Researchers from Columbia University and Politecnico di Milano used an atomically thin material to build microscopic color converters. The advancement is a first step toward replacing the standard materials used in today’s tunable lasers, which are measured in millimeters and centimeters. “Nonlinear optics is currently a macroscopic world, but we want to make it microscopic,” said Chiara Trovatelloa, a postdoctoral student who worked on the research in the lab of James Schuck, an associate professor of mechanical engineering at Columbia. The device — a fraction of the size of conventional color converters — could produce new kinds of ultrasmall optical circuit chips and advance quantum optics, the researchers said. Devices that use lasers often need to be able to deploy different colors of laser light. For example, a green laser pointer is produced by an infrared laser that is converted to a visible color by a macroscopic material. Although researchers use nonlinear optical techniques to change the color of laser light, conventionally used materials must be relatively thick for color conversion to occur efficiently. Molybdenum disulfide (MoS2) is one of the most studied transition metal dichalcogenides, which are 2D materials that can be peeled into atomically thin layers. Single layers of MoS2 can convert light frequencies efficiently, but they are too thin to be used to build devices. Larger crystals of MoS2 tend to be more stable in a noncolor converting form. To fabricate the necessary crystals, known as 3R-MoS2, the team worked with the commercial 2D-material supplier HQ Graphene. The researchers characterized how efficiently devices built from stacks of MoS2 less than 1 μm thick convert light frequencies at telecom wavelengths to produce different colors. Tiny crystals made from molybdenum disulfide (MoS2) efficiently changed the color of light, which could help researchers shrink laser-based devices to microscopic scales. Courtesy of Nicoletta Barolini/Columbia University. Using 3R-MoS2, the researchers tested how efficiently samples of varying thickness converted the frequency of light. Special sensors are usually needed to register the light produced by a sample, and it takes a long time for them to do so, said Xinyi Xu, a doctoral student in Schuck's lab. “With 3R-MoS2, we could see the extremely large enhancement almost immediately," he said. The team recorded these conversions at telecom wavelengths. In one scan, Xu focused on a random edge of a crystal and saw fringes that suggested that waveguide modes were present inside the material. Waveguide modes keep different color photons in sync that might otherwise move at different speeds across the crystal, and they could possibly be used to generate entangled photons. Currently, the most popular crystal for waveguided conversion and generating entangled photons is lithium niobate, a hard and stiff material that needs to be fairly thick for achieving useful conversion efficiencies. 3R-MoS2 is equally efficient but is 100x smaller and flexible enough to be combined with silicon photonic platforms to create optical circuits on chips, following the trajectory of ever-smaller electronics. The challenge to realizing real-world applications of the color converters is large-scale production of 3R-MoS2 and high-throughput structuring of the devices, the researchers said. The research was published in Nature Photonics (www.doi.org/10.1038/s41566-022-01053-4).