An international group of researchers, led by a team at MIT, has developed a programmable, wireless device that can control light, such as by focusing a beam in a specific direction or manipulating the light’s intensity and doing it orders of magnitude more quickly than commercial devices. Their device, called spatial light modulator, could create super-fast lidar (light detection and ranging) sensors for self-driving cars, which could image a scene about a million times faster than existing mechanical systems. It could also accelerate brain scanners, which use light to “see” through tissue.
The scanners could produce higher-resolution images with less noise from dynamic fluctuations in living tissue, such as flowing blood, by being able to image tissue more quickly.
A spatial light modulator (SLM) manipulates light by controlling its emission properties. It transforms a passing beam of light, focusing it in one direction or refracting it to many locations for image formation.
A two-dimensional array of optical modulators inside the SLM controls the light. Since light’s wavelengths are only a few hundred nanometers, a very dense array of nanoscale controllers is required to control light at high speeds adequately. To do this, the scientists used a variety of photonic crystal microcavities. These photonic crystal resonators provide wavelength-scale controllable light storage, processing, and emission.
Before escaping into space, light is kept inside a cavity for nearly a millisecond and bounces more than 100,000 times. One billionth of a second, or a nanosecond, is required for the device to alter the light accurately. Scientists can regulate the amount of light that escapes by changing the reflectance of a hollow. To quickly and precisely direct a beam of light, the scientists simultaneously manipulate the array to alter a full light field.
Christopher Panuski Ph.D. ’22, who recently graduated with his Ph.D. in electrical engineering and computer science, said, “One novel aspect of our device is its engineered radiation pattern. We want the reflected light from each cavity to be a focused beam because that improves the beam-steering performance of the final device. Our process essentially makes an ideal optical antenna.”
Scientists achieved this goal thanks to a newly developed algorithm to design photonic crystal devices that form light into a narrow beam as it escapes each cavity.
The SLM was controlled by the team using a micro-LED display. One LED can tune a single microcavity because the LED pixels align with the silicon chip’s photonic crystals. When a laser strikes the triggered microcavity, the cavity reacts to the laser differently depending on the LED’s light.
Michael Strain, professor at the Institute of Photonics of the University of Strathclyde, said, “This application of high-speed LED-on-CMOS displays as micro-scale optical pump sources is a perfect example of the benefits of integrated photonic technologies and open collaboration. We have been thrilled to work with the team at MIT on this ambitious project.”
“The use of LEDs to control the device means the array is programmable and reconfigurable, but also completely wireless.”
“It is an all-optical control process. Without metal wires, we can place devices closer together without worrying about absorption losses.”
Englund said, “Getting a device architecture that would be manufacturable was one of the huge challenges at the outset. I think it only became possible because Chris worked closely for years with Mike Fanto and a wonderful team of engineers and scientists at AFRL, AIM Photonics, and our collaborators, and because Chris invented a new technique for the machine vision-based holographic trimming.”
The scientists use a laser to “trim” the microcavities during this operation. The silicon is heated by the laser to a temperature of more than 1,000 °C, resulting in silicon dioxide or glass. To add a layer of glass that aligns the resonances, or the natural frequencies at which the cavities vibrate, the scientists developed a mechanism that simultaneously blasts all of the cavities with the same laser.
Panuski says, “After modifying some properties of the fabrication process, we showed that we could make world-class devices in a foundry process with very good uniformity. That is one of the big aspects of this work — figuring out how to make these manufacturable.”
The device demonstrated near-perfect control — in both space and time — of an optical field with a joint “spatiotemporal bandwidth” 10 times greater than existing SLMs. Being able to control a huge bandwidth of light precisely could enable devices that can carry massive amounts of information extremely quickly, such as high-performance communications systems.
Journal Reference:
- Panuski, C.L., Christen, I., Minkov, M. et al. A full degree-of-freedom spatiotemporal light modulator. Nat. Photon. 16, 834–842 (2022). DOI: 10.1038/s41566-022-01086-9