The core building blocks of mirasol displays, the Interferometric Modulator (IMOD) element is a simple MEMS (micro-electro-mechanical system) device that is composed of two conductive plates.
One is a thin film stack on a glass substrate, the other is a reflective membrane suspended over the substrate. There is a gap between the two that is filled with air.
The IMOD element has two stable states. When no voltage is applied, the plates are separated, and light hitting the substrate is reflected as shown above. When a small voltage is applied, the plates are pulled together by electrostatic attraction and the light is absorbed, turning the element black.
This is the fundamental building block from which Qualcomm's mirasol displays are made.
At the most basic level, a mirasol® display is an optically resonant cavity. The device consists of a self-supporting deformable reflective membrane and a thin-film stack (each of which acts as one mirror of an optically resonant cavity), both residing on a transparent substrate.
When ambient light hits the structure, it is reflected both off the top of the thin-film stack and off the reflective membrane. Depending on the height of the optical cavity, light of certain wavelengths reflecting off the membrane will be slightly out of phase with the light reflecting off the thin-film structure. Based on the phase difference, some wavelengths will constructively interfere, while others will destructively interfere.
The human eye will perceive a color as certain wavelengths will be amplified with respect to others. The image on a mirasol display can switch between color and black by changing the membrane state. This is accomplished by applying a voltage to the thin-film stack, which is electrically conducting and is protected by an insulating layer. When a voltage is applied, electrostatic forces cause the membrane to collapse.
The change in the optical cavity now results in constructive interference at ultraviolet wavelengths, which are not visible to the human eye. Hence, the image on the screen appears black.
A full-color display is assembled by spatially ordering IMOD elements reflecting in the red, green and blue wavelengths.
Since visible light wavelengths operate on the nanometer scale (i.e., 380nm to 780nm), the deformable IMOD membrane only has to move a short distance—a few hundred nanometers—in order to switch between two colors. This switching happens extremely fast, on the order of tens of microseconds. This switching speed directly translates to a video rate-capable display with no motion-blur effects.
One of the key advantages of the mirasol® display’s design is its bistable nature, which allows for near-zero power usage in situations where the display image is unchanged.
IMOD elements, which make up the IMOD pixels, possess electro-mechanical memory called hysteresis. The hysteresis effect works somewhat like the pull top on an aluminum can. Once the reflective membrane has been pulled down, it requires less energy to hold it than was exerted in pulling it down.
This bistability not only allows mirasol displays to replace the non-linearity of an active matrix device, it can also act as a real memory element. The bistability of mirasol displays comes from the inherent hysteresis derived from the technology’s electro-mechanical properties - the inherent imbalance between the linear restorative forces of the mechanical membrane and the non-linear forces of the applied electric field.
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