Plasmonic nanorods as ultra-fast switching colour pixels
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.
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.
“Upon illumination, each individual plasmonic NR can support collective electron oscillations along its transverse and longitudinal dimensions. These localized surface plasmon resonances result in enhanced extinction at certain wavelengths of light” explains the paper.
Light polarized along the long axis of a NR induces excitation of the long-wavelength LSP mode, while orienting the short dimension along the polarization direction excites the short-wavelength TSP mode. In a suspension of NRs that are randomly oriented with respect to the incident light, both the LSP and TSP modes of each individual NR are excited. But the relative proportion of LSP and TSP response can be controlled by inducing NR alignment, which is how the researchers modulate the ensemble NR extinction spectrum between unaligned and aligned NRs, hence modulating the colour and brightness of the light transmitted through the suspension.
Next, the researchers tested various nanorod chemical compositions and geometries (of different aspect ratios) to establish a library of tunable colours. They also fabricated a plasmonic NR seven-segment to demonstrate the use of NR-based plasmonic pixels in a practical application. Another advantage of plasmonic nanorods over other metasurfaces, is their ease of preparation through colloidal synthesis, bringing costs down. The authors also point out that in contrast to conventional liquid crystal displays where half of the illumination intensity is lost through polarization, the plasmonic pixels do not require polarized illumination, potentially increasing a display’s overall energy efficiency.
United States Naval Research Laboratory – www.nrl.navy.mil
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