Scientists at Caltech have created an optomechanical nanodevice capable of detecting single photons passing through it. Because of this extreme sensitivity in a tiny package, the technology may find itself as a primary component in future imaging or diagnostic modalities.
The fact that photons of light, despite having no mass, nonetheless carry momentum and can interact with mechanical objects is an idea that dates back to Kepler and Newton. The mechanical properties of light are also known to limit the precision with which one can measure an object’s position, since simply by using light to do the measurement, you apply a force and disturb the object.
It was important to consider these so-called back-action effects in the design of devices to measure weak, classical forces. Such considerations were part of the development of gravity-wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO). These sorts of interferometer-based detectors have also been used at much smaller scales, in scanning probe instruments used to detect or image atomic surfaces or even single electron spins.
To get an idea of how these systems work, consider a mirror attached to a floppy cantilever, or spring. The cantilever is designed to respond to a particular force-say, a magnetic field. Light shining down on the mirror will be deflected when the force is detected-i.e., when the cantilever moves-resulting in a variation in the light beam’s intensity that can then be detected and recorded.
They did this, he explains, because as these interferometer-based detectors are scaled down, the mechanical properties of light become more pronounced, and interesting interactions between light and mechanics can be explored.
“To this end, we made our cantilevers many, many times smaller, and made the optical interaction many, many times larger,” explains Painter.
To create their zipper cavity device, the researchers made two nanobeams from a silicon chip, poking holes through the beams to form an effective optical mirror. Instead of training a light down onto the nanobeams, the researchers used optical fibers to send the light “in plane down the length of the beams,” says Painter. The holes in the nanobeams intercept some of the photons, circulating them through the cavity between the beams rather than allowing them to travel straight through the device.
Such an interaction is possible, he adds, because the structure is precisely designed to maximize the transfer of momentum from the input laser’s photons to the mechanical nanobeams. Indeed, a single photon of laser light zipping through this structure produces a force equivalent to 10 times that of Earth’s gravity. With the addition of several thousand photons to the cavity, the nanobeams are effectively suspended by the laser light.
Changes in the intensity and other properties of the light as it moves along the beams to the far end of the chip can be detected and recorded, just as with any large-scale interferometer.
The potential uses for this sort of optomechanical zipper cavity are myriad. It could be used as a sensor in biology by coating it with a solution that would bind to, say, a specific protein molecule that might be found in a sample. The binding of the protein molecule to the device would add mass to the nanobeams, and thus change the properties of the light traveling through them, signaling that such a molecule had been detected. Similarly, it could be used to detect other ultrasmall physical forces, Painter adds.