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Title: When Every Photon Matters: OpticalUV Imaging Spectrophotometers


1
When 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
2
A 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

3
Faint, 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

4
Imaging 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
5
Planet 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)

6
Imaging 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
7
Simulation of Speckle Suppression
  • Simulation from Soummer Remi and Ben Oppenheimer
    at AMNH

8
General 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)
9
General 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

10
Microwave 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)

11
Microwave 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
12
Microwave 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

13
Microwave 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

14
Phase 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
15
Quasiparticle 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

16
MKID 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.

17
MKID Testbed at Caltech
  • Kelvinox 25 outfitted for MKID measurements

18
Optical/UV/X-ray MKID Arrays
  • Strips can be stacked to form a high fill factor
    array

19
X-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)

20
Ta/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!

21
Optical 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

22
New Ta/Al Strip Detector
  • 150 nm Ta, 50 nm SiO2 protect, 50 Al

23
New Array Frequency Noise
  • New optical/UV array frequency noise (40nm Al)

24
Excellent Frequency Accuracy
  • 0.8 MHz resonator to resonator frequency jitter!

25
Al/Ti Strip Detectors
  • New hybrid array design (200 nm Al/100 nm Ti on
    Sapphire)

26
Titanium 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!
27
Microwave Kinetic Inductance Detectors
  • IQ readout of amplitude and phase using homodyne
    mixing
  • Standard microwave technique with both analog and
    digital implementations

28
Software 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

29
Software 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
30
Up 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

31
Software 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.

32
SDR 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

33
SDR Demonstration
  • We have demonstrated SDR readouts in the lab and
    at the telescope

Mazin et al., Proceedings of LTD-11, 559 (2006)
34
MKID 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

35
Multicolor Submillimeter MKIDs
  • Antenna coupled multi-color submillimeter
    detectors

36
4x4 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)
37
4x4 Pixel Antenna-Coupled MKID photos
38
Frequency Response and Beam Maps
Band1
Band2
39
DemoCam
  • We brought a submillimeter MKID camera
    (DemoCam) with a SDR readout to the Caltech
    Submillimeter Observatory (CSO) in April, 2007

40
DemoCam
  • 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

41
The 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)
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