## 3 Experiments and Results
### Evaluation of Novel Fast Tunable Interference Color Filter
Each resonance peak is split into two mutually perpendicular linearly-polarized peaks; one polarized along the long and the other polarized along the short molecular axis of the NLC. The distance between the two peaks is determined by the birefringence of the NLC-alignment layer  
Figure 14: Overview of the basic configuration of the nematic liquid crystal (NLC) Fabry–Perot color filter and the principle of the switching in an electric field E.  
stack. If a voltage above \(U_{\text{th}}\) is applied in the direction of the layer normal, a reorientation of the NLC into a homeotropic alignment takes place (Figure 14). This results in the disappearance of the peak splitting. For thin cavities of the Fabry-Perot filter, the distributed reflection of the dielectric mirrors has to be taken into account to calculate the positions of the resonance peaks [30]. This effect leads to a reduction of the peak distances with decreasing cavity thickness and, thus, to a reduction of the shifting range. As a compromise of an optimized shifting range and a sufficient FSR in the visible range, the 2\(\lambda\) resonance peak was selected to operate the color filters. This corresponds to an optical path length of 800-1600 nm. [31] Taking the refractive index of the NLC into account being typically in the range of about 1.5-1.75, cavity thicknesses of about 0.5-1 \(\upmu\)m are required. For the experimental verification of the electrically-tunable color filter performance, three test devices were constructed, which operate in the red, green or blue regions of the visible range. The specific device parameters are listed in Table 3.  
Spacer thickness and the dielectric mirror of each device were adapted to the desired spectral operating range. Patterned sub-micrometer-thick spacers with a specific thickness were manufactured by a coating process in which SiO\({}_{2}\) was sputtered through a shadow mask on top of the respective substrate. The spacer thickness thus could be controlled with an accuracy of 1 nm. The optical properties of the devices were characterized by Vis-spectroscopic measurements. As an example, the spectra of Device I in the off-state and on-state are displayed in Figure 15. In the off-state, a split of the 2\(\lambda\) resonance peak into two equally-sized peaks is observed. Both peaks are mutually perpendicular linearly-polarized as displayed when inserting a linear polarizer into the path of the beam of the spectrometer. By rotating this polarizer, one or the other peak can be completely eliminated while the other reaches maximum intensity. This result indicates a homogeneously planar orientation of the NLC. One peak occurs at \(\lambda_{\text{max}}=596\) nm, corresponding to the refractive index along the short molecular axis of the NLC (\(n_{\text{o}}\)), and the other at \(\lambda_{\text{max}}=627\) nm, corresponding to the refractive index along the long molecular axis of the NLC (\(n_{\text{e}}\)). Applying a DC voltage of 30 V between the ITO layers leads to a switching into the on-state and, thus, to the disappearing of the resonance peak splitting. The peak at \(\lambda_{\text{max}}=596\) nm  
Figure 15: Transmission of Device I in the off-state (\(\longrightarrow\)) and in the on-state (... ) measured with non-polarized (**a**) and with linear polarized (**b**) incident light. \begin{table}
\begin{tabular}{c c c c} \hline
**Device** & **Mirror Design (1)** & **Mirror Materials SiO\({}_{2}\)/Ta\({}_{2}\)O\({}_{5}\)** & **Electrode ITO (Indium Tin Oxide)** & **Spacer SiO\({}_{2}\)** \\ \hline I & (HL)4 H & 106 nm/73 nm & 40 nm & 817 nm \\ II & (HL)4 H & 96 nm/64 nm & 40 nm & 753 nm \\ III & (HL)4 H & 76 nm/51 nm & 40 nm & 583 nm \\ \hline \end{tabular}
\end{table} 
Table 3: Cell parameters of the devices: mirror design and layer thicknesses. remains unaffected by the electric field, while at the same time, the peak at \(\lambda_{\text{max}}=627\) nm shifts to \(\lambda_{\text{max}}=597\) nm, where both peaks overlap. These findings indicate a homeotropic alignment of the NLC.