Thread: IMOD basics

Basically, an IMOD unit is the modified version of an optical device called the Fabry-Perot interferometer or etalon. Etalon is basically a cavity formed by two reflective surfaces aligned in parallel. When light passes into the cavity through the top surface, which is translucent, it reflects from the bottom surface and then bounces back and forth between the surfaces infinitely, with a little bit of light leaking out of the top translucent mirror each time. All this bouncing causes most wavelengths of light to cancel themselves out due to a phenomenon called 'interference.' But the rebounding waves actually reinforce the reflection of wavelengths that just happen to fit precisely in the gap between the top and bottom reflective surfaces.

So in effect, the etalon as a whole functions as a mirror that reflects only one specific colour, which can be chosen simply by adjusting the spacing between the mirrors. Like etalon, the basic unit of IMOD displays is a tiny mechanism that consists of two mirrored surfaces in parallel with a gap between them. In essence, it is a microscopic etalon in which the parallel reflecting surfaces are thin-film layers crea ed using MEMS technique.

A micromechanical system, abbreviated as MEMS, is a system in which micromechanisms are coupled .with microelectronics. MEMS are the foundation of a broad range of products like optoelectronic devices, sensors, switches and resonators fabricated as integrated circuits on silicon, glass, quartz as well as plastic wafers. MEMS technology encompasses a variety of processes enabling three-dimensional (3D) shape of wafers or stacks of wafers. MEMS-based micromirrors operate faster and have lower mass. During the fabrication process, the spacing between films could be set to reflect a specific wavelength or colour. The visi¬ble range extends from 700 nanometres (redLto about 400 nanometres (violet) from crest to crest, with all other colours of the rainbow in between.

shows a red-coloured unit cell of IMOD with a set air spacing between two parallel mirrored surfaces suitable for reflection of red colour. When light hits the structure, some of it reflects off the top and some passes through the translucent top mirror into the gap where it reflects internally. A little light, however, leaks out with each upw.ard bounce. Many of the green and blue escapipg light waves in this red unit cell will be slightly out of phase with those rebounding off the top as well as with other escaping waves. These waves will cancel out one another via destructive interference. But reflecting light waves that are in phase with red will amplify and become visible.




Because the bottom layer is flexible, a coloured unit cell can be turned off very easily by applying a very brief voltage between the two mirrors. When a voltage is applied, it produces an electrostatic attraction between the mirrors. This would make the bottom layer bulge upward, thereby collapsing the air gap. The shrunk gap shifts the reflected light into the invisible ultraviolet ray, which appears black. Moreover, the unit cell stays black, without consuming any further power, until the'time comes to flip it back to coloured. Flipping it back is quite simple: you just have to apply another voltage pulse.

The unit cell in black state is shown in. For high-resolution display, it is necessary to array each unit coloured cell across a screen surface by the millions, with small groups of them forming a single pixel (picture element). An IMOD colour pixel features arrays of red, green and blue subpixels, each of which consists of two columns of seven unit cells. The hue and brightness of a pixel depend on the number and colour of activated cells. Gap depth determines whether the cells are red, green or blue.

For colour display, it is necessary that each of the pixels actually consists of three subpixels: one for each of the three primary colours of red, green and blue. And each of these subpixels has to have several unit cells or sub-subpixels, which can be turned on or off independently by the electronic circuit of the gadget so as to produce a range of colours and brightness. For good-quality display, the individual IMOD unit cells are made of IOO-micron size across. The electronic circuit of pixels is more or less similar to that of the LCD explained earlier.

Prospects

IMODs are powerful photonic devices that can be used as excellent displays for manipulation of light not only in cellphones but also in camcorders, electronic books, computer monitors, wall-mounted TVs, etc. The market for IMOD displays is very huge. Nokia makes about 36 million mobile handsets a year. If we take into consideration all phone and other handheld electronics manufacturers, the worldwide output will be a couple of billions. So it will not require much of effort to penetrate such a huge market. All in all, in the coming days, IMOD displays are likely to replace other devices.