A lossless optical structure cannot absorb light. By definition, it has no material dissipation — no mechanism to convert electromagnetic energy into heat. Light that enters must leave. The scattering matrix is unitary: every photon that goes in comes out, perhaps redirected or phase-shifted but never consumed. This is a theorem, not an approximation. A dielectric metasurface with no imaginary part in its permittivity is transparent in the energy-balance sense, regardless of how complicated its geometry is.
Rustomji, Mohammadi Estakhri, and Estakhri (arXiv 2602.23328, February 2026) show that such structures can absorb light temporarily through a mechanism called coherent virtual absorption — a storage-release cycle that looks like absorption at the input and looks like emission at the output, with a controllable delay between them.
The key is that the scattering zeros of a lossless structure lie at complex frequencies. A scattering zero is a frequency at which the structure's scattering matrix has zero output — complete absorption. For a lossy structure, these zeros can sit on the real frequency axis, and a real-frequency monochromatic wave can excite perfect absorption. For a lossless structure, the zeros are displaced into the complex plane — they correspond to exponentially growing or decaying signals, not to any physical steady-state excitation.
But a temporally shaped pulse can access a complex-frequency zero. If the input signal is tailored to match the temporal profile of the complex-frequency mode — an exponentially modulated pulse at the right carrier frequency — the structure's scattering output vanishes during the excitation period. Energy enters the structure and does not leave. The structure is not absorbing in the thermodynamic sense — no energy is being dissipated — but it is storing the energy in its electromagnetic modes. The field inside builds up.
When the excitation stops, the stored energy radiates out. The structure releases what it captured, on a timescale set by the quality factor of the mode. The entire cycle — capture, hold, release — conserves energy perfectly. No photon is lost. But during the hold phase, an external observer measuring only the scattered field would see nothing coming out. The structure looks absorbing. It isn't. It's holding.
The authors demonstrate this for dielectric metasurfaces — arrays of subwavelength dielectric resonators with no material loss — using full-wave electromagnetic simulations. The complex-frequency zeros are identified from the rational approximation of the scattering matrix, and the temporal pulse shape needed to access them is computed. The approach works for arbitrary metasurface configurations, not just simple geometries.
The applications are in optical memory and sensing. A lossless structure that can capture a pulse, hold it, and release it on demand functions as a photonic memory without the losses that plague plasmonic or absorptive approaches. The hold time is finite — set by the quality factor, not by a material lifetime — but for high-Q dielectric resonances, the delay can be significant. The structure doesn't absorb. It pauses.