Voltage programmable liquid optical devices

• Main Author: Wells, Gary George
• Format: Dissertation
• Language: English
• Published: Nottingham Trent University 2009

In this study I have looked at two types of voltage programmable liquid optical device. The first is a rotatable liquid crystal waveplate and the second is an amplitude programmable liquid phase grating. The rotatable waveplate was created by confining a droplet of E7 nematic liquid crystal between two glass substrates in a photolithographically produced square well of 60x60x15µm dimensions. The droplet forms a circular disc shape with two nematic defects at opposite edges of the disc. The optical texture has been examined and the most likely n-director orientation is shown to be Bipolar. Electrodes etched into the sandwiching substrate allow an in-plane electric field to be applied and because of the E7's positive dielectric anisotropy the n-director of the droplet aligns itself with the field. The speed at which this rotation occurs is shown to follow the relationship Ø = 0.0472гV 2 and rotation speeds of 450° /s have been achieved. The droplet is multistable and can be rotated to any arbitrary angle. The temperature of the droplet affects its switching ability by changing the shape and size of the droplet within the well. The switching time with voltage at different temperatures has been investigated and shows that at higher temperatures switching is faster. Rounded square droplets have also been seen which exhibit six stable states which can be switched from one state to another by applying various electric fields. The switching speed of these states has been investigated for several different switching regimes. A voltage programmable phase grating has also been demonstrated by inducing an undulation in the surface of a thin film of either hexadecane or 1-decanol oil. The thin film of oil is the result of a dielectrophoretic force created by a non-uniform electric field above a set of interdigitated electrodes. The thickness of the film is almost independent of the applied voltage. At higher voltages, an undulation in the oil air interface occurs which is static and stable with the same period as the electrode pitch. Measurements of hexadecane and 1-decanol oil for a variety of layer thickness on several different electrode pitches show that the amplitude of the undulation is proportional to the voltage squared. This scaling relationship and the exponential dependence on the ratio of oil thickness and the electrode pitch can be explained in terms of the balance between the dielectrophoretic forces created by the non-uniform electric field and the increase in surface energy associated with the surface deformation. For a thin layer of oil with low dielectric constant the higher order spatial harmonics in the electric field cause the undulation to become non-sinusoidal. The extent to which these harmonics shape the surface of the oil is explored and their relationship to oil thickness is shown. The relative contributions of these higher order harmonics are also shown to be independent from the applied voltage. As an example of the devices applications it has been shown to operate as an amplitude programmable phase grating and diffracts the energy of an incoming 543nm laser from the zero straight through order into the higher orders. The diffraction efficiency of 32.8% is very close to that predicted by theory. The device is polarization insensitive and switching times in excess of 40µs have been achieved in switching the first diffraction order from its minimum intensity to its maximum.