Observation of Floquet states and their dephasing in colloidal nanoplatelets driven by visible pulses

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.