Seeing in nanometers: Laser and gratings make tiny motions visible to human eye
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A scanning electron micrograph of the motion detector shows the top grating layer suspended by four springs and actuated by two sidecomb drives. The second, fixed grating layer is hidden below the first. The area of the grating itself is 10 X 10 mm. The scale bar at bottom represents 10 mm. |
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A researcher studies tiny motions formerly invisible to the human eye, using a laser beam interacting with a diffraction grating fabricated at Sandia's Microelectronic Development Lab. |
They use a formerly unrecognized property of optics: Light diffracted from small gratings that move miniscule lateral distances undergoes a relatively large, and thus easily measurable, change in reflection because of subwavelength interference.
The device is fabricated out of polysilicon using standard lithography techniques for MEMS devices. It is a kind of accelerometer, about the size of the inexpensive sensors that activate automobile air bags. Two comb-like structures are laid one over the other. The bottom comb is locked in place. The top comb is secured by horizontal springs. Any motion sends the top comb skittering over the bottom one, laterally deforming the grating. A laser shining on the grating as it moves undergoes an amplitude change in the visible and near-IR.
Researchers postulate such sensors could be used to detect earthquakes and their predictive geophysical movements. Another use would be for skid and traction control in cars, detecting sideways movements of a car's backend.
"This is the first time anyone has tried to manipulate the optical near-field region to affect changes in the far-field qualities of a grating," says James Walker, an independent consultant who once headed up Advanced Technologies at Tellium Inc., and the MEMS Network Element Subsystems Group at Lucent's Bell Laboratories. "The ability to do this is a direct result of the nanoscale nature of the device.Its high responsivity-to-displacement ratio should give it far-reaching application in areas as diverse as chemical sensing, infrared imaging, accelerometry, and displays."