The realization of photonic time crystals is a major opportunity but also comes with considerable challenges. The most pressing one, potentially, is the requirement for a substantial modulation strength in the material properties to create a noticeable momentum bandgap. Reaching that noticeable bandgap in optics is highly demanding with current, and possibly also future materials platforms because their modulation strength is small by tendency. Here we demonstrate that by introducing temporal variations in a resonant material, the momentum bandgap can be drastically expanded with modulation strengths in reach with known low-loss materials and realistic laser pump powers. The resonance can emerge from an intrinsic material resonance or a suitably spatially structured material supporting a structural resonance. Our concept is validated for resonant bulk media and optical metasurfaces and paves the way towards the first experimental realizations of photonic time crystals.

光子时间晶体的实现是一个重大机遇，但也带来了相当大的挑战。最紧迫的可能是材料属性中需要相当大的调制强度，以产生明显的动量带隙。在当前的以及未来的材料平台上，达到明显的带隙要求很高，因为它们的调制强度趋势很小。在这里，我们证明了通过在谐振材料中引入时间变化，动量带隙可以急剧扩展，并且具有已知的低损耗材料和真实的激光泵浦功率的调制强度。共振可以来自固有的材料共振或支持结构共振的适当空间结构的材料。我们的概念在共振体介质和光学超表面中得到了验证，并为光子时间晶体的首次实验实现铺平了道路。