World's Thinnest Known Optical Crystal Arrives!

On a quartz substrate lies a twisted rhombohedral boron nitride crystal mere 1-3 micrometers thick—thinner than a cicada's wing—yet its energy efficiency surpasses traditional optical crystals by 100 to 10,000 times. This breakthrough, developed by Chinese scientists, represents the world's thinnest known optical crystal. It was unveiled as a major achievement at the opening ceremony of the 2024 Zhongguancun Forum on April 25.

Optical crystals are the "heart" of laser technology. "Laser technology underpins modern technological civilization, playing pivotal roles in micro-nano manufacturing, quantum light sources, and biomedical monitoring," explains Professor Liu Kaihui from Peking University's School of Physics. Breakthroughs in laser technology rely heavily on optical crystals, a critical material.

The future of lasers hinges on integration, miniaturization, and multifunctionality. However, traditional optical crystals struggle to efficiently generate laser output within limited thickness, making the development of thinner crystals a global scientific priority.

After extensive experimentation, Chinese researchers identified boron nitride as the optimal material due to its lightweight properties. Yet, simply stacking boron nitride layers caused "phase mismatch"—like an out-of-sync orchestra—when laser light passed through, blocking efficient laser output and rendering it unusable for laser devices.

A team led by Academician Wang Enge and Professor Liu Kaihui from Peking University's Quantum Materials Science Center, along with Associate Researcher Hong Hao, pioneered a novel crystal design: twisting individual rhombohedral boron nitride layers at specific angles before stacking them. This "twisted" structure reduces energy loss during laser transmission, enabling highly efficient laser generation.

By combining this groundbreaking design with advanced fabrication techniques, the team slimmed the optical crystal to 1-3 micrometers—a fraction of the millimeter-to-centimeter thickness of traditional crystals.

The researchers summarized their approach as the "interfacial twist angle theory of 2D materials." "This theory could shrink laser devices to micrometer scales and revitalize materials previously deemed unsuitable for optical crystals by adjusting stacking angles," Liu told reporters.

This innovation not only advances laser technology but also opens new avenues for ultra-compact, high-performance optical devices.