Microwave liquid-crystal variable phase grating

Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 85 (2004) and may be found at http://link.aip.org/link/?APPLAB/85/2041/1A voltage-controlled variable phase grating, at microwave frequencies, is described and its response characterized. It comprises a stack of 71 aluminium strips of 1 mm thickness separated by 75 µm spaces, filled with aligned nematic liquid crystal. For microwaves polarized normal to the grating strips there are a set of resonant transmitted frequencies. By varying the voltages applied across the liquid crystal layers and their distribution, a variable phase microwave grating is realized. This allows low-voltage control of output beam profile and intensity

Exploration of the surface director profile in a liquid crystal cell using coupling between the surface plasmon and half-leaky optical guided modes

Copyright © 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 92 (2008) and may be found at http://link.aip.org/link/?APPLAB/92/151103/1The half-leaky guided mode geometry with a thin metal tunnel barrier as one cladding layer is used to explore the distribution of the director very near to the surface in a hybrid aligning nematic liquid crystal cell. From theoretical analysis together with numerical modeling, it is shown how the coupled p-polarized surface plasmon/s-like guided modes excited in the geometry leads to extremely sensitive to the surface director tilt profile near the metal wall—a sensitivity which is even higher than that of the surface plasmon resonance alone. The experimental results have fully confirmed the model predictions

Delay effect of switch-on in a supertwisted nematic cell

Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 85 (2004) and may be found at http://link.aip.org/link/?APPLAB/85/5070/1By using a convergent beam system and the fully leaky guided mode technique the switch-on dynamics of an 180° supertwisted nematic have been studied. Using the Ericksen–Leslie theory and analyzing the guided mode data taken from the cell, the director structure in the cell at different times during switch-on is obtained. For three different applied voltages it is found that the switch-on time is strongly dependent on the applied field—the higher voltage corresponds to faster switching, with no evidence of backflow. A delay at the beginning of the switch-on process has been found and explored for different applied fields. This leads to a suggestion for increasing the switch-on speed of such devices by 25%

Dynamical process of switch-off in a supertwisted nematic cell

Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 96 (2004) and may be found at http://link.aip.org/link/?JAPIAU/96/310/1The switch-off dynamics of a 180° supertwisted nematic (STN) are explored in detail by using a convergent beam system and the fully leaky guided-mode technique. From the dynamical guided-mode data and using the Ericksen–Leslie theory, the director structure in the cell at different times during switch-off is obtained. We have studied the dynamic switch-off from different applied voltages and have found that the relaxation time is independent of the voltage. From fitting the optical data, the effective viscosities are obtained. It appears that two viscosity coefficients, γ and η1, largely control the dynamics of switch-off. From the analysis of the director tilt profile in the cell, it is found that there is no backflow-induced increased tilt at the cell center in the STN cell during the switch-off. From liquid crystal hydrodynamics, this no-backflow dynamic process in the STN cell is well explained

Voltage dependent director of a homeotropic negative liquid crystal cell

Copyright © 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 93 (2008) and may be found at http://link.aip.org/link/?APPLAB/93/031909/1Thin layers of obliquely (60° to normal) thermally evaporated SiOx lead to homeotropic alignment of a nematic liquid crystal (LC) having negative dielectric anisotropy. Under application of an ac voltage the director, as characterized by the fully leaky waveguide technique, is found to realign with a voltage controlled tilt along the evaporation direction. This behavior is in complete contrast with that of a LC having positive dielectric anisotropy and may have important implications for modern LC display technology

Otto-coupled surface plasmons in a liquid crystal cell

Copyright © 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 95 (2009) and may be found at http://link.aip.org/link/?APPLAB/95/171102/1Surface plasmons on silver in the visible domain are excited using a several micron thick layer of liquid crystal as the optical tunnel barrier. This thickness is possible when the orientation of the director in the liquid crystal varies from homeotropic at the entrance surface (against the coupling prism) to homogeneous on the thick silver layer at the other side of the cell, with the director tilting in a plane normal to the plane of incidence. This geometry also allows the excitation of guided modes, which mixes with the surface plasmon resonance. Both types of mode are then explored as a function of applied voltage

Microwaves: thin metal slits and liquid crystals

Copyright © 2004 Society of Photo-Optical Instrumentation Engineers. This paper was published in Proceedings of SPIE Volume 5618, pp. 1-14 and is made available with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.Using liquid crystals to control the propagation of microwaves is a potentially interesting technology. By incorporating small amounts of liquid crystal in thin slat metal structures through which the microwaves may resonantly pass a whole new range of voltage tuned microwave devices are becoming available. Metallic sub-wavelength slit structures at microwave frequencies have been constructed which show Fabry-Perot type resonances in very thin slits. If the dielectric in such thin slits is an aligned liquid crystal it is found possible to voltage-control the resonant frequencies. Novel selective filters and structures for microwave beam steering have been fabricated leading to a new generation of liquid crystal controlled devices

Resonant Transmission of Microwaves through a Narrow Metallic Slit

Fuzi Yang, and J. Roy Sambles, Physical Review Letters, Vol. 89, article 063901 (2002). "Copyright © 2002 by the American Physical Society."Strong resonant transmission of microwave radiation through a very narrow (much less than the radiation wavelength) metallic slit is recorded. The results show that a set of resonant self-coupled surface plasmons are excited within the small gap, giving a Fabry-Pérot-like behavior in accord with analytical results published earlier [Y. Takakura, Phys. Rev. Lett. 86, 5601 (2001)]. The metallic slit, formed by two thick metal plates spaced apart by tens of microns, is inserted in a wavelength-sized aperture. On resonance the transmissivity through the metal slit is more than 2 orders of magnitude greater than the radiation impinging on the slit area

Coupled surface plasmons and optical guided wave exploration of near-surface director profile

Copyright © 2007 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft. This is the published version of an article published in New Journal of Physics Vol. 9, article 49. DOI: 10.1088/1367-2630/9/3/049For a liquid crystal (LC) cell with thin silver claddings it is possible, using a high index coupling prism, to excite both surface plasmon modes and ordinary optical guided modes. In a situation where the tilt of the director varies from homogeneous to homeotropic through the cell, then for p-polarized incident radiation the p-polarized surface plasmon mode and the ordinary guided waves may couple to each other. When the plane containing the director is normal to the incident plane, there is also polarization conversion leading to strong coupling between the p-polarized surface plasmon and s-like guided modes. From theoretical analyses together with numerical modelling it is shown how this coupling between the surface plasmon mode and guided waves gives a high sensitivity to the surface director tilt profile near the walls, higher than that of the surface plasmon mode alone. Experimental confirmation of this has been realized using a hybrid aligned nematic (HAN) LC cell with the director in a plane normal to the incident plane. The results fully confirm the model predictions showing that this coupling of surface plasmons to guided waves provides a powerful tool for near-surface director studies