Ultrafast nanophotonics is an emerging research field aimed at the development of nanodevices capable of light modulation with unprecedented speed^1,2,3,4. A promising approach exploits the optical nonlinearity of nanostructured materials (either metallic or dielectric) to modulate their effective permittivity via interaction with intense ultrashort laser pulses. Although the ultrafast temporal dynamics of such nanostructures following photoexcitation has been studied in depth⁵, sub-picosecond transient spatial inhomogeneities taking place at the nanoscale have been overlooked so far. Here, we demonstrate that the inhomogeneous spacetime distribution of photogenerated hot carriers induces a transient symmetry breaking in a highly symmetric plasmonic metasurface. The process is fully reversible and results in a broadband transient dichroism with a recovery of the initial isotropic state in less than 1 ps, overcoming the speed bottleneck caused by slower (electron–phonon and phonon–phonon) relaxation processes. Our results pave the way to ultrafast dichroic devices for high-speed modulation of light polarization.

超快纳米光子学是一个新兴的研究领域，旨在开发能够以前所未有的速度^1,2,3,4进行光调制的纳米器件。一种有前途的方法利用了纳米结构材料（金属或电介质）的光学非线性，通过与强超短激光脉冲的相互作用来调节其有效介电常数。尽管已经在深度^5中研究了这种纳米结构在光激发后的超快速时间动力学迄今为止，在纳米级发生的亚皮秒瞬态空间不均匀性一直被忽略。在这里，我们证明光生热载流子的不均匀时空分布在高度对称的等离子体超表面中诱导了瞬态对称性破坏。该过程是完全可逆的，并导致宽带瞬态二向色性，并在不到1 ps的时间内恢复了初始各向同性状态，克服了较慢的（电子-声子和声子-声子）弛豫过程所引起的速度瓶颈。我们的研究结果为超高速二向色性设备进行了光偏振的高速调制铺平了道路。