Set a new world record! Optical crystals as thin as cicada wings increase energy efficiency by over a hundred times

On quartz sheets, the angular rhombic boron nitride crystals with a thickness of only 1 to 3 microns are as thin as cicada wings, but their energy efficiency is 100 to 10000 times higher than traditional optical crystals. At the opening ceremony of the 2024 Zhongguancun Forum Annual Conference held on April 25th, the world's thinnest known optical crystal was listed as one of the top ten technological achievements.

The rhombohedral boron nitride crystal laser technology on fused silica has gone through more than 60 years of development since it was invented by scientists. "Laser technology is the cornerstone of our current technological civilization. In fields such as micro and nano processing, quantum light sources, and biological monitoring, laser technology is shining brightly." Professor Liu Kaihui from the School of Physics at Peking University said that breakthroughs in laser technology highly rely on a special material - optical crystals. The implementation of functions such as laser frequency conversion, pulse compression, and information processing cannot be achieved without optical crystals. "It can be said that optical crystals are the 'heart' of laser technology."

If a laser is carefully dissected, there are a total of three steps required for energy to go through from the input instrument to the output laser. Electric energy enters the seed light source generated by the instrument excitation, passes through specific laser crystals and optical crystals, and "runs back and forth" in the resonant cavity to form resonance, ultimately producing various types of lasers with different functions. The miniaturization, integration, and functionalization of lasers are one of the core directions for the development of future laser technology, but traditional optical crystals are difficult to efficiently produce lasers within a limited thickness. Aiming at the goal of preparing thinner optical crystals, Wang Enge, an academician of the CAS Member, together with Liu Kaihui, led the team to tackle key problems for more than 10 years.

Their raw materials are lighter than others, and the relative molecular weight of light elements such as boron, carbon, and nitrogen is relatively small. After repeated combination attempts, lightweight boron nitride has become the most preferred. The single-layer thickness of the rhombic boron nitride material prepared with it is 0.34 nanometers, which is only one hundred thousandth of the diameter of an ordinary human hair. However, some of its properties can be comparable to traditional centimeter level optical crystal materials.

However, single-layer boron nitride molecules cannot be used as optical crystals for laser manufacturing. "We need to make it grow and thicken in a specific direction," said Liu Kaihui. However, experiments have found that if layers of boron nitride molecules are simply stacked like building blocks, the laser will have "inconsistent steps" when passing through, resulting in phase mismatch, which leads to the inability of the laser to output successfully and efficiently.

In the traditional crystal research system, the emergence of this phenomenon almost declares the failure of this material, and can only be replaced with new materials for re development. But the R&D team did not give up on this. Through complex theoretical derivation, they discovered a new crystal design method - when each rhombic boron nitride material is rotated at a specific angle like a Rubik's cube, the stacked optical crystals can reduce the energy "internal friction" of the laser passing through and efficiently produce the required laser.

"We summarize this rule as the interface angle theory of two-dimensional materials. This is another major original breakthrough in optical crystal theory since the invention of laser technology." Liu Kaihui sighed, "The combination of new crystal design theory and preparation methods has successfully" slimmed down "optical crystals. Traditional optical crystals have thicknesses ranging from millimeters to centimeters, while the thickness of angle boron nitride optical crystals is only 1 to 3 micrometers.". The application of this theory is expected to reduce the size of lasers to the millimeter or even micrometer level in the future. Many materials that were once considered incapable of manufacturing optical crystals may also be revitalized by the rotation of material stacking angles.

At the Huairou Science City, the cross platform of light element quantum materials jointly built by Peking University and Beijing provides greater growth space for optical crystals. "The maximum diameter of crystals produced in the laboratory is only five or six centimeters. To achieve the industrialization of laser technology, larger crystals need to be made." Liu Kaihui said that currently, the cross platform is undergoing production equipment debugging, and it is expected that optical crystals with a diameter of tens of centimeters will flourish here in the future.

"Optical crystals are the cornerstone of laser technology development. Whoever masters the design theory and preparation technology of optical crystals will grasp the future of laser technology." Wang Enge said that ultra-thin and high-energy efficient angle boron nitride optical crystals have laid the theoretical and material foundation for the new generation of laser technology. Its excellent integrability and new functions are expected to achieve new application breakthroughs in fields such as quantum light sources, quantum communication, photonic chips, and artificial intelligence in the future.

Source: Yangtze River Delta Laser Alliance