Programmable device enables optical inspection at unprecedented speeds

Programmable device enables optical inspection at unprecedented speeds

CAMBRIDGE, Mass., Nov. 30, 2022 — An international team led by MIT researchers has developed a spatial light modulator (SLM) that promises better control of light orders of magnitude faster than commercial devices. The team also developed a manufacturing process to ensure consistent quality of the device when manufactured at scale.

The device could be used to create ultra-fast lidar sensors for autonomous vehicles that could image a scene about a million times faster than existing mechanical systems. It could also be used to speed up brain scans that use light to see through tissue. By being able to image tissue faster, scanners could generate higher resolution images that are unaffected by the noise of dynamic fluctuations in living tissue, such as flowing blood.

Spatial light modulators manipulate light by controlling its emission properties. Similar to an overhead projector or computer screen, an SLM transforms a passing beam of light, focusing it in one direction or refracting it in many places for image formation. Inside the SLM, a two-dimensional array of optical modulators control the light. However, since wavelengths are only a few hundred nanometers, to precisely control light at high speeds, the device needs an extremely dense array of nanoscale controllers.

In recent work, the researchers used an array of photonic crystal microcavities to achieve this goal. These photonic crystal resonators allow light to be stored, manipulated and emitted in a controlled manner on the wavelength scale.

Scientists have developed a wireless, programmable spatial light modulator that can manipulate light on the wavelength scale with orders of magnitude faster response than existing devices.  Courtesy of Sampson Wilcox, MIT.

Scientists have developed a wireless, programmable spatial light modulator that can manipulate light on the wavelength scale with orders of magnitude faster response than existing devices. Courtesy of Sampson Wilcox/MIT.

In the work, by varying the reflectivity of a cavity, the researchers controlled how light escaped. Simultaneously control the modulated array an entire light field so that researchers can quickly and precisely direct a beam of light.

Christopher Panuski, lead author of the paper and recent Ph.D. graduate, said, “An innovative aspect of our device is its designed radiation pattern. We want the light reflected from each cavity to be a focused beam as this improves the beam steering performance of the final device. Our process essentially makes an ideal optical antenna.

The researchers developed an algorithm to design photonic crystal devices that shape light into a narrow beam as it escapes from each cavity, Panuski said.

The team used a micro-LED display to control their SLM. The LED pixels were aligned with the photonic crystals on the silicon chip, so that turning on an LED tuned a single microcavity. When a laser hit this activated microcavity, the cavity responded differently to the laser depending on the light from the LED.

Using LEDs to control the device ensures that the matrix is ​​not only programmable and reconfigurable, but also completely wireless, Panuski said. “It is an entirely optical control process. Without metal wires, we can bring devices closer together without worrying about absorption losses.

To make the device, the researchers sought to use the same techniques used to create integrated circuits for computers, allowing them to mass-produce the device. They partnered with the Air Force Research Laboratory to develop a highly precise mass manufacturing process that stamps billions of cavities on a 12 inch. silicon wafer. Then they incorporated a post-processing step to ensure that the microcavities all operate at the same wavelength.

“Getting a device architecture that would actually be manufacturable was one of the huge challenges initially,” said Dirk Englund, lead author and associate professor of electrical engineering and computer science at MIT. According to Englund, the manufacturing approach benefited from a technique for holographic slicing based on machine vision developed by Panuski. For this “cutting” process, the researchers shined a laser on the microcavities. The laser heated the silicon to over 1000 ºC, creating silicon dioxide. The researchers created a system that cavities with the same laser at a time, adding a layer of glass that perfectly aligned the resonances, ie the natural frequencies at which the cavities vibrate.

The device demonstrated near-perfect control – both in space and time – of an optical field with a common spatio-temporal bandwidth 10 times that of existing SLMs. Being able to precisely control an enormous bandwidth of light could enable devices capable of transporting huge amounts of information extremely quickly, such as high-performance communications systems.

Now researchers are working on making larger devices for quantum control or ultrafast sensing and imaging.

The paper was a collaboration between researchers at MIT, Flexcompute Inc., University of Strathclyde, State University of New York Polytechnic Institute, Applied Nanotools Inc., Rochester Institute of Technology and the US Air Force Research Laboratory.

The research has been published in Nature Photonics (

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