Basic Detection Techniques Front-end Detectors for the Submm

1. Basic Detection TechniquesFront-end Detectors for the Submm Andrey Baryshev Lecture on 21 Sept 2006

2. Outline • Stability measurements of practical receiver system, Allan variance plot, calibration intervals • Direct detectors (principle) • Photo-detectors • Bolometers • Other types (pyro-detectors, Golay cell) • Noise in direct detectors • NEP -- noise equivalent power • Photon noise • Electronics noise • Low noise detectors in submm THz region • Transition edge sensors • Kinetic inductance detectors • SIS junction as direct detector • Practical measurement of NEP Basic Detection Techniques – Submm receivers (Part 3)

3. Practical receiver at a telescope/or any lab Front-end Calibrator Receiver Back-end Thot Tcold Tsys, Gsys Tsky(t) Source spectrum a 1 - bin Telescope 100 K f Signal amplitude = 1 K on top of 100 K background Spectrometer bin bandwidth = 1 MHz, Tsys = 100 K What to do to detect with accuracy 5 σ (S/N=5)? Basic Detection Techniques – Submm receivers (Part 3)

4. Integrate (wait)! How long? Uncertainty dT Tsky dT = Radiometer equation τ B Tsky2 NOTE: Tsky = Tbkg+Tsys = 200 K, τ = dT2 B 201 K for the line! Not Tsys! dT = 1 K / 5 (S/N) = 0.25 K Ideally after continuous integration of 0.64 s accuracy is achieved! Basic Detection Techniques – Submm receivers (Part 3)

5. Why radiometer equation? Fundamental noise is photons -> statistics is “white” noise: uniform spectral density Fourier -> transform White photon noise statistics results in radiometer equation Basic Detection Techniques – Submm receivers (Part 3)

6. Real life receivers Ideal Real System instability: Standing waves, drift, 1/f noise, ambient temperature, Atmosphere, many more … How often do we need to calibrate (loose time)? Basic Detection Techniques – Submm receivers (Part 3)

7. Allan variance Measurement sequence with minimum integration time tmin s1,s2 … sn … sN 1 <(yn+1-yn)2> where yn is the average of subset of sn over integration time t σ(t)2 = 2 2t 4t … Basic Detection Techniques – Submm receivers (Part 3)

8. Allan time real ideal Maximum integration time between recalibrations Basic Detection Techniques – Submm receivers (Part 3)

9. Direct detector principles Direct detector gives signal proportional to the power of incoming radiation or amount of photons. Usually detector pixel is much simpler than heterodyne counterpart, so large arrays are possible • Photo detector (electronic) • Bolometric principle (Thermal detectors) • Coherent detectors (diode) • Other principles Basic Detection Techniques – Submm receivers (Part 3)

10. Parameters of direct detectors • Quantum efficiency • Noise • Linearity • Dynamic range • Number and size of pixels • Time response • Spectral response • Spectral bandwidth Basic Detection Techniques – Submm receivers (Part 3)

11. NEP NEP is input power at the input of the detector to produce SNR=1 One can add the contributions of different noise sources in square fascion as in the formula ebove for optics noise contribution Basic Detection Techniques – Submm receivers (Part 3)

12. Photon noise and Johnson noise Detector is limited by statistics of incoming photons 2hc(1/t)1/2 NEP = λη1/2 Detector is limited by Johnson noise (thermal fluctuations) 2hc(kT)1/2 NEP = ηλqGR1/2 Basic Detection Techniques – Submm receivers (Part 3)

13. Black body facts Uncertainty in photon numbers Photon occupation numbers Photon NEP Φ = 4 π R2 L L= e (2 h f3)/(c/n)2 /(Exp(hf/(kT))-1) M = σT4 Stefan-Boltzmann law Basic Detection Techniques – Submm receivers (Part 3)

14. hF > Egap Photo detectors Arriving photon generate/modify free charge carriers distribution • Classical semiconductor (utilizing band gap) • It has a lower frequency limit hF > Egap • Typical semiconductor work in IR region • By applying stress to the crystal, it is possible to decrease Egap Like in stressed germanium • SIS junction • No low frequency limit (effective band gap modified by bias point) • High frequency limit due to gap structure • Kinetic inductance detectors • Photons break Cupper pairs • It has low frequency limit E Basic Detection Techniques – Submm receivers (Part 3)

15. Example Detectors PACS instrument on Herschel, Stressed germanium Basic Detection Techniques – Submm receivers (Part 3)

16. 1 2 100 m Antenna CPW ¼  Resonator L= 5 mm @ 6 GHz Al ground plane Coupler CPW Through line substrate Central conductor 1 2 Readout signal ~GHz KID arrays for AstronomyPrinciple of Kinetic Inductance Detection Pair breaking detector • Superconductor ~ LKIN at T<Tc/3 • LKIN ~ Nqp ~ power absorbed • Use LKIN to measure absorbed power KID a SC material in resonance circuit • read out at F0 ~ 4 GHz • resonance feature is function of Nqp • signal in S21 or R and θ Basic Detection Techniques – Submm receivers (Part 3)

17. 1 2 100 m KID arraysKID radiation coupling Antenna Antenna in focus of Si lens Herschell band 5 & 6 Radiation from sky FRF >>2Δ/h -> increases Nqp -> change in S21 or R and θ F0 << FRF antenna << resonator F0 << 2Δ/h No qp creation due to readout Most sensitive area CPW ¼  Resonator L= 5 mm @ 6 GHz Si Lens Al ground plane Radiation Coupler CPW Through line substrate Central conductor 1 2 Readout signal ~GHz Basic Detection Techniques – Submm receivers (Part 3)

18. KID arraysPrinciple of KID arrays • Resonances @ F0 • F0 set by geometry (length) • Intrinsic FDM Basic Detection Techniques – Submm receivers (Part 3)

19. 0.48 mm KID arrays for astronomyGeneral idea for the FP • Optical Interface flies eye array of Si lenses, size 20Fλ/2. 90.6% packing efficiency in hexoganal • Array Detectors printed on back Si lens array • Readout 4 SMA coax connectors 2 full chains -> redundancy ~5000 pixel Basic Detection Techniques – Submm receivers (Part 3)

20. KID focal plane for NIKA400 pixel test array for 2 mm antenna KID Through line Basic Detection Techniques – Submm receivers (Part 3)

21. Pair breaking detector: fundamental sensitivity limit # quasiparticles DOS quasiparticle lifetime e-p coupling Pmax/NEP>10.000 1 sec Basic Detection Techniques – Submm receivers (Part 3)

22. Measure bare resonators Measure all ingredienst of NEP Quasiparticle lifetime qp noise Sx Quasiparticle response δx/δNqp For R and θ Noise Signal qp roll-off θ or R Measuring Dark NEP Cryostat Shorted end Quadrature mixer analyses Synthesizer Open end, coupler Superconductor Re ADC IQ 1 2 ~ LNA Im Basic Detection Techniques – Submm receivers (Part 3)

23. SIS photon detector ・High sensitive in far-IR – sub-mm region SIS junction Bias voltage, V q: elementary charge h:plank constant ν:frequency Isg: subgap current η:quantum efficiency Superconductor Insulator Superconductor photon E Density of states Δ qV Current status: 10-16 ～10-17 W/√Hz EF Our goal: @ 600 GHz 23 Basic Detection Techniques – Submm receivers (Part 3)

24. Comparison with theoretical value(1) Tinkham (1975) D(E):density of states, F(E): Fermi function, Δ: gap energy Nb/Al-AlN/Nbjunction 4.2 K 1.6 K Current [ A ] Theoretical curves ? 24 Bias voltage [ V ] 3/11/2014 Basic Detection Techniques – Submm receivers (Part 3)

25. Transition edge sensor principle Thin superconducting film as thermometer Square law power detector thermal time constant t = C/GC: thermal capacitanceG: thermal conductivity Basic Detection Techniques – Submm receivers (Part 3)

26. Procedure of an NEP measurement • Determine the signal power • It is given by Planck formula • Need temperature of calibrator black-bodies • Frequency coverage of the detector (measured by FTS) • Knowledge of solid angle of antenna beam pattern • Determine the responsively • Measure response from hot/cold radiators • Calibrate detector output in input power units • Determine the background noise • Block connect the detector beam to as little background –possible • Measure time trace and using responsively and integration time express it in NEP Wt/Hz1/2 Basic Detection Techniques – Submm receivers (Part 3)