Optical Signatures of Dynamical Excitonic Condensates

We theoretically study dynamical excitonic condensates occurring in bilayers with an imposed chemical potential difference and in photodoped semiconductors. We show that optical spectroscopy can experimentally identify phase-trapped and phase-delocalized dynamical regimes of condensation. In the weak-bias regime, the trapped dynamics of the order parameter's phase lead to an in-gap absorption line at a frequency almost independent of the bias voltage, while, for larger biases, the frequency of the spectral feature increases approximately linearly with bias. In both cases, there is a pronounced second-harmonic response. Close to the transition between the trapped and freely oscillating states, we find a strong response upon application of a weak electric probe field, compare the results to those found in a minimal model description for the dynamics of the order parameter's phase, and analyze the limitations of the latter.