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2Physics Quote:
"Can photons in vacuum interact? The answer is not, since the vacuum is a linear medium where electromagnetic excitations and waves simply sum up, crossing themselves with no interaction. There exist a plenty of nonlinear media where the propagation features depend on the concentration of the waves or particles themselves. For example travelling photons in a nonlinear optical medium modify their structures during the propagation, attracting or repelling each other depending on the focusing or defocusing properties of the medium, and giving rise to self-sustained preserving profiles such as space and time solitons or rapidly rising fronts such as shock waves." -- Lorenzo Dominici, Mikhail Petrov, Michal Matuszewski, Dario Ballarini, Milena De Giorgi, David Colas, Emiliano Cancellieri, Blanca Silva Fernández, Alberto Bramati, Giuseppe Gigli, Alexei Kavokin, Fabrice Laussy, Daniele Sanvitto. (Read Full Article: "The Real-Space Collapse of a Two Dimensional Polariton Gas" )

Sunday, October 27, 2013

Nanosecond Electro-Optics of Liquid Crystals

Volodymyr Borshch (top) , 
Sergij V. Shiyanovskii 
(bottom left), 
Oleg D. Lavrentovich 
(bottom right)

Volodymyr Borshch, Sergij V. Shiyanovskii, and Oleg D. Lavrentovich

Affiliation: Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent State University, USA

Nematic Liquid Crystals (NLC) brought a revolution in the way we present information nowadays, enabling an entire industry of portable displays. The principle of operation is simple and is based on the anisotropic character of molecular ordering in NLC. A typical nematic is formed by elongated rod-like molecules that tend to align parallel to each other. There is no long-range positional order so that the molecules can glide with respect to each other. The direction of predominant molecular orientation is called the director , which is also the optic axis of the material. A uniformly aligned nematic is optically similar to a uniaxial birefringent monocrystal. Unlike the case of a solid crystal, however, the optic axis of the nematic can be easily realigned, say, by the electric field, thanks to the material’s dielectric anisotropy.

Reorientation of molecular orientation caused by external electromagnetic fields has been discovered in late 1920-ies by Vsevolod Frederiks [1] who worked at the National Physical and Technical Laboratories in Leningrad, Soviet Union. It is precisely the Frederiks effect that is used in display applications, Fig.1. The cell is placed between two crossed linear polarizers and is addressed by the electric field that reorients the optic axis. The scheme is chosen in such a way that the field on and field off states differ in light transmittance through the nematic slab and the two polarizers.
Fig. 1. Frederiks effect in a nematic with a negative dielectric anisotropy: Reorientation of the director by an electric field (a) homeotropic alignment, with the director (represented by black lines) initially oriented normal to substrates; (b) Electric field reorients away from the vertical direction .

For example, in the popular approach pursued by companies such as Samsung Displays, Inc., in the off state the optic axis is parallel to propagation of light, and in the on state the optic axis is tilted along the direction between the polarizer and analyzer. The nematic in this scheme has a “negative” dielectric anisotropy, which means that the optic axis prefers to align perpendicularly to the applied field, Fig.1. The off state is “black” while the on state is “bright”. The remaining element of a modern display is just a panel of color filters; the nematic pixels serve as shutters that decide which color would become bright and which would be dimmed in the pattern.

The principal weakness of the nematic-based displays is their relatively slow relaxation into the ground state when the field is switched off. The ground state typically represents a monocrystalline slab, orientation of which is helped by a special treatment of the glass plates that confine the liquid crystal. When the field is switched off, it is the relatively weak elastic forces that are responsible for relaxation of the director in the entire bulk. As a result, the process is slow, taking about 5 milliseconds for a 5 micron thick cell. Although this response time is sufficient for many display applications, it is not enough for newer technologies, such as 3D television.

In our recent publication in Physical Review Letters [2], we explored a new way of switching the nematic liquid crystal, in which the direction of the optic axis remains intact. In the experiment, we used a commercially available nematic material with negative dielectric anisotropy. In contrast to the standard display mode shown in Fig.1, the director is aligned in a planar fashion, Fig.2, and remains in this state regardless of whether the field is on or off.
Figure 2. Electrically modified order parameter effect. (a) Nematic with a negative dielectric anisotropy is aligned parallel to the bounding plates. The ellipsoid of revolution with axes parallel to the director represents the refractive indices of the material. (b) The applied electric field modifies the order parameter and induces uniaxial and biaxial changes to the tensor of refractive indices, shown by red segments. When viewed between two crossed polarizers, the material shows an electrically-induced change in birefringence.

Although the applied electric field does not realign the optic axis, it modifies the refractive indices of the slab, by modifying the order parameter of the liquid crystal. The order parameter is a measure of how well the molecules are aligned along the director . The applied field causes not only the changes in the birefringence of the originally uniaxial slab, but also induces a biaxial character of the molecular order. Since the changes occur at the microscopic level, their dynamics is much faster that the collective reorientation of molecules in the Frederiks effect. As our experiments demonstrated, a typical nematic cell can be switched within tens of nanoseconds, Fig.3.
Figure 3. Experimental demonstration of nanosecond switching in electrically modified order parameter effect for both the field-on and field-off driving. -(a) Optical response (blue squares) is modulated by repeated voltage pulses (red circles) with nanosecond rise and fall fronts. (b) The optical response (grey dots) induced by voltage pulse (red) fits well with the theoretical model (black solid line) [2]. The blue dashed line shows the biaxial component of effect, see Ref. [2] for more details.
Figure 4. Experimental set up.

This work is only the first step in nanosecond switching of liquid crystals. Our original experiments were conducted with a commercially available nematic that has a modest dielectric anisotropy at optical frequencies (birefringence) in its natural field-free state Δn = 0.03. As a result, the driving electric voltage was very high (hundreds of volts) and the changes in birefringence rather modest, around 0.001. These deficiencies, however, appear to be minor if considered in the context of the principal advantage of the new approach, which is the nanosecond response time to both field on and field off switching. The issue of the large driving voltage and small birefringence changes can be addressed by designing new nematic materials and optic schemes. As in any technology development, the effect needs to be explored at deeper levels with a variety of materials so that the effect can produce an optimum performance. Such a work is in progress, and is being currently supported by our research grants from the Department of Energy (grant DEFG02-06ER46331) and National Science Foundation (grant DMR 1104859).

[1] A. Repiova and V. Frederiks, “On the Nature of the anisotropic liquid state of mater”, Journal of Russian Physico-Chemical Society, 59, 183-20 (1927); V. Frederiks and V. Tsvetkov, Soviet Physics, 6, 490 (1934); V. Fréedericksz and V. Zolina, “Forces causing the Orientation of an Anisotropic Liquid”, Transactions of the Faraday Society, 29, 919-930 (1933). Abstract.
[2] Volodymyr Borshch, Sergij V. Shiyanovskii, and Oleg D. Lavrentovich, “Nanosecond Electro-Optic Switching of a Liquid Crystal”, Physical Review Letters, 111, 107802 (2013). Abstract.



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