Structural Color vs Pigmentation
(or Chemical vs Structural Color)
All the colors you see originate from two distinct phenomenon: pigmentation and structural color. In pigmentation, molecules absorb part of the visible spectrum while leaving the rest to be scattered back to the viewer. Structural color, on the other hand, is produced through combinations of reflection, scattering and interference due to a structured material. The majority of reflective display technologies are based on absorptive pigments, employing polymer color filters or charged pigmented beads. However, these pigment based color technologies are susceptible to high intensity light exposure, require multilayer processing for individual pixels, and are limited in resolution due to macroscopic pixel size. In response, structural color from engineered plasmonic metasurfaces is a durable and high resolution candidate for future display technologies. A key aspect of these devices is tuning of the optical response with structure dimensions. However, the ability to actively tune (post-fabrication) the optical response with the surrounding media’s index truly sets structural color apart from typical pigmented color..
Tuning Structural Color with Liquid Crystals
Nematic liquid crystals are long rod-shaped molecules that have extraordinary properties, most importantly: long range order and alignment with an electric field. This allows liquid crystals to actively control the color reflected from metallic nanostructures.
Types of Reflective Color Generation
A wide range of reflective displays have recently been developed, and Fig. XX compares several of the most prominent. Much like color generation in animals, reflective displays can be separated into the same two main categories; pigmentation and structural color. Under the tent of pigmentation are products such as e-ink based “E-readers”, and color filter based liquid crystal displays. E-readers use the translocation of 3 charged pigmented beads, and as such, require seconds to switch between images. Due to the macroscopic size of each pixel, resolution and color reproduction are also limited. Reflective liquid crystal displays are much quicker, taking only milliseconds to switch states, but are limited in brightness as polarizers immediately halve the amplitude of the reflected light. Many structural color based displays are currently in development and have only recently entered the market. One such device is an interferometric modulator produced by Qualcomm where within each pixel, a cavity is formed between a Bragg stack and a MEMs mirror. By controlling the cavity spacing, the reflected light experiences either constructive or destructive interference resulting in a bright color or dark state. While producing the signature bright vivid colors of Bragg reflection, the device is inherently angle sensitive and limited to rigid substrates. Another emerging structural color based device uses a photonic crystal made from silica spheres submerged within an electro-active polymer, and is branded Photonic Ink (P-ink). The polymer stretches as a field is applied, increasing the period of the photonic crystal and therefore the wavelength of reflected light. Though the colors are vivid and tunable, the response time of the polymer is tens of seconds, making video impossible. Cholesteric and blue phase LC displays behave in a similar manner. Helixes of LC form periodic nanostructures which produce Bragg reflections at desired wavelengths. The LC structures can be switched through an external field thereby producing dark and light states. While producing vivid color, these devices are limited in brightness as the helical structures only reflect circular light of the same handedness of the LC. By assessing current technologies we determine there is much to understand and develop in order to truly mimic color generation in nature. A fast response, angle independent LC-metasurface based display which can actively shift the color of its pixels from RGB to black holds the promise for development of truly thin-film flexible displays.
Schematic of the plasmonic-liquid crystal device with impinging white light. Light transmits through the superstrate and liquid crystal layers to interact with the reflective plasmonic surface. The surface selectively absorbs light while reflecting the rest back out of the device. The wavelength of this absorption depends on the liquid crystal orientation near the interface which in turn depends on the voltage applied across the cell. Voltage across the cell reorients the liquid crystal and changes the wavelengths of absorbed light.
Compatibility and Scale
The heart of E-Skin Display’s technology resides at the nanostructured plasmonic surface. Fabricated through roll-to-roll nano-imprint lithography (NIL), the layer can be laminated onto existing products or used alone to create ultra-thin reflective displays. This versatility allows the technology to be easily integrated into existing manufacturing lines with little to no changes in machinery.