Coherent interaction between light and matter generates photon-dressed replicas of the original electronic levels (that is, Floquet states). This opens up a so-called field of Floquet engineering that applies electromagnetic fields to create new non-equilibrium phases of solid-state materials. The direct observation of such Floquet states, which often requires low-temperature, high-vacuum housing of the interrogated materials and low-energy infrared photons or microwaves as the driver, has been challenging. Here we report the observation of Floquet states in CdSe nanoplatelets, which are the colloidal analogue of quantum wells, under ambient conditions using femtosecond transient absorption. A sub-bandgap visible photon dresses a heavy-hole state (|hh₁〉) to a Floquet state (|hh₁ + ℏω_L〉) that can hybridize with the first quantized electron state (|e₁〉). This enables us to probe the Floquet state using a near-infrared photon through its transition to the second quantized electron state (|e₂〉). Dephasing of the Floquet state into the real population of |e₁〉 is also directly observed with a dephasing time of a few hundred femtoseconds, which is well reproduced by our density matrix simulations.

光与物质之间的相干相互作用生成原始电子能级（即弗洛奎特态）的光子修饰复制品。这开辟了所谓的 Floquet 工程领域，该领域应用电磁场来创建固态材料的新非平衡相。直接观察这种 Floquet 态通常需要低温、高真空的被询问材料外壳以及低能红外光子或微波作为驱动器，一直具有挑战性。在这里，我们报告了在环境条件下使用飞秒瞬态吸收对 CdSe 纳米片（量子阱的胶体类似物）中 Floquet 态的观察。子带隙可见光子将重空穴态 (| hh ₁ 〉) 修饰为可与第一量子化电子态 (| e ₁ 〉) 杂化的 Floquet 态 (| hh ₁ + ℏ ω _L 〉)。这使我们能够利用近红外光子通过其跃迁到第二量子化电子态 (| e ₂ >) 来探测 Floquet 态。将 Floquet 状态分解为真实总体 | e ₁ > 也可以在几百飞秒的移相时间下直接观察到，这可以通过我们的密度矩阵模拟很好地再现。