KAIST

< (From left) Professor Bumki Min, Ph.D. Candidate Kyungmin Lee >

A groundbreaking discovery reveals that spontaneous emission, a key phenomenon in the interaction between light and atoms, manifests in a new form within a Photonic Time Crystal (PTC). This research, led by a KAIST team, not only overturns existing theory but further predicts a novel phenomenon: spontaneous emission excitation.

Professor Bumki Min's research team from the KAIST Department of Physics, in collaboration with Professor Jonghwa Shin of the Department of Materials Science and Engineering, Professor Wonju Jeon of the Department of Mechanical Engineering, Professor Gil Young Cho of the Department of Physics, and researchers from IBS, UC Berkeley, and the Hong Kong University of Science and Technology, announced that they have proven that the spontaneous emission decay rate in a Photonic Time Crystal is, on the contrary, enhanced rather than being "extinguished," as suggested by a paper published in Science in 2022. Furthermore, they predicted a new process—spontaneous emission excitation—where an atom transitions from its ground state to an excited state while simultaneously emitting a photon.

< Spontaneous emission decay rate (left) and spontaneous emission excitation rate (right) when a quantum emitter (or atom) is placed in an environment that changes very rapidly and periodically in time >

Spontaneous emission is the process by which an atom intrinsically emits a photon and is fundamental to quantum optics and photonic device research. Until now, control over spontaneous emission has been achieved by designing spatial structures like resonators or photonic crystals. However, the advent of Photonic Time Crystals, which periodically modulate the refractive index of a medium over time, has drawn attention to the potential for control along the time axis.

Previous theory predicted that the spontaneous emission decay rate in a Photonic Time Crystal would completely vanish at a specific frequency. In contrast, this study is the first to prove that the decay rate is significantly enhanced. This is attributed to the non-orthogonal mode effect, highlighting the importance of research into non-Hermitian optics.

The research team also predicted and reported a new process, 'spontaneous emission excitation,' where an atom gains energy and transitions from its ground state to an excited state while simultaneously emitting a photon. This is a non-equilibrium process made possible by the time-crystal medium supplying external energy, representing a new light-matter interaction phenomenon that cannot be explained by conventional equilibrium optics.

The findings fundamentally shift the paradigm of spontaneous emission research and hold promise for broad applications in fields such as quantum light source design and non-equilibrium quantum optics.

Professor Bumki Min stated, "This achievement re-establishes the fundamental theory describing spontaneous emission in a rapidly time-varying environment. The enhancement of spontaneous emission decay and the 'spontaneous emission excitation' phenomenon have the potential to change the paradigm of light-matter interaction research."

Ph.D. candidate Kyungmin Lee participated as the first author of this research. The results were published online in the international academic journal Physical Review Letters on September 23, 2025, simultaneously highlighted on Physics.org, and selected as an Editors' Suggestion paper. The research was supported by the National Research Foundation of Korea and the Samsung Science and Technology Foundation.

Note: The paper is titled “Spontaneous emission decay and excitation in photonic time crystals.”