of small changes in the overall functionality of the prepolymer syrup was observed. An increase in the amount of monofunctional compound resulted in small, nearly spherical domains (<100 nm in diameter) confined in well-defined lamellae for samples with a 0.49 μm Bragg spacing. A decrease in concentration (an effective increase in monomer functionality) at this Bragg spacing resulted in larger domains (100-200 nm) with much greater distribution of sizes and shapes. The local volume fractions of discrete liquid crystalline (LC) domains was considerably larger in the latter case. An increase in the Bragg spacing to 1.35 μm also resulted in well-defined lamellae of LC domains, although much more coalescence into irregularly shaped individual domains was observed. Surprisingly, the local volume fraction of LC increased in the larger Bragg spacing samples. The morphology results are discussed qualitatively in terms of liquid-gel demixing where the inherent crosslink density and elasticity of the polymeric host must be considered in phase separation processes on the nanoscale.

Morphology of reflection holograms formed in situ using polymer-dispersed liquid crystals

The morphology of a reflection grating formed using a polymer-dispersed liquid crystalline material system is examined using low-voltage high-resolution scanning electron and transmission electron microscopy. The grating is formed by establishing a fringe pattern in the intensity profile of an argon-ion laser line (λ = 488 nm) leading to a periodic anisotropic cure through the thickness of the film. The in situ, one-step procedure produces periodic layers of polymer- and LC-rich planes lying parallel to the film surface. Droplet diameters are very small (<100 nm) and little coalescence of individual droplets is observed. The grating spacing measured from electron micrographs (153 nm) nearly corresponds to the expected spacing from the observed reflection notch at λ = 472 nm.

The effect of fluorine-substituted acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystals

The effects of fluorinated acrylate monomers on the electro-optical and morphological properties of polymer dispersed liquid crystal (PDLC) films are reported. The partial fluorination of host polymer matrices resulted in improved optical properties and better defined morphologies. An enhancement in contrast ratio was observed for fluorinated systems containing trifluoroethyl acrylate (TFEA) and hexafluoroisopropyl acrylate (HFIPA). Conversely, the incorporation of methyl acrylate (MA), a chemically similar non-fluorinated acrylate, resulted in no appreciable change in contrast ratio and an increase in relaxation time. Scanning electron microscopy morphological studies were conducted to understand further the influence of fluorinated monomers in PDLC systems.

Electrically induced bandwidth broadening in polymer stabilized cholesteric liquid crystals

The reflection notch bandwidth of a cholesteric liquid crystal (CLC), equal to the product of the liquid crystal (LC) birefringence (Δn), and the pitch length (po), is typically on the order of 50-100 nm in the visible portion of the electromagnetic spectrum. Static bandwidths greater than 100 nm can be observed in CLCs that possess a pitch gradient throughout the thickness of the cell. In this work, we report on polymer stabilized CLC (PSCLC) systems that exhibit electrically controllable, dynamic bandwidths governed by the strength of a direct current (DC) electric field applied across the sample. Symmetric notch broadening which increases linearly with field and reaches a maximum value at 4 V/μm is observed. Removal of the field returns the PSCLC cell to its original optical properties. A seven fold increase in bandwidth was observed for 22 μm thick cells which contained LCs with a small birefringence (∼0.04). A variety of CLC mixtures with small positive or negative dielectric anisotropies are shown t...

Electromechanical tuning of cholesteric liquid crystals

The electrical tuning of negative dielectric anisotropy (−Δe) cholesteric liquid crystals (CLCs) under the influence of ac and dc electric fields was studied. Unlike (+Δe) CLCs, these materials align their helical axis along the applied electric field, allowing the preservation of the optical band gap. Our results show band gap shifts greater than 20% of the original notch position with little change in the quality of the band gap, including the bandwidth and notch depth. These results can be understood by using an electromechanical model, which shows that the Maxwell’s stresses are sufficiently strong to distort the ITO glass substrates. Simple beam and plate elasticity theory is shown to adequately describe the observed behavior. The electromechanical effect is then used to create CLC cells, which can both red and blue tune. This mechanism may have interesting applications in tunable optical filters, optical pressure/stress sensors, and tunable laser technologies.