Researchers from Aalto University have demonstrated a new manufacturing approach that allows atomically thin materials to hold light on a chip for millions of cycles – a result that, in the words of the authors, surpasses previous achievements by about three orders of magnitude. The work, published on April 13 in the journal "Nature Materials" and carried out by an international team, overcomes one of the most persistent obstacles to the creation of photonic chips based on the so-called van der Waals materials.
Microscopic "armor" for fragile crystals
Van der Waals materials are ultra-thin layered crystals from the same broad family to which graphene belongs. They attract enormous interest in photonics because of their atomically smooth surfaces, which minimize light loss. Standard nano-processing tools, however – such as focused ion beam lithography – usually damage their delicate crystal lattice and thus limit real applications.
The team, led by Aalto University, offers an elegant solution: a thin layer of aluminum is applied to the material before processing. "This aluminum layer acts as a microscopic armor," explains researcher Andreas Liapis. "It absorbs the destructive effect of the ion beam and allows us to process the material with sub-nanometer precision below 100 nanometers, while preserving the quality of the crystal."
Using this "shield", researchers have created miniature circular structures – microdiscs that hold light with extremely small losses. The devices show a quality factor (Q-factor) above one million, which means that only about one millionth of the light is lost per cycle. In practice, this allows the light to go around the disc along its trajectory millions of times before it fades.
Record efficiency in light conversion
Because light is held so efficiently in microdiscs, it interacts with the material much more intensely, which greatly enhances nonlinear optical effects. In tests of second harmonic generation – a process in which light is converted from one frequency to another – the team recorded an increase in efficiency by four orders of magnitude, i.e. about 10,000 times, compared to previous records for such systems.
"These indicators surpass previous resonant systems based on van der Waals materials by three orders of magnitude, which is a huge breakthrough in the field," emphasizes Professor Zhipeng Sun, head of the photonics group at Aalto University.
The achievement opens up the prospect of creating reconfigurable photonic circuits, quantum light sources and highly sensitive optical sensors, integrated directly on a chip. It shows that materials that were recently considered too fragile for engineering solutions can become key building blocks for next-generation photonic devices.