Midinfrared Birefringence of Liquid Crystals, Polarimetry, and Intensity Modulators

The Stokes parameters of transmitted light and the birefringence were measured for several liquid crystal mixtures in the midinfrared (MIR) spectral range (≈1900–5500 cm–1). A MIR intensity modulator has been fabricated and characterized. Germanium substrates show both sufficient conductivity and transparency in the MIR range to serve as transparent electrodes. The modulator makes use of the well-known principle of a twisted nematic cell, which rotates the plane of polarization of a transmitted electromagnetic wave in the field-off state and does not change the state of polarization at sufficiently high voltage. For this application, alignment layers made of poly(vinyl alcohol) and wire grid polarizers on a substrate made of 1,1,2,2-tetrafluorethen (Teflon) are used. The modulator shows high contrast ratios.