Transparent conductors for Mid-infrared liquid crystal spatial light modulators

Abstract

Transparent conductors (TCs) are required for liquid crystal spatial light modulators(LC-SLMs) in order to set up an electric eld across the LC layer. In the middle infrared (Mid-IR) range ( = 2 to 5 m), LC-SLMs can of- fer a low-cost, non-mechanical, random-access and compact alternative to the gimbaled mirrors used currently for Mid-IR laser beam-steering. Indium tin oxide(ITO) is the industry standard for applications in the visible spec- trum but it performs poorly in the IR, with a transmittance < 20% for Mid-IR wavelengths. Little work has been done to develop a comparable material which ful ls the required properties in the Mid-IR: A sheet resis- tance allowing operation at typical frequencies ( 1kHz) and, if patterned, with minimal voltage drop along the electrode, a transmittance > 50% in the target range, chemical, thermal and mechanical robustness which can endure subsequent processing, and ability to be patterned at low-cost to a resolution comparable to the wavelengths investigated. Ni and Cu ultra thin metallic lms (UTMFs), CuxO thin lms (TFs), and chemical vapour de- position (CVD) grown mono-layer graphene were investigated. Ni UTMFs and graphene were found to have the best performance with sheet resistance values of 747 86 = and 360 34 = respectively for samples having a transmittance of 65% and 97% at = 2:3 m. Both Ni UTMFs and CVD mono-layer graphene were found to be suitably stable with age. An increase of sheet resistance after baking was recorded due to oxidation and desorption of contaminating dopants respectively. Ni UTMFs were found to be patternable down to a 3 m resolution, limited by the mask, using a standard photo-lithographic lift-o process. Transmissive LC cells with a maximum phase shift of 3 at = 2:3 m were assembled with both Ni UTMFs and mono-layer graphene as TCs on sapphire, with the former having transmittance of 18.7% and contrast ratio of 25.0, and the latter having transmittance of 81.1% and contrast ratio of 50.7.