From Event: SPIE Organic Photonics + Electronics, 2018
Azobenzene-based complex dyes (PAAD) have found many applications, in particular in liquid crystal based waveplates and flat lenses [1-2]. We investigated PAAD layers, both as individual thin films and as an alignment layer in liquid crystal waveplates, for modulating and steering laser beams.
Thin (15-35 nm), stand-alone films of PAAD, illuminated by an interference fringe pattern, show considerable photo-sensitivity, in spite of limited thickness. We exploited this effect to record polarization patterns and observed considerable diffraction efficiency, with gratings formed through the refractive index change rather than surface relief. Diffraction efficiency depended strongly on the relative polarization of the pump versus the probe beam. The ratio between diffraction efficiencies in the two polarization states reached two orders of magnitude. These measurements were used to carry out preliminary modelling to investigate grating formation and optical axis distribution in the PAAD layer, thus opening the possibility of using optical measurements to study the PAAD thin film molecular dynamics.
We also developed an optically controlled, half-wave plate based on a twisted nematic liquid crystal cell with a single PAAD alignment layer. The cell was bistable, with the two states controlled by one-step illumination of with visible light. The photo-alignment properties of the layer led to reversible switching between two perpendicular alignment states at the cell surface, resulting in controllable polarization manipulation. Reproducible modulation of the transmitted polarization with an optical contrast of up to 90-100% was achieved for different wavelengths of the probe beam, from visible to near infrared.
Malgosia Kaczmarek, Eleni Perivolari, Jordan Gill, Nina Podoliak, Elena Mavrona, Sakellaris Mailis, Vasilis Apostolopoulos, Giampaolo D'Alessandro, and Nelson Tabiryan, "Bistable and photo-switchable thin films for efficient visible and near-infrared light manipulation (Conference Presentation)," Proc. SPIE 10735, Liquid Crystals XXII, 107350D (Presented at SPIE Organic Photonics + Electronics: August 19, 2018; Published: 18 September 2018); https://doi.org/10.1117/12.2320491.5836581505001.
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