## 2 Materials and Methods
This characteristic makes the green light particularly relevant for applications where minimizing such disruptions is crucial, including military and aviation industries where pilot safety is paramount. Furthermore, the peak sensitivity of CCD cameras in the green region of the spectrum makes them susceptible to potential damage or interference from green lasers, which our optical limiter aims to prevent. By demonstrating the effectiveness of our device under green light conditions, we emphasize its potential for protecting sensitive optical components in diverse operational circumstances. However, due to the high transparency of NLC over a wide spectrum range, the results presented here can easily be scaled to other wavelengths. The research study was conducted in several stages to investigate the underlying mechanisms of optical power limiting with nematic liquid crystal (NLC) materials. In the first stage, undoped NLC was used to study pure reorientational nonlinearity, which was essential in achieving power limitation. In the next stage, the NLC was precisely doped with a selected dye that allowed us to exploit both reorientational and thermal nonlinearities. The interplay between these nonlinear effects improved the efficiency and responsiveness of the power-limiting process. Finally, plastic polarizers were used to take advantage of the pure thermal nonlinearity of the NLC material, which played a crucial role in optimizing the device's performance for applications requiring a robust response to varying optical intensities. Each phase of the study built upon the previous findings, providing a comprehensive understanding of the dynamic interactions within the NLC-based optical limiter. The nonlinear transmission experiments were performed in a setup according to the scheme shown in Figure 1c. The TN NLC cell was placed between two crossed polarizers (Glan-Taylor calcite polarizers). The first polarizer, i.e., the polarization direction of the input laser beam, was parallel to the rubbed direction of the incident glass substrate of the TN NLC cell. According to the Mauguin theorem, the transmitted light will be rotated 90\({}^{\circ}\); therefore, the transmission through the analyzer is expected to be at maximum. The reference and transmitted beams were measured simultaneously by two identical detectors. Figure 1e shows experimental results for typical (undoped NLC) TN cells with thicknesses of 12 \(\upmu\)m (circles), 50 \(\upmu\)m (diamonds), and 75 \(\upmu\)m (triangles), for a [-4-(trans-4\({}^{\prime}\)-exylcyclohexyl)]isothiocyanatobenzene (6 CHBT) NLC [48; 49]. Illumination of the optical system with a beam of intensity below the threshold value results in a fixed level of light transmission T, which depends mainly on the specific absorption of the NLC and decreases with the cell thickness. The operation principle of the proposed NLC optical power limiter system is based on reorientational nonlinearity. It is known that for homogenous planar or TN-textured NLC subjected to an external electric field directed perpendicular to the director (or parallel to the TN twist helix axis), there is a threshold voltage below which the internal elasticity of the liquid crystal exceeds the electric forces such that the system remains undeformed from its initial configuration [50]. This effect is known as a Freederick's transition or an optical Freederick's transition when the external field is caused by an intense light beam propagating through an NLC layer [51]. When the threshold power value is exceeded, the reorientation of the molecules begins, simply untwisting the NLC director axis, thus diminishing the birefringence (\(\Delta n\)) experienced by the polarized light and inhibiting the polarization rotation process through the NLC cell, which is now not fully aligned with the optical axis of the analyzer. This causes attenuation in transmitted light and the whole system's transmission to decline as the optical power increases.