Spatial photonic crystals (SPCs) are unique structures for light–matter interactions because they achieve a large and spatially periodic dielectric contrast on wavelength scales^1,2,3,4. Their temporal analogues, photonic time crystals (PTCs), promise similar advances by periodically modulating optical properties in time^{5,6,7,8,9,10,11}, but require strong, ultrafast modulation, which is challenging to obtain experimentally^{5,12,13,14,15}. Driven metamaterials have been considered as a route to realize PTCs, yet all-optical implementations have remained unknown because of the challenge of achieving modulation on such short timescales. Here we demonstrate the all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at terahertz frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the kinetic energy of the carriers and effective mass reaching up to 80% of their rest mass. Our spectroscopic measurements show a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% (refs. ^16,17), and we predict plasmonic lasing to be within experimental reach. These results establish a robust platform for time-domain photonics in plasmonic systems.

Guo, T., Sueiro, J., Andolina, G.M. et al. Plasmonic metamaterial time crystal.
Nature (2026). https://doi.org/10.1038/s41586-026-10825-9

https://doi.org/10.1038/s41586-026-10825-9

At the heart of this innovation lies the concept of bound states in the continuum—peculiar optical resonances that, despite overlapping spectrally with radiating waves, remain localized and non-radiative due to symmetry-protected interference. These states exhibit extremely high quality factors (Q-factors), making them ideal for enhancing light-matter interaction and thus valuable for sensing, lasing, and nonlinear optics. However, the very symmetry that protects these states imposes a fundamental limitation: their activation and modulation traditionally require breaking or perturbing this delicate symmetry, often resulting in slower switching speeds or reduced efficiency.

The team led by Crotti, Schirato, and Pashina overcame this challenge by demonstrating a method to induce ultrafast temporal modulation of a metasurface quasi-BIC. Their approach revolves around an ingenious use of transient optical symmetry breaking, enabled by an ultrafast optical pump. When the metasurface is illuminated by a carefully timed pump pulse, it imposes a temporal asymmetry on the structure’s optical response, rapidly switching the quasi-BIC on and off within femtosecond timescales.

Technically, the metasurface architecture utilized in the study is designed to support a quasi-BIC resonance characterized by a sharp spectral feature indicative of its high Q-factor. The researchers employed ultrafast laser pulses to perturb the refractive index and symmetry properties of the metasurface through nonlinear optical effects. This dynamic perturbation transiently disrupts the symmetry conditions necessary for sustaining the quasi-BIC, effectively switching the resonance state in real time without physically altering the structure. The recovery to the original symmetry and state occurs rapidly once the pump pulse subsides, allowing for repeated and reversible switching.

This ultrafast switching mechanism opens exciting avenues for the development of active photonic devices that require rapid and efficient control of light, such as high-speed optical modulators, optical switches, and components in integrated photonic circuits. Unlike conventional optical modulators that often rely on electronic control or slower thermal effects, the all-optical symmetry-breaking approach leverages purely photonic processes, achieving speeds governed by the pulse duration itself and intrinsic material response times.

Furthermore, the use of a quasi-BIC mode amplifies the modulation depth—thanks to the intense, localized electromagnetic fields—and simultaneously preserves low losses, a critical factor for practical device application. The interplay between strong light confinement and ultrafast dynamic symmetry manipulation thus presents a paradigm shift in how optical resonances can be controlled with unprecedented speed and precision.

The experimental setup involved time-resolved pump-probe measurements to monitor the temporal evolution of the resonance feature in response to the pump pulses. The team observed a substantial and reversible dip in the transmission spectra corresponding to the rapid on/off switching of the quasi-BIC resonance within sub-picosecond intervals. This remarkable temporal precision underscores the feasibility of integrating such metasurface-based components into platforms demanding picosecond or faster optical switching.

The implications extend beyond mere switching speed. The demonstrated technique could impact nonlinear optical processes, enabling dynamic tuning of phenomena such as harmonic generation or four-wave mixing. By swiftly toggling the resonance state, it becomes possible to engineer time-dependent nonlinear interactions, leading to novel functionalities in signal processing, frequency conversion, and quantum photonics.

Moreover, transient optical symmetry breaking provides a versatile and contactless control modality, which is particularly relevant for miniaturized and integrated photonic systems where electrical interconnections pose limitations. Photonic circuits leveraging this nonlinear optical control could achieve significantly enhanced bandwidth and energy efficiency compared to electrical counterparts, addressing critical bottlenecks in data communication and optical computing technologies.

This research also holds promise for sensing applications. High-Q quasi-BIC modes are extremely sensitive to environmental changes; by incorporating ultrafast switching, sensors can achieve rapid response times and dynamic range modulation, facilitating real-time monitoring of chemical, biological, or physical systems. The ability to swiftly toggle the resonance could allow selective enhancement or suppression of signals, thereby improving sensitivity and selectivity.

On a fundamental level, the study enriches the understanding of light-matter interaction in symmetry-protected systems. It highlights how temporal modulation of symmetry, rather than static structural changes, can be harnessed to modify photonic states dynamically and reversibly. This insight could inspire future designs of metamaterials and metasurfaces with programmable, ultrafast optical functionalities that adapt on-the-fly to external stimuli.

Equally important is the material platform used to realize these effects. The metasurface was fabricated from materials exhibiting strong nonlinear optical coefficients and ultrafast response times, enabling the observed transient symmetry breaking. Such materials are crucial in ensuring that the ultrafast pump-probe approach produces distinct and reproducible switching without deleterious thermal or irreversible effects.

The researchers further elaborated on the theoretical framework underpinning their observations, employing coupled-mode theory and numerical simulations to model the transient behavior of the quasi-BIC resonance under optical pumping. The calculations corroborated the experimental data, confirming the link between transient refractive index modulation, symmetry perturbation, and resultant resonance switching dynamics.

Looking ahead, the integration of such metasurface-based ultrafast switches into larger photonic architectures presents both challenges and opportunities. Scaling the fabrication of high-quality metasurfaces with precise control over resonant features will be vital. Additionally, engineering pump configurations compatible with chip-scale devices or using alternative excitation schemes such as electrical or all-optical modulation will be avenues for future exploration.

In conclusion, the groundbreaking work led by Crotti and colleagues opens a transformative chapter in nanophotonics by demonstrating that transient optical symmetry breaking can serve as a powerful tool for controlling quasi-bound states in the continuum on ultrafast timescales. This advance not only pushes the fundamental understanding of symmetry-protected photonic states but also lays a robust foundation for next-generation ultrafast optical switches and modulators with diverse applications across communications, sensing, and beyond.

Subject of Research: Ultrafast optical switching of quasi-bound states in the continuum in metasurfaces through transient optical symmetry breaking

Article Title: Ultrafast switching of a metasurface quasi-bound state in the continuum via transient optical symmetry breaking

Article References:
Crotti, G., Schirato, A., Pashina, O. et al. Ultrafast switching of a metasurface quasi-bound state in the continuum via transient optical symmetry breaking. Light Sci Appl 14, 240 (2025). https://doi.org/10.1038/s41377-025-01885-z

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41377-025-01885-z