The first practical approach to creating photonic time crystals at optical frequencies, developed by an international research team, could lay the groundwork for faster, more compact lasers, sensors, and other optical devices. The team comprising scientists from Aalto University, the University of Eastern Finland, Karlsruhe Institute of Technology, and Harbin Engineering University previously demonstrated photonic time crystals at microwave frequencies. However, designing the crystals at optical frequencies has remained a challenge for the researchers, due to the need for a fast, large-amplitude variation of properties in the material platforms for these crystals. Unlike traditional crystals, which have spatially repeating structures, photonic time crystals are uniform in space, but exhibit a periodic oscillation in time. This temporal oscillation creates a momentum bandgap in the crystal, an unusual state during which light pauses inside the crystal while its intensity grows exponentially over time. The momentum bandgaps in photonic time crystals can lead to exotic light-matter interactions. To achieve a momentum bandgap that is large enough to noticeably amplify light, the material platforms for photonic time crystals require substantial modulation strength. The modulation strength in most material platforms tends to be low. The researchers devised a way to expand the momentum bandgaps in photonic time crystals through resonances. By introducing temporal variations in a resonant material, the team was able to expand the momentum bandgap in the material and produce a modulation strength in reach with known low-loss materials and realistic laser pump powers. The resonance came from an intrinsic material resonance or by using a material that was spatially constructed to support a structural resonance. Rather than seeking out new materials with improved nonlinear characteristics, the researchers capitalized on artificial composites that could support high-quality resonances. The team validated its concept for resonant photonic time crystals for bulk materials and optical metasurfaces through theoretical models and electromagnetic simulations. The team’s findings indicate that momentum bandgap size can be enhanced considerably by exploiting the structural resonances in the metasurfaces of photonic time crystals. The researchers achieved a momentum bandgap size that was 350 x wider than the same metasurface operating far away from the structural resonances, with a modulation strength as small as 1%. In principle, a stronger resonance has the potential to decrease the required modulation strength even further, the team said. Potential uses for photonic time crystals include applications in communication, imaging, and sensing. For example, light amplification with photonic time crystals could enhance nanosensing applications. Photonic time crystals — optical materials that exponentially amplify light — exhibit a periodic oscillation in time while remaining uniform in space, a quality which creates momentum bandgaps that enable light to pause while it is inside the crystal and grow in intensity. Courtesy of Xuchen Wang. “Imagine we want to detect the presence of a small particle, such as a virus, pollutant, or biomarker for diseases like cancer,” Aalto University professor Viktar Asadchy said. “When excited, the particle would emit a tiny amount of light at a specific wavelength. A photonic time crystal can capture this light and automatically amplify it, enabling more efficient detection with existing equipment.” The new approach to light amplification in photonic time crystals could lead to the design of more complex photonic time and space-time crystals. The geometry developed by the team, which does not require the emitter to be immersed inside a solid material, could be used to amplify the spontaneous emission of light from emitters near the structure. This approach to creating photonic time crystals could also be used to design lenses. Although the photonic time crystals developed by the team operate in the IR spectrum, the crystals can be implemented for the visible spectrum using other materials. “This work could lead to the first experimental realization of photonic time crystals, propelling them into practical applications and potentially transforming industries,” Asadchy said. “From high-efficiency light amplifiers and advanced sensors to innovative laser technologies, this research challenges the boundaries of how we can control the light-matter interaction.” The research was published in Nature Photonics (www.doi.org/10.1038/s41566-024-01563-3).