Cooper Pairs 2Δ. Quasiparticles

Transcription

1 The quasiparticle generation efficiency in a superconductor measured over a broad frequency band Cooper Pairs 2Δ hν Pieter de Visser Quasiparticles SRON: Stephen Yates, Jochem Baselmans, Andrey Baryshev Delft: Teun Klapwijk, Nuria Llombart, Andrea Neto Cambridge: Tejas Guruswamy, David Goldie, Stafford Withington Superconductivity and Photons

2 From light to signal Cooper Pairs 2Δ hν Quasiparticles S21 [db] δf F 0 Incoming photons break Cooper pairs => quasiparticles => Higher resistance and inductance => Resonance shifts and gets shallower Microwave readout, energies far below the gap F [Ghz] P. Day, et al., Nature 425, 817 (2003)

3 From light to signal Cooper Pairs 2Δ hν Quasiparticles S21 [db] δf F 0 Incoming photons break Cooper pairs => quasiparticles => Higher resistance and inductance => Resonance shifts and gets shallower Microwave readout, energies far below the gap F [Ghz] P. Day, et al., Nature 425, 817 (2003)

4 From light to signal Cooper Pairs 2Δ hν Quasiparticles Superconducting pair breaking detectors: far below T c, paired electrons (energy 2 ) Observables (KID, STJ, QCD) mainly sensitive to quasiparticles at E= For hf=2 only 2 quasiparticles at E= : 100% efficient For hf between 2 and 4 still only 2 qp s per photon Above 4 it depends on the phonon trapping

5 From light to signal Cooper Pairs 2Δ hν Quasiparticles Superconducting pair breaking detectors: far below T c, paired electrons (energy 2 ) Observables (KID, STJ, QCD) mainly sensitive to quasiparticles at E= For hf=2 only 2 quasiparticles at E= : 100% efficient For hf between 2 and 4 still only 2 qp s per photon Above 4 it depends on the phonon trapping

6 Pair breaking efficiency close to the gap Phonon trapping factor Guruswamy, Goldie, Withington, SuST 27, (2014)

7 Pair breaking efficiency close to the gap Phonon trapping factor Commonly used number 0.59 (or 0.57) was derived for thick films and very high energies, Kozorezov et al. PRB 61 (2000) Guruswamy, Goldie, Withington, SuST 27, (2014)

8 Quasiparticle efficiency: why relevant? Number of quasiparticles per photon Detector responsivity: scales linearly with η pp, most sensitive close to Recombination noise, dd dd = η oooη pp τ qq Δ dd dd qq In generation-recombination noise also in photon noise limit! Optical NEP vs dark NEP, Visit poster G1.34 by Reinier Janssen

9 Detector sensitivity NNN GG = 2 η ooo η pp N qq τ qq Sensitivity limited by GR noise from excess quasiparticles, scales with η pp Nature Comms. 5, 3130 (2014)

10 Recombination noise vs photon noise NNN pp NNN R = hfη pp / (= 2 fff 2 < hf < 4) NEP R /NEP photon hf/ NEP R /(NEP photon +NEP R ) hf/

11 Neto, IEEE Trans. Antennas and Prop. 58, 2238 (2010) Neto et al. IEEE Trans. THz Sci. Tech. 4, 26 (2013) Broadband leaky wave antenna + lens Ta KID, energy gap at 324 GHz

12 FTS response of Tantalum KID 1 Detector phase response (a.u.) Frequency (GHz) KID is the detector in the FTS (measure absorption) Corrected for FTS frequency dependence

13 FTS response of Tantalum KID 1 Detector phase response (a.u.) Antenna efficiency Absorption superconductor Response superconductor Frequency (GHz)

14 Absorption vs frequency Antenna efficiency (CST) Energy gap Ta: 324 GHz

15 Absorption vs frequency Pair-breaking in Superconducting CPW, travelling wave absorption Crucial to remove f-dependence of R sheet! Energy gap Ta: 324 GHz

16 CPW absorption * antenna efficiency Detector phase response (a.u.) Energy gap Ta: 324 GHz Frequency (GHz)

17 Steady state f(e) Non-equilibrium quasiparticle distribution QP creation efficiency σ 2 N qq η pp Constant power, only effect is F-dependence through f(e) Frequency (GHz)

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19 Summary Appl. Phys. Lett. 106, (2015)

20 Summary Appl. Phys. Lett. 106, (2015) Follow up: Microwave power dependence (here P read <<P opt ) Materials: - substrate (phonon trapping) - superconductor: Al expected as Ta, disordered TiN

21 extra

22 Non-equilibrium FTS response Not planar absorption but waveguide absorption, full absorption above 2Δ Phonon losses, Cooper pairs have energy 2Δ => nonequilibrium f(e)

23 Corrections to raw FTS response

24 Noise levels Optical Power Temperature S A P SGG = hfη oooη pp A 2 = 8 dd

25 Connecting η pb to observables

26 Measured beam patterns (350 GHz) Co - pol Cross - pol

27 Source vs beampattern contours