| A detailed study of several novel infrared optical limiting and switching devices using nematic liquid crystal films is presented. The limiting and switching characteristics of the devices result from two different nonlinear optical processes, including thermally mediated transverse self-phase modulation and nonlinear interface switching near the total internal reflection state. Both transmission to total internal reflection and total internal reflection to transmission switching are considered. Detailed theoretical models for these devices are developed to aid in device design and optimization. The theoretical models take into account the laser beam parameters (intensity, polarization, wavelength, etc.), the material parameters (absorption, refractive index, etc.) and device geometry (placement of lenses, liquid crystal films, detectors, etc.). The relevant optical and thermal properties of the nematic liquid crystal BDH-E7 have been measured in the 9-11 {dollar}mu m{dollar} spectral region. Many of these parameters were unknown at these wavelengths. The effective value of {dollar}chisp{lcub}(3){rcub}{dollar} was measured using a cw CO{dollar}sb2{dollar} laser ({dollar}lambda{dollar} = 10.6 {dollar}mu m{dollar}) and is on the order of 10{dollar}sp{lcub}-4{rcub}{dollar} esu. The results of a detailed experimental study of transverse self-phase modulation and limiting devices based on this effect are given. The data indicate that the limiting threshold for these devices is on the order of 20 W/{dollar}cmsp2{dollar}. The dynamics of transmission to total internal reflection and total internal reflection to transmission switching were studied using a CO{dollar}sb2{dollar} laser. The experimental results indicate that submicrosecond switching times may be possible. The results of this study show that practical limiting and switching devices for use in IR sensor protection applications can be fabricated using nematic liquid crystal films. |
