Alexander Potts: Finite-momentum Cooper plasmons in superconducting terahertz microcavities

Abstract: The phase mode of a superconductor's order parameter encodes fundamental information about pairing and dissipation, but is typically inaccessible at low frequencies due to the Anderson-Higgs mechanism. Superconducting samples thinner than the London penetration depth, however, support a gapless phase mode whose dispersion can be reshaped by a proximal screening layer. Here, we theoretically and experimentally show that this screened phase mode in a superconducting thin film integrated into on-chip terahertz circuitry naturally forms a superconducting microcavity that hosts resonant finite-momentum standing waves of supercurrent density, which we term Cooper plasmons. We measure two Cooper plasmons in a superconducting NbN microcavity and demonstrate that their resonance frequencies and linewidths independently report the density of participating carriers and plasmon's dissipation at finite momenta. Our results reveal an emergent collective mode of an integrated superconductor–circuit system and establish design principles for engineering or suppressing Cooper plasmons in superconducting terahertz devices and circuits.

Bio: Alex Potts is a postdoctoral fellow at Columbia University and the Max Planck Institute for the Structure and Dynamics of Matter. He completed his Ph.D. at UCSB in the group of Andrea Young by developing and using on-chip THz spectroscopy to study superconductors. He is now applying these techniques and ideas to probe the cavity electrodynamics of 2D materials, where device engineering, THz circuitry, and ultrastrong light-matter interaction can dynamically reshape the electrodynamic properties of the ground state.