Boffins' beam forming kit opens the door to more realistic holograms

A research team led by MIT has developed a novel piece of kit that can drastically increase the speed and capability of optical beam forming and is easy to produce at scale.

If commercialized, the team's new type of spatial light modulator (SLM) could open the way to super-fast LiDAR imaging for autonomous vehicles, improved medical scanners and even developing free-standing 3D holograms akin to those from Star Wars. Admittedly we'll be waiting on that last one for a while.

According to MIT, the project has been a four-year endeavor leading up to a paper published in Nature this week. Lead author Dr Christopher Panuski described the findings as a "major step toward the ultimate goal of complete optical control — in both space and time — for the myriad applications that use light." 

An SLM upgrade

SLMs are devices that modulate the wavelengths of beams of light to produce colors or shapes; the most basic example of this is the overhead projector transparency sheet. 

More advanced SLMs use two-dimensional arrays of things like liquid crystals and digital micromirrors to change the colors of light passing through them, but those are still limited in bandwidth and pixel density.

To get around these limitations, the MIT team opted for an array of "photonic crystal microcavities" they call a PhC-SLM. According to the researchers, their design achieves a ten-fold improvement over older 2D SLMs. 

Cavities in the PhC-SLM trap light for around a half a nanosecond - just long enough for the cavity to be tuned to manipulate it and form a beam as needed. To maximize the effectiveness of the PhC-SLM, the team designed an algorithm to map out the best way to form a narrow beam of light. 

"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," Panuski said. 

A micro-LED array is used to control the PhC-SLM, with each cavity paired with a single LED. The LEDs tune the cavities, and the cavities are used to modulate beams of light as required for a given application. According to Panuski, that means the devices can be placed incredibly close together without absorption loss.

Commercially viable research results?

While the MIT team didn't give any indications that commercialization of their product is forthcoming, Panuski said that making the PhC-SLMs manufacturable was a major part of the project. With each cavity just around a micron in size, and an entire PhC-SLM being manufactured on a 12-inch silicon wafer, any small deviation in manufacturing could affect performance. 

To circumvent this problem, the team developed a method of "machine vision-based holographic trimming" that uses a superheated laser to create a layer of silicon dioxide on the surface of each cavity. MIT said that the laser was created to hit all the cavities at the same time, ensuring the silica aligns the resonances of each cavity.

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The MIT team now plans to build larger devices for testing some of the many potential applications of their new SLMs. As for the Star Wars holograms, "high-definition, high frame-rate holographic displays" that allow for "full-DoF spatiotemporal modulation" could also be created using the tech, the team said in the paper, but with some restrictions like the necessity of a back-reflector. 

Still, it's a first step in creating the fast, precise control of light needed for it and other photonic marvels.

No word on whether that new beamformer could also be used to aid in the creation of that other long-sought piece of Star Wars technology that relies on forming light into spatiotemporally-controlled beams, so lay those Jedi aspirations back to rest for now. ®