The solution that the Stanford team, a collaboration between the Laboratory for Integrated Nano-Quantum Systems (LINQS) and ArbabianLab, came up with relies on a phenomenon known as acoustic resonance. The team built a simple acoustic modulator using a thin wafer of lithium niobate – a transparent crystal that is highly desirable for its electrical, acoustic and optical properties – coated with two transparent electrodes.

Critically, lithium niobate is piezoelectric. That is, when electricity is introduced through the electrodes, the crystal lattice at the heart of its atomic structure changes shape. It vibrates at very high, very predictable and very controllable frequencies. And, when it vibrates, lithium niobate strongly modulates light – with the addition of a couple polarizers, this new modulator effectively turns light on and off several million times a second.

“What’s more, the geometry of the wafers and the electrodes defines the frequency of light modulation, so we can fine-tune the frequency,” Atalar says. “Change the geometry and you change the frequency of modulation.”

In technical terms, the piezoelectric effect is creating an acoustic wave through the crystal that rotates the polarization of light in desirable, tunable and usable ways. It is this key technical departure that enabled the team’s success. Then a polarizing filter is carefully placed after the modulator that converts this rotation into intensity modulation – making the light brighter and darker – effectively turning the light on and off millions of times a second.

“While there are other ways to turn the light on and off,” Atalar says, “this acoustic approach is preferable because it is extremely energy efficient.”

Practical outcomes

Best of all, the modulator’s design is simple and integrates into a proposed system that uses off-the-shelf cameras, like those found in everyday cellphones and digital SLRs. Atalar and advisor Amin Arbabian, associate professor of electrical engineering and the project’s senior author, think it could become the basis for a new type of compact, low-cost, energy-efficient lidar – “standard CMOS lidar,” as they call it – that could find its way into drones, extraterrestrial rovers and other applications.

The impact for the proposed modulator is enormous; it has the potential to add the missing 3D dimension to any image sensor, they say. To prove it, the team built a prototype lidar system on a lab bench that used a commercially available digital camera as a receptor. The authors report that their prototype captured megapixel-resolution depth maps, while requiring small amounts of power to operate the optical modulator.

Better yet, with additional refinements, Atalar says the team has since further reduced the energy consumption by at least 10 times the already-low threshold reported in the paper, and they believe several-hundred-times-greater energy reduction is within reach. If that happens, a future of small-scale lidar with standard image sensors – and 3D smartphone cameras – could become a reality.

Additional Stanford authors include Amir H. Safavi-Naeini, associate professor of applied physics, and postdoctoral fellow Raphael Van Laer. This work was funded in part by Stanford SystemX Alliance, the Office of Naval Research and the National Science Foundation.