Transparent conductors for Mid-infrared liquid crystal spatial light modulators

• Published in: Thin solid films Vol. 660; pp. 411 - 420
• Main Authors: Micallef, Fabian G., Shrestha, Pawan K., Chu, Daping, McEwan, Kenneth, Rughoobur, Girish, Carey, Tian, Coburn, Nigel, Torrisi, Felice, Txoperena, Oihana, Zurutuza, Amaia
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30.08.2018
• Subjects: Copper; CVD graphene; Mid-infrared; Nematic liquid crystals; Nickel; Spatial light modulation; Transparent conductors; Ultra-thin metallic films; Spatial light modulation; Nematic liquid crystals; Mid-infrared; Ultra-thin metallic films; Nickel; Transparent conductors; Copper; CVD graphene
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2018.05.037

Transparent conductors (TCs) are required for liquid crystal spatial light modulators (LC-SLMs) in order to set up an electric field across the LC layer. In the middle infrared (Mid-IR) range (λ = 2 to 5 μm), LC-SLMs can offer 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 spectrum 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 fulfils the required properties in the Mid-IR: A sheet resistance allowing operation at typical frequencies (≈1 kHz) 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 films (UTMFs), CuxO thin films (TFs), and chemical vapour deposition (CVD) grown mono-layer graphene were investigated. Ni UTMFs and graphene were found to have the best performance with sheet resistance values of 747±86Ω/□ and 360±34Ω/□ respectively for samples having a transmittance of 65% and 97% at λ = 2.3 μm. 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 μm resolution, limited by the mask, using a standard photo-lithographic lift-off process. Transmissive LC cells with a maximum phase shift of 3π at λ = 2.3 μm 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. •Ni, Cu and CuxO thin films are compared as transparent conductors in the Mid-IR.•Key optical and electrical properties are modelled based on thin film morphology.•Mono-layer CVD graphene is proposed as a better transparent conductor in the Mid-IR.•Graphene alignment is achieved without a dedicated polyimide alignment layer.•Graphene LC cell has a higher contrast ratio and efficiency than Ni UTMF cell.