An international group of researchers, led by a team at MIT, has developed a programmable, wireless device that can control light, for example by directing a beam in a specific direction or by manipulating the intensity of the light and doing it much faster than commercial devices. Their device, called spatial light modulator, could create high-speed lidar (light detection and ranging) sensors for self-driving cars, which can frame a scene about a million times faster than existing mechanical systems. It could also speed up 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 imaging.
A two-dimensional array of optical modulators in the SLM controls the light. Since the wavelengths of light are only a few hundred nanometers, a very dense array of nanoscale controllers is required to adequately control light at high speeds. To do this, the scientists used several photonic crystal microcavities. These photonic crystal resonators provide controllable wavelength-scale light storage, processing and emission.
Before escaping into space, light is held in a cavity for almost a millisecond and bounces more than 100,000 times. The device takes a billionth of a second, or a nanosecond, to accurately change the light. Scientists can control the amount of light that escapes by changing the reflection of a cavity. In order to focus a beam of light quickly and accurately, the scientists simultaneously manipulate the array to change an entire field of light.
Christopher Panuski Ph.D. ’22, who recently graduated with his Ph.D. in electrical engineering and computer science, said: “A new aspect of our device is the manipulated 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 from 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 are aligned with the photonic crystals of the silicon chip. When a laser hits the triggered microcavity, the cavity responds differently to the laser depending on the light from the LED.
Michael Strain, Professor at the University of Strathclyde’s Institute of Photonics, 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 are excited to be working with the team at MIT on this ambitious project.”
“Using LEDs to control the device means the array is programmable and reconfigurable, as well as completely wireless.”
“It is a complete 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 in the beginning. I think it only became possible because Chris spent years working closely with Mike Fanto and a great team of engineers and scientists at AFRL, AIM Photonics and our associates, and because Chris invented a new technique for 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 blasts all the cavities simultaneously with the same laser.
Panuski says, “After adjusting some properties of the manufacturing process, we showed that we can make world-class devices in a foundry process with very good uniformity. That’s one of the big parts of this job, figuring out how to make it manufacturable.”
The device demonstrated near-perfect control — both in space and time — of an optical field with a collective “spatiotemporal bandwidth” 10 times greater than that of existing SLMs. Being able to precisely control a huge bandwidth of light enables devices that can transport massive amounts of information extremely quickly, such as high-powered communication systems.
- Panuski, CL, Christen, I., Minkov, M. et al. A full degree of freedom spatiotemporal light modulator. Wet. Photon. 16, 834-842 (2022). DOI: 10.1038/s41566-022-01086-9