Title: When Every Photon Matters: OpticalUV Imaging Spectrophotometers
1When Every Photon MattersOptical/UV Imaging
Spectrophotometers
- Benjamin Mazin, June, 2008
The Caltech/JPL MKID team Caltech Jonas
Zmuidzinas, Sunil Golwala, Tasos Vayonakis,
Jiansong Gao, David Moore, Omid Noozorian, Jack
Sayers JPL Bruce Bumble, Peter Day, Rick LeDuc,
Ben Mazin
2A New View on the Universe
- CCDs are powerful tools, but have some weaknesses
- Poor time resolution
- Energy resolution requires dispersive optics or
filters - Read noise, dark current, and cosmic rays degrade
performance - Low Temperature Detectors (LTDs) can measure the
energy and arrival time of every incoming photon - Microsecond time resolution
- Energy resolution RE/?E20-50 or higher, wide
bandwidth - No read noise or dark current, near perfect
cosmic ray rejection - Limited count rate
- Large arrays are difficult to make
- Arrays of near-IR/optical/UV LTDs allow us to
address scientific questions that are difficult
or impossible with conventional focal planes
3Faint, Rapidly Variable Compact Objects
- Optical counterparts are seen for a handful of
pulsars - North Crab, PSR B065614, Geminga, PSR B192910,
etc. - South PSR B0540-69, Vela, PSR B1055-52, etc.
- Optical pulsations have been seen in a subset of
these objects - The source of the optical/UV emission from
pulsars is unknown - Polar Cap Model
- Outer Gap Model
- And more
- Determining the source of the optical/UV emission
may help us understand pulsar emission at other
wavelengths and explore one of the most energetic
and interesting natural physics labs
4Imaging Spectrophotometer forCompact Object
Science
- Many of the difficulties in the CCD-based pulsar
camera could be overcome by using a
photon-counting, energy-resolving focal plane - spectral changes
- number of peaks
- separation between peaks
- lag between optical, gamma-ray, and radio peaks
- ratio of the flux in the peaks
- polarization
- Many other interesting observations of compact
objects (LMXBs, Am Hercs, AXPs, SGRs, etc.)
Crab Pulsar Romani et al. 1999
5Planet Finding
- Coronagraphs from the near-IR to optical are one
of the best ways to directly image terrestrial
planets around nearby stars - Suppress the starlight by a factor of 10 billion
and image a planet as close in as the second or
third Airy ring - Harder than it sounds, and it sounds pretty hard!
- Ground-based (TMT) Space-based (TPF-C)
6Imaging Spectrophotometer for Planet Finding
- Coronagraphs are limited by speckles from
scattered and diffracted light - Energy-resolving focal planes could increase
sensitivity by a factor of up to 100 (!) - Lower mirror and coronagraph tolerances
- Removes requirement of a separate spectrograph
- Gives the spectra of all planets in the dark box
Energy Resolved Photon Counting at Each Pixel
7Simulation of Speckle Suppression
- Simulation from Soummer Remi and Ben Oppenheimer
at AMNH
8General Astrophysics inthe Near-IR/Optical/UV
- Integral Field Units are becoming popular
instruments for diverse general astrophysical
problems - Even low energy resolution (RE/?E 20-50)
allows determination of redshifts - Galaxy age/metallicity/extinction variations with
morphology - Larger fields of view allows survey science
- Strong and Weak Lensing
- Microlensing
- Gamma-ray bursts afterglows
- UV missions
- Order sorter for Echelle spectrograph
- Much more!
Redshift determination of simulated 300-800 nm
observations of spiral galaxies, R55, 1 hour on
Keck (Mazin and Brunner, AJ 2000)
9General Astrophysics at Other Wavelengths
- Basic technology behind MKIDs applicable from the
millimeter to the X-ray - CMB Polarization
- Submillimeter Galaxies
- Iron Lines from Black Hole Accretion Disks
- Also good for some particle detection and
industrial applications - Cryogenic Dark Matter Search (CDMS)
- Neutrino mass through Rhenium beta decay (MARE)
- X-ray microanalysis
10Microwave Kinetic Inductance Detectors
- Microwave Kinetic Inductance Detectors (MKIDs)
are pair-breaking superconducting detectors - Superconductivity is caused by bound pairs of
electrons known as Cooper Pairs. Free electrons
not bound in pairs are known as quasiparticles. - There is a gap energy separating the ground state
of Cooper Pairs from the quasiparticles. This is
know as the gap of the superconductor, and is
lt1 meV. (Panel a) - Energy is required to accelerate or decelerate
Cooper pairs in a superconductor, leading to a
extra inductance known as kinetic inductance. - Changing the population of Cooper pairs by
breaking them with photons or increasing the
temperature changes the kinetic inductance. - In an MKID we use a superconductor as the
inductor in a high quality factory (Q104106) LC
resonant circuit. (Panel b)
11Microwave Kinetic Inductance Detectors
- Breaking Cooper Pairs increases the kinetic
inductance and surface resistance - The resonance shifts to lower frequency and gets
shallower - This can be observed as a change in the phase and
amplitude of a probe signal centered on the low
temperature resonant frequency - A resonant circuit can be designed to have nearly
perfect transmission off resonance and reflect
power on resonance (Panel c) - Built in frequency domain multiplexing!
- For more details, see
Nature, vol. 425, pp. 817-821, 2003Mazins PhD
Thesis, Caltech, 2004App. Phys. Lett., 89,
222507, 2006
12Microwave Kinetic Inductance Detectors
- We chose to pursue coplanar waveguide (CPW)
transmission line MKIDs - Quarter wavelength long, one end open, the other
shorted to ground - Simple, one layer design
- No lossy thin-film dielectrics
- Elbow coupler allow accurate control of Q
- Position dependent response
- hybrid architecture all Nb except the last 100
microns of center strip, which is usually Al, Re,
or Ti
13Microwave Kinetic Inductance Detectors
- Transmission line and lumped element
implementations possible - Excite from a common feedline
- Lithographically set each resonator to a
different resonant frequency (by changing the
length, L, or C) - Dump quasiparticles into shorted end for maximum
responsivity - Excite with a comb on frequencies generated at
room temperature - Use a wide bandwidth HEMT to amplify the signal
from all resonators - Use room temperature electronics to sort out the
signals
14Phase and Amplitude
- We can represent the transmission past the
resonator, S21, in the complex plane - We reference phase and amplitude changes to the
center of the lowest T resonance loop
O
Points on the resonance loop are equally spaced
in frequency
15Quasiparticle Trapping
- Quasiparticles that diffuse from a higher gap
metal into a lower gap metal quickly emit a
phonon and become trapped - Decouples photons absorption from energy
detection - Allows large absorbers with small, sensitive
detectors
16MKID Strip Detectors
- A linear array of strips provides a pixel array
- ABSORPTION
- Each photon breaks many Cooper pairs in the
absorber. - DIFFUSION
- Quasiparticles diffuse about the absorber,
reaching the ends in microseconds. - TRAPPING
- MKIDs made of a material with lower
superconducting gap energy are attached to the
ends of the strip the quasiparticles emit
phonons and collapse to the new gap energy,
becoming trapped in the MKID. - DETECTION
- Photon energy is determined by summing the
response from the MKIDs on both sides of the
absorber -
- RESOLVE X-POSITION
- The relative ratio of signals at the two sensors
provides the position along the strip, so the
energy resolving power determines the strip
aspect ratio and pixel size. - MULTIPLEXING
- All resonators are coupled to a single drive
line.
17MKID Testbed at Caltech
- Kelvinox 25 outfitted for MKID measurements
18Optical/UV/X-ray MKID Arrays
- Strips can be stacked to form a high fill factor
array
19X-ray Strip Detector Results
- Working X-ray strip detector demonstrated in 2006
- 600 nm Ta absorber
- 200 nm Al MKID
- Illuminated with 55Fe X-rays
- ?E 62 eV at 6 keV
- Data from a poor quality sapphire substrate,
should be able to do significantly better - Mazin et al., App. Phys. Lett., 89, 222507 (2006)
20Ta/Al Optical/UV Strip Detector
- 4 Layer Device on Sapphire
- 80 nm epi-Ta
- 40 nm SiO2 Ta protect
- 20 nm Al resonator
- 100 nm Nb resonator body
- 1 micron center strip!
21Optical Strip Detector Results
- Optical strip detector demonstrated in late 2007
- ?E 40 nm at 250 nm
- Working now on refining array fabrication to
improve energy resolution - Arrays of many thousands of pixels are feasible
now
22New Ta/Al Strip Detector
- 150 nm Ta, 50 nm SiO2 protect, 50 Al
23New Array Frequency Noise
- New optical/UV array frequency noise (40nm Al)
24Excellent Frequency Accuracy
- 0.8 MHz resonator to resonator frequency jitter!
25Al/Ti Strip Detectors
- New hybrid array design (200 nm Al/100 nm Ti on
Sapphire)
26Titanium Hybrid MKIDs
- Titanium hybrids have very long penetration
depths and quasiparticle lifetimes, making
extremely sensitive detectors.
NEP 4 x 10-19 W Hz-0.5 At 100 Hz!
27Microwave Kinetic Inductance Detectors
- IQ readout of amplitude and phase using homodyne
mixing - Standard microwave technique with both analog and
digital implementations
28Software Radio Readout
- Software Defined Radio (SDR) Overview
- Generate frequency comb and upconvert to
frequency of interest - Pass through MKID and amplify
- Downconvert and Digitize
- Extract signals in a powerful FPGA
29Software Radio Signal Generation
- Generate an comb of frequencies at baseband with
a fast D/A - Modulation possible
- Waveform buffer size important
- No phase jumps!
f1
f3
f2
30Up and Down Conversion
- Shift the comb to microwave frequencies with
mixer (SSB, IQ modulator, etc.) - Using the same synthesizer for up and down
conversion removes the synthesizer phase noise
contribution (to first order) - Pass the comb through the detector
- Amplify the signal with a low noise amplifier
(HEMT, SQUID Amp, etc.) - Amplifier 1 dB compression can limit the number
of probe signals - Depends on detector readout power, number of
detectors - Weinreb HEMTs can handle -40 dBm
- Carrier suppression can help with this
- Downconvert back to baseband
31Software Radio Demodulation
- Sample with a fast A/D
- A/D dynamic range important!
- Figure out how to get the amplitude and phase
modulation imprinted by the detectors out - Lots of possibilities
- Direct digital downconverters on dedicated IC
- Direct digital downconverters on FPGA
- Polyphase filterbank or FFT on FPGA
- Hybrid FFT/DDC solutions
- Careful of dynamic range, output sample rates,
phase skips, etc.
32SDR Hardware
- Innovative Integration x5-400m board
- Dual 400 MHz 16-bit D/A
- Dual 400 MHz 14-bit A/D
- Xilinx Virtex 5 (SX95T) FPGA
- Complete 144 resonator, 400 MHz bandwidth readout
on a PMC module! - 75/resonator, 7.5/optical pixel (due to strip
multiplexing) - Other boards (Pentek, CASPER) are also
possibilities
33SDR Demonstration
- We have demonstrated SDR readouts in the lab and
at the telescope
Mazin et al., Proceedings of LTD-11, 559 (2006)
34MKID Camera for Palomar 200
- Lens coupled 20x64 pixel array in cryogen-free
ADR, 0.33 pixels - 350 nm to 850 nm simultaneous bandwidth
- Energy resolution of 20 at 400 nm
- On-the-sky QE of 30-40
- Maximum count rate/pixel 200 cts/sec due to
strip multiplexing - Only for faint objects (V gt 20)
- Dark time only
- All digital room temperature readout
- Possibility of future operation with PALM-3000 AO
system - Preliminary design is for Coudé focus
- Camera Team Ben Mazin, Bruce Bumble, Sunil
Golwala, David Moore - Collaborators J. Zmuidzinas, C. Martin, P. Day,
R. LeDuc
35Multicolor Submillimeter MKIDs
- Antenna coupled multi-color submillimeter
detectors
364x4 Pixel Antenna-Coupled MKIDs
Fabricated and tested prototype 4x4
antenna-coupled MKID array for 200-400 GHz.
Nb pixel ground plane and microstrip
resonators
Al feedline and resonators
feedline
3.3 mm
20 mm
Lithographic lumped-element bandpass
filters (multicolor pixel)
374x4 Pixel Antenna-Coupled MKID photos
38Frequency Response and Beam Maps
Band1
Band2
39DemoCam
- We brought a submillimeter MKID camera
(DemoCam) with a SDR readout to the Caltech
Submillimeter Observatory (CSO) in April, 2007
40DemoCam
- We detected an object outside of the solar
system! - Plan to return this summer with a 6x6 pixel, 4
color array with 144 channel SDR readout
41The Future Microstrip MKIDs
- Funding started in late 07 from ROSES-APRA2 grant
- Goal is to develop megapixel optical/UV imagers
using microstrip MKIDs - Thin dielectrics (5-20 nm) yield high kinetic
inductance fraction, short resonators - Can be made on top of absorbers and
back-illuminated for high fill factors in 2-d - Use amplitude readout to avoid excess phase noise
problems - High Q thin-film dielectrics needed, develop in
conjunction with Martinis at UCSB, Pappas at NIST
Martinis et al. (2005)