According to a report in the latest issue of Nature Nanotechnology, researchers from Graz University of Technology in Austria, Harvard University, and the University of Texas at Austin have developed a new type of metasurface structure that combines semiconductor layers with nanostructures, increasing light conversion efficiency by approximately 72,000 times compared to previous materials. This breakthrough is expected to advance the development of telecommunications and quantum technologies.

Modern fiber-optic communication networks transmit information by leveraging the linear propagation properties of light. However, for applications such as complex computing, quantum technologies, quantum cryptography, and optical frequency combs used in high-precision measurement, relying solely on linear light propagation is insufficient. It is necessary to utilize nonlinear polarization to enable photons to interact with one another and exchange information.

Traditionally, crystal structures such as lithium niobate have been used to achieve nonlinear coupling of light waves. However, this approach requires large material volumes and high optical power, which limits the miniaturization and application of related devices. The starting point of this research was a special semiconductor material developed by the team. Using molecular beam epitaxy, they fabricated nanoscale semiconductor layers of gallium arsenide and aluminum gallium arsenide, incorporating asymmetrically coupled quantum wells. Quantum wells confine the motion of electrons, causing them to form quantized energy levels. Due to the asymmetric design of the quantum wells, electrons tend to move in one direction when exposed to light. The resulting nonlinear electron oscillations enable more efficient interactions between different light waves.

However, to fully harness the potential of this material, light needs to propagate in a direction parallel to the semiconductor layers. To address this issue, the team constructed a metasurface atop the semiconductor layers. The metasurface consists of titanium dioxide nanopillars arranged in a checkerboard pattern, with each pillar measuring only a few hundred nanometers in size. It redirects the propagation direction of light, allowing it to travel along the semiconductor layers and thereby fully exploit the material's nonlinear properties.

Experiments revealed that when light strikes the device perpendicularly, the optical fields cancel each other out due to the geometric symmetry, making second-harmonic generation nearly impossible. By simply tilting the device by approximately 0.3 degrees, the researchers were able to break this symmetry, enabling efficient nonlinear polarization generation.

Higher conversion efficiency implies the potential to manufacture smaller and lower-energy optical devices in the future, such as reducing energy consumption for information transmission between computers within data centers. This technology may also find applications in integrated photonics and large-scale data processing.