Photosensitive Liquid Crystal Research Articles

Light-powered sustained chaotic jumping of a liquid crystal elastomer balloon

Chaotic systems comprising of active materials have the capability to absorb energy directly from the external environment to preserve their own sustained motion, holding tremendous potential for utilization in diverse fields, for instance, controllable sensors, energy harvesting, motion behavior control of smart materials and more. In this paper, a sustained jumping system consisting of photosensitive liquid crystal elastomer (LCE) balloon is developed innovatively, in which the LCE balloon can transition from a static state to a sustained jumping state under periodic illumination. Given the dynamic constitutive model of LCE and Hertz contact theory, the nonlinear dynamics model of the sustained jumping LCE balloon system is established. Numerical calculations demonstrate the existence of two jumping modes for the LCE balloon, i.e., periodic jumping and chaotic jumping. The mechanism of sustained jumping that counterbalances the energy dissipation by the net work combining contact expansion and buoyancy variation, as well as the mechanism of chaotic phenomena, are revealed. In addition, the influences of vital parameters on the jumping mode of the balloon are examined, and the conversion of the jumping modes with parameters is illustrated by bifurcation diagrams. This work can broaden the knowledge of the dynamic behavior of smart materials, which is of guiding significance for the research on chaos control and chaotic system design.

Opto‐regulation for the 2D to 3D transformation of a liquid crystal network membrane

AbstractLight loading mode plays essential roles in the deformation morphology of the photosensitive liquid crystal network (LCN) membrane. However, owing to the strong non‐linearity of membrane deformation and the constraint effect of membrane boundary, the precise surface pattern control of a LCN membrane through pre‐designing the light loading pattern have not been fully carried out. In this article, a second light‐engraving strategy is proposed to decouple the local and integral deformation of the LCN membrane, and the accurate opto‐regulation of the 2D to 3D pattern transformation could be realized. Firstly, the opto‐deformation governing equations for the LCN membrane under nonuniform illumination are established, and solved by the finite difference method. The deformation characteristics of the LCN membrane under nonuniform illumination has been discussed, and the relation between the light loading mode and the spontaneous deformation in the LCN membrane is clarified. Then a second‐engraving technology based on the deformed membrane is proposed, and the precise opto‐tuning of the 2D to 3D deformation for a LCN membrane is realized. “Robert face” and three‐dimensional word printing is presented as example. This work provides theoretical and numerical guidance on the LCN membrane based three dimensional modeling technology.

Photosensitive Bent-Core Liquid Crystals with Laterally Substituted Azobenzene Unit

Photosensitive liquid crystals represent an important class of functional materials that experience rapid development. Hereby, we present novel bent-core liquid crystals bearing a lateral substitution on the central core and in the vicinity of the photosensitive unit—an azo group. The azo group enables fast (E)-to-(Z)-isomerization upon irradiation with UV-light and visible light, while the substitution facilitates the high stability of the photochemically formed (Z)-isomer. The effectiveness of the irradiation and the composition of photostationary states was determined by UV/Vis and 1H NMR spectroscopy. A nematic phase formed by the materials was characterized by differential scanning calorimetry and optical polarizing microscopy. We show that the materials easily change their relative configuration of the N=N double bond not only in solution, but also in the mesophase, which leads to fast isothermal phase transition from the nematic phase to isotropic liquid.

Light beam induced finger instability in a photosensitive liquid crystal cell

ANID-PFCHA Doctorado Nacional Grant 2017-21171672 ANID Millenium Science initiative program ICN17_012 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1210353

Optical-trapping of a red Bessel beam controlled by co-propagating green Gaussian beam in azobenzene liquid crystal

Simultaneous co-propagation of red nondiffracting Bessel beam and green Gaussian beam at milliwatt powers in a photosensitive azobenzene liquid crystal has been studied. The fine displacement of the narrow-width Gaussian beam relative to the Bessel beam central maximum showed the Bessel beam distortion and controlled bending of its core in the direction opposite to the location of green Gaussian beam. After switching off the green beam, the dynamics of restoration of Bessel beam demonstrated the pronounced optical-trapping feature of its central core and transient soliton-like propagation with reduced diameter and increased intensity and propagation depth thus forming a waveguiding channel.

Refractive Bessel lattice in azobenzene liquid crystal

ABSTRACTA refractive Bessel lattice with micrometric periodicity is induced optically in a photosensitive azobenzene liquid crystal cell of 3 µm thickness by a green 532 nm, 30 mW Bessel beam and with simultaneous illumination by a red 632.8 nm, 15 mW Gaussian beam. The uninterrupted action of both beams plays a key role in the complete mechanism of the refractive lattice formation. The lattice formation is investigated in real-time by the measurement of forward diffracted ring powers from both the red Gaussian and the green Bessel beams. The diffraction efficiency is investigated depending on the green Bessel beam intensity and on the mutual relation between the green beam polarization and the rubbing direction of the cell. A maximum diffraction efficiency of 1.1% is obtained which corresponds to a refractive index change of 6 × 10−3. Simulations are performed and a physical model to explain the experimental results is discussed.

Super-twist generation and instabilities in photosensitive liquid crystal cells

In planar nematic liquid crystals cells twist deformation was generated through photoalignment. By increasing the twist angle gradually, supertwisted cells were constructed in the range of 2π–3π twist angle. The supertwist relaxed through the formation of either π or 2π inversion loops, depending on the character of the photosensitive substrate. The difference in the relaxation process can be related to the zenithal anchoring strength on the photosensitive plate.

Modeling a photoinduced planar-to-homeotropic anchoring transition triggered by surface azobenzene units in a nematic liquid crystal.

The performance of light-controlled liquid crystal anchoring surfaces depends on the nature of the photosensitive moieties and on the concentration of spacer units. Here, we study the kinetics of photosensitive liquid crystal cells that incorporate an azobenzene-based self-assembled monolayer. We characterize the photoinduced homeotropic-to-planar transition and the subsequent reverse relaxation in terms of the underlying isomerization of the photosensitive layer. We show that the response time can be precisely adjusted by tuning the lateral packing of azobenzene units by means of inert spacer molecules. Using simple kinetic assumptions and a well-known model for the energetics of liquid crystal anchoring we are able to capture the details of the optical microscopy experimental observations. Our analysis provides fitted values for all the relevant material parameters, including the zenithal and the azimuthal anchoring strength.

Light-addressable liquid crystal polymer dispersed liquid crystal

Scattering-free liquid crystal polymer-dispersed liquid crystal polymer (LCPDLC) films are fabricated by combining a room temperature polymerizable liquid crystal (LC) monomer with a mesogenic photosensitive LC. The morphological and photosensitive properties of the system are analysed with polarized optical microscopy and high resolution scanning and transmission electron microscopy. A two-phase morphology comprised of oriented fibril-like polymeric structures interwoven with nanoscale domains of phase separated LC exists. The nanoscale of the structures enables an absence of scattering which allows imaging through the LCPDLC sample without optical distortion. The use of a mesogenic monomer enables much smaller phase separated domains as compared to non-mesogenic systems. All-optical experiments show that the transmitted intensity, measured through parallel polarizers, can be modulated by the low power density radiation (31 mW/cm2) of a suitable wavelength (532 nm). The reversible and repeatable transmission change is due to the photoinduced trans-cis photoisomerization process. The birefringence variation (0.01) obtained by optically pumping the LCPDLC films allow their use as an all-optical phase modulator.

Photosensitive bent-core liquid crystals based on methyl substituted 3-hydroxybenzoic acid

Photosensitive liquid crystals are of contemporary interest not only from the scientific point of view but also for various applications. The chemical structure of materials is used to tune their mesomorphic and photosensitive properties.

Electro‐/All‐Optical Light Extraction in Gold Photonic Quasi‐crystals Layered with Photosensitive Liquid Crystals

Electro‐/All‐Optical Light Extraction in Gold Photonic Quasi‐crystals Layered with Photosensitive Liquid Crystals

All-optically reconfigurable and tunable fiber surface grating for in-fiber devices: a wideband tunable filter

A fiber surface grating (FSG) formed from a photosensitive liquid crystal hybrid (PLCH) film overlaid on a side-polished fiber (SPF) is studied and has been experimentally shown to be able to function as an all-optically reconfigurable and tunable fiber device. The device is all-optically configured to be a short period fiber surface grating (SPFSG) when a phase mask is used, and then reconfigured to be a long period FSG (LPFSG) when an amplitude mask is used. Experimental results show that both the short and long period FSGs can function as an optically tunable band-rejection filter and have different performances with different pump power and different configured period of the FSG. When configured as a SPFSG, the device can achieve a high extinction ratio (ER) of 21.5dB and a wideband tunability of 31nm are achieved. When configured as a LPFSG, the device can achieve an even higher ER of 23.4dB and a wider tunable bandwidth of 60nm. Besides these tunable performances of the device, its full width at half maximum (FWHM) can also be optically tuned. The reconfigurability and tunability of the fiber device open up possibilities for other all-optically programmable and tunable fiber devices.

Nanosecond switching of photo-responsive liquid crystal diffraction gratings

The diffraction efficiency of periodic microstructures composed of photoresponsive liquid crystals and polymer gratings is modulated using visible CW or pulsed radiation. A polymeric template (static grating) enabled by holographic photopatterning was infiltrated by capillary flow with a high performance photosensitive (green) liquid crystal after the initial formation step. The infiltration realizes high quality diffractive structures containing well-aligned photosensitive LC molecules and layers separated by polymer walls in a transmission grating geometry. The optical anisotropy present due to the oriented liquid crystals induces a strong polarization dependence of the grating diffraction efficiency. Exposure of the periodic structure to green CW or pulsed excitation modulates both the spectral and amplitude transmission behavior of the grating structure. In particular, under the influence of single pulse excitation, the diffraction efficiency can be modulated from 87% to 17% in 4 ns with a spontaneous back relaxation time three orders of magnitude (microsecond) faster than the one observed for the same material conventionally aligned in standard glass cells. The ability to remotely and quickly change the diffraction properties of soft-composite periodic microstructures marks a breakthrough towards the realization of ultra-fast all-optical devices.

A fiber-optic violet sensor by using the surface grating formed by a photosensitive hybrid liquid crystal film on side-polished fiber

A fiber-optic violet sensor is demonstrated by using the surface grating formed by a photosensitive liquid crystal (LC) hybrid film on the flat area of side-polished fiber (SPF). The surface grating is constructed through a periodic intensity illuminating the hybrid LC film, where the periodic intensity is created by a phase mask through which the violet light passes. Experiment shows that a loss peak appears in the transmission spectrum between 1520 and 1620 nm, and this loss peak shifts toward shorter wavelength as the 405 nm light power increases. The wavelength shift of the peak shows very good linearity with the irradiation power between 30 and 80 mW cm−2. The very high sensitivity of the light power sensor is measured to be 1.154 nm (mW cm−2)−1, which implies that the minimum change of power intensity that can be detected is 0.866 µW cm−2 for this sensor under the limited wavelength resolution of 0.001 nm of the optical spectrum analyzer. For UV light, much higher sensitivity will be further obtained, as the LC hybrid is more sensitive to UV light than to violet light.

Photo-sensitive liquid crystals for optically controlled diffraction gratings

We report on the fabrication and characterization of diffraction gratings based on a polymeric microstructure including a light responsive material, which enables an all-optical control of the diffraction of light. The enhanced performances of a mesogenic azo dye are combined with the outstanding properties of reconfigurability and fluidity of nematic liquid crystals; the mesogenic affinity between the two substances enables realization of a photosensitive liquid crystal whose optical properties can be controlled by means of low power light. Long range order and high mechanical stability of the photosensitive liquid crystal composition are obtained by using the universal organizing capability of a channeled polymeric template, without the need for any kind of surface treatment. The structure exhibits strong contrast ratio and a high diffraction efficiency, both optically controlled.

Information Recording in Photosensitive Photonic Cholesteric Liquid Crystal

In this paper we propose a new method of information recording based on the photo controllable shift of the selective reflection wavelength in photo sensitive cholesteric liquid crystals.

Full Optical Control of Holographic Gratings Realized in Composite Materials Containing Photosensitive Liquid Crystals

We report on the characterization of an all optical switching effect that takes place when a UV light beam irradiates an azo-POLICRYPS diffraction grating. This high quality polymeric structure is utilized to confine and stabilize a well aligned nematic liquid crystal (NLC) film doped with a small percentage of azo-NLC molecules. The optical control of the grating diffraction efficiency is obtained by exploiting the photochemical phase transition between the nematic and the isotropic phases of the liquid crystal, based on the photoisomerization of the azobenzene guest molecules. The switch ON-OFF of the grating diffraction efficiency is triggered by a UV pump laser and is investigated by means of a low power red probe beam.

Dielectric and electro-optical properties of a photosensitive liquid crystal

The temperature dependencies of the dielectric and electro-optical properties of a pure photo-ferroelectric liquid crystal have been investigated, in the chiral smectic C (SmC*) phase, with and without ultraviolet (UV) illumination. The SmC* dielectric Goldstone mode characteristics, the spontaneous polarization, the tilt angle and the rotational viscosity are affected by UV irradiation. Under increasing UV light intensities the dielectric strength ΔϵG increases whereas the relaxation frequency fG , the spontaneous polarization P s, the electro-optical rotational viscosity γeo and the tilt angle θ decrease. The twist elastic constant K 3 and the Goldstone mode rotational viscosity γG of the studied compound are deduced from dielectric experimental results. Good agreement was found between both viscosities. The observed dielectric behaviours are controlled by the elastic constant and the rotational viscosity variations.

Optically reconfigurable patterning for control of the propagation characteristics of a planar waveguide

We demonstrate an easily fabricated all-optical and freely reconfigurable method of controlling the propagating characteristics of the optic path within a planar waveguide with low insertion losses by employing the optical patterning of the refractive index of an erasable and rewriteable photosensitive liquid crystal polymer cladding layer.

Azobenzene photoswitches in bulk materials

A nonadiabatic two-scale quantum-classical dynamics method is developed to allow for dynamical simulation of photoinduced processes and reactions in chemically complex condensed-matter systems such as photosensitive liquid crystals. A first application to photoswitching of azobenzene in the bulk reveals not only valuable insights into the excited-state dynamics underlying the cis to trans transformation, but also into the influence of the condensed phase environment on mechanism and relaxation time scales compared to vacuum.