A research team including scientists at the Institute for Basic Science (IBS) in South Korea has uncovered a groundbreaking way for atoms to emit and absorb light using time-periodic photonic structures, known as photonic time crystals (PTCs). Their findings challenge the conventional understanding of light-matter interaction and reveal a new physical process called spontaneous emission excitation, where atoms absorb energy from temporal modulation while emitting light. This phenomenon is impossible in static environments.​

For decades, scientists have controlled spontaneous emission by engineering the spatial environment around atoms, such as using resonant cavities or photonic crystals. However, no prior work had shown how time-varying optical environments affect the fundamental nature of spontaneous emission.

The team applied Floquet analysis and classical light-matter theory to photonic time crystals (PTCs), materials whose optical properties vary periodically in time rather than space. They discovered:

• - Substantial enhancement of spontaneous emission near the momentum-gap frequency in PTCs
• - Spontaneous emission excitation, where atoms absorb energy from temporal modulation and emit a photon simultaneously
• - A critical role of non-Hermitian physics and exceptional points in determining emission behavior

Notably, the study overturns earlier predictions that spontaneous emission should vanish at the time-crystal band edge. Instead, the team showed that the non-orthogonality of Floquet modes, quantified by the Petermann factor, counteracts this effect to yield a finite and even enhanced emission rate.

“Our work demonstrates that time modulation provides a previously unexplored lever for controlling light-matter interaction,” the authors explain. “Unlike traditional photonic crystals, photonic time crystals provide an intrinsically nonequilibrium environment where gain, loss, and mode non-orthogonality give rise to entirely new physical processes

This discovery could open new directions in quantum optics, time-varying photonic devices, and energy-efficient light sources. Future research will explore the full quantum electrodynamic description of time-modulated photonic platforms.

Figure 1. (a) Time-periodic modulation of the permittivity ε(t) forms a momentum-gap region. (b) In the high-loss regime, positive kDOS enhances spontaneous emission. (c) In the low-loss regime, negative kDOS indicates spontaneous emission excitation, where atoms absorb energy from time modulation while emitting photons. This process arises from non-Hermitian dynamics and non-orthogonal Floquet modes near exceptional-point boundaries.