Liquid Crystal (LC) devices with photosensitive elements have incredible scope for creating unique photo-induced optical devices. The use of azobenzene based materials, which undergo a trans-cis isomerisation when irradiated with light of a specific wavelength, is firmly established in LC research. The trans conformation is an elongated rod-like shape, similar to LC mesogens, whilst the cis conformation is closer to a spherical (bent) shape, disrupting to the LC order. When these materials are doped into LC materials they are able to produce light induced responses, and therefore their application to photo-switchable optics and devices is undeniable. In this research paper the light induced order modification, rather than light induced reorientation, is utilized to produce an all-optical switchable laser protection device. Upon irradiation of an azo-doped LC system with a continuous, low power (0.5 mW), laser threat (λ=405 nm) the transcis photoisomerisation process is triggered. This results in the trans-cis conformal shape change, lowering of the LC order, and causing the system to switch from the LC nematic phase (transmitting between crossed polarisers) to the isotropic liquid phase (blocking/dark between crossed polarisers). The optical properties of the azo-doped LC materials have been characterized and the response time dependence on azo-dopant concentration, system temperature, and laser threat intensity is thoroughly investigated.
Optical tweezers can be used as a valuable tool to characterize electrophoretic display (EPD) systems. EPDs are ubiquitous with e-readers and are becoming a commonplace technology where reflective, low-power displays are required; yet the physics of some features crucial to their operation remains poorly defined. We utilize optical tweezers as a tool to understand the motion of charged ink particles within the devices and show that the response of optically trapped electrophoretic particles can be used to characterize electric fields within these devices. This technique for mapping the force can be compared to simulations of the electric field in our devices, thus demonstrating that the electric field itself is the sole governor of the particle motion in an individual-particle regime. By studying the individual-particle response to the electric field, we can then begin to characterize particle motion in ‘real’ systems with many particles. Combining optical tweezing with particle tracking techniques, we can investigate deviations in many particle systems from the single-particle case.
The inability of the eye to focus on nearby objects, presbyopia, is suffered by ~100% of people over the age of 50. Liquid crystal (LC) spectacle lenses have shown great potential for correcting presbyopia. However, correcting presbyopia in contact lens users has proven elusive and existing commercial options suffer significant compromises in vision and comfort. We describe a novel contact lens that includes a liquid crystal element that offers to correct presbyopia without the compromises associated with other technologies. We fabricated variable focus lenses using a balanced optical system, providing the additional optical power presbyopes require for near vision (typically +1.00 D to +2.00 D). The system uses positive optical power from the two substrates and variable negative optical power from the LC layer to form a balanced optical system which, when unpowered, corrects distance vision. Upon voltage application, the liquid crystal layer decreases in refractive index, resulting in additional optical power in the system, offering correction equivalent to reading glasses. Our new technology is based on a traditional contact lens material which could be placed directly on the eye. The liquid crystal lens employed is well suited to the small optical areas associated with contact lenses. We compare several different LC materials and geometries which are suitable for our application, and discuss the influence of material and geometry on switching times, optical quality and operating voltage. Our contact lenses typically switch ±2.00D in response to < 10 V<sub>rms</sub> with response times of the order of a second.
Exciting new directions for liquid crystals (LCs) are emerging on the length scale of the wavelength of light. Two complementary micron-sized systems are formed by LC droplets and by dispersions of colloidal particles in LCs. The dimensions of each of these systems are ideal for laser tweezer manipulation, allowing a new range of photon-addressed LC systems to be envisaged. Trapping and moving micron-sized particles in LCs is a beautiful approach that can build novel colloidal photonic materials. However, it is also a unique way of studying fundamental LC properties, particularly anisotropic viscosity coefficients in the low Ericksen regime, which can be accessed by laser trapping. Rather few nematic materials have been studied using laser traps; we describe two different approaches to deduce the viscosity coefficients of nematic mixtures. Micron-sized LC droplets are emerging as intriguing photonic systems in their own right. Angular momentum can be transferred from laser traps to droplets, with specific polarization properties and droplet geometries resulting in a variety of novel photon-driven effects. Fast optical switches, rotating at speeds >1kHz, can be produced from nematic droplets in circularly polarized beams. Both droplet geometry and beam polarization influence the droplet rotation, allowing control of the phenomenon. Surprisingly, a chiral nematic droplet can sometimes undergo continuous rotation in a <i>linearly</i> polarized trap, a phenomenon caused by optically-induced changes in chirality. We describe this remarkable effect which demonstrates how the control of chirality through polarization can result in an optically driven transducer.
The dynamic response characteristics of a liquid crystal (LC) device are dependent upon its viscosity coefficients.
Local shear viscosity coefficients, or Miesowicz viscosity coefficients, η<sub>i</sub>, are of particular importance for backflow
effects and their optimisation allows for faster LC device response times. With such a wide range of LC
materials available, information regarding their viscous properties is often incomplete. Micromanipulation with
laser tweezers offers an alternative method for determining shear viscosity coefficients. Micron sized dielectric
particles are dispersed in homeotropically and planarly aligned nematic LC, sandwiched between two coverslips.
The microfluidic behaviour of the LC is investigated using a computer controlled laser tweezer system where
particle tracking is performed using a high speed CMOS camera to record bead displacement for power spectral
density analysis. We investigate the effective viscosity coefficients parallel and perpendicular to the director
<i>n</i>, η<sup>II</sup><sub>eff</sub> and η⊥<sub>eff</sub> respectively. These are directly related to the Miesowicz viscosity coefficients for homeotropic
alignment η1, and homogenous alignment η2 and η3. The results infer practically pertinent details about the viscoelastic properties of liquid crystals, and particles in LC systems.
Many applications of laser tweezers rely on the accurate measurement of the transverse or axial trapping force. We have concentrated on the transverse trapping force and the most common method used to measure it, applying a viscous drag force. A trapped sphere was subjected to a viscous drag force via a Stokesian flow. The flow was achieved by oscillating the sample stage at a constant speed of 750 microns/second. A Zeiss oil-immersion (N.A. equals 1.3) objective was used to focus a 1064 nm Nd:YVO<SUB>4</SUB> laser beam in order to trap 6 microns diameter polystyrene spheres suspended in distilled water. The minimum power needed to hold the particle in the trap at a particular viscous drag force was then measured. The influence of trap depth, oscillation amplitude used and particle concentration have been investigated, in particular the effects caused by the characteristics of the function used to create the oscillation. The minimum laser power needed to trap a sphere was found to increase with a rise in oscillation amplitude. The velocity profile through the fluid, the rotation of the trapped particle and the effect of interactions with other particles is considered when explaining these effects.
The transverse force of an optical trap is usually measured by equating the trapping force to the viscous drag force applied to the trapped particle according to Stokes' Law. Under normal conditions, the viscous drag force on a trapped particle is proportional to the fluid velocity of the medium. In this paper we show that an increase of particle concentration within the medium affects force measurements. In order to trap the particle, 1064 nm light from a Nd:YVO<SUB>4</SUB> laser was brought to a focus in a sample slide, of thickness around 380 microns, by using an inverted Zeiss microscope objective, with NA equals 1.3. The slide was filled with distilled water containing 6 micron diameter polystyrene spheres. Measurements were taken at a fluid velocity of 0.75 microns/sec, achieved by moving the sample stage with a piezo-electric transducer whilst a particle was held stationary in the trap. The laser power required to hold a sphere at different trap depths for various concentrations was measured. Significant weakening of the trap was found for concentrations >0.03% solids by weight, becoming weaker for higher trap depths. These results are explained in terms of aberrations, particle-particle interactions and distortion of the beam due to particle-light interactions.
This study examines the optical response and physical properties of the homologous series 4-n-butyl-4'-n-alkoxyazobenzene. The members of this series all exhibit liquid crystalline phase behavior, and have also been used as dopants in 4-cyano-4'- n'pentylbiphenyl (5CB), a room temperature nematic liquid crystal. The guest-host system and the azobenzene series have been characterized using optical microscopy and UV-vis spectrophotometry. Illumination of these systems with light of a suitable wavelength induces a trans-cis isomerization of the azo- dye molecules which results in a reorientation of the liquid crystal director, often to such an extent that the liquid crystal phase is disrupted, causing an in situ isothermal phase transition. The response of the liquid crystal system to a linearly polarized beam of exciting radiation is examined with the use of a probe He:Ne laser. Changes in light transmission are then detected with a photodiode. Responses are discussed in terms of homologue, cell thickness and temperature.