Photon upconversion: Steering light with supercritical coupling

Photon upconversion, the process of converting low-energy photons into high-energy photons, is an important technique with a wide range of applications, ranging from super-resolution imaging to advanced photonic devices. Despite considerable progress, the search for efficient photon upconversion has faced challenges due to the inherent limitations in irradiation of lanthanide-doped nanoparticles and the critical conditions of optical resonance coupling.

The concept of “supercritical coupling” plays an important role in addressing these challenges. This fundamentally new approach, proposed by a research team led by Prof. LIU Xiaogang from the NUS Department of Chemistry and his colleague, Dr. Gianluigi ZITO from the National Research Council of Italy, called “bound states in a continuum” ( BICs) take advantage of the physics. ). BICs are phenomena that enable light to be trapped in open structures with theoretically infinite lifetimes, transcending critical pair boundaries. These phenomena differ from the normal behavior of light. By coupling 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. .

Their findings are published in the journal The nature.

Experimental validation of supercritical coupling marks a significant leap forward, leading to eight orders of magnitude increase in upconversion luminescence. The experimental setup includes a photonic-crystal nanolab with upconversion nanoparticles. These nanoparticles act as microscale sources and lasers. The unique properties of BICs, characterized by negligible light scattering and microscale dimensions of light spots, were exploited to achieve precision in focusing and directional control of emitted light. This opens up new avenues for controlling lighting conditions.

“This breakthrough is not only a fundamental discovery, but represents a paradigm shift in the field of nanophotonics, transforming our understanding of light manipulation at the nanoscale,” said Professor Liu. “The implications of critical coupling extend beyond photon conversion and offer potential breakthroughs in quantum. photonics and various systems based on coupled resonators.”

“As the research community grapples with the implications of this work, the door is open to a future where light, one of the most fundamental elements of our universe, can be measured with unprecedented precision and efficiency,” added Professor Liu. can be controlled.”