Analysis of nematic-liquid-crystal binary gratings with high spatial frequency

The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. Director profiles of nematic-liquid-crystal phase gratings are simulated for different orientations of the liquid-crystal director field with respect to the electrode grating. With increasing spatial frequency of the grating the fringing electric field becomes important and the diffraction pattern changes. The optical properties are analyzed and measurements are performed.

Silicon oxide deposition for enhanced optical switching in polydimethylsiloxane-liquid crystal hybrids

Julien Cuennet, Luciano De Sio, Demetri Psaltis, Andreas Vasdekis

We report an optical switch based on a diffraction grating by combining PDMS microstructures with a photo-responsive Nematic Liquid Crystal (NLC). The grating was realized via replica molding and was subsequently coated with a thin SiO layer. SiO induced a full planar alignment of the liquid crystal. The induced parallel alignment of the LC reduces the response time of the structure by approximately an order of magnitude compared to the same structures without SiO. We explored the effect of the pump intensity on the transmission properties and time response of the switch and identified a strong dependence on the probe polarization, due to the full planar alignment in this structure. The aforementioned inclusion of the SiO layer enables enhanced performance of optical devices based on the fusion of nematogens with soft and flexible substrates. (C) 2011 Optical Society of America

2011

Liquid Crystal Alignment Chemical Nanopatterns : Control over Azimuthal and Polar Alignment

Toralf Scharf

We discuss alignment properties of nematic liquid crystals on surfaces that contain homeotropic and planar alignment areas on the same surface substrate. The basic idea is to locally pattern homeotropic agents (silane) on surfaces that usually lead to planar alignment (SiO2). The patterns used are linear gratings with periods smaller than 1 μm, which were produced via nanoimprint lithography on glass substrates. As the main parameters the period of the gratings and the coverage ratio between silane and SiO2 areas were varied. It is shown that simultaneous control of both polar and azimuthal orientation of liquid crystal is possible using the chemical nanopatterns as alignment layer. While the liquid crystal azimuthal angle orients along the direction of the silane lines, the polar orientation depends on the coverage ratio of the homeotropic/planar surface potential areas. Simulations for different configurations are used to explain experimental findings.

2005

Advanced passive micro-optical elements

Micro-optical elements are optical elements having size or details in the range of the micrometer scale. The emergency of such elements is strongly linked to the development made in the fabrication techniques during the nineties to follow the still actual trend of miniaturization. From this time it has been convenient to separate micro-optical element having refractive or reflective optical design from those ones having strictly diffractive design. In the present thesis we are in between this two fields and from conventional refractive optical elements, different features of physical optics like diffraction, polarization or interference effects are added. We start by adding the birefringent property of liquid crystal polymers (LCP) to microlenses. The interest to employ LCP is linked to the fact that bulky birefringent structures of various shapes can easily be obtained by embossing. Moreover the choice of the optical axis of this material can be chosen by boundary conditions imposed by the different surfaces embedding the liquid crystal. Using this material a micro-optical device allowing the increase of the efficiency of conventional polarizer has been designed, simulated and realized. We continue by studying the influence of the aperture size on the position of the peak irradiance of microlenses. This diffractive effect of the aperture limitation is known as the focal shift which tends to decrease the real focal length obtained from paraxial geometric optics. In the frame of this thesis, we pursue the investigation and show that this effect could be advantageously used to obtain achromatic microlenses. In the frame of beam shaping applications we realized diffusers with very narrow angular distribution. These diffusers allow high energy throughputs and homogeneity distribution for highly coherent sources employed in conventional Fly's eye condensers. The consequences of coherent sources used in such beam shaper are discused in detail and rules are given to employ our diffusers in such systems. To finish, we developed and characterized a new method to fabricate concave microlenses with diffraction limited property and applied them in the realization of optical diffusers.

Institut de Microtechnique, Université de Neuchâtel2008

Advanced passive micro-optical elements

Institut de Microtechnique, Université de Neuchâtel2008