Nanosecond Electro-Optics of Liquid Crystals

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



















Authors: 
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 n̂, 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 n̂. 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).

References: 
[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.

A Current-Driven Single-Atom Memory

Top row: (left to right): Christian Schirm, Manuel Matt, Fabian Pauly. Bottom row (left to right): Elke Scheer, Juan Carlos Cuevas, Peter Nielaba.

Authors: Christian Schirm¹, Manuel Matt¹, Fabian Pauly¹, Juan Carlos Cuevas², Peter Nielaba¹, Elke Scheer¹

Affiliation:
¹Department of Physics, University of Konstanz, Germany.
²Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, Spain.

Building functional devices at the atomic scale is a central vision of nanotechnology. Recently we demonstrated a two-state electrical switch with basically a single atom acting as moving part [1]. The switch consists of an aluminum atomic contact that is connected to electrodes at each side. Applying positive or negative currents above a certain threshold will change the state. Lower currents can be used to read out the switching status by simply measuring the resistance. This qualifies the device for future potential applications in high-density solid-state memories.

The single-atom switch was analyzed in close collaboration between experimental and theoretical physicists. Quantum mechanical transport channels of the electrical conductance served as an important link between experiment and theory. (The conductance is the inverse of the electrical resistance.) On the one hand these channels could be determined experimentally by measuring non-linear current-voltage characteristics in the superconducting state at very low temperatures [2]. On the other hand the channels could be calculated through a combination of molecular dynamics simulations, electronic structure theory, and transport theory expressed in terms of non-equilibrium Green's functions [4]. The theory enabled us to identify atomic configurations for both states of the experimentally realized switches. The comparison showed that in some cases the observed switching effect can be attributed to the geometrical rearrangement of a single atom.

Fig 1: Aluminum bridge with a 100 nm constriction, fabricated by electron beam lithography (see left panel). Due to the use of an elastic substrate, the aluminum structure can be stretched by carefully bending the sample. Since we measure the conductance continuously, we can stop the breaking procedure at a constriction narrowed to a single atom (see sketch in the right panel).

In the experiment we created an atomic contact of aluminum by the mechanically controllable break-junction technique using a lithographically fabricated bridge with a 100 nm wide constriction [3]. Fig. 1 shows a scanning electron micrograph (in false colors) of such an aluminum sample on an elastic substrate and illustrates the breaking process to reach an atomic-sized contact. Next, as shown in the graph in Fig. 2, we applied a slowly increasing electrical current and measured the conductance simultaneously. At some threshold current an electromigration process takes place, which can be observed as a jump in the conductance. The current is now decreased until the next jump is detected, then increased and so on. After some rearrangements the system may reversibly switch between two conductance states. A comparison of the conduction channel signatures verifies that these states remain exactly the same.

Fig 2: Applying a careful electromigration protocol when an atom-size contact has formed, a two-state atomic switch may develop. For this the current through the contact is increased slowly with time. When a jump in the conductance is detected, the current is reversed. After some time the system may switch reversibly between two conductance states. The inset shows the rectangular conductance-current hysteresis of a successfully generated two-level switch. The unit of conductance is G₀=2e²/h, also called the “quantum of conductance”.

In molecular dynamics simulations we performed a stretching procedure similar to the preparation of the contact in the experiment. The simulations generate realistic contact geometries and transport data as we verified by comparing experimental and theoretical conductance histograms [4,5]. Fig. 3 shows two atomic geometries as obtained in a stretching process. The calculated total conductance and individual conductance channels were in good agreement with several experimentally realized switches.

Fig 3: Molecular dynamics simulation of geometries corresponding to conductance values and channel signatures found in the experiment. The primary difference between both geometries is a relocation of the central atom and the breaking of two bonds to move from state “1” to state “0” (symbolized by the red scissors). The conductance and the individual channels are given in units of G₀.

The project demonstrates how theory and experiment can work hand in hand to advance nanotechnology. The atomic switch fulfills several technological requirements. For instance, it is relatively easy to fabricate due to the two-terminal configuration. Our work can be considered as a feasibility study of a one-bit atomic-size memory [6]. Potentially, an array of such switches can be implemented in a cross-bar architecture of solid-state memories when further technological issues, such as the stable operation at room temperature, can be solved.

References:
[1] C. Schirm, M. Matt, F. Pauly, J. C. Cuevas, P. Nielaba, E. Scheer, "A current-driven single-atom memory", Nature Nanotechnology 8, 645–648 (2013). Abstract. 
[2] E. Scheer, P. Joyez, D. Esteve, C. Urbina, M. H. Devoret, “Conduction channel transmissions of atomic-size aluminum contacts”, Physical Review Letters, 78, 3535–3538 (1997). Abstract. 
[3] J. M. van Ruitenbeek, A. Alvarez, I. Piñeyro, C. Grahmann, P. Joyez, M. H. Devoret, D. Esteve, C. Urbina, "Adjustable nanofabricated atomic size contacts", Review of Scientific Instruments, 67, 108 (1996). Abstract. 
[4] F. Pauly, M. Dreher, J. K. Viljas, M. Häfner, J. C. Cuevas, P. Nielaba, "Theoretical analysis of the conductance histograms and structural properties of Ag, Pt, and Ni nanocontacts", Physical Review B, 74, 235106 (2006). Abstract. 
[5] See supplementary information in reference [1].
[6] Sense Jan van der Molen, "Single-atom switches: Toggled with electrical current", Nature Nanotechnology, 8, 622 (2013). Abstract. 

Physics Nobel Prize 2013: Higgs Boson

Peter Higgs (left) and François Englert (right)

The Nobel Prize in Physics 2013 was awarded jointly to François Englert and Peter W. Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider"

In 1964, they proposed the theory independently of each other (Englert together with his now deceased colleague Robert Brout) [1,2]. In 2012, their ideas were confirmed by the discovery of a so called Higgs particle at the CERN laboratory outside Geneva in Switzerland.

François Englert, Belgian citizen. Born 1932 in Etterbeek, Belgium. Ph.D. 1959 from Université Libre de Bruxelles, Brussels, Belgium. Professor Emeritus at Université Libre de Bruxelles, Brussels, Belgium.

Link to Prof. Englert's homepage >>

Peter W. Higgs, UK citizen. Born 1929 in Newcastle upon Tyne, UK. Ph.D. 1954 from King’s College, University of London, UK. Professor emeritus at University of Edinburgh, UK.

Link to Prof. Higg's homepage >>

The awarded theory is a central part of the Standard Model of particle physics that describes how the world is constructed. According to the Standard Model, everything, from flowers and people to stars and planets, consists of just a few building blocks: matter particles. These particles are governed by forces mediated by force particles that make sure everything works as it should.

The entire Standard Model also rests on the existence of a special kind of particle: the Higgs particle. This particle originates from an invisible field that fills up all space. Even when the universe seems empty this field is there. Without it, we would not exist, because it is from contact with the field that particles acquire mass. The theory proposed by Englert and Higgs describes this process.

On 4 July 2012, at the CERN laboratory for particle physics, the theory was confirmed by the discovery of a Higgs particle. CERN’s particle collider, LHC (Large Hadron Collider), is probably the largest and the most complex machine ever constructed by humans. Two research groups of some 3,000 scientists each, ATLAS and CMS, managed to extract the Higgs particle from billions of particle collisions in the LHC [3,4].

Even though it is a great achievement to have found the Higgs particle — the missing piece in the Standard Model puzzle — the Standard Model is not the final piece in the cosmic puzzle. One of the reasons for this is that the Standard Model treats certain particles, neutrinos, as being virtually massless, whereas recent studies show that they actually do have mass. Another reason is that the model only describes visible matter, which only accounts for one fifth of all matter in the cosmos. To find the mysterious dark matter is one of the objectives as scientists continue the chase of unknown particles at CERN.

References:
[1] F. Englert and R. Brout, “Broken symmetry and the mass of gauge vector mesons”. Physical Review Letters, 13, 321 (1964). Abstract. Read full paper in Google Books.
[2] P. W. Higgs, "Broken symmetries, massless particles and gauge fields". Physics Letters, 12, 132-133 (1964). Abstract. 
[3] ATLAS Collaboration, "Observation of a New Particle in the Search for the Standard Model Higgs Boson with the ATLAS Detector at the LHC", Physics Letters B, 716, 1-29 (2012). Full Paper.
[4] CMS Collaboration, "Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC", Physics Letters B, 716, 30-61 (2012). Full paper.