This demonstration highlights the potential for RE elements to produce stable multiband lasing. While erbium and ytterbium were used in this study, other RE elements, such as thulium, holmium, and neodymium, may enable adaptable pump schemes and a wealth of lasing wavelengths.
The ultrahigh-Q doped microcavity may also provide a good platform for ultrahigh-precision sensing and research into cavity-matter-light interactions, in addition to its benefits for laser applications.
In the 60-some years since they were invented, lasers have absolutely transformed our lives.
Giulio Cerullo, Nonlinear Optics Researcher, Politecnico di Milano
Today, laser-using gadgets are ready to go much smaller thanks to new research from Cerullo and partners at Columbia University, which was published in Nature Photonics.
Xinyi Xu, a Ph.D. student, and postdoc Chiara Trovatello investigated a 2D substance called molybdenum disulfide while working in engineer James Schuck’s lab at Columbia (MoS2).
They described the effectiveness of devices made from stacks of MoS2 that are less than one-millimeter thick—100 times thinner than a human hair—in converting light frequencies at telecom wavelengths to produce various hues.
According to Trovatello, who recently finished her Ph.D. with Cerullo in Milan, this new research is a first step toward replacing the typical materials used in today’s tunable lasers, which are estimated in millimeters and centimeters.
“Nonlinear optics is currently a macroscopic world, but we want to make it microscopic,” she said.
Coherent light is a unique type of light that lasers emit, meaning that every photon in the beam has the same frequency and color.
Lasers can only function at a limited range of frequencies, yet systems frequently require the ability to use a variety of laser light colors. A green laser pointer, for instance, is actually an infrared laser that a macroscopic substance transforms into a visible hue.
Nonlinear optical techniques are employed by researchers to alter the color of laser light; however, the traditional materials utilized must be relatively thick for color conversion to happen effectively.
One of the most investigated transition metal dichalcogenides—a new family of materials that can be peeled into atomically thin layers—is MoS2. Despite being too thin to be utilized in the construction of devices, single layers of MoS2 are capable of efficiently converting light frequencies.
On the other hand, larger MoS2 crystals are typically more stable in their non-color converting state. Working with the commercial 2D material source HQ Graphene, the team created the required crystals, known as 3R-MoS2.
Xu started peeling off samples of various thicknesses of 3R-MoS2 to gauge how well they translated light frequency. The outcomes were remarkable right away.
“Rarely in science do you start on a project that ends up working better than you expect—usually it’s the opposite. This was a rare, magical case,” commented Schuck.
According to Xu, in most cases, specialized sensors are required and take some time to register the light emitted by a sample.
“With 3R-MoS2, we could see the extremely large enhancement almost immediately,” he said. The fact that the team captured these conversions at telecom wavelengths is noteworthy since it is an important aspect of possible optical communications applications like the delivery of internet and television services.
By chance, Xu concentrated on a random crystal edge during one scan and noticed fringes that indicated waveguide modes existed inside the material.
Waveguide modes can potentially be utilized to produce so-called entangled photons, a crucial element of quantum optics applications, by keeping various color photons in sync, which would otherwise move at different speeds across the crystal.
The group delivered their equipment to physicist Dmitri Basov’s lab, where postdoc Fabian Mooshammer verified their suspicion.
Lithium niobate is now the most often used crystal for waveguided conversion and producing entangled photons—it is a hard, rigid material that requires a substantial amount of thickness to achieve effective conversion efficiencies.
Following the trend of ever-smaller electronics, 3R-MoS2 is equally efficient but 100 times smaller and sufficiently flexible to be integrated with silicon photonic platforms to produce optical circuits on chips.
With this solid evidence finding, 3R-MoS2 production on a wide scale and high-throughput device structuring are the barriers to real-world applications. The team claims that the industry will have to take control there. They believe they have shown the potential of 2D materials with this effort.
“I’ve been working on nonlinear optics for more than thirty years now. Research is most often incremental, slowly building on what came before. It is rare that you do something completely new with big potential. I have a feeling that this new material could change the game,” said Cerullo.
Xu, X., et al. (2022) Towards compact phase-matched and waveguided nonlinear optics in atomically layered semiconductors. Nature Photonics. doi.org/10.1038/s41566-022-01053-4.