## 3 Experiments and Results
### Evaluation of Novel Fast Tunable Interference Color Filter
By switching off the electric field, the NLC returns to the planar orientation, and again, each resonance peak splits into two equally-sized peaks. In summary, the \(2\lambda\) resonance peaks of Device I can be reversibly shifted through an applied electric field by 30 nm within the red region of the visible range. Switching time measurements reveal that the peak shift in both directions can appear in the millisecond regime or even faster. It responds to an applied DC voltage of 30 V for \(t_{\text{on}}=0.11\) ms, and \(t_{\text{off}}=1.1\) ms was obtained. These switching times are much faster than reported for NLC Lyot filter stacks or NLC Fabry-Perot filters with cavity thicknesses of several \(\upmu\)m being in the range of tens to hundreds of milliseconds [32]. Similar electro-optical properties were obtained for Devices II and III. Peak positions (\(\lambda_{\text{max}}\)) of the \(2\lambda\) resonance peaks, as well as the \(t_{\text{on}}\) and \(t_{\text{off}}\) switching times are listed in Table 4.  
As can be seen, the performed variation of the cavity thickness of the devices shifts the positions of their resonance frequencies without significantly affecting their operation range and switching times. Whereas Device I operates in the red region, Devices II and III operate in the green or blue region of the visible range, respectively. Illuminating the devices with a monochromatic backlight whose wavelength matches the device specific peak position of the electrically-tunable \(2\lambda\) resonance peak in the off-state, the devices can operate as a fast switchable barrier filter. This operation mode is displayed in Figure 16. In the off-state, the backlight can pass the devices, and thus, they shine red (Device I), green (Device II) or blue (Device III). Switching the devices from the off-state into the on-state shifts their \(2\lambda\) resonance peaks out of the spectrum of the backlight. Accordingly, the devices appear dark (see the on-state in Figure 16). The same effect could be realized by using a narrowband LED emitting at \(\lambda_{\text{max}}=574\) nm with a full width at half maximum of 11 nm as the backlight for Device II. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Device** & **Switching On-State** & **Switching Off-State** & **Switching Times (\(U=30\) V)** \\ \hline
**No.** & \(\lambda_{\text{max}}\) **of the \(2\lambda\) Resonance Peak** & \(\lambda_{\text{max}}\) **of the \(2\lambda\) Resonance Peaks** & \(t_{\text{on}}\) & \(t_{\text{off}}\) \\ \hline I & \(597\) nm & \(596\) nm & \(627\) nm & \(0.11\) ms & \(1.1\) ms \\ II & \(545\) nm & \(544\) nm & \(576\) nm & \(0.11\) ms & \(1.1\) ms \\ III & \(479\) nm & \(479\) nm & \(501\) nm & \(0.10\) ms & \(1.1\) ms \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measured electro-optical properties of the devices for red, green and blue transmission. Figure 16: Transmission and visual appearance for the devices dedicated to a monochromatic incident light for red (**a**); green (**b**) and blue (**c**). Again, the device operates as a fast switchable barrier filter, and a green to dark switching is observed. Although the anisotropy of the birefringence of the used NLC is rather small (\(\Delta n=0.11\)) compared to high birefringence NLC (\(\Delta n\) up to 0.7) [33; 34], the resulting operation range of about 30 nm is already large enough for the demonstrated barrier filter function. Illuminating the devices with linear polarized light enables switching the wavelength of the transmitted light. This operation mode of Device I is displayed in Figure 15b. In this mode, the polarization direction of the incident light is aligned in the direction of the long molecular axis of the NLC, and thus, only the electrically-switchable transmittance peak appears in the off-state. By applying an electric field, the transmittance peak is shifted by about 30 nm. This shift takes place within the free spectral range of the off-state. The performance of the presented filter design depends to a large extent on the choice of the NLC.