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

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 pairbreaking 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 nonequilibrium quasiparticle distribution functions at each frequency, is in agreement with the measurements.In a superconductor at low temperature, most of the electrons are bound in Cooper pairs. These pairs can be broken into quasiparticles by absorbing photons with an energy larger than the binding energy. This mechanism is frequently used to detect submillimetre and terahertz radiation using conventional superconductors such as aluminium. Pair-breaking detectors are usually assumed to measure the number of quasiparticles created by the absorbed radiation. The observable that measures the number of quasiparticles varies from the complex conductivity for microwave kinetic inductance detectors 1 (MKIDs), the current through a tunnel junction 2 to the capacitance of a small superconducting island 3 . These observables are mainly sensitive to quasiparticles with an energy close to the gap energy of the superconductor, ∆. The working principle of these detectors is usually explained in terms of an effective number of quasiparticles which is maintained by a balance between the radiation power and electron-phonon interaction (recombination) 4 . To convert the power (P ) into a number of quasiparticles (N qp ), the quasiparticle creation efficiency η pb is introduced, which compares the actual N qp with the maximum possible N qp when all created quasiparticles would have an energy ∆. Since Cooper pairs have a binding a) Electronic mail: p.j.devisser@tudelft.nl; Current address: Department of Quantum Matter Physics, University of Geneva, Geneva 1211, Switzerland energy of 2∆, a photon with an energy in between 2∆ and 4∆ can still only create two quasiparticles. The rest of the energy is lost through electron-phonon scattering, hence η pb < 1. For higher energies η pb depends on the phonon trapping factor, which determines whether high energy phonons are directly lost or can break an additional pair. η pb is therefore not an efficiency in the sense that photons are lost, but it reduces the detector responsivity. MKIDs are superconducting microwave resonators which sense the number of quasiparticles through the complex conductivity of the superconductor. The phase response (θ) of such a resonator can be approximated bywhere σ 2 is the imaginary part of the complex conductivity. For the last propo...

Phonon-Trapping-Enhanced Energy Resolution in Superconducting Single-Photon Detectors

“…The present experiment should not be taken as a measurement of η pb (as in Ref. [47]), because of the large range of energies involved, but mainly as a verification that the single-photon pulse heights in Al MKIDs are consistent with our current understanding of the response.…”

Effects of nonequilibrium quasiparticles in a thin-film superconducting microwave resonator under optical illumination

“…TLS loss exhibits a distinct power-dependent saturation behavior 3,7,8 that leads to a characteristic decrease in loss with an increase in temperature or microwave power. Third, dissipation can be caused by quasiparticles which can be created by thermal effects, stray light, radiation, or other mechanisms [9][10][11][12][13][14][15] . Quasiparticle generation from ionizing radiation or photons provides the physical basis for superconducting radiation detectors 5,6,10,14 , and is also relevant to some proposed hybrid quantum systems in which a superconducting device must function in close proximity to optically trapped atoms 16,17 .…”

Low-Temperature Detectors for CMB Imaging Arrays

“…4 is a non-trivial function of the photon frequency to gap ratio-unity at the gap edge, scaling as 1/ν in the range 2∆ < hν < 4∆ , and plateaus to a constant and material dependent number ∼ 0.6 at hν > 4∆ [84]. This detailed picture has been experimentally verified in BCS superconductors [85].…”