Chalmers U Tech combines nonlinear photonics and high-index nanophotonics

17 Sep 2024

Nanodisk of MoS2 preserves “broken inverse symmetry” to maintain optical nonlinearity.

Scientists at Chalmers University of Technology, Gothenburg, Sweden, have for the first time succeeded in combining two major research fields in photonics by creating a nanoobject with special optical qualities.

The researchers at the Department of Physics have succeeded in combining two major research fields – nonlinear and high-index nanophotonics – into a single disk-like nanoobject.

Professor Timur Shegai, who led the study at Chalmers, said, “My feeling is that this breakthrough has great potential in the development of efficient and compact nonlinear optical devices.”

“The disk-looking structure is much smaller than the wavelength of light, yet it’s a very efficient light frequency converter. It is also at least 10,000 times more efficient than the unstructured material of the same kind, proving that nano structuring is the way to boost efficiency,” said Dr. Georgii Zograf, lead author of the explanatory article in Nature Photonics.

No loss of properties

Essentially, the development 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 – molybdenum disulfide – an atomically thin material with significant optical properties at room temperature. The problem with the material is however that it is 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,” said Dr. 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 nano structure is also very efficient in localizing electromagnetic field and generating doubled frequency light; second-harmonic generation. This is a nonlinear optical phenomenon, 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, says the Chalmers team.

‘Big step’

“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,” said Dr. Zograf.

“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 utilize them are typically on the centimeter scale. In contrast, the scale of our object is about 50 nanometers,” said research leader Prof. Shegai.

The researchers believe that the nanodisk development will push photonics research forward. In the long term, TMD materials’ compact dimensions, combined with their special properties, could potentially be used in applications in optical circuits, or miniaturization of photonics systems.

Prof. Shegai concluded, “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 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.”