The non-equilibrium response of a superconductor to pair-breaking radiation measured over a broad frequency band

TL;DR

This study measures how a superconductor absorbs terahertz radiation across a broad frequency range, revealing the non-equilibrium dynamics of pair-breaking and phonon trapping effects.

Contribution

First experimental measurement of frequency-dependent quasiparticle creation efficiency in a superconductor due to pair-breaking radiation.

Findings

Resonator response peaks at the superconductor's gap edge.

Response decreases at twice the pair-breaking energy.

Response increases again at higher frequencies due to phonon trapping.

Abstract

We have measured the absorption of terahertz radiation in a BCS superconductor over a broad range of frequencies from 200 GHz to 1.1 THz, using a broadband antenna-lens system and a tantalum microwave resonator. From low frequencies, the response of the resonator rises rapidly to a maximum at the gap edge of the superconductor. From there on the response drops to half the maximum response at twice the pair-breaking energy. At higher frequencies, the response rises again due to trapping of pair-breaking phonons in the superconductor. In practice this is the first measurement of the frequency dependence of the quasiparticle creation efficiency due to pair-breaking in a superconductor. The efficiency, calculated from the different non-equilibrium quasiparticle distribution functions at each frequency, is in agreement with the measurements.