Tunable filter using birefringent plasmonic structures and liquid crystals
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An electrically tunable filter based on a plasmonic phase retarder and liquid crystal cells is reported. The plasmonic phase retarder consists of a periodic array of deep-subwavelength metallic nanostructures. A first entrance polarizer prepares the incident light in a polarization state oriented at 45° from the nanowires orientation. A strong phase retardation between TM and TE polarizations is induced by the plasmon resonances. A polarization analyzer based on liquid crystal cells allows to project the transmitted light onto a polarization state whose orientation depends on the applied voltage. Using this approach, a range of 8V is enough to span more than 50% of the area covered by standard RGB filters in CIE color coordinates with a single filter. In order to ensure the applicability to large area production, UV nanoimprint lithography (UV-NIL) and thin film coatings have been used to fabricate the plasmonic phase retarder. The nanoimprint master consists in a periodic binary grating with a sub-wavelength period below 200nm in order to avoid diffraction effects in the visible range and maximize the angular stability. The grating master is imprinted and coated with a silver thin film and encapsulated. The evaporation is performed with an angle, so that a self-shadowing effects prevents full coverage of the surface. The resulting structure consists in a periodic array of silver nanowires of total width 50nm, with a cross section forming an inverted U-shape. This particular shape shows a high degree of tunability of the plasmon resonance position given the constraints of a sub-wavelength periodicity. Multiple interfering resonances are observed so that the nominal transmission can reach >70%. Placed between a polarizer oriented at 45° from the nanowires orientation and a liquid crystal cell, the transmission spectrum of the plasmonic phase retarder can be tuned with the applied voltage. For a low voltage, the polarization transmitted through the liquid crystal cell is oriented along the gratings lines. For higher voltage, the light transmitted through the liquid crystal cell is oriented across the grating lines and the resulting spectrum has a dip in transmission, which is the signature of a plasmon resonance. At a voltage of 8V, a full rotation of the polarization by 180° has been applied. Different colors can be obtained within this range, including orange, magenta, purple, blue, turquoise, green and yellow with the same tunable filter. Other designs have been investigated in order to obtain more saturated blue, green or red using this approach.
SPIE digital optical technologies, June 2019.