The Future of Far-Infrared
Detectors for Space
Jamie Bock
Jet Propulsion Laboratory
Far-Infrared Astronomy from Space
Pasadena, 28-30 May 2008
Space-Ready Far-Infrared Detectors Today
HERSCHEL
- 3.5 m telescope (80 K)
- L2 halo orbit
- 4.5 year lifetime
- 2008 launch
- Three instruments
• SPIRE (bolometers)
• photometer at 250, 350, 500 um
• FT spectrometer 200 – 350, 350 – 670 um
• PACS (photoconductors & bolometers)
• imaging at 70/100, 170 um
• grating spectroscopy 60 – 210 um
• HIFI (heterodyne)
• high-resolution spectroscopy 610 – 270 um
PLANCK
- All-sky map of the CMB
• 5’ resolution
• 30 - 850 GHz coverage
• polarization capability
SPIRE Bolometer Arrays
Detector Array Assembly
Single ‘spiderweb’ bolometer
300 mK Assembly
• NEPs designed to meet Herschel photon background
• Small format – 16:1 “feedhorn multiplexed”
• Low-frequency stability for drift-scanned observations
325 detectors in 5 arrays
NEP = 4e-17 W/√Hz (dark)
QE = 75 %
T0 = 300 mK
See Mark Devlin’s talk on BLAST this afternoon !!!
Sparse beams on sky
Griffin, Bock & Gear (2002) Applied Optics 41, 6543
JFET Amplifiers for SPIRE
Low Frequency Noise Stability
System noise voltage
n
V [ nV Hz
-1/2
N
mod 10 ]
10
10
10
-5
20 nV Hz-1/2
-6
20 nV Hz-1/2
-7
20 nV Hz-1/2
JFET Membrane (24 channels)
10
-8
0.001
0.001
Circuit type
Differential source follower
JFET temperature
140 K
Module temperature
10-50 K
Median noise voltage
7 nV/√Hz at 100 Hz
Power per channel
185 μW
0.01
0.01
0.1
0.1
Freq [Hz]
1
1
10
10
Frequency (Hz)
• Nice noise properties
• Well understood systems issues
• But wiring and power limit array formats
PACS Bolometer Arrays
• Large format - CMOS multiplexed
• Close packing for Nyquist sampling
• Higher NEPs suited for Herschel background
2560 detectors in 2 arrays
NEP = 2e-16 W/√Hz (dark)
T0 = 300 mK
See Poster 38 Billot et al.
PACS Photoconductor Arrays
800 detectors in 2 arrays
NEP = 5e-18 W/√Hz (dark)
QE = 30 %
T0 = 1.7 K
MIPS 32x32 Unstressed Ge:Ga
Ge:Ga Stress Block
PACS Photoconductor Focal Plane Array
Sensitivity Requirements for Far-IR Detectors
Sub-orbital photometry*
R=2
Sub-orbital spectroscopy*
R = 1000
CMB ground / space
5 K Telescope R = 2
5 K Telescope R = 1000
*ε=10
%, T=300 K, η=25%, AΩ=λ2
Readiness for Space-Borne Instruments
Space-Borne Instruments Are Different from Sub-Orbital Instruments
1. Set Instrument Specifications
2. Design System Defenses at the Beginning
- EMI/EMC Mitigation
- Temperature Stability
- Magnetic Shielding
- Wiring
- Known unknowns
3.
4.
5.
6.
- Straylight and Spectral Leaks
- Microphonics
- Crosstalk
- Array Process Control
- Unknown unknowns
Define Subsystems Interfaces
Test Subsystems and Key Interfaces
Verify Instrument System is inflexible to changes
Verify System
Large expensive test, hardware changes virtually impossible
Historical Example: NTD Ge Bolometers
Ge Bolometer Low et al. 1961
Si JFET Preamps IRAS 1983
DC-Stable Readouts SuZIE 1990
NEP = 1e-18 W/√Hz Demonstrated 1996
First Arrays Bolocam 1998
Selected for Herschel 2000
Flight on Herschel 2008+
No Substitute for the Experience Gained with Sub-Orbital Instruments
Example of a Systems Solution
2-D Grating
Parallel-plate
waveguide
Input
• Compact for long-wavelengths
• Shields detectors from straylight
• Forms part of a Faraday cage
Detector
Surface
Transition Edge Superconducting (TES) Bolometers
Advantages
• High Sensitivity
NEP = 5e-19 W/√Hz demonstrated
NEP = 3e-20 W/√Hz achievable
• Large Formats
10,000 detector instruments
• High Efficiency
Optimized absorber - X-ray to millimeter-wave
• “Fast” Response
• 1/f Noise Stable
• Radiation Insusceptible
• No Memory Effects
• Many Applications
FIR/mm-wave - cameras in the field
CMB polarization
X-ray astronomy
Dark matter
Weaknesses
• Need 50-300 mK for Space
• Excess Noise Mechanisms
• Limited Multiplexing
• System Issues
B-Fields, EMI/EMC, Straylight, etc.
Space qualification
Time-Division Multiplexing
SCUBA2 Camera
Focal Plane Readout
40x32 Multiplexer Array
SCUBA2 Instrument
Focal Plane Readout
Focal Plane Assembly
Total detectors = 8x40x32 = 10,240
Based on 40x32 mux array
Frequency Domain Multiplexing
South Pole Telescope
SPT Sub-array
180˚ Hybrid
Absorber and TES
Dual Polarization, Single Band
P1
Bandpass Filter
CMB Detector Development
P2
Back-Illumination
AR
Silicon
Advantages for CMB Polarization
- Antennas define beams
- RF filters spectral band
- Polarization with matched pair differencing
Nb
- Stokes I, Q & U in one pixel
μ-strip - Compact inter-changeable detector
- TES’s already meet space sensitivities
Applicability for the Far-Infrared?
Competing groups actively
fielding instruments
NEPs Are Higher
~ 1e-17 W/√Hz
Formats Are Smaller
~ 2000 detectors
Antennas?
Losses large in Far-IR
Very demanding systems
Noise properties
Overall stability
Shielding
40 mHz
Ti TES Low Frequency Noise Performance
Science Opportunity: Galactic Polarization
QUAD 150 GHz Stokes I
QUAD 150 GHz Stokes U
Kinetic Inductance Detectors
New Emerging Concepts
First Astronomical Light @CSO!
See Jonas Zmuidzinas’ talk
P40 Day et al. Distributed Slot TES Detectors
Array of micro-TES read in parallel
NEP = 4e-19 W/√Hz at 50 mK measured
P41 Echternach et al. Quantum Capacitor Detector
Variable capacitance from quasi-particles
RF multiplexed
Ultra-low NEPs possible
Far-Infrared Spectroscopy
Line ratios measure:
Æ Gas mass, temperature, density
Æ UV field strength and hardness
Æ Metal abundances
Æ Starburst / AGN contributions
Æ Stellar type, starburst age.
Æ Degree of ISM processing
SAFARI Instrument for SPICA
Photoconductors
TES Bolometers
Si Bolometers
Downselect
Fall 2009
K. Inductance Detectors
Imaging FTS
SPICA 3.5m 5 K Telescope
Scientific Case for Extragalactic Spectroscopy
BLISS = Background-Limited Infrared-Submillimeter Spectrometer
Scientific Emphasis
• Physical processes in high-z FIR galaxies
Instrument Description
• Dispersive Spectrometer
- λ/Δλ ~ 1000
- Covers 40 – 600 μm simultaneously
Bolometric Detectors
- 8000 detectors
- NEP = 3e-20 W/√Hz
• ADR Cooler
• Beam Modulator
NEP = 5e-19 W/√Hz
NEP = 5e-20 W/√Hz
Salient Points
• Large gains possible for spectroscopy
• Most gain comes from temperature not size
• Need background limited detectors
ZSPEC: A Hard-Working Millimeter-wave Spectrometer
Spectrometer at 50 mK
Focal
Surface
Good performance across full 1:1.6 band
Grating
Surface
End-to-End
Instrument Efficiency
Frequency [GHz]
3He/4He
Fridge
ADR
Bolometer
Filters
Input Feed
Waveguides
Grating
Detector
Z-Spec Spectra from the CSO
Millimeter-wave Example of
Broadband Spectroscopy
• Complete spectrum obtained
• Molecular rotational transitions
• Expect some surprises!
Flux Density [mJy]
Z-Spec Spectra of IRAS 17208-0014
Si3N4 Beam Isolation
NEP = √4kT2G
τ = (C/G)/(L+1)
L = αP/GT; α = dlnR/dlnT
2 legs x 120 x 900 x 1 μm3
4 legs x 9 x 900 x 1 μm3
4 legs x 3 x 900 x 1 μm3
4 legs x 3 x 8300 x .5 μm3
4 legs x 0.35 x 700 x .5 μm3
G~T3
G~T2
G~T
Measured NEP
Phonon NEP
Phonon NEP
Low-G Beam-Isolated TES Bolometers
Low thermal conductance with long thin legs
700 x 0.5 x 0.5 μm
Suitable for 1D or 2D arrays
No real mechanical problems!
More extreme geometries fabricated
NEP=√4kT2G
Fabricated Full TES with Tc = 220 mK
Electrical NEP = 4e-19 W/√Hz
Agrees with phonon NEP = √4kT2G
Fabricated full TES with Tc = 70 mK
Phonon NEP = 6e-20 W/√Hz
Electrical NEP measurements underway
Close to R=1000 photon noise!
Matt Kenyon et al. (see poster 39)
Si3N4 Heat Capacity Measurements
Si3N4 ‘absorber’
IR Area = 200 x 200 μm
2 μm beams, 20 μm grid (20 % ff)
0.25 μm thick
No metal deposited
Dry XeF2 release
Si3N4 supports
4 x 700 μm x 0.5 μm x 0.25 μm
Heater & noise thermometer
40 μm x 10 μm x 60 nm +
60 μm x 20 μm x 60 nm
Total heat capacity
τ = 0.8 s
G = 20 fW/K at 60 mK*
Ctot ~ 25 fJ/K fast enough for slow chop
*Assumed due to some problems with heater wiring
†Calculated
Roadmap to a Future Space Mission
Capabilities determined by available technology
Far-Infrared detectors are largely developed by astronomers
- Small collaborations between universities and government labs
The good news
- Required NEPs are in reach
- We know how to multiplex 1000’s of detectors
- New detectors emerging (and maturing)
- Larger multiplexing techniques emerging
- Leverage sub-orbital to advance some TRLs: multiplexing, optical coupling
- Leverage technical developments in other scientific fields
The bad news
- Space background are very low: no representative environment except lab
- No NASA program for advancing detector TRL for space
- How do we build and maintain systems expertise for space?
We need a low-cost, realistic and scientifically compelling space platform for
this decade to sustain technologies for a future US mission in the next decade
Z-Spec Spectra of the Cloverleaf (z=2.56)
CO 6-5
CO 7-6
CO 8-7
CO 9-8