Nanoflashlight Could Turn Mobile Devices into Powerful Biosensors

Researchers at MIT built a device that they describe as a “nanoscale flashlight” on a chip. The device overcomes a limitation of spectroscopy, in that spectrometers are relatively large instruments. Much like a spectrometer, the MIT team’s nanoflashlight shines a beam of light on a material, analyzes the light after it has passed through the material at several wavelengths, and captures the interactions of light with the material for each color.

The researchers believe the compact flashlight could turn cellphones into sensors that would detect viruses and other miniscule objects. The approach they used to design the light beam on a chip, they said, could also be used to create a variety of other nanoflashlights with different beam characteristics for different applications.

Though researchers have made strides in the miniaturization of sensors that can detect and analyze light that has passed through a given material, a miniaturized light beam and appropriately shaped light beam — or flashlight — has remained a challenge. Nanoscale equipment, such as laser systems that are not built into the chip itself as the sensors are, are often used as light sources.

The team used multiple computer modeling tools in the work. These included conventional approaches based on the physics involved in the propagation and manipulation of light, and machine-learning techniques. Researchers showed the computer model many examples of nanoflashlights so that it could learn to make better flashlights.

Schematic of three different nanoflashlights for the generation of focused, wide-spanning, and collimated beams (left to right). Each could have different applications. Courtesy of Robin Singh.

The combination of modeling tools yielded a final, optimal design that the team successfully adapted to create flashlights with different kinds of light beams. With that design, the researchers created a specific flashlight with a collimated beam, one in which the light rays are perfectly parallel to one another. The instrument ultimately involved roughly 500 rectangular nanoscale structures of different dimensions, which the team’s modeling predicted would enable a collimated beam. The nanostructures of different dimensions would lead to different kinds of beams and, as a result, enable other applications.

The tiny flashlight with a collimated beam provided a beam 5× more powerful than is possible with conventional structures — in part because of the high level of control that the instrument delivered. The ability to better control the light meant that less was scattered and lost.

In addition to the model-based approach, Robin Singh, lead author of two papers that describe the advancement, created the device with fabrication technologies that were already familiar to the microelectronics industry. This increases the probability that the approach could be deployed at a mass scale, with the lower cost that implies. As the work supports the ability of industry to create a complete sensor on a chip with both light source and detector, it represents an advance in the use of silicon photonics for the manipulation of lightwaves on microchips for sensor applications.

“Silicon photonics has so much potential to improve and miniaturize the existing bench-scale biosensing schemes. We just need smarter design strategies to tap its full potential,” Singh said. “This work shows one such approach.”

The research was supported in part by the MIT Skoltech Initiative, and was published in Scientific Reports (www.doi.org/10.1038/s41598-020-76225-9), (www.doi.org/10.1038/s41598-021-84841-2).