Period‐Halving Effect in Floquet Photonic Structures

ABSTRACT Floquet or periodic drives, widely utilized in a multitude of fields for controlling quantum states, compel the wave function of the states to be recurrent with the drive period. The associated probability is thus commonly deemed to oscillate with no more than the drive frequency, and to date, a faster oscillation exceeding this maximum frequency has never been reported to the best of our knowledge. In this work, we demonstrate a frequency‐doubling (or period‐halving) effect in Floquet systems based on a photonic platform. By setting a Floquet Hamiltonian and its counterpart lagged by half a period to meet a chiral‐symmetric relation, the probability of the Floquet eigenstates in this system is able to oscillate twice as fast as the drive. Such exotic dynamics is unique for the zero‐quasi‐energy Floquet eigenstate, yet absent for the other Floquet eigenstates with non‐zero quasi‐energies. The period‐halving effect is observed experimentally in the intensity oscillation of light along longitudinally modulated waveguides. Our results provide a deep insight into the chiral symmetry manifested in Floquet systems and may initiate attempts to generate super‐harmonic effects through engineering periodic drives.