Controlling optical non-linearity: A new technique with a twist

Non-linear optics, the study of how intense light interacts with matter, plays an important role in optical applications such as ultrafast optical signal processing, ultrafast switches, lasers and sensors. Through the discovery of new uses for light, non-linear optics is promoting advances in areas such as medical testing, communications and solar energy harvesting.

Researchers supported by the EU-funded QSpec-NewMat and PeSD-NeSL projects have now developed a novel method for modulating an important non-linear optical process called second harmonic generation (SHG). In SHG, also known as frequency doubling, two photons with the same frequency interact with a non-linear material and are combined to produce a new photon with twice the energy of the initial photons. The 2D material the researchers used for this optical process was hexagonal boron nitride. “Our work is the first to exploit the dynamically tunable symmetry of 2D materials for nonlinear optical applications,” stated Assoc. Prof. James Schuck of Columbia University in a news item posted on the ‘EurekAlert!’ website. The study was published in the journal ‘Science Advances’.The electrical properties of 2D materials have been found to change significantly depending on the angle between their layers. In their work, the research team members showed that this concept of rotating or twisting one layer relative to another – twistronics – also applied to optical properties. “We are calling this new research area ‘twistoptics’,” remarked study co-author Assoc. Prof. Schuck. “Our twistoptics approach demonstrates that we can now achieve giant nonlinear optical responses in very small volumes--just a few atomic layer thicknesses--enabling, for example, entangled photon generation with a much more compact, chip-compatible foot print. Moreover, the response is fully tunable on demand.”

Most conventional non-linear optical crystals have rigid structures, making it difficult to control the material’s optical properties. The team’s solution to gaining this needed control was through the use of van der Waals crystals. The crystals’ weak interlayer force makes it easy to manipulate crystal orientation between layers. Thin films of boron nitride (known for its weak van der Waals interaction between layers) were therefore stacked on top of each other at different twist angles.The researchers were able to precisely tune optical SHG with micro-rotator devices. They were also able to greatly enhance SHG with van der Waals vertical superlattice structures with multiple twisted interfaces. “We showed that the nonlinear optical signal actually scales with the square of the number of twisted interfaces,” observed first author Kaiyuan Yao of Columbia University. “So this makes the already large nonlinear response of a single interface orders of magnitude stronger still.” The enhanced non-linear response produced from this process could point to a new atomically precise manufacturing method for efficient non-linear optical crystals. “We hope that this demonstration provides a new twist in the ongoing narrative aimed at harnessing and controlling the properties of materials,” Assoc. Prof. Schuck concluded.

The QSpec-NewMat (Quantum Spectroscopy: exploring new states of matter out of equilibrium) and PeSD-NeSL (Photo-excited State Dynamics and Non-equilibrium States under Laser in Van der Waals Stacked Two-dimensional Materials) projects are coordinated by Germany’s Max Planck Institute for the Structure and Dynamics of Matter. QSpec-NewMat ends in September 2021 and PeSD-NeSL in June 2022.

For more information, please see:

QSpec-NewMat project web page

PeSD-NeSL project