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4.4. Optical Switching Based on MEMS
No diffracted light
Aluminum
Silicon
nitride
Air gap
Movable
microbridge
Switched state
Fig. 4.33. Deformable diffraction gratings.
The array is addressed by a set of row and column electrodes. Every other
microbridge in the pixel is addressable. The others are held at a fixed-bias
voltage so that they cannot be switched. When a pixel is selected by the
combined effect of the row and column address voltages, the air gap voltage of
the selected microbridges exceeds a threshold level. The movable bridges
deflected through one-quarter of the wavelength of the incident light and touch
down onto the substrate. They remain there, electromechanically latched, as
long as a minimum holding voltage is maintained by the electrode.
Light incident onto a switched pixel is strongly diffracted because the optical
path difference upon reflection between the pairs of microbridges is one-half the
wavelength. For the nonswitched state, the microbridges are coplanar and the
light is specularly reflected.
