November R&D Round Up

E-mail Melanie Martella

This month we've got a pair of tiny but mighty projects: a nanoscale accelerometer that uses laser light to achieve high sensitivity and an amazingly sensitive photon detector that combines a CMOS image sensor with a single-photon avalanche photodiode.

Accelerometers with a Light Touch
Accelerometers are remarkably handy devices, cropping up in a wide variety of applications, from testing the vibration of a machine or a vehicle to telling your smartphone which way is up. But depending on what you're trying to do, you'll choose a specific type of accelerometer to fit those application needs. If the researchers at the California Institute of Technology and the University of Rochester have their way, you'll soon have a new option for sensing acceleration.

The researchers have developed an ultrasensitive accelerometer that uses laser light to detect very fast and very small motions.

Accelerometers work by generating an electrical signal as their proof mass moves in response to an applied acceleration. What the researchers have done is to shrink the motion-sensitivity of a laser interferometer down to the nanoscale and they've done this by placing the inertial structure (two silicon nanobeams, one of which is attached to the proof mass) inside a tiny optical cavity (20 µm long by 1 µm wide by a few tenths of a micron thick). Laser light that enters the cavity, it's guided by the nanobeams, bouncing back and forth between holes in the nanobeams. When the proof mass moves, the gap between nanobeams changes and thus changes the intensity of the light reflected out of the cavity. The device can measure proof mass displacements as small as a few femtometers and can do so in a second. To quote the article "Developing the Next Generation of Microsensors",

"It turns out that because the cavity and proof mass are so small, the light bouncing back and forth in the system pushes the proof mass—and in a special way: when the proof mass moves away, the light helps push it further, and when the proof mass moves closer, the light pulls it in. In short, the laser light softens and damps the proof mass's motion."

The result? A highly sensitive device that can sense large accelerations while still being mind-bogglingly tiny.

An Image Sensor That Makes Good Use of Scarce Photons
While current image sensors perform far better in extreme conditions—too much or too little light—there are still occasions where what you want is good images in very low-light conditions. Researchers at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg, working with partners of the Microelectronic Single-Photon 3D Imaging Arrays for low-light high-speed Safety and Security Applications (MiSPiA) project consortium have created an ultrasensitive image sensor by combining CMOS and single-photon avalanche photodiode technology. (You can read more in the article, "Ultrasensitive photon hunter.") Avalanche photodiodes can detect very low levels of light; an incident photon triggers an avalanche cascade, which acts as a photoelectric amplification method. By combining the detector and camera on a single chip, the resulting image sensor can count individual photons within a few picoseconds and can take images even with extremely weak light sources. In addition, there's no need for additional analog signal processing, because the researchers can do image processing directly on the sensor microchip. So, we get a smaller, faster, and much more sensitive image sensor!


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