Timothy J. Bunning

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.

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.

Polarization-controlled, photodriven bending in monodomain liquid crystal elastomer cantilevers

We report on the fast and optically controlled angular bending of cantilevers made from liquid crystal polymer networks functionalized with azobenzene moieties (azo-LCN). For potential applications such as adaptive optics, photoresponsive cantilevers should rapidly deform to controlled angles while maintaining a modulus that can accomplish appreciable mechanical work. This work demonstrates cantilevers made of monodomain azo-LCN containing pendant (side chain) azobenzene mesogens with a storage modulus of 1.4 GPa bend 85° in less than 300 ms upon exposure to 442 nm irradiation. Moreover, the bending angle of these monodomain azo-LCN cantilevers can be controlled by the polarization angle of the source relative to the long-axis of the cantilever. The bending performance (deformation angle and speed) of this monodomain system is compared to a polydomain analogue. The impact of azobenzene concentration, laser intensity, and thickness on these parameters is also presented.

Three-dimensional control of the helical axis of a chiral nematic liquid crystal by light

Chiral nematic liquid crystals—otherwise referred to as cholesteric liquid crystals (CLCs)—are self-organized helical superstructures that find practical application in, for example, thermography, reflective displays, tuneable colour filters and mirrorless lasing. Dynamic, remote and three-dimensional control over the helical axis of CLCs is desirable, but challenging. For example, the orientation of the helical axis relative to the substrate can be changed from perpendicular to parallel by applying an alternating-current electric field, by changing the anchoring conditions of the substrate, or by altering the topography of the substrate’s surface; separately, in-plane rotation of the helical axis parallel to the substrate can be driven by a direct-current field. Here we report three-dimensional manipulation of the helical axis of a CLC, together with inversion of its handedness, achieved solely with a light stimulus. We use this technique to carry out light-activated, wide-area, reversible two-dimensional beam steering—previously accomplished using complex integrated systems and optical phased arrays. During the three-dimensional manipulation by light, the helical axis undergoes, in sequence, a reversible transition from perpendicular to parallel, followed by in-plane rotation on the substrate surface. Such reversible manipulation depends on experimental parameters such as cell thickness, surface anchoring condition, and pitch length. Because there is no thermal relaxation, the system can be driven either forwards or backwards from any light-activated intermediate state. We also describe reversible photocontrol between a two-dimensional diffraction state, a one-dimensional diffraction state and a diffraction ‘off’ state in a bilayer cell.

Polymer film with optically controlled form and actuation

A low power laser beam is used to induce large and fast variations in the shape of a polymer film due to photoinduced contraction and expansion of the polymer film surface subject to the beam. The direction of the photoinduced bend or twist of the polymer can be reversed by changing the polarization of the beam. Thus the film orientation could be varied within +/-70 masculine. The phenomenon is a result of optically induced reorientation of azobenzene moieties in the polymer network.

Light-activated shape memory of glassy, azobenzene liquid crystalline polymer networks

Rapidly reconfigurable, adaptive materials are essential for the realization of “smart”, highly engineered technologies sought by aerospace, medicine, and other application areas. Shape memory observed in metal alloys and polymers (SMPs) is a primary example of shape change (adaptation). To date, nearly all shape adaptations in SMPs have been thermally triggered. A desire for isothermal, remotely cued shape adaptations of SMP has motivated examinations of other stimuli, such as light. Only a few reports document so-called light-activated SMP, in both cases exploiting photoinduced adjustments to the crosslink density of a polymer matrix with UV light of 365 nm (crosslinking) and <260 nm (decrosslinking). This work presents a distinctive approach to generating light-activated SMP by employing a glassy liquid crystal polymer network (LCN) material that is capable of rapid photo-fixing with short exposures (<5 min) of eye-safe 442 nm light. Here, linearly polarized 442 nm light is used to photo-fix temporary states in both cantilever and free-standing geometries which are then thermally or optically restored to the permanent shape. The combination of thermal and photo-fixable shape memory presented here yields substantial functionality in a single adaptive material that could reduce part count in applications. As a demonstration of the opportunities afforded by this functional material, the glassy, photoresponsive LCN is thermally fixed as a catapult and subsequently used to transduce light energy into mechanical work, demonstrated here in the “photo-fueled” launching of an object at a rate of 0.3 m s−1.

Dynamic color in stimuli-responsive cholesteric liquid crystals

ROY G. BIV is the acronym used around the English-speaking world to aid children in the memorization of the traditional colors of the rainbow (red, orange, yellow, green, blue, indigo, and violet). Color surrounds us and the ability to change color by external stimuli (heat, force, light exposure, magnetic or electric field) continues to be leveraged for many present day applications. This review focuses on the state of the art in the use of cholesteric liquid crystals (CLCs) as color changing optical materials. After a brief summary of thermal and electrically induced color changes, the bulk of the article describes recent efforts in photoresponsive CLCs, materials in which light is used to control the color output.

Liquid crystalline polymer cantilever oscillators fueled by light

We report on the laser and sunlight driven, fast and large oscillation of cantilevers composed of photoresponsive liquid crystal polymer materials. The oscillation frequency, driven with a focused 100 mW laser of multiple wavelengths (457, 488, 514 nm), is as high as 270 Hz and is shown to be strongly correlated to the physical dimensions of the cantilever. The experimental frequency response is accurately described by the calculated natural resonant frequency for a non-damped cantilever. To further understand the conversion efficiency of light energy to mechanical work in the system, the oscillatory behavior of a 2.7 mm × 0.7 mm × 0.04 mm cantilever was examined at pressures ranging from 1 atm to 0.03 atm. A large increase in amplitude from 110° at STP to 250° at low pressure was observed. A first approximation of the system efficiency was calculated at 0.1%. The large increase in amplitude at low pressure indicates strong hydrodynamic loss and thus, the material efficiency is potentially much greater. Using a simple optical setup, oscillatory behavior was also demonstrated using sunlight. This work indicates the potential for remotely triggered photoactuation of photoresponsive polymer cantilevers from long distances with lasers or focused sunlight.

Light-Driven Reversible Handedness Inversion in Self-Organized Helical Superstructures

We report here a fast-photon-mode reversible handedness inversion of a self-organized helical superstructure (i.e., a cholesteric liquid crystal phase) using photoisomerizable chiral cyclic dopants. The two light-driven cyclic azobenzenophanes with axial chirality show photochemically reversible trans to cis isomerization in solution without undergoing thermal or photoinduced racemization. As chiral inducing agents, they exhibit good solubility, high helical twisting power, and a large change in helical twisting power due to photoisomerization in three commercially available, structurally different achiral liquid crystal hosts. Therefore, we were able to reversibly tune the reflection colors from blue to near-IR by light irradiation from the induced helical superstructure. More interestingly, the different switching states of the two chiral cyclic dopants were found to be able to induce a helical superstructure of opposite handedness. In order to unambiguously determine the helical switching, we employed a new method that allowed us to directly determine the handedness of the long-pitched self-organized cholesteric phase.

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.