Vincent P. Tondiglia

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.

Voxelated liquid crystal elastomers

Making small actuators more effective Liquid-crystal molecules orient locally in response to external fields. When long-chain liquid-crystalline molecules are crosslinked together, changes in local orientation can lead to significant volume changes. Ware et al. made efficient microactuators that can change their shape from flat to three-dimensional structures (see the Perspective by Verduzco). By patterning volume elements so that each has a different preferred alignment for the liquid-crystalline molecules, they could fine-tune the volume changes. Science, this issue p. 982; see also p. 949 Liquid crystal elastomers are spatially patterned to create microactuators with controlled local volume changes. [Also see Perspective by Verduzco] Dynamic control of shape can bring multifunctionality to devices. Soft materials capable of programmable shape change require localized control of the magnitude and directionality of a mechanical response. We report the preparation of soft, ordered materials referred to as liquid crystal elastomers. The direction of molecular order, known as the director, is written within local volume elements (voxels) as small as 0.0005 cubic millimeters. Locally, the director controls the inherent mechanical response (55% strain) within the material. In monoliths with spatially patterned director, thermal or chemical stimuli transform flat sheets into three-dimensional objects through controlled bending and stretching. The programmable mechanical response of these materials could yield monolithic multifunctional devices or serve as reconfigurable substrates for flexible devices in aerospace, medicine, or consumer goods.

A high frequency photodriven polymer oscillator

High frequency and large amplitude oscillations are driven by laser exposure in cantilevers made from a photosensitive liquid crystal polymer.

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.

Topography from Topology: Photoinduced Surface Features Generated in Liquid Crystal Polymer Networks

Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.

Phenomenological model of anisotropic volume hologram formation in liquid-crystal-photopolymer mixtures

The real time formation of anisotropic volume holographic reflection gratings in a liquid-crystal/photopolymer mixture is studied. We develop a phenomenological model of grating formation that incorporates the photophysics and photochemistry of the initiator dye, reaction-diffusion kinetics of the monomer-polymer system, phase separation of the liquid crystal, nematic order evolution of liquid-crystal droplets, and volume shrinkage of the polymer. We then test this model by experimentally monitoring the diffraction efficiency for s and p polarization, Bragg wavelength, and laser scattering in real time as the grating is formed. The model yields good agreement with experimental data for different recording intensities and exposure times. We discuss the physics of the system as it evolves in time and explain the major features of anisotropic grating formation in acrylate-based holographic polymer-dispersed liquid crystals.

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.

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%.