Reflection spectra of distorted cholesteric liquid crystal structures in cells with interdigitated electrodes

We studied the appearance of second- and third-order Bragg reflections in cholesteric liquid crystals (CLCs) in cells where the electric field was perpendicular to the helical axis. Second-order reflections with reflectance values as large as 80% of the first-order one were observed in the gap regions of alignment cells with interdigitated electrodes for CLC mixtures with pitches in the range 0.5-1.0 μm upon application of a field. The characterization was enabled by local probing of the CLC using a microspectrophotometer. LC cells that are transparent in the visible spectrum in the off-state and become colored upon application of a field due the second- or third-order reflection band appearance were demonstrated. The spectral position of the higher-order Bragg reflections can also be tuned by adjusting the magnitude of the electric field.

Light-induced color changes of microgel-based etalons

Poly(N-isopropylacrylamide) (pNIPAm) microgel-based etalons were used to fabricate systems that change visual color in response to light exposure. These systems were fabricated by adding pH responsive microgel-based etalons to a solution composed of the photoacid o-nitrobenzaldehyde (o-NBA). Upon exposure of this system to ultraviolet (UV) irradiation, the photoacid released a proton, lowering the pH of the solution. Since the pNIPAm microgel-based etalon was responsive to pH, the etalon changed its optical properties and, hence, visual color. We went on to show that patterned etalons could be fabricated, which only contained pH-responsive microgels in specific regions. These etalons only changed color in the pH-responsive regions, to yield patterns that change color upon UV light exposure. Finally, the color of the etalon was shown to be fully reversible and could be switched multiple times. These unique systems could potentially be used for display technologies, and as a controlled/triggered drug delivery system.

Photostimulated control of laser transmission through photoresponsive cholesteric liquid crystals

Cholesteric liquid crystals (CLCs) are selectively reflective optical materials, the color of which can be tuned via electrical, thermal, mechanical, or optical stimuli. In this work, we show that self-regulation of the transmission of a circularly polarized incident beam can occur upon phototuning of the selective reflection peak of a photosensitive CLC mixture towards the pump wavelength. The autonomous behavior occurs as the red-shifting selective reflection peak approaches the wavelength of the incident laser light. Once the red-edge of the CLC bandgap and incident laser wavelength overlap, the rate of tuning dramatically slows. The dwell time (i.e., duration of the overlap of stimulus wavelength with CLC bandgap) is shown to depend on the radiation wavelength, polarization, and intensity. Necessary conditions for substantial dwell time of the CLC reflection peak at the pump beam wavelength include irradiation with low intensity light (~1mW/cm²) and the utilization of circularly polarized light of the same handedness as the helical structure within the CLC. Monitoring the optical properties in both reflection and transmission geometries elucidates differences associated with attenuation of the light through the thickness of the CLC film.

Functional organic materials based on polymerized liquid-crystal monomers: supramolecular hydrogen-bonded systems

Functional organic materials are of great interest for a variety of applications. To obtain precise functional properties, well-defined hierarchically ordered supramolecular materials are crucial. The self-assembly of liquid crystals has proven to be an extremely useful tool in the development of well-defined nanostructured materials. We have chosen the illustrative example of photopolymerizable hydrogen-bonding mesogens to show that a wide variety of functional materials can be made from a relatively simple set of building blocks. Upon mixing these compounds with other reactive mesogens, nematic, chiral nematic, and smectic or columnar liquid-crystalline phases can be formed that can be applied as actuators, sensors and responsive reflectors, and nanoporous membranes, respectively.

Printable optical sensors based on H-bonded supramolecular cholesteric liquid crystal networks

A printable H-bonded cholesteric liquid crystal (CLC) polymer film has been fabricated that, after conversion to a hygroscopic polymer salt film, responds to temperature and humidity by changing its reflection color. Fast-responding humidity sensors have been made in which the reflection color changes between green and yellow depending on the relative humidity. The change in reflection band is a result of a change in helix pitch in the film due to absorption and desorption of water, resulting in swelling/deswelling of the film material. When the polymer salt was saturated with water, a red-reflecting film was obtained that can potentially act as a time/temperature integrator. Finally, the films were printed on a foil, showing the potential application of supramolecular CLC materials as low-cost, printable, battery-free optical sensors.

All-optical diffractive/transmissive switch based on coupled cycloidal diffractive waveplates

Pairs of cycloidal diffractive waveplates can be used to doubly diffract or collinearly propagate laser radiation of the appropriate wavelength. The use of a dynamic phase retarder placed in between the pair can be utilized to switch between the two optical states. We present results from the implementation of an azo-based retarder whose optical properties can be modulated using light itself. We show fast and efficient switching between the two states for both CW and single nanosecond laser pulses of green radiation. Contrasts greater than 100:1 were achieved. The temporal response as a function of light intensity is presented and the optical switching is shown to be polarization independent.

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.