Basic Detection Techniques Front-end Detectors for the Submm Andrey Baryshev Lecture on 17 Oct 2011

Outline 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 TES bolometer Practical measurement of NEP Basic Detection Techniques – Submm receivers (Part 3)

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)

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)

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

Photon noise and Johnson noise Detector is limited by statistics of incoming photons 2hc(1/t)1/2 NEP = λ η1/2 Detector is limited by Johnson noise (thermal fluctuations) 2hc(kT)1/2 NEP = ηλqGR1/2 Basic Detection Techniques – Submm receivers (Part 3)

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)

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 hF > Egap Basic Detection Techniques – Submm receivers (Part 3)

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

KID arrays for Astronomy Principle of Kinetic Inductance Detection 2 1 CPW Through line CPW ¼  Resonator Coupler Antenna substrate Central conductor 100 m L= 5 mm @ 6 GHz Al ground plane Readout signal ~GHz 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)

KID arrays KID radiation coupling 2 1 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 100 m Radiation CPW Through line Coupler substrate Central conductor 1 Readout signal ~GHz 2 Basic Detection Techniques – Submm receivers (Part 3)

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

KID arrays for astronomy General 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 0.48 mm Basic Detection Techniques – Submm receivers (Part 3)

KID focal plane for NIKA 400 pixel test array for 2 mm antenna KID Through line Basic Detection Techniques – Submm receivers (Part 3)

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)

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

SIS photon detector Δ E Quantum type detector with superconductor ・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 SIS photon detector that is quantum type detector with superconductor. Under the condition that bias voltage is applied to the SIS junction, photons can excite quasi-particles to empty states. We can observe this event as photo-current. Sensitivity of the detector is defined by responsivity divided by shot noise of subgap current. Current states of Noise Equivalent Power, NEP is about 10 to the -16th to 10 to the -17th W/√Hz. Our goal is 10 to the -19th W/√Hz at six hundred GHz. In order to achieve this level, subgap current should be reduced to 10 fA with quantum efficiency of 0.5. Δ qV Current status: 10-16 ～10-17 W/√Hz EF Our goal: @ 600 GHz 17 Basic Detection Techniques – Submm receivers (Part 3) 17

Comparison with theoretical value (1) Tinkham (1975) D(E):density of states, F(E): Fermi function, Δ: gap energy Nb/Al-AlN/Nb junction 4.2 K Current [ A ] 1.6 K Theoretical curves Then, we compared measured I-V curves with theoretical ones at 4.2 K and 1.6 K. Theoretical curves are calculated by BCS theory as shown in this equation. D, F, and delta show density of states, fermi function, and gap energy, respectively. At 4.2 K, we can see the difference between the two near the gap voltage. The experimental curve shows round shape. On the other hand, at 1.6 K, in addition to the difference, we can see the excess current on the theoretical curve. In order to investigate the origin of the excess current, … 18 Bias voltage [ V ] 11/17/2018 Basic Detection Techniques – Submm receivers (Part 3) 18

ò ln ÷ ø ö ç è æ × = D df NEP E Direct Detection of Radiation Bandpass Filter (Absorber, Antenna) Detector Amplifier Signal Filter Output Noise: Noise Equivalent Power (NEP )  T Energy Resolution: 2 1 ln - ¥ ÷ ø ö ç è æ × = D ò df NEP E Best Sensitivity  direct Detection & low Temperatures

Thermal Conductance G to bath TBath = const. Incident Radiation EPh, FPh, tPh Thermometer DT = f ( C, G, EPh, FPh) tTh = f ( C, G,… ) Absorber with Heat Capacitance C Thermal Conductance G to bath TBath = const. C ( T/  t) + G (TTES - TBath) = PAbs (t) + PMeas (t) Spectral Bandwidth :  Absorber and Coupling to Thermometer  very high: l ~ 10-3 m...10-12 m EPh ~ meV...MeV

“Quantum Calorimeter“ Incident Radiation EPh, FPh, tPh Thermometer DT = f ( C, G, EPh, FPh) tTh = f ( C, G,… ) Absorber with Heat Capacitance C Thermal Conductance G to bath TBath = const. C ( T/  t) + G (TTES - TBath) = PAbs (t) + PMeas (t) tTh  tPh : “Quantum Calorimeter“ tTh  tPh : “Bolometer“

Operation at low Temperatures  4.2 K Quantum Calorimeter Bolometer DT = (EPh / C )  1/(1 - i / w tTh) DT = (FPh / G )  1/(1 + i w tTh) Energy dispersive detection of Power sensitive detection rapid temperature change of quasi-dc photon flux for high signal & fast detector for low noise power: DT  , tTh   C  NEP = 4 kB T 2 G  G  C , G  Tn, n > 1 Operation at low Temperatures  4.2 K

Transition Edge Sensor (TES) TES Thermometer Thermometer: high sensitivity & high dynamic range compatible with T  4.2 K & low power dissipation transducer temperature  electrical signal Thermometer Absorber C  Superconducting Phase Transition Thermometer Transition Edge Sensor (TES) TBath Low-Tc Superconductors Nb  9K Al  1K Mo  0.92 K Ti  0.4 K Ir  150 mK W  15 mK...150mK

Resistive Thermometers TES Thermometer Resistive Thermometers R = R0Tn a = (T/R)  R /  T = n T < 4.2K: TES: a  10...1000 Normal Metals: a  0.001 Semiconductor: a  -0.1..-1 TES Thermometers: high & positive a  sensitive temperature-to-resistance transducers  low ohmic, i.e., voltage bias & current readout  negative Electro-Thermal-Feedback

TES Thermometer with ETF „Voltage Bias“ VTES=const. C ( T/  t) + G (TTES - TBath) = PABS (t) + PJoule (t) ITES in transition region: DPABS  DT  DRTES PJoule = - ITES VTES a T / TTES RTES TTES PJoule / T < 0  negative ETF Negative Electro-Thermal Feedback  Linearized TES response: DPABS = -DPJoule (for very strong n-ETF)  Faster TES response: tThETF  3 (C/G) / a (Calorimeter count rate )

 TES readout with SQUIDs IBias Voltage Bias: RBias << RTES RTES  1mW…100mW (Cryogenic) current sensor: ZNoise  RTES RBias RTES DITES  Elektrisches Signal @ 300K <4K SQUIDs : current sensor for TESs low temperature compatible, low power dissipation (<1nW) highly sensitive (pA resolution) & high bandwidth (dc – MHz)

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

LABOCA (as example) Basic Detection Techniques – Submm receivers (Part 3)

Space TES detectors (SPICA, SAFARI) Low G TES High G TES Basic Detection Techniques – Submm receivers (Part 3)

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)