Jelle De Smet

Electrically Controllable Liquid Crystal Component for Efficient Light Steering

In this paper, we present an electrically controllable microoptical component for light beam steering and light intensity distribution built on the combination of nematic liquid crystal (LC) and polymer microprisms. Polymer microprism arrays are fabricated using soft embossing with elastic polydimethylsiloxane molds and ultraviolet curable resins. Surface profiling measurements show that the dimensions of the replicated prisms closely approximate those of the master prism. Two different LC alignment techniques were employed: hybrid rubbing alignment and obliquely evaporated SiO2 alignment, both of which result in proper alignment of the LC molecules along the prism groove direction. The operation voltage of the LC components is relatively low (10 Vrms). The steering angle of a green laser beam was experimentally studied as a function of applied voltage, and a steering range of 3° was found. The active LC components also effectively deflect a collimated white light beam over a steering angle of about 2° with an efficiency of 27%-33%. All the optical measurements are in agreement with theoretical calculations based on Snells law.

On the effect of alignment layers on blue phase liquid crystals

In the present work, the effect of alignment layers on blue phase liquid crystals was investigated. It was found that homogeneous alignment layers have profound selective influence on blue phase II (BPII). In the absence of alignment layers, BPII domains were randomly oriented and showed weak Bragg reflection in the UV, whereas with assistance of anchoring uniform domains with sharp Bragg reflection in the visible range appeared. On the other hand, the magnitude of Bragg shift in response to alignment layers in BPI is negligible. Domains of BP with alignment layers exhibit sharp Bragg reflection peaks (with FWHM < 15 nm), with very vivid colors and possessing fast switching speeds (<5 ×10−4 s). This simple method of selectively assisting one of the cubic phases is expected to be advantageous in the comparative studies of the two phases.

Efficient disparity vector prediction schemes with modified P frame for 2D camera arrays

An efficient disparity estimation algorithm for multi-view video sequences, recorded by a two-dimensional camera array in which the cameras are spaced equidistantly, is presented. Because of the strong geometrical relationship among views, the disparity vectors of a certain view can for most blocks be derived from the disparity vectors of other views. A frame constructed using that idea is called a D frame in this work. Three new prediction schemes which contain D frames are proposed for encoding 5x3 multi-view video sequences. The schemes are applied to several multi-view image sequences taken from a camera-array and they are compared in terms of quality, bit-rate and complexity. The experimental results show that the proposed prediction schemes significantly decrease the complexity of the encoder at a very low cost of quality and/or bit-rate.

Efficient disparity vector coding for multi-view 3-D displays

Disparity estimation can be used for eliminating redundancies between different views of an object or a scene recorded by an array of cameras which are arranged both horizontally and vertically. However, estimation of the disparity vectors is a highly time consuming process which takes most of the operation time of the multi-view video coding. Therefore, either the amount of data that is to be processed or the complexity of the coding method needs to be decreased in order to encode the multi-view video in a reasonable time. It is proven that the disparities of a point in the scene photographed by cameras which are spaced equidistantly are equal. Since there is a strong geometrical correlation of the disparity vectors, the disparity vector of a view can for most blocks be derived from the disparity vector of another view or views. A new algorithm is presented that reduces the amount of processing time needed for calculating the disparity vectors of each neighboring view except the principal ones. Different schemes are proposed for 3x3 views and they are applied to several image sequences taken from a camera-array. The experimental results show that the proposed schemes yield better results than the reference scheme while preserving the image quality and the amount of encoded data.

Tunable Optical Beam Deflection Via Liquid Crystal Gradient Refractive Index Generated By Highly Resistive Polymer Film

An effective tunable optical beam deflection device based on a periodic gradient refractive index (GRIN) profile is reported, which is capable of achieving a 5° beam deflection with an angle resolution of 0.5°. The operation voltage is around 6 Vpp, and the maximum operation frequency is 2 kHz. The GRIN beam deflection device is induced by periodically varying liquid crystal orientations. A highly resistive and patterned polymer film with a resistivity of 11 GΩ/sq is deposited to achieve a linear potential distribution between the two addressing electrodes in every unit cell. The resulting electric field pattern across the device is sawtooth-like. Both the device fabrication and the driving interconnection circuit are very simple. The advanced materials and the techniques used in this device are also applicable to other tunable optical and photonic components (such as adaptive lenses and light intensity modulators) by adapting the electrode design and the voltage signals.

Microfabrication of a spherically curved liquid crystal display enabling the integration in a smart contact lens

Abstract A spherically curved liquid crystal display based on a guest-host liquid crystal configuration is fabricated. An asymmetric display design is introduced: two flexible display substrates with varying surface area are used, allowing for compact integration of powering and driving electronics. A matrix of spacer structures composed of photosensitive adhesive and defined by photolithography, simultaneously provides the uniform cell gap while acting as glue between both display substrates. Using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as transparent, conductive electrode layer and obliquely evaporated SiO2 as alignment layer, a functional single-pixel display was fabricated. The display can be embedded in a smart contact lens, thereby enabling applications within the emerging biomedical field. An artificial iris, for example, could be designed to help people suffering from iris deficiencies that involve hypersensitivity to light.

An active artificial iris controlled by a 25-μW flexible thin-film driver

We show an active artificial iris based on solely thin-film components, wherein several LCD elements are controlled a metal-oxide TFT and by powered by thin-film photovoltaics (TFPV). Key aspects for the driver are size and low power consumption. We demonstrate power consumption down to 25μW for the full iris.

Laser ablation of micro-photonic structures for efficient light collection and distribution

In this work we report the fabrication of polymer micro-photonic gratings for use in liquid-crystal based actively tunable electro-optic components. The gratings are produced by moving the sample surface sideways across a perpendicularly impinging KrF excimer laser beam (λ = 248 nm), which is shaped by specially designed triangular and trapezoidal masks. To obtain correctly dimensioned and smooth grating surfaces, different materials (SU-8, polycarbonate, Epoclad and Epocore) are subjected to the laser ablation with optimized laser processing parameters. The resulting grating structures on Epocore exhibit the best surface roughness and dimensional fidelity. Optionally, spacers for maintaining the cell gap of the superimposed liquid crystal layer can also be fabricated in the same process. Two different methods were demonstrated: overlapping ablation and double mask ablation. Micro-grating structures were produced that deflect a monochromatic (543 nm) laser beam to the theoretically predicted 11th order with an angle of 7°.

Replicating micro-optical structures using soft embossing technique

In this article we report the fabrication of large arrays of micro-optical gratings using soft embossing with elastic Polydimethylsiloxane (PDMS) molds and ultra-violet (UV) curable resins. Three different kinds of resins are used to replicate the master gratings in a process akin to a roll to roll process. The optical surface profiling measurements show that the dimensions of the replicated gratings closely approximate those of the master gratings. Optical diffractions of these gratings are also measured and analyzed.

Novel analog pulse‐width‐modulated 15‐μm SiGe micromirrors

A novel six-electrode SiGe micromirror pixel enabling analog pulse-width modulation (PWM) in display applications is presented. Instead of utilizing bitplanes, arbitrary duty cycles can be realized, resulting in an analog gray-level distribution. This eliminates the posterization (contouring) effect that is typical for digital micromirrors. Moreover, the pixel design does not require the incorporation of electronic comparators and the gamma compensation can be processed externally. Two out of the six electrodes are used as landing electrodes. The other four attracting electrodes are driven by two anti-phase saw-tooth signals and two fixed analog voltage signals. By applying this signal scheme, the duty cycle of the mirror is modulated in an analog manner. Test vehicles were manufactured where SiGe is the microelectromechanical systems (MEMS) material. The use of SiGe as a structural layer is advantageous to build MEMS since the CMOS and MEMS layers can be deposited in a monolithic manner. Measurements using a Laser Doppler Vibrometer (LDV) have confirmed the feasibility of analog PWM for 15-μm SiGe micromirrors.