# Fast Ferroelectric Liquid Crystal Based Optical Switch: Simulation and Experiments
## 4 Results and Discussions  
### Electro-Optic Measurements  
The KCFLC 10R material under study exhibits an optimal molecular tilt angle of 22.5\({}^{\circ}\) at room temperature (Figure 6a). The molecular tilt angle as a function of temperature (Figure 6a) is measured in a 6 \(\upmu\)m cell using the application of an external field. While cooling down from isotropic temperature, \(\theta\) exhibits optimal angle at room temperature. The phase transition to the ferroelectric phase \(\theta\) is considered as the primary order parameter, and it is the angle between the smectic layer and the principal director \(n\). The switching time of FLC material under investigation has been characterized as a function of an applied electric field, shown in Figure 6b. Fast response time is one of the important peculiarities of FLCs together with bi-stability when compared to conventional nematic LC switches. At room temperature, the 6 \(\upmu\)m cell exhibits much faster switching time (<10 \(\upmu\)s for 4 V/\(\upmu\)m) and it shows an increasing trend of response time with an increasing electric field. LC shutters are limited by the switching time to rotate the LC director under the application of electric field and the recovery time after the field is removed. While the recovery time is entirely dependent upon the properties of LC, switching time also depends on the applied field. For optical systems, as with flat panel displays, both switching and recovery times are important, but for recording one time images only the switching time is more important than recovery time. The contrast ratio (K) as a function of temperature in crossed and parallel polarized-state measured at room temperature is depicted in Figure 7. The measurements were performed by KCFLC 10R material with a cone angle of 45\({}^{\circ}\) to obtain the maximum contrast. Figure 6: (a) Temperature-dependent molecular tilt angle (\(\theta\)) and (b) the electric field-dependent response time (\(\tau\)) for the KCFLC 10R sample in a planar aligned cell with cell thickness of 6 \(\upmu\)m. The measurement was performed at 24 \({}^{\circ}\)C.  
Experimentally, it is convenient to express the cross-talk in terms of measured K and the results in Figure 7 shows maximum contrast is achieved with 6 \(\upmu\)m cell for both crossed and parallel polarized states. The calculated cross-talk in both polarized states is 19 dB. The simulated results were confirmed by experiments with a FLC binary switch. When compared to all other optical switches, LC-based switches exhibit slightly excessive cross-talk because of the cross-talk sources, two of which are from FLCs and two from beam splitters. This is evident in the schematic shown in Figure 8.  
Here, \(A\)p and \(A\)s are the cross-talk sources of passed and shifted signals by LBS and \(A1\) (due to the depolarization of the light beam passed through the switched FLC state) and \(A0\) (which shows how closely the light rotates at 90\({}^{\circ}\) while passing through the FLC material) are from the FLCs. The measured cross-talk of the FLC binary switch is presented in Table 1.  
Figure 8: Different cross talk sources: two are from FLCs and two from lateral beam splitters (LBSs). Figure 7: Temperature dependent contrast ratio of SmC* phase (K) in (**a**) crossed and (**b**) parallel polarized states. Measurements are carried out by the application of 40 V in planar-aligned cells filled with KCFLC 10R material, in a cooling process. It can clearly be seen in Table 1 that the main cross-talk in the binary FLC switching stage under study is from the beam splitter (\(A\)p). In ideal conditions, the vertically polarized light must completely pass straightforward through the LBS (see Figure 4), however practically some part of the beam is reflected from the air split and is shifted by the beam splitter (\(A\)s), as can be seen in Figure 8. Similarly, some part of the horizontally polarized light (\(A\)p) passes straight forward through the LBS.