Optical activity from exciton Aharonov-Bohm effect: a Floquet engineering approach

We theoretically demonstrate that an originally achiral molecular system can exhibit nonzero circular dichroism (CD), through Floquet engineering, when it is driven with elliptically polarized light. More specifically, we consider an isotropic ensemble of small cyclic molecular aggregates in solution whose local low-frequency vibrational modes are driven by a continuous-wave infrared pump. We attribute the origin of the nonzero CD to time-reversal symmetry breaking due to an excitonic Aharonov-Bohm (AB) phase arising from laser-driving and coherent interchromophoric exciton hopping. The obtained Floquet-engineered excitonic AB phases are far more tunable than their analogous electronic AB phases in the nanoscale, highlighting a virtually unexplored potential that excitonic AB phases have in the coherent control of molecular processes and simultaneously introducing new analogues of magneto-optical effects in molecular systems which bypass the use of strong magnetic fields.