Chaotic optical microcavities provide a versatile platform for exploring the interplay between classical ray dynamics and wave behavior, including phenomena such as directional emission, wave localization, and dynamical tunneling. In these cavities, long-lived resonances can concentrate around specific classical trajectories. How these classical trajectories leave fingerprints on wave patterns is a fundamental question in wave chaos. However, the intracavity field is tightly confined and cannot be accessed directly from the far field, so that previous studies have relied mainly on indirect probes such as transmission spectra and far-field pattern, leaving the spatial structures of the resonances challenging to image directly.
A team led by Professor Yun-Feng Xiao at Peking University has now developed a new multiphoton fluorescence imaging strategy to directly visualize these light paths, which identifies paired resonances in a wave-chaotic microcavity and demonstrates dynamical tunneling in real space.
Morphology imaging revealing paired resonances
To realize this imaging scheme, the team deposited a 10-nanometer erbium-doped fluorescent layer on the asymmetric microcavity surface. The evanescent infrared field of a cavity mode is weakly absorbed by erbium ions and converted through multiphoton upconversion into incoherent green fluorescence. The resulting fluorescence pattern directly maps the local mode intensity, allowing the intracavity wave patterns to be observed from the far field with sub-micrometer resolution.
For the first time, the team directly visualize paired resonances in which light undergoes the same number of boundary bounces but forms staggered spatial patterns (Fig. 1). In each pair, an island mode is localized on a stable periodic orbit, whereas a scar-like mode is localized by a partial barrier in the chaotic sea. The researchers observed such paired morphologies for period-7, period-6, and period-5 orbits.
Figure 1. Schematic of direct imaging of paired optical resonances in an asymmetric microcavity, showing representative period-6 and period-5 mode pairs, each consisting of an island mode (left) and a scar-like mode (right).
Demonstration of selective excitation and dynamical tunneling
Owing to the high-resolution images, the team selectively excited different mode families by tuning the nanofiber coupling, bringing the targeted resonance to critical coupling selectively. They further observed dynamical tunneling between classically separated resonances, where distinct spatial mode patterns evolve into hybridized field distributions with clearly resolved fine spatial structures near resonance, accompanied by a clear anti-crossing in the transmission spectra.
Figure 2. Observation of tunneling-mediated mode hybridization. (a) Transmission spectra at different relative mode indices 
, revealing an anticrossing between the two resonances. (b) Experimentally imaged hybridized mode morphology at the anti-crossing, providing direct real-space evidence of dynamical tunneling.
“Seeing the wave pattern directly changes how we study optical microcavities,” said Professor Yun-Feng Xiao, the corresponding author of this work. “Instead of inferring the underlying dynamics only from spectra, we can now watch how distinct resonances are selected, interact, and hybridize in real space.”
By linking observed real-space mode patterns to the underlying ray dynamics, this approach provides a new route to studying localization, tunneling, and transport in chaotic wave systems. It may also help diagnose and control optical modes in nonlinear, topological, and integrated photonic devices.
The work, entitled “Orbit-resolved imaging of paired resonances in a wave-chaotic microcavity”, has been published in Physical Review Letters.
Link: https://doi.org/10.1103/p4qp-wnhr
