Unique Nanodisc Accelerates Photonics Research

Researchers at Chalmers University of Technology in Sweden have succeeded for the first time in combining two major research areas in photonics, creating a nano-object with unique optical properties. Because the object is a thousand times thinner than a human hair, yet very strong, this breakthrough has huge potential for the development of efficient and compact nonlinear optical devices. “In my opinion, this discovery has huge potential,” says Professor Timur Shegai, who led the study at Chalmers.

Photonic applications harness the power of light-matter interactions to generate a variety of intriguing phenomena. This has enabled significant advances in communications, medicine, and spectroscopy, among others, and is also used in laser and quantum technologies. Now, scientists from 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 delighted with what we were able to achieve. The disk-like structure is much smaller than the wavelength of light, yet it is a very efficient transducer of light frequencies. It is also 10,000 times, or even more, more efficient than the same type of unstructured material, proving that nanostructuring is a way to increase efficiency,” says Dr. Georgii Zograf, lead author of the paper in Nature Photonics where the research results are presented.

New production without loss of properties

In simple terms, it is a combination of material and optical resonances with the ability to convert light frequencies through the nonlinearity of the crystal, which the researchers combined in the nanodisk. To produce it, they used a transition metal dichalcogenide (TMD), namely molybdenum disulfide, a material with an atomically thin structure that has exceptional optical properties at room temperature. The problem with the material, however, is that it is very difficult to stack without losing its nonlinear properties due to the symmetry constraints of the crystal lattice.

“For the first time, we have created a nanodisk made of specially arranged molybdenum disulfide, which preserves broken inverse symmetry in its volume and therefore maintains optical nonlinearity. Such a nanodisk can preserve the nonlinear optical properties of each individual layer. This means that the material effects are both preserved and enhanced,” says Georgii Zograf.

The material has a high refractive index, which means that light can be compressed more efficiently in this medium. In addition, the material has the advantage that it can be transferred to any substrate without having to match the atomic lattice to the base material. The nanostructure is also very efficient at localizing an electromagnetic field and generating double-frequency light from it, an effect called second-harmonic generation. This is a so-called nonlinear optical phenomenon, for example similar to the sum and difference frequency generation effects used in high-energy laser systems.

As a result, the nanodisc combines extreme nonlinearity with a high refractive index in a single, compact structure.

A major step forward in optics research

“Our proposed material and design are state-of-the-art due to their extremely high inherent nonlinear optical properties and remarkable linear optical properties – a refractive index of 4.5 in the visible range. These two properties make our research so innovative and potentially attractive even for industry,” says Georgii Zograf.

“This is truly a milestone, especially given the extremely small size of the disk. Second-harmonic generation and other nonlinearities are used in lasers every day, but the platforms that use them are typically on the centimeter scale. In contrast, our object is on the scale of about 50 nanometers, which is about 100,000 times thinner,” says lead researcher Professor Timur Shegai.

Scientists believe that the nanodisc work will push photonics research forward. In the long term, the extremely compact dimensions of TMD materials combined with their unique properties could potentially find use in advanced optical and photonic applications. For example, these structures could be integrated into various types of optical circuits or used in photonics miniaturization.

“We believe this could contribute to future experiments in nonlinear nanophotonics of all kinds, both quantum and classical. By having the ability to nanostructure this unique material, we could dramatically reduce the size and increase the efficiency of optical devices such as nanodisk arrays and metasurfaces. These innovations could be used in applications in nonlinear optics and the generation of entangled photon pairs. This is a small first step, but a very important one. We are just scratching the surface,” says Timur Shegai.