When the length of an optical cavity is modulated, theory predicts exponential concentration of energy around particular space-time trajectories. Viewed stroboscopically, photons in such a driven cavity propagate as if in a curved spacetime, with black hole and white hole event horizons corresponding to unstable and stable fixed points of the evolution. Such phenomena have resisted direct experimental realization due to the challenges associated with achieving effective near-relativistic accelerations of massive cavity mirrors. We report results of an experiment that overcomes this limitation by exchanging the roles of light and matter. A matter wave endowed with quasirelativistic dispersion is confined between two barriers made of light, one of which is periodically translated at speeds comparable to the matter wave group velocity. In this strongly modulated cavity we observe the emergence of the predicted bright and dark fixed point trajectories, and we furthermore demonstrate that changing the modulation waveform can vary the number of fixed points and exchange their stability character. We observe signatures of nontrivial dynamics beyond those predicted for photons and attribute them to residual curvature in the dispersion relation. In addition to experimentally realizing and characterizing cavity dynamics in the ultrastrong driving regime, these results point the way to implementations of related dynamics in electro-optic materials, with potential applications in pulse generation and signal compression.

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• Atoms, ions, & molecules in cavities
• Cold gases in optical lattices
• Dispersion engineering
• Geometrical & wave optics
• Pulse compression
• Quantum aspects of black holes
• Quantum fields in curved spacetime