Coherence onset in PT-symmetric organic microcavities: towards directional propagation of light

For the investigation of non-Hermitian effects and physics under parity-time (PT) symmetry, photonic systems are ideal model systems for both experimental and theoretical research. We investigate a fundamental building block of a potential photonic device, consisting of coupled organic microcavities. The coupled system contains cavities with gain and loss and respects parity-time symmetry, leading to non-Hermitian terms in the corresponding Hamiltonian. Experimentally, two coupled cavities are realized and driven optically using pulsed laser excitation up to the lasing regime. We show that above the lasing threshold, when coherence evolves, the coupled-cavity system starts to operate asymmetrically, generating more light on one side of the device, being characteristic of non-Hermitian PT-symmetric systems. Calculations and simulations on a Su-Schrieffer-Heeger (SSH) chain composed of these PT-symmetric unit cells show the emergence of non-trivial topological features.