Abstract
We theoretically investigate collective modes in a minimal model of a superconducting bilayer with Rashba spin-orbit coupling. The additional layer degree of freedom allows for novel odd-parity s-wave superconducting pairing states. Starting with a uniform BCS ground state, we determine a gauge-invariant Gaussian effective action for the electromagnetic (EM) field which interacts with the bilayer through minimal coupling. The inclusion of quantum fluctuations in a subdominant unconventional odd-parity channel leads to a novel Bardasis-Schrieffer-like phase mode with excitation energy within the superconducting gap and a finite coupling to the external EM field. Accounting for the long-range Coulomb interaction, we evaluate the possibility to observe the fluctuating odd-parity pairing channel.