Finite-momentum Cooper plasmons in superconducting terahertz microcavities

TL;DR

This paper uncovers and characterizes finite-momentum Cooper plasmons in superconducting microcavities, revealing their potential for controlling supercurrent dynamics in terahertz superconducting circuits.

Contribution

It introduces the concept of Cooper plasmons in superconducting microcavities and demonstrates their experimental observation and tunability.

Findings

Two Cooper plasmons observed in NbN microcavity

Resonance frequencies report carrier density and dissipation

Design principles for superconducting terahertz devices established

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.