Microflow sensing and control using an in-channel birefringent biomembrane.

This study describes the function, optimization, and demonstration of a new class of passive, low-cost microfluidic flow meters based on birefringent chitosan biomembranes analyzed by polarized microscopy. We subjected the membrane to dynamic flow conditions while monitoring the real-time response of its optical properties. We obtained figures of merit, including the linear response operating range (0 to 65 µL min-1), minimum response time (250 ms), sensitivity (2.03% × 10-3 µL-1 min), and minimum sensor longevity (1 week). In addition, possible sources of interference were identified. Finally, we demonstrate the membrane as a low-cost flow rate measurement device for the close loop control of a commercial pressure-driven pump. Preliminary experiments using a basic PID controller with the membrane-based flow rate measurement device showed that stable control could be achieved and the system could reach steady-state behavior in less than 15 seconds. Analysis of fundamental limits to sensor response time indicate the potential for faster steady-state behaviour.

Tunable liquid crystal lens with symmetric bipolar operation.

We describe an electrically tunable liquid crystal lens that can dynamically generate symmetric wavefront profiles. The curvature of these profiles may be inversed, enabling a bipolar response (focusing and defocusing). Different wavefronts, including non-monotonic, are predicted theoretically and demonstrated experimentally. The optical performance of the devices is characterized experimentally in an imaging scheme.

Towards a mini-endoscope design with spatially selective excitation and imaging.

We describe a mini-endoscope design that uses a new type of electrically tunable liquid crystal lens array enabling the dynamic increase of spatial resolution by adjusting the working distance in various zones of interest over a relatively large field of view (FoV) without mechanical movement. The characterization of the system is performed by using uniform fluorescent films, fluorescent micro spheres and a tissue sample expressing the fluorescent calcium indicator GCaMP6s. Lateral resolution of up to 2 µm over the FoV between 300 µm - 400 µm is experimentally demonstrated.

Wavefront control capability in a modal lens with segmented circular peripheral electrodes.

We report the detailed investigation of the capability of an electrically tunable liquid crystal lens (TLCL) to dynamically generate various wavefront shapes. The TLCL operates in the modal-control mode with a peripheral circular electrode divided into eight individually controlled segments. This segmentation allows producing a rather rich set of influence functions. We characterize these functions and the crosstalk between them by adjusting the voltage and the frequency of electrical signals applied to different electrode segments. Various wavefronts are produced in a closed-loop control mode and described using Zernike polynomials. The dynamical response of the lens is also briefly investigated. Obtained results may be used to design different adaptive optical systems where a dynamic wavefront control is required.

Dynamic beam shaping for LED lighting.

We describe an electrically tunable liquid crystal microlens array with a 65 mm clear aperture that allows the dynamic symmetric broadening of white light without mechanical movements. The fundamental mechanism of broadening is demonstrated to involve a spatial twist of molecular orientation. Continuous variations of the light divergence angle are obtained in a very large dynamic range (from 8° to 50°). Particular attention is paid to maintain the desired intensity and color temperature distributions over the angle. The developed lens may be used in various broadband illumination applications based on light-emitting diode-pumped phosphor sources.

Simple electrically tunable liquid crystal spatial phase modulator.

We describe multiple optical functionalities obtained with a simple electrically tunable liquid crystal element that can be controlled by 4 electrodes, which are connected to a serpentine shaped transparent indium tin oxide layer. We experimentally demonstrate that the device is capable of dynamically generating refractive index distributions corresponding to a standard spherical lens, axicon, cylindrical lens, and prism. The dynamic switching of the device between these different operation modes is done in a very simple electronic way. We think that this element has a significant potential for applications in adaptive imaging, optogenetics, photonic integrated circuits, etc.

Adaptive lens for foveal vision, imaging, and projection over large clear apertures.

We report an electrically tunable liquid crystal device that enables the generation of lenses the diameters of which may be dynamically changed from sub-millimeter to multiple millimeter sizes. These lenses can be created in different regions of interest over very large (above 50 mm) optical clear apertures. The approach is based on the activation of periodically spaced contacts on a single serpentine-shaped electrode with phase-shifted electrical signals. It enables a highly reconfigurable operation of locally created lenses with variable position, diameter, optical power (OP) and aberrations. The preliminary demonstration of the capabilities of the proposed device is presented here by creating a local lens, moving its center over an area of 25 mm x 25 mm, gradually changing its diameter from 1.3 mm to 4.55 mm as well as by tuning its OP value from zero up to, respectively, ≈ 40 and ≈3.5 diopters. Typical driving signals are at the order of 3.5 V. We think that such lenses can be used for ophthalmic or augmented reality applications as well as in microscopy, adaptive panoramic cameras with large distorted field of view, dynamic projection, etc.

Large diameter electrically tunable lens for ophthalmic distance accommodation.

Electrically tunable liquid crystal lens with 30 mm diameter is presented based on the refractive Fresnel concept. Relatively large optical power variation range (from - 0.74 to +0.71 Diopters) is demonstrated along with very low root mean square aberrations (≤0.15 µm). Optical characterizations, including with Snellen chart, show that good vision recovery may be obtained with fast response time (under 500 msec) and relatively low haze. The proposed design is very simple and may be fabricated by using single step lithography. Perspectives on its applications are discussed.

Electrically tunable lens with a non-monotonic wavefront control capability.

We describe an electrically tunable liquid crystal lens that can produce a rich variety of wavefronts, including sombrero-type (non-monotonic) phase modulation, enabling the focusing of light into a ring-shaped intensity distribution. The lens can also generate axicons or standard spherical lenses with a bipolar response (providing both positive and negative optical powers). The design of the lens requires only a single-step lithography process, dramatically simplifying its manufacturing. We describe various driving modes of this lens and present the first experimental results and discuss its possible applications in miniature cameras and microscopes. We think that this device can revolutionize the optical design in many areas of photonics.

Electrically tunable liquid crystal lens with a serpentine electrode design.

The design and operational principle of a new electrically tunable gradient index liquid crystal lens are described. The approach is based on linear serpentine electrodes and does not require a semiconductor layer. A preliminary validation is done for a lens with a 2 mm clear aperture, demonstrating 9.5 diopters of optical power and a root-mean-square wavefront error of 0.16 µm. The developed lens is tested with a miniature camera and the image quality improvement is demonstrated experimentally.

Reducing the light scattering impact in liquid-crystal-based imaging systems.

We show an experimental method of quantifying the effect of light scattering by liquid crystals (LCs) and then apply rather simple image processing algorithms (Wiener deconvolution and contrast-limited adaptive histogram equalization) to improve the quality of obtained images when using electrically tunable LC lenses (TLCLs). Better contrast and color reproduction have been achieved. We think that this approach will allow the use of thicker LC cells and thus increase the maximum achievable optical power of the TLCL without a noticeable reduction of image quality. This eliminates one of the key limitations for their use in various adaptive imaging applications requiring larger apertures.

Dynamic control of polarization mismatch and coma aberrations in rod-GRIN assemblies.

We describe the use of stacked electrically tunable liquid crystal lenses (TLCLs), along with rod gradient index (GRIN) fixed focus lenses, for endoscopic applications. Architectural and driving conditions are found for the optimization of total aberrations of the assembly. Particular attention is devoted to the coma and polarization aberrations. The coma aberration is reduced by stacking two TLCLs with "opposed" pre-tilt angles (all molecules are in the same plane), and then two such doublets are used with cross oriented molecules (in perpendicular planes) to reduce the polarization dependence. The obtained adaptive rod-GRIN lens enables a focus scan over 80µm (with exceptionally low RMS aberrations ≤0.16µm), making possible the high-quality observation of neurons at various depths.

Dynamic compensation of gradient index rod lens aberrations by using liquid crystals.

An electrically variable liquid crystal lens is used to compensate the aberrations of commercial gradient index rod lenses used for deep brain endoscopy. This is achieved by the use of a weakly conductive layer in the so-called "modal control" lens approach with a segmented peripheral electrode. The root mean square aberrations of the system are reduced by a factor of 4.3. The proposed solution can be used in many photonic applications using fixed optical components with high aberrations that have significant sample-to-sample variability.

Liquid crystal lens with corrected wavefront asymmetry.

We propose a simple technique allowing the correction of the inherent wavefront asymmetry in electrically variable liquid crystal lenses with relatively small diameters. This is achieved by splitting the peripheral hole patterned electrode of the lens in the direction of the ground-state anisotropy axis. Optical aberrations are measured before and after the split. It is demonstrated that a significant correction is achieved with a trade-off of trefoil increase in the direction of the split. This increase is then eliminated by the further segmentation of the peripheral electrode.

Modulation transfer function of liquid crystal microlenses and microprisms using double dielectric layer.

We investigate electrically tunable liquid crystal (LC) microlenses and microprisms based on double dielectric optically hidden (DDOH) layers. Comparative theoretical study of the spatial resolution limits in the creation of a spatially modulated electric field by the DDOH layer is conducted. Both the depth of the resulting optical phase modulation and its deviation from the desired wavefront are obtained for sine and sawtooth geometries of the DDOH layer's structure. A comparison is provided with the standard LC reorientation approach using patterned electrodes.

Transient locking of the hook procures enhanced motility to flagellated bacteria.

Flagellated bacteria often proliferate in inhomogeneous environments, such as biofilms, swarms and soil. In such media, bacteria are observed to move efficiently only if they can get out of "dead ends" by changing drastically their swimming direction, and even to completely reverse it. Even though these reorientations are ubiquitous, we have only recently begun to describe and understand how they happen. In the present work, we visualized the flagella of bacteria swimming in a soft agar solution. The surprising observation that the filaments do not rotate while being flipped from one side of the cell to the other suggests that reversals are driven directly by the motor rather than by the thrust created by the rotating filament. This was confirmed by observing bacteria in a liquid crystal, where the linear movement of bacteria greatly simplifies the analysis. These observations suggest that the reversal and reorientation processes involve a temporary locking of the flagellum's hook, which is the normally flexible joint between the rotary motor and the long helical filament that propels the cell. This newly described locked-hook mode occurs only when the motor switches to a clockwise rotation. That correlates with other phenomena that are triggered by a switch in one direction and not the other.

Diffusion of chiral molecules and propagation of structural chirality in anisotropic liquids.

Diffusion in nature is usually considered as a smooth redistribution process. However, it appears that the diffusion of chiral molecules and the propagation of chirality may proceed in quite different ways. Indeed, in the present work, unexpected quantization of the spatial concentration of chiral molecules is discovered in self-aligned molecular liquids. It is shown that the interpenetration of two liquids is forming discrete diffusion barrier walls resulting in steplike concentration distribution of chiral molecules in space. The concentration gradient is at least an order of magnitude stronger from both sides of the barrier wall compared to the gradient between those walls. It is also shown that this microscopic diffusion process may be controlled by macroscopic boundary conditions imposed on the host molecular system. Both of those phenomena are related to the collective long-range orientational "elastic" interactions of molecules of the host. The observed phenomena may radically change our understanding of diffusion of chiral molecules, among others, in biological tissue, which contains many examples of self-aligned molecular liquids. This, in turn, has the potential to revolutionize drug design and delivery techniques.

Theoretical analyses of a liquid crystal adaptive lens with optically hidden dielectric double layer.

In this work we theoretically analyze the performance trends of a liquid crystal lens based on the optically hidden dielectric double layer principle. We demonstrate possible ways to optimize the lens as a function of the material and geometric parameters used. The impact of relative dielectric constants, conductivities, and dimensions of the components of the hidden dielectric layer, as well as the thickness and the temperature of the liquid crystal material, are demonstrated. Corresponding trade-offs are briefly discussed.

Motion-free endoscopic system for brain imaging at variable focal depth using liquid crystal lenses.

We present a motion-free system for microendoscopic imaging of biological tissues at variable focal depths. Fixed gradient index and electrically tunable liquid crystal lenses (TLCL) were used to build the imaging optical probe. The design of the TLCL enables polarization-independent and relatively low-voltage operation, significantly improving the energy efficiency of the system. A focal shift of approximately 74 ± 3 µm could be achieved by electrically controlling the TLCL using the driving frequency at a constant voltage. The potential of the system was tested by imaging neurons and spines in thick adult mouse brain sections and in vivo, in the adult mouse brain at different focal planes. Our results indicate that the developed system may enable depth-variable imaging of morpho-functional properties of neural circuitries in freely moving animals and can be used to investigate the functioning of these circuitries under normal and pathological conditions.

Bacterial Motility Reveals Unknown Molecular Organization.

The water solubility of lyotropic liquid crystals (LCs) makes them very attractive to study the behavior of biological microorganisms in an environment where local symmetry is broken (as often encountered in nature). Several recent studies have shown a dramatic change in the behavior of flagellated bacteria when swimming in solutions of the lyotropic LC disodium cromoglycate (DSCG). In this study, the movements of Escherichia coli bacteria in DSCG-water solutions of different concentrations are observed to improve our understanding of this phenomenon. In addition, the viscosity of DSCG aqueous solutions is measured as a function of concentration at room temperature. We also experimentally identify a previously undescribed isotropic pretransition zone where bacteria start sticking to each other and to surfaces. Simple estimations show that the unbalanced osmotic pressure induced depletion force might be responsible for this sticking phenomenon. An estimate of the bacteria propulsive force and the DSCG aggregates length (versus concentration) are calculated from the measured viscosity of the medium. All these quantities are found to undergo a strong increase in the pretransition zone, starting at a threshold concentration of 6±1 wt % DSCG that is well below the known isotropic-LC transition (∼10 wt %). This study also shines light on the motility of flagellated bacteria in realistic environments, and it opens new avenues for interesting applications such as the use of motile microorganisms to probe the physical properties of their host or smart bandages that could guide bacteria out of wounds.