Electronic paper is a form of displays that use ambient light rather than backlights to generate an image. Although such displays have only captured niche markets as they have been unable to generate high resolution color images or refresh at video rates. However interferometric modulation (IMOD), a new technology based on a bio-mimicry model, is revolutionizing that. These bio-mimetic displays are poised to become the power-saving technology of choice for displays in mobile phones and laptop computers.
The inspiration for these new displays comes from the Blue Morpho butterfly. [2] Many butterfly species, including the Blue Morpho, use light-interacting structures on their wings in order to produce color. [2] The cuticle on the scales of these butterfly species' wings are made of transparent, chitin-and-air layered nano-structures that don't just statically absorb and reflect certain light wavelengths as pigments and dyes do. [2, 5] They instead selectively cancel out certain colors (wavelengths/ frequencies) through interference and reflect others. This interference simply depends on the exact structure and interspatial distance between diffracting layers. [2,3] This system of producing color allows for a dynamic control of light flow and wavelength interaction, which butterflies rely upon for camouflage, thermoregulation, and signaling. [2, 5]
Structurally-produced color has now been translated into designs that create high-performance electronic color displays. The most developed of these technologies is called interferometric modulation (IMOD) and is in the market under the brand name Mirasol. [1, 2] This was originally invented by Iridigm Display, a private company founded in San Francisco in 1988. Iridigm Display was then acquired by Qualcomm in 2004. [1] The Mirasol display is both extremely low power, and highly reflective. Its reflectivity allows the display to be seen even in direct sunlight which is something that traditional LCD displays lack. [1, 3]
In an IMOD display, each subpicture element (subpixel) is made up of two reflective layers, one on top of the other, to form an optical cavity. [4] The position of the upper partially reflective layer is fixed. However the lower one sits on a microelectromechanical-system (MEMS) switch that can be switched between an upper and lower position. The lower position is carefully spaced so that the optical cavity formed between the two reflective layers causes constructive interference for either red, green or blue light. [3] When light reflects off the subpixel, only one primary color is reflected. When a pulse of voltage is applied to the MEMS switch and it flips to its higher position, the optical cavity is reduced to the point that only UV light is reflected, making it appear black, or off. IMOD can switch states in tens of microseconds. Interferometric modulation also produces much brighter colors because, unlike most other displays that use filters, it doesn't rely on absorption to generate these colors. [4]
Bistability: This nature inspired MEMS-based innovation is bistable. This allows near-zero power usage in situations where the display image is unchanged. This bistability derived from the inherent hysteresis of the material causes considerable power savings, especially compared to displays that continually refresh, such as LCDs. [3, 4]
Speed:
Since visible light wavelengths lie in the nanometer scale (i.e. 380nm to 780nm), the deformable IMOD membrane only has to move a short distance (~100nm) in order to switch between two colors. This switching happens extremely fast, on the order of tens of microseconds. [2] This is 1000 times faster than that of traditional displays. Higher switching speed directly translates to a video rate-capable display with no motion-blur effects. [4]
Robustness:
In addition to microsecond switching, mirasol displays maintain their switching speed across a wide temperature range. In contrast, the switching speeds of organic liquid-crystal-based displays decrease as temperatures go into low environmental ranges. [1, 4]
Readability:
A mirasol display offers a superior contrast ratio in brightly lit environments. Qualcomm's mirasol displays offer reflectivity on the order of 60 percent and contrast ratios greater than 10:1. By comparison, the Wall Street Journal newspaper offers a reflectivity of 60 percent and a contrast ratio of around 4:1. [4]
Reflective technology based on bio-mimetic models hold much promise. However, in order for commercial use, reflective displays they need to be viewed in both daylight and in the dark. The commercialization of this technology is thus dependent on combining the benefits of backlit transmissive displays with the outdoor readability of reflective technologies. [1, 2, 3] These new displays will mean faster response times, better color reproduction, and higher resolutions - a revolution in display technology. [4, 5]
© Nruthya Kavadichanda Madappa. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Graham-Rowe, "Electronic Paper Targets Colour Video," Nature Photonics 2, 204 (2008).
[2] S. Kinoshita et al., "Photophysics of Structural Color in the Morpho Butterflies," Forma 17, 103(2002).
[3] J. B Sampsell, "MEMS-Based Display Technology Drives Next-Generation FPDs for Mobile Applications," Information Display 22, No. 6, 24 (2006).
[4] C.-D. Liao, "The Evolution of MEMS Displays," IEEE Trans. Industrial Electronics 56, 1057 (2009).
[5] P. Vukusic, et al. "Quantified Interference and Diffraction in Single Morpho Butterfly Scales," Proc. Roy. Soc. B 266, 1403 (1999).