Redefining optical limits: Engineers discover enhanced nonlinear optical properties in 2D materials

Recently, according to a paper published in Nature Communications titled "Phonoenhanced nonlinearities in hexagonal boron nitride," engineers from Columbia University collaborated with theoretical experts from the Max Planck Institute of Material Structure and Dynamics to discover that pairing lasers with lattice vibrations can improve the nonlinear optical properties of layered two-dimensional materials.

Cecilia Chen, a doctoral student in engineering at Columbia University and co-author of the latest paper, and colleagues from her Alexander Gaeta quantum and nonlinear photonics group used hexagonal boron nitride (hBN). HBN is a two-dimensional material similar to graphene: its atoms are arranged in a honeycomb like repeating pattern, which can be peeled off into thin layers with unique quantum properties. Chen pointed out that hBN is stable at room temperature, and its constituent elements - boron and nitrogen - are very light. This means they vibrate very quickly.

Understanding atomic vibrations
Atomic vibrations occur in all materials above absolute zero. This motion can be quantized as quasi particles called phonons, with specific resonances; In the case of hBN, the team is interested in optical phonon modes that vibrate at 41 THz, with a wavelength of 7.3 μ m. Located in the mid infrared region of the electromagnetic spectrum.

Although the mid infrared wavelength is considered short and therefore has high energy, in images of crystal vibrations, they are considered long and have low energy in most laser optical studies, with the vast majority of experiments and studies conducted in the visible to near-infrared range, approximately 400nm to 2um.

experimental result 
When they tune the laser system to match 7.3 μ When m corresponds to the hBN frequency, Chen, his doctoral student Jared Ginsberg (now a data scientist at Bank of America), and postdoctoral researcher Mehdi Jadidi (now the team leader of quantum computing company PsiQuantum) are able to simultaneously drive phonons and electrons in the hBN crystal, effectively generating new optical frequencies from the medium, which is a fundamental goal of nonlinear optics. The theoretical work led by Professor Angel Rubio from the Max Planck Institute helped the experimental team understand their results.

They used commercial desktop mid infrared lasers to explore the phonon mediated nonlinear optical process of four wave mixing, in order to generate light close to even harmonics of optical signals. They also observed that the number of third-order harmonics produced increased by more than 30 times compared to the case where phonons were not excited.

Dr. Chen said, "We are pleased to demonstrate that amplifying natural phonon motion through laser driving can enhance nonlinear optical effects and generate new frequencies.". The team plans to explore how to use light to modify hBN and similar materials in future work.

This study was funded by the US Department of Energy, the European Research Council, and the German Research Association.

Source: Sohu