Hidden excitonic quantum states with broken time-reversal symmetry

The spontaneous breaking of time-reversal symmetry due to purely-orbital mechanisms (i.e.~not involving spin degrees of freedom) yields extremely exotic phases of matter such as Chern insulators and chiral superconductors. In this Letter, we show that excitonic insulators, by exploiting the transition from the excitonic ground state to a purely-orbital time reversal symmetry broken hidden state, can realize another notable example of this class. The transition to the hidden state is controlled by engineered geometrical constraints which enable the coupling between the excitonic order parameter and the free-space electromagnetic field. These results pave the way towards exotic orbital magnetic order in quantum materials and are also relevant for disentangling excitonic phase transitions from trivial structural ones.