Topological defects are a consequence of broken symmetry in ordered systems and are important for understanding a wide variety of phenomena in physics. In liquid crystals (LCs), defects exist as ...points of discontinuous order in the vector field that describes the average orientation of the molecules in space and are crucial for explaining the fundamental behaviour and properties of these mesophases. Recently, LC defects have also been explored from the perspective of technological applications including self-assembly of nanomaterials, optical-vortex generation and in tunable plasmonic metamaterials. Here, we demonstrate the fabrication and stabilisation of electrically-tunable defects in an LC device using two-photon polymerisation and explore the dynamic behaviour of defects when confined by polymer structures laser-written in topologically discontinuous states. We anticipate that our defect fabrication technique will enable the realisation of tunable, 3D, reconfigurable LC templates towards nanoparticle self-assembly, tunable metamaterials and next-generation spatial light modulators for light-shaping.
Suggestions of collapse in small herbivore cycles since the 1980s have raised concerns about the loss of essential ecosystem functions. Whether such phenomena are general and result from extrinsic ...environmental changes or from intrinsic process stochasticity is currently unknown. Using a large compilation of time series of vole abundances, we demonstrate consistent cycle amplitude dampening associated with a reduction in winter population growth, although regulatory processes responsible for cyclicity have not been lost. The underlying syndrome of change throughout Europe and grass-eating vole species suggests a common climatic driver. Increasing intervals of low-amplitude small herbivore population fluctuations are expected in the future, and these may have cascading impacts on trophic webs across ecosystems.
The artefact known as speckle can plague numerous imaging applications where the narrow linewidth of laser light is required, which includes laser projection and medical imaging. Here, we report on ...the use of thin-film chiral nematic liquid crystal (LC) devices that can be used to mitigate the influence of speckle when subjected to an applied electric field. Results are presented which show that the speckle contrast (a quantitative measure of the presence of speckle) can be significantly reduced by decreasing the pitch of the chiral nematic LC from 2700 to 244 nm. Further reduction in the speckle contrast can be observed by operating the diffuser technology at a temperature close to the chiral nematic to isotropic transition. At such temperatures, we observe a simultaneous improvement in the transmission of light through the device and a decrease in the electric field amplitude required for the minimum speckle contrast value. We conclude by presenting a laser projected image of the 1951 USAF target with and without the LC device to demonstrate the visual improvement as a result of the speckle reduction.
In this letter, we present a non-mechanical 2D beam-steering system suitable for optical wireless communication. Steering is achieved using polarization gratings combined with nematic liquid crystal ...cells operating as voltage-controlled polarization shifters. These steering elements are combined with a holographic diffuser matched to the discrete steering angles of the polarization grating, enabling continuous angular coverage. Beam-steering is also used at the receiver, allowing a large collection area receiver with a relatively narrow field of view to be used. The approach presented here could, in principle, be applied to a broad range of wavelengths (including visible light and near-infrared wavelengths). Furthermore, the technique does not inherently limit the transmission data rate. Besides, it improves the link margin and offers the potential for a bidirectional steered link using the same beam-steering elements. Details of the approach are set out in the letter, followed by experimental results from a 50 Mbit/s optical link operating over one meter. Future directions are then discussed.
Lasers possess many attractive features (e.g., high brightness, narrow linewidth, well-defined polarization) that make them the ideal illumination source for many different scientific and ...technological endeavors relating to imaging and the display of high-resolution information. However, their high-level of coherence can result in the formation of noise, referred to as speckle, that can corrupt and degrade images. Here, we demonstrate a new electro-optic technology for combatting laser speckle using a chiral nematic liquid crystal (LC) dispersed with zwitterionic dopants. Results are presented that demonstrate when driven at the optimum electric field conditions, the speckle noise can be reduced by >90% resulting in speckle contrast (C) values of C = 0.07, which is approaching that required to be imperceptible to the human eye. This LC technology is then showcased in an array of different display and imaging applications, including a demonstration of speckle reduction in modern vectorial laser-based imaging.
Systematic tuning of chemical and physical structure allows fine control over desired electronic and optical properties, including those of conjugated polymer semiconductors. In the case of physical ...structure, orientation via liquid crystalline alignment allows access to fundamental optical anisotropies and the associated refractive index modification offers great potential for fabrication of photonic structures. In this paper, photoalignment is used to orient the liquid crystalline conjugated polymer poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT), specifically involving two‐photon infrared laser writing of patterns in an azobenzene sulphonic dye (SD1). These patterns are transferred into the overlying film by thermotropic orientation in the nematic melt, then frozen in place by quenching to a room temperature nematic glass. Optimization of laser power and scan speed allows features with linewidths ≤ 1 µm. Photoluminescence (PL) peak anisotropy values reach PLII/PL⊥ = 13 for laser writing, compared with PLII/PL⊥ = 9 for polarized ultraviolet light emitting diode exposure of the same SD1 layer. These two approaches also result in different film microstructures; evidenced by characteristic changes in PL spectra. The anisotropic PL spectra provide information on emissive excited states that complements previous studies on non‐oriented F8BT and related copolymers, also suggesting two emissive states.
High resolution, sub‐micron patterning of liquid crystalline conjugated polymers (LCCP) is demonstrated using two‐photon laser writing, offering great flexibility for the construction of optical structures for a variety of applications. The image displays a college crest patterned by laser writing on a uniformly aligned photoalignment film on top of which the LCCP has been oriented as a nematic glass.
Conventional polymer dispersed liquid crystal (PDLC) films have been successful as electrically‐switchable screens for privacy applications. However, spatial patterning of the films so as to generate ...a visually appealing design, logo, or image typically requires intricate fabrication processes, such as the use of prefabricated photomasks that do not allow for on‐demand designs. Herein is reported on the fabrication and characterization of spatially patterned PDLC “pixels” using drop‐on‐demand inkjet printing, and it is demonstrated how these materials can be used to form a new generation of smart windows that consist of embedded images or company logos, which can be made to disappear with the application of a voltage. Following refinements to the material rheology and the subsequent successful deposition of individual PDLC droplets, arrays of PDLC pixels are printed at a resolution of 250 pixels per inch with an individual pixel size of 130 µm operating at an electric field strength (E) of E = 1.4 V μm−1. Finally, using the approach developed herein, these printed PDLC pixels are arranged to form a college emblem that is embedded within a smart window that can be made to disappear with the application of a voltage.
Polymer dispersed liquid crystal (PDLC) printed droplet arrays are demonstrated for smart window technologies that consist of dynamically switchable images. By adjusting the PDLC ink rheology, logos and emblems can be formed using a drop‐on‐demand printing process. In the presence of an electric field, the printed logos embedded in the smart windows can be made to disappear.
We present dynamic time-resolved measurements of a multi-pixel analog liquid crystal phase modulator driven at a 1 kHz frame rate. A heterodyne interferometer is used to interrogate two pixels ...independently and simultaneously, to deconvolve phase modulation with a wide bandwidth. The root mean squared optical phase error within a 30 Hz to 25 kHz bandwidth is <0.5° and the crosstalk rejection is 50 dB. Measurements are shown for a custom-built device with a flexoelectro-optic chiral nematic liquid crystal. However, the technique is applicable to many different types of optical phase modulators and spatial light modulators.
Direct laser writing is demonstrated by two‐photon polymerization of multi‐element diffractive optical components that can be switched on and off with an applied voltage. By exploiting the 3D ...capabilities of the laser microfabrication technique, multiple diffractive optical elements are written into a single liquid crystal (LC) layer. The switching behavior of the diffractive optical elements is controlled by simply changing the write‐voltage of the anisotropic polymer structures during fabrication. Initially, 2D diffraction gratings are written at different depths within the LC layer. Each element is then activated by applying a voltage of sufficient amplitude that causes the second diffractive optical element to become inactive. This is then followed by a demonstration of multi‐element computer generated holograms that are written at different depths within the LC layer. By altering the magnitude of the applied voltage, different images/patterns can be observed in the replay field using a simple electrode configuration. These compact and transmissive LC optical components could excel in applications where a degree of switchability is required but a highly pixelated fully programmable device is excessive.
This paper demonstrates switchable multi‐component diffractive optical elements in polymerizable liquid crystal devices. These switchable elements have been fabricated at different depths within the liquid crystal layer using two‐photon polymerization direct laser writing. By changing the voltage amplitude applied to the liquid crystal device, different computer generated holograms can be accessed resulting in different images in the replay field.
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