Researchers from Peking University developed a high-efficiency ultrathin optical crystal based on boron nitride. According to Liu Kaihui, director of the Institute of Condensed Matter and Material Physics at Peking University, the research provides a brand-new design model and material system, opening avenues in basic optics theory, materials science, and potential technologies. Due to the limitations of traditional theory models and material systems, existing crystals have struggled to meet the requirements for developing future laser devices, such as miniaturization, high integration, and functionalization. Optical phase matching involves establishing a proper phase relationship between the fundamental excitation and generated waves to enable efficient optical parametric processes. It is typically achieved through birefringence or periodic polarization. The optical crystal developed at Peking University is based on rhombohedral boron nitride The interlayer twist angle in the material creates a nonlinear geometric phase which compensates for phase mismatch and enables the material to be utilized for second harmonic generation. Courtesy of Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.233801. According to the researchers, the interlayer twist angle in 2D materials creates a nonlinear geometric phase that can compensate for the phase mismatch. The vertical assembly of the 2D layers with a proper twist sequence generates a nontrivial “twist-phase-matching” regime, the researchers said. “The laser generated by optical crystals can be viewed as a marching column of individuals. The twist mechanism can make everyone's direction and pace highly coordinated, greatly improving the energy conversion efficiency of the laser,” said Kaihui, who also serves as deputy director of the Interdisciplinary Institute of Light-Element Quantum Materials at the Beijing Huairou National Comprehensive Science Center. The team developed their crystal from twisted rhombohedral boron nitride films with a thickness of 3.2 μm. The crystal was capable of producing a second-harmonic generation with conversion efficiency of ~8% and enabled facile polarization controllability, which is absent in conventional crystals. “The TBN crystal's thickness ranges from 1 to 10 microns. The thickness of optical crystals we had known before is mostly at the level of a millimeter or even centimeter,” Kaihui said. The methodology, the researchers said, establishes a platform for the rational design and atomic manufacturing of nonlinear optical crystals based on abundant 2D materials. The size, integration potential, and new functionalities enabled by the material are expected to impact quantum light sources, photonic chips, and other fields in the future, said Wang Enge, a professor in Peking University’s School of Physics. The team is currently applying for patents in the U.S., Britain, Japan, and other countries. They have developed a laser prototype and are continuing research on the material’s applications in laser technology. The research was published in Physical Review Letters (www.doi.org/10.1103/PhysRevLett.131.233801).
