  Bardasis-Schrieffer modes in superconductors are fluctuations in subdominant
pairing channels, e.g., d-wave fluctuations in an s-wave superconductor. This
Rapid Communication shows that these modes also generically occur in excitonic
insulators. In s-wave excitonic insulators, a p-wave Bardasis-Schrieffer mode
exists below the gap energy, is optically active and hybridizes strongly with
photons to form Bardasis-Schrieffer polaritons, which are observable in both
far-field and near-field optical experiments.
