Researchers at the National University of Singapore (NUS) have unveiled a novel concept termed supercritical coupling that enables a several-fold increase in photon upconversion efficiency. This discovery not only challenges existing paradigms but also opens a new direction in the control of light emission, the team said. Photon upconversion, the process of converting low-energy photons into higher-energy ones, is a crucial technique with broad applications, ranging from superresolution imaging to advanced photonic devices. Despite considerable progress, the quest for efficient photon upconversion has faced challenges due to inherent limitations in the irradiance of lanthanide-doped nanoparticles and the critical coupling conditions of optical resonances. Illustration of the principle of “supercritical coupling” and directive upconversion emission through supercritical edge BIC coupling, depicting the layout of the photonic-crystal nanoslab with unit cell geometry and demonstrating collimated upconversion achieved through supercritical coupling tuned at the edge. Courtesy of Nature. The concept of supercritical coupling plays a pivotal role in addressing these challenges. This approach, proposed by a research team led by Liu Xiaogang a professor in the department of chemistry at NUS and his collaborator, Gianluigi Zito from the National Research Council of Italy, leverages the physics of bound states in the continuum (BICs). BICs are phenomena that enable light to be trapped in open structures with theoretically infinite lifetimes, surpassing the limits of critical coupling. These phenomena are different from the usual behavior of light. By manipulating the interaction between dark and bright modes within these structures, similar to the classical analogue of electromagnetically induced transparency, the researchers not only enhanced the local optical field but also precisely controlled the direction of light emission. According to the researchers, the experimental validation of supercritical coupling marks a significant leap forward, demonstrating an eight-orders of magnitude increase in upconversion luminescence. The experimental setup involves a photonic-crystal nanoslab covered with upconversion nanoparticles. These nanoparticles serve as microscale sources and lasers. The unique properties of BICs, characterized by negligible light dispersion and microscale dimensions of the light spots, were harnessed to achieve precision in focusing and directional control of the emitted light. This, the team said, opens up new avenues for controlling the state of the light. According to Liu, the work represents not just a fundamental discovery, but a paradigm shift in the field of nanophotonics. The implications, he said, extend beyond just photon upconversion, and could usher in advances in quantum photonics as well as systems based on coupled resonators. “As the research community grapples with the implications of this work, the door stands open to a future where light, one of the most fundamental elements of our universe, can be controlled with unparalleled precision and efficiency,” Liu said. The research was published in Nature (www.doi.org/10.1038/s41586-023-06967-9).