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

2. 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)

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)

4. 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)

5. 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)

6. 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)

7. 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)

8. 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)

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

10. KID arrays for Astronomy Principle of Kinetic Inductance Detection2 1
CPW Through line
CPW ¼  Resonator
Coupler
Antenna
substrate
Central conductor
100 m
L= 5 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)

11. KID arrays KID radiation coupling2 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 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)

12. KID arrays Principle of KID arraysF0
F0 set by geometry (length)
Intrinsic FDM
Basic Detection Techniques – Submm receivers (Part 3)

13. 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)

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

15. 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)

16. 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)

17. 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: ～ W/√Hz EF
Our goal:
@ 600 GHz 17
Basic Detection Techniques – Submm receivers (Part 3) 17

18. 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

19. 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

20. 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 m
EPh ~ meV...MeV

21. “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“

22. 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

23. 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

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

25. 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 )

26.  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)

27. 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)

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

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

30. 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)