Plasmonic nanorods as ultra-fast switching colour pixels

March 27, 2019 //By Julien Happich
Plasmonic nanorods as ultra-fast switching colour pixels
While plasmonic colours are often associated with metasurfaces made of fixed nanostructures, researchers from the United States Naval Research Laboratory have given a boost to plasmonic-based displays by demonstrating dynamic plasmonic pixels, whereby the position of the actual plasmonic nanostructures can be controlled electronically to impact the transmittance and colour of an incident back-light.

Publishing their work in the ACS Nano journal under the title “Dynamic Plasmonic Pixels”, the researchers dispersed gold nanorods (NRs) in organic suspensions, which very much like anisotropic liquid crystal molecules in LCD displays, can be aligned on demand by applying an electric-field to modulate light. But the similarity ends there. The authors explain that NR suspensions offer a switching time at least 3 orders of magnitude smaller than conventional LC devices (for lack of neighbour interactions), about 30μs versus tens of millisecond for LC-based displays.

Schematic depiction of the dynamic plasmonic pixel
when the applied electric field is OFF
(unaligned: 0V/μm) and ON (aligned: 6V/μm).

Also, by tailoring the geometry and composition of the nanorods (gold and silver-coated gold), the researchers were able to demonstrate light modulation across a significant portion of the visible and infrared spectrum (from 630 to 2230nm), which they say could be used to create high-performance optical devices with a fast switching time or fast colour displays where compound pixels would be architected in sub-pixels of different NR types to perform appropriate colour mixing. Placed between two transparent conducting electrodes in a non-polar solvent, the polymer-coated gold nanorods can be aligned along their long axes, parallel to the applied field (ON state), or left randomly dispersed in the OFF state (unaligned).

It is not just the optical anisotropy of the alignment that modulates light, but the combination of long-wavelength longitudinal surface plasmon (LSP) and short-wavelength transverse surface plasmon (TSP) modes, which can be controlled using an electric field.

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