Mid-wavelength IR (MWIR) polarizers from glassy cholesteric liquid crystals

The investigation of glass-forming liquid crystalline materials for mid wave infrared polarization applications is driven by their low melt viscosity and ability to vitrify order, and thus functionality, into films with a wide range of thicknesses. Commercially available polarizers that function in the mid-wave infrared region suffer from poor polarization contrast, high cost, and limited size. In this work, we explore the feasibility of using cyclic siloxane-based liquid crystalline materials with chiral mesogens to form circular polarizers in the midwave infrared spectrum (3-5 mu m) Specifically, we have designed a cholesteric molecular blend to possess the proper helical twisting power to exhibit a selective reflection notch in the 4.0-4.5 mu m region. We have fabricated circular polarizers using shear under a number of processing conditions, and explored their performance as measured by polarization contrast. Processed films with reflection notches at the proper wavelength and near theoretical reflection performance have been prepared. A proper balance between film thickness and cooling rate has led to the demonstration of optically transparent and uniform films on 1 inch diameter substrates. The use of simple alignment layers was demonstrated to yield consistent formation of Grandjean monodomains, by reducing the tendency of the large pitch blends to spontaneously form a fingerprint molecular orientation observed in cells with untreated surfaces. The measured polarization contrast of > 70:1 exceeds the values obtained from state-of-the art commercial polarizers in this wavelength region.