In a scene from “Star Wars: Episode IV – A New Hope”, R2D2 projects a three-dimensional hologram of Princess Leia making a desperate plea for help. This scene, filmed over 45 years ago, involved a bit of cinematic magic – even today we don’t have the technology to create such realistic and dynamic holograms.
Generating a stand-alone 3D hologram would require extremely precise and fast control of light beyond the capabilities of existing technologies, which are based on liquid crystals or micromirrors.
An international group of researchers, led by a team from MIT, spent more than four years solving this high-speed optical beamforming problem. They have now demonstrated a programmable wireless device that can control light, for example by focusing a beam in a specific direction or manipulating the intensity of light, and do so orders of magnitude faster than commercial devices.
They also pioneered a manufacturing process that ensures that the quality of the device remains near perfect when manufactured at scale. This would make their device easier to implement in real-world settings.
Known as the Spatial Light Modulator, the device could be used to create ultra-fast lidar (light detection and ranging) sensors for self-driving cars, which could image a scene around a million times faster than existing mechanical systems. It could also speed up brain scanners, which 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.
“We focus on the control of light, which has been a recurrent research theme since ancient times. Our development is another major step towards the ultimate goal of complete optical control, both in space and over time, for the myriad of applications that use light,” says lead author Christopher Panuski, who recently earned his PhD. in electrical engineering and computer science.
The paper is a collaboration between researchers at MIT; Flexcompute, Inc.; the University of Strathclyde; the Polytechnic Institute of the State University of New York; Applied Nanotools, Inc.; the Rochester Institute of Technology; and the US Air Force Research Laboratory. The lead author is Dirk Englund, an associate professor of electrical engineering and computer science at MIT and a research scientist at the Electronics Research Laboratory (RLE) and the Microsystems Technology Laboratories (MTL). The research is published today in Nature Photonics.
A spatial light modulator (SLM) is a device that manipulates 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. But the wavelengths of light are only a few hundred nanometers, so to precisely control light at high speeds, the device needs an extremely dense array of nanoscale controllers. 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.
When light enters a cavity, it is held for about a nanosecond, bouncing more than 100,000 times before escaping into space. While a nanosecond is only a billionth of a second, that’s enough time for the device to precisely manipulate light. By varying the reflectivity of a cavity, researchers can control how light escapes. Simultaneously controlling the array modulates an entire light field, so researchers can quickly and precisely direct a beam of light.
“A new aspect of our device is its technical radiation pattern. We want the reflected light from each cavity to be a focused beam, as this improves the beam steering performance of the final device. Our process is essentially an optical antenna ideal,” Panuski said. said.
To achieve this goal, the researchers developed a new algorithm to design photonic crystal devices that shape light into a narrow beam as it escapes from each cavity, he explains.
Use light to control light
The team used a micro-LED display to control the SLM. The LED pixels align with the photonic crystals on the silicon chip, so turning on an LED sets a single microcavity. When a laser hits this activated microcavity, the cavity reacts differently to the laser depending on the light from the LED.
“This application of high-speed LED-on-CMOS displays as micro-scale optical pump sources is a perfect example of the benefits of integrated photonics technologies and open collaboration. team at MIT on this ambitious project,” says Michael Strain, a professor at the University of Strathclyde’s Institute of Photonics.
Using LEDs to control the device means the matrix is not only programmable and reconfigurable, but also completely wireless, says Panuski.
“It’s an all-optical control process. Without metal wires, we can bring devices closer together without worrying about absorption losses,” he adds.
Figuring out how to fabricate such a complex device in a scalable way has been a years-long process. The researchers wanted to use the same techniques that create integrated circuits for computers, so that the device could be mass-produced. But microscopic deviations occur in any manufacturing process, and with micron-sized cavities on the chip, those tiny deviations could cause huge fluctuations in performance.
The researchers partnered with the Air Force Research Laboratory to develop a highly precise mass fabrication process that stamps billions of cavities onto 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 at the start. I think it only became possible because Chris worked closely for years with Mike Fanto and a wonderful team of designers. engineers and scientists from AFRL, AIM Photonics and with our other collaborators, and because Chris invented a new holographic slicing technique based on machine vision,” Englund explains.
For this “trimming” process, the researchers shine a laser on the microcavities. The laser heats the silicon to over 1,000 degrees Celsius, creating silicon dioxide or glass. The researchers created a system that explodes all the cavities with the same laser at once, adding a layer of glass that perfectly aligns the resonances, ie the natural frequencies at which the cavities vibrate.
“After modifying some properties of the fabrication process, we showed that we were able to fabricate world-class devices in a foundry process that had very good uniformity. This is one of the great aspects of this work: figuring out how to make them manufacturable,” says Panuski.
The device demonstrated near-perfect control – both spatially and temporally – of an optical field with a common “spatio-temporal bandwidth” 10 times greater than 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 that they’ve perfected the fabrication process, the researchers are working to fabricate larger devices for quantum control or ultrafast sensing and imaging.
Christopher L. Panuski et al, A Full Degree of Freedom Spatiotemporal Light Modulator, Nature Photonics (2022). DOI: 10.1038/s41566-022-01086-9
Provided by Massachusetts Institute of Technology
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