In a study published Nov. 24 in Nature Communications, Columbia Engineering researchers and collaborators at the Max Planck Institute for the Structure and Dynamics of Matter find that pairing laser light to crystal lattice vibrations can enhance the nonlinear optical properties of a layered 2D material. The finding could lead to new ways of using light to modify materials.
Cecilia Chen, a Columbia Engineering PhD student and co-author of the paper, and her colleagues from the Quantum and Nonlinear Photonics group, led by Alexander Gaeta, David M. Rickey Professor of Applied Physics and Materials Science, used hexagonal boron nitride (hBN). A 2D material similar to graphene, hBN’s atoms are arranged in a honey-combed-shaped repeating pattern and can be peeled into thin layers with unique quantum properties. Chen noted that hBN is stable at room temperature, and its constituent elements—boron and nitrogen—are very light. That means they vibrate very quickly.
Atomic vibrations occur in all materials above absolute zero. That movement can be quantized into quasiparticles called phonons with particular resonances; in hBN's case, the team was interested in the optical phonon mode corresponding to a wavelength of 7.3 μm, in the mid-infrared regime of the electromagnetic spectrum. While mid-IR wavelengths are considered short, and thus, high energy, in the picture of crystal vibrations, they are considered very long and low energy in most optics research, where the overwhelming majority of experiments and studies are performed in the visible to near-IR range of approximately 400 nm to 2 um.
When they tuned their laser system to hBN’s frequency, 7.3 μm, Chen, along with Jared Ginsberg PhD’21 (now a data scientist at Bank of America) and postdoc Mehdi Jadidi (now a team lead at PsiQuantum), were able to coherently drive the resonance of the hBN crystal and generate new optical frequencies from the medium—a goal of nonlinear optics. Theoretical work led by Angel Rubio’s group at Max Planck helped the experimental team understand their results.
Using commercially available, table-top mid-infrared lasers, they explored the phonon-mediated four-wave mixing around even harmonic orders. The crystal could withstand up to 10% atomic displacement without damage. They also observed a 30-fold increase in third-harmonic generation.
“We’re excited to show that amplifying the natural phonon motion with laser driving can enhance nonlinear optical effects and generate new frequencies,” said Chen. In future work, the team plans to explore how to modify hBN and materials like it using light.
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