Microscopic Color Converters May Yield Small Laser-Based Devices

Columbia University, New York, NY

Researchers have used an atomically thin material to build a device that can change the color of laser beams. (Image: engineering.columbia.edu)

New research suggests that laser-based devices are poised to become a lot smaller. Researchers at Columbia University and Politecnico di Milano studied a 2D material called molybdenum disulfide (MoS₂) and characterized how efficiently devices built from stacks of MoS₂ less than one micron thick — 100 times thinner than a human hair — convert light frequencies at telecom wavelengths to produce different colors.

“Nonlinear optics is currently a macroscopic world, but we want to make it microscopic,” said Giulio Cerullo, a Nonlinear Optics Researcher at Politecnico di Milano in Italy.

Lasers radiate a special kind of coherent light, which means all the photons in the beam share the same frequency and color. Lasers operate only at specific frequencies, but devices often need to be able to deploy different colors of laser light. For instance, a green laser pointer is actually produced by an infrared laser that’s converted to a visible color by a macroscopic material. Researchers use nonlinear optical techniques to change the color of laser light, but conventional materials need to be relatively thick for color conversion to occur efficiently.

MoS₂ is one of the most-studied examples of an emerging class of materials — transition metal dichalcogenides — which can be peeled into atomically thin layers. Single MoS₂ layers can convert light frequencies efficiently but are too thin to be used to build devices. Larger crystals of MoS₂, however, tend to be more stable in a non-color converting form. To fabricate the necessary crystals, 3R-MoS₂, the team worked with the commercial 2D-material supplier HQ Graphene.

With 3R-MoS₂ in hand, researchers began peeling off samples of varying thickness to test how efficiently they converted the frequency of light and saw extremely large enhancement almost immediately.

The new crystal is as efficient but 100 times smaller than lithium niobate — currently the most-popular crystal for wave-guided conversion and generating entangled photons — and flexible enough that it can be combined with silicon photonic platforms to create optical circuits on chips, following the trajectory of ever-smaller electronics.

With the result, the bottleneck toward real-life applications is large-scale production of 3R-MoS₂ and high-throughput structuring of devices — at which point the industry will need to take over.

“I’ve been working on nonlinear optics for more than 30 years now,” said Cerullo. “Research is most often incremental, slowly building on what came before. It’s rare that you do something completely new with big potential. I have a feeling that this new material could change the game.”

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