雅惠
Symmetric H state formation in π-cell devices Introduction
The π-cell [1] has long been considered to be a potentially fast-switching nematic liquid crystal (LC) device. Originally, this device was investigated in the V state (Vs), also known as the optically compensated bend mode (OCB). In this mode the devices have faster switching times than conventional LC displays, and were ud as high speed optical shutters. With the advent of flat panel televisions, which require fast-switching electo-optic devices to display high quality video information, there has been renewed interest in π-cells. Recently, an even faster switching mode, known as the symmetric H state (Hs), has been discovered in π-cells. The Hs, however, is a transient state with a short lifetime, and so further development is required to understand the conditions under which it forms and stabili it before a marketable display can be constructed.
Nematics π cells
愚人节快乐用英语怎么说>nickyThe π-cell is a nematic device is a tunable birefringence device. This device us parallel pretilts, and the commonly known states that form
prajnaparamitawithin the device are shown in Figure 1.
pha是什么意思
Figure 1: The commonly known states that form in nematic π-cell devices. Dotted lines indicate nucleation process.
Below the Freedericksz threshold, there is a splayed ground state (if K11 = K33, then the splay is linear from one side of the device to the other). Above the Freedericksz threshold (Vca ≈ 1 Vrms), the director begins to align with the field (for ∆ε > 0), forming the asymmetric H states (Ha). The two Ha states
are of equal energy (they are simply mirror-images of each other), so each state forms in different places within the device (as discusd in Section 1.3.2). When the voltage reaches Vcv, the V state (Vs) becomes the lowest energy state (Vcv ≈ 3.5 Vrms). This state is not topologically similar to the H states, and so the transition from one to the other is via a nucleation process. Areas of the Vs grow from defects in the device (this is a slow process and takes in the order of a few conds). If the voltage is now reduced below Vcv, the 180o twisted state (Ts) forms, but this state is not stable. Over a few conds, the splayed ground state forms via a nucleation process.
ugly是什么意思
Operation of π cells in the V state
Changing the voltage while in the V state allows the birefringence of the device to be tuned (e Fig.
2). This device can be placed at 45 degrees between crosd polarirs, and then be ud in a similar manner to Freedericksz cells. As the voltage is incread, so the birefringence will be reduced, changing the transmission through the device. Switching speeds of around 2.5 ms] can be achieved using Vs switching, when an appropriate retarder is inrted between the π-cell and one of the polarirs (a normal Freedericksz cell switches in around 5 ms). The retarder is require to cancel the residual birefringence of the device in the higher voltage state in order to produce a dark state at a finite voltage, improving the contrast, and also to improve the viewing angle.
Figure 2: Switching between low (left) and high (right) voltage V states. The birefringence is reduced at higher voltages as the director aligns more and more with the applied field.
The major problem when using π-cells in the Vs is that in the gaps between the pixels of thedisplay, there is no applied voltage. Within the ctions, V <Vcv, so and H state is prent. This can start to nucleate into the pixel area when the pixel is in the lower voltage Vs, causing a visible coloured defect. One solution to this problem is to u high values of pretilt. Most LC devices u pretilts of around 1 degrees to 7 degrees. If the pretilt is incread to above around 50 degrees, the Ha threshold becomes lower than the Vs threshold (Vca < Vcv), so the Vs is stable even when there is no applied voltage (the ground state becomes a bend state, rather than a splay state). In this situation, the nucleation problems described above would not be an issue. Incread pretilts, howev
er, slow the switching of the device significantly. At around 30 degrees pretilts, the relaxation time is incread to nearly 5 ms, which is not a significant improvement on a conventional Freedericksz device.
The symmetric H state
Figure 3: The symmetric H state (Hs): V = 0 (left), V >Vcs (center and right). Right: simplification of the Hs as two half-thickness Freedericksz devices.
It was originally believed that when the voltage applied to a π-cell is between Vca and Vcv then the Ha states would always form. However, Towler and Raynes [5] found that a transient high voltage sy
mmetric H state (Hs) is formed under certain conditions. Under puld loadings, the Hs can be obrved. The director configuration of the Hs is shown in Figure 3, and is similar to two halfthickness Freedericksz devices (this is a simplification, since it adds a no-slip boundary condition at the centre of the device). The relaxation times of LC devices are much slower than the switch-on times, and so it is the relaxation time that is the limiting factor. Since the relaxation time of a Freedericksz device, assuming no flow and zero pretilt, is given by
the Hs should relax approximately four times faster than a Freedericksz device, potentially producing sub-millicond switching. Using the same simplified model, we can also draw some conclusions about the threshold voltage of the Hs. From Equation 1.1, the threshold voltage of a Freedericksz device is independent of the thickness. Thus, if a voltage Vca is to be applied across each of the two half-thickness devices, the total applied voltage must be 2 × Vca. This would indicate that the Hs threshold is Vcs = 2Vca.mike pence
In a ideal device at absolute zero with precily balanced pretilts, no thermal fluctuations and with the electric field exactly perpendicular to the glass plates, the Hs would be expected to form (there is
nothing to push the device into one or other of the Ha states). In a real device at real temperatures, however, the Hs is an unstable equilibrium state, and decays into one or other of the Ha states over time.scandal
The work carried out by Brimicombe [6] ud puld loading experiments and static modelling to examine the properties of the Hs and Ha. It was obrved that the transmission through a π-cell device as it responds to a step input from zero applied voltage to a voltage above Vcs consists of two portions (e Figure 4). If the voltage is removed during the plateau, the device relaxes quickly (Hs). If the voltage
is not removed until after the droop in transmission, then the device relaxes much more slowly (Ha). This indicates that during switch-on the Hs forms initially, with the Ha forming after a period of time. The work using puld loadings allowed the calculation of the Hs and Ha threshold voltages (Vcs and Vca), and the results indicated that Vcs ≈ 2Vca, as expected.
中考复习资料
Figure 4: Switch-on of a π-cell device from 0 Vrms to 4.5 Vrms, showing fast Hs relaxation when voltage is removed during the transmission plateau, and slower Ha relaxation when voltage is removed after the transmission droop.
The static modelling by Brimicombe indicates that the energy of the Ha is always lower than that of the Hs. This means that at high voltages the Hs is never the long-term stable, regardless of material or device parameters. The energies can be made clor together by increasing the pretilt, but the Hs
energy is never below that of the Ha, and this will also slow down the relaxation. The advantages to developing a display using the Hs are numerous. Such a display would u the nematic pha, unlike FLCs, which is well-understood and easy to work with. There is no need for a nucleation to produce the required state, and is potentially faster-switching than π-cells ud in the Vs. The device construction is simple, and similar devices have been mass-produced for many years now. There are problems, however, the main one being that the Hs is a transient state and work is required to understand the conditions under which it forms, and to stabili it.
Rearch results and aims
The aim of this project has been firstly to gain an understanding about the formation and properties of the Hs, and then subquently to attempt to stabli the Hs, or at least increa its lifetime. In order to gain a better understanding of the dynamic behaviour of π-cells, a model has been developed to understand the influence of flow in the devices. This modelling has produced further evidence that the fast-switching state recently obrved within π-cells [5] [6] is indeed the Hs. It has been shown
that this state does not suffer from backflow (over-rotation of the director), and that the flow prent
when switching in this state is similar to that prent in the Vs. Investigation into the influence of the viscosity parameters has indicated that only three of the Miesowicz viscosities (η1, η2, and γ1) have any significant effect on the switching. Initial attempts to fit the modelling results to experimental data shows that a very good fit is possible, but the experiments must be repeated using monochromatic light before any conclusions can be drawn from this work. The transition from Hs to Ha is the key process that must be understood before stabilisation of the Hs can be attempted (this transition governs the lifetime of the Hs). Experimental evidence suggested that the lifetime of the Hs is incread by using higher applied voltages, and this was confirmed when modelling the transition. The modelling has also produced evidence that the Hs lifetime can be incread by the u of materials with low values of K33 K11, and this has been corroborated through experimentation. The explanation for the increas of the Hs lifetime is becau both of the techniques introduce a more homeotropic region between the surfaces and the centre of the device. Regions that are more homeotropic have a greater de-coupling effect, and when the surfaces are de-coupled from the centre of the device, there is little to drive the Hs to Ha transition, making it slower.
The likely nature of the transition from Hs to Ha has been discusd, and it has been propod that there are likely to be two process involved: Ha domain formation followed by Ha domain growth. U
sing the model developed here, only domain growth can be simulated (and that only in a simplified manner, since the interaction across the domain wall cannot be included). Regardless of this, however, this modelling of the transition has allowed some lengthening of the Hs lifetime.
This report was created Paul Brimicombe.
prepare的用法
