Photonic appli­cations harness the power of light-matter interactions to generate various intriguing phenomena. This has enabled major advances in communi­cations, medicine, and spectroscopy, among others, and is also used in laser and quantum technologies. Now, researchers at the Department of Physics at Chalmers University of Techno­logy have succeeded in combining two major research fields – nonlinear and high-index nano­photonics – in a single disk-like nanoobject.

“We were amazed and happy by what we managed to achieve. The disk looking structure is much smaller than the wavelength of light, yet it's a very efficient light frequency converter. It is also 10,000 times, or maybe even higher, more efficient than the unstructured material of the same kind, proving that nanostruc­turing is the way to boost efficiency,” says Georgii Zograf. Somewhat simpli­fied, it is a combination of material and optical resonances with the ability to convert light frequency through crystal’s non-linearity that the researchers have combined in the nanodisk.

In its fabri­cation, they have used transition metal dichalco­genide (TMD), namely molybdenum disulfide, an atomi­cally thin material that has outstanding optical properties at room temperature. The problem with the material is however that it is very difficult to stack without losing its nonlinear properties due to its crystal­line lattice symmetry constraints. “We have fabricated for the first time a nanodisk of specifically stacked molybdenum disulfide that preserves the broken inverse symmetry in its volume, and therefore maintains optical non­linearity. Such a nanodisk can maintain the nonlinear optical properties of each single layer. This means that the material's effects are both maintained and enhanced," says Georgii Zograf.

The material has a high refractive index, meaning that light can be more effectively compressed in this medium. Further­more, the material has the advantage of being trans­ferable on any substrate without the need to match the atomic lattice with the underlying material. The nano­structure is also very efficient in localizing electro­magnetic field and genera­ting doubled frequency light out of it, a second-harmonic generation. It's a nonlinear optical phenomenon, for example, similar to the sum- and difference-frequency generation effects used in high-energy pulsed laser systems. Thus, this nanodisk combines extreme non­linearity with high-refractive index in a single, compact structure.

“Our proposed material and design are state-of-the-art due to extremely high inherent nonlinear optical properties and notable linear optical properties – a refractive index of 4.5 in the visible optical range. These two properties make our research so novel and potentially attrac­tive even to the industry,” Georgii Zograf says. “It really is a milestone, particularly due to the disk's extremely small size. Second harmonic generation and other non-linearities are used in lasers every day, but the platforms that utilise them are typically on the centimeter scale. In contrast, the scale of our object is about 50 nanometers, so that's about a 100,000 times thinner structure,” says research leader Timur Shegai.

The researchers believe that the nanodisk’s work will push photonics research forward. In the long term, TMD materials’ incredibly compact dimensions, combined with their unique properties, could potentially be used in advanced optical and photonic appli­cations. For example, these structures could be integrated into various kinds of optical circuits, or used in miniaturizations of photonics. “We believe it can contribute towards future nonlinear nano­photonics experiments of various kinds, both quantum and classical. By having the ability to nano structure this unique material, we could dramati­cally reduce the size and enhance effi­ciency of optical devices, such as nanodisk arrays and metasurfaces. These inno­vations could be used for appli­cations in nonlinear optics and the generation of entangled photon pairs. This is a first tiny step, but a very important one. We are only just scratching the surface,” says Timur Shegai. (Source: Chalmers U.)

Reference: G. Zograf et al.: Combining ultrahigh index with exceptional nonlinearity in resonant transition metal dichalcogenide nanodisks, Nat. Phot. 18, 751 (2024); DOI: 10.1038/s41566-024-01444-9

Link: Nano- and Biophysics, Dept. of Physics, Chalmers University of Technology, Göteborg, Sweden