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

Abstract

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 siloxanebased liquid crystalline materials with chiral mesogens to form circular polarizers in the midwave infrared spectrum (3-5mum). Specifically, we have designed a cholesteric molecular blend to possess the proper helical twisting power to exhibit a selective reflection notch in the 4.0-4.5mum 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.