A superconducting focal plane array for ultraviolet, optical, and near-infrared astrophysics.
ABSTRACT Microwave Kinetic Inductance Detectors, or MKIDs, have proven to be a powerful cryogenic detector technology due to their sensitivity and the ease with which they can be multiplexed into large arrays. A MKID is an energy sensor based on a photon-variable superconducting inductance in a lithographed microresonator, and is capable of functioning as a photon detector across the electromagnetic spectrum as well as a particle detector. Here we describe the first successful effort to create a photon-counting, energy-resolving ultraviolet, optical, and near infrared MKID focal plane array. These new Optical Lumped Element (OLE) MKID arrays have significant advantages over semiconductor detectors like charge coupled devices (CCDs). They can count individual photons with essentially no false counts and determine the energy and arrival time of every photon with good quantum efficiency. Their physical pixel size and maximum count rate is well matched with large telescopes. These capabilities enable powerful new astrophysical instruments usable from the ground and space. MKIDs could eventually supplant semiconductor detectors for most astronomical instrumentation, and will be useful for other disciplines such as quantum optics and biological imaging.
- SourceAvailable from: ltd12.grenoble.cnrs.fr[show abstract] [hide abstract]
ABSTRACT: The DROID (Distributed Read-Out Imaging Detector) is being developed to overcome the limitation in sensitive area with the use of single STJ’s (Superconducting Tunnel Junctions). The DROID configuration allows the reconstruction of the position of the photon absorption and therefore it can replace a number of single STJ’s in a detector array. The position resolution dictates how many STJ the DROID can replace. We present direct measurements of the position resolution in DROIDs, using a 10µm spot of visible light which illuminates the DROID locally and which is scanned along the absorber length. The DROIDs used for the measurements have 100nm thick Ta absorber strips with Ta/Al/AlO x /Al/Ta STJ’s on either end. The STJ’s are square with the same size as the absorber width and the base Ta layer are adjacent to the absorber. The position resolution is measured for absorber length ranging from 200 to 400µm and 30µm width.Journal of Low Temperature Physics 01/2008; 151(1):298-303. · 1.18 Impact Factor
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ABSTRACT: We are developing time- and energy-resolved near-IR/optical/UV photon detectors based on sharp superconducting-normal transition edges in thin films. We report observations of the Crab pulsar made during prototype testing at the McDonald 2.7 m with a fiber-coupled transition-edge sensor (TES) system. These data show substantial (δα ~ 0.3), rapid variations in the spectral index through the pulse profile, with a strong phase-varying IR break across our energy band. These variations correlate with X-ray spectral variations, but no single synchrotron population can account for the full spectral energy distribution. We also describe test spectrophotopolarimetry observations probing the energy dependence of the polarization sweep; this may provide a new key to understanding the radiating particle population.The Astrophysical Journal 12/2008; 563(1):221. · 6.73 Impact Factor
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ABSTRACT: Our research group has developed and characterized transition-edge sensor (TES) arrays for near IR-optical-near UV astrophysical observations. These detectors have a time-stamp accuracy of and an energy resolution of 0.16 eV for 2.33 eV photons at very high rates (30 kHz). We have installed a 6×6 array of these TESs in an adiabatic demagnetization refrigerator equipped with windows for direct imaging. We discuss new instrumentation progress and current data in all aspects related to successful operation of this camera system, including: detector and array performance, position dependence and cross-talk, low-temperature and readout electronics, quantum and system efficiency, IR filtering, and focus and imaging.Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/2006; 559(2):525-527. · 1.14 Impact Factor
A superconducting focal plane array for
ultraviolet, optical, and near-infrared
Benjamin A. Mazin,1∗Bruce Bumble,2Seth R. Meeker,1Kieran
O’Brien,1Sean McHugh,1and Eric Langman1
1Department of Physics, University of California, Santa Barbara, California 93106, USA
2NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, California 91109, USA
proven to be a powerful cryogenic detector technology due to their sensi-
tivity and the ease with which they can be multiplexed into large arrays.
A MKID is an energy sensor based on a photon-variable superconducting
inductance in a lithographed microresonator, and is capable of functioning
as a photon detector across the electromagnetic spectrum as well as a
particle detector. Here we describe the first successful effort to create a
photon-counting, energy-resolving ultraviolet, optical, and near infrared
MKID focal plane array. These new Optical Lumped Element (OLE)
MKID arrays have significant advantages over semiconductor detectors
like charge coupled devices (CCDs). They can count individual photons
with essentially no false counts and determine the energy and arrival time
of every photon with good quantum efficiency. Their physical pixel size
and maximum count rate is well matched with large telescopes. These
capabilities enable powerful new astrophysical instruments usable from
the ground and space. MKIDs could eventually supplant semiconductor
detectors for most astronomical instrumentation, and will be useful for other
disciplines such as quantum optics and biological imaging.
Microwave Kinetic Inductance Detectors, or MKIDs, have
© 2011 Optical Society of America
OCIS codes: (040.1240) Detector Arrays; (350.1270) Astronomy and Astrophysics
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Cryogenic detectors are currently the preferred technology for astronomical observations over
most of the electromagnetic spectrum, notably in the far infrared through millimeter (0.1–
3 mm) [1, 2, 3], X-ray , and gamma-ray  wavelength ranges. In the important ultravi-
olet, optical, and near infrared (0.1–5 µm) wavelength range a variety of detector technologies
based on semiconductors, backed by large investment from both consumer and military cus-
tomers, has resulted in detectors for astronomy with large formats, high quantum efficiency,
and low readout noise. However, these detectors are fundamentally limited by the band gap of
the semiconductor (1.1 eV for silicon) and thermal noise sources from their high (∼100 K)
operating temperatures . Cryogenic detectors, with operating temperatures on the order of
100 mK, allow the use of superconductors with gap parameters over 1000 times lower than typ-
ical semiconductors. This difference allows new capabilities. A superconducting detector can
count single photons with no false counts while determining the energy (to several percent or
better) and arrival time (to a microsecond) of the photon. It can also have much broader wave-
length coverage since the photon energy is always much greater than the gap energy. While a
CCD is limited to about 0.3–1 µm, the new arrays described here are sensitive from 0.1 µm in
the UV to greater than 5 µm in the mid-IR, enabling observations at infrared wavelengths vital
to understanding the high redshift universe.
This approach has been pursued in the past with two technologies, Superconducting Tunnel
Junctions (STJs) [7, 8] and Transition Edge Sensors (TESs) [9, 10]. While both of these tech-
nologies produced functional detectors, they are limited to single pixels or small arrays due to
the lack of a credible strategy for wiring and multiplexing large numbers of detectors, although
recently there have been proposals for larger TES multiplexers .
Microwave Kinetic Inductance Detectors, or MKIDs, are an alternative cryogenic de-
tector technology that has proven important for millimeter wave astrophysics[13, 14] due to
their sensitivity and the ease with which they can be multiplexed into large arrays. MKIDs use
frequency domain multiplexing  that allows thousands of pixels to be read out over a single
microwave cable. While the largest STJ array is 120 pixels  and the largest optical TES
array is 36 pixels , the MKID arrays described below are 1024 pixels, with a clear path to
Megapixel arrays. The ability to easily reach large formats is the primary advantage of MKID
In this paper we describe the first photon-counting, energy-resolving ultraviolet, optical,
and near infrared MKID focal plane array. These Optical Lumped Element (OLE) MKID
arrays have significant advantages over semiconductor detectors like charge coupled devices
(CCDs) . They can count individual photons with essentially no false counts and determine
the energy and arrival time of every photon with good quantum efficiency. Their physical pixel
size and maximum count rate is well matched with large telescopes. These capabilities enable
powerful new astrophysical instruments usable from the ground and space.
2.Detector design and fabrication
MKIDs work on the principle that incident photons change the surface impedance of a su-
perconductor through the kinetic inductance effect . The kinetic inductance effect occurs
because energy can be stored in the supercurrent of a superconductor. Reversing the direction of
port 1port 2
Fig. 1. Left: The basic operation of an MKID, from . (a) Photons with energy hν are ab-
sorbed in a superconducting film, producing a number of excitations, called quasiparticles.
(b) To sensitively measure these quasiparticles, the film is placed in a high frequency pla-
nar resonant circuit. The amplitude (c) and phase (d) of a microwave excitation signal sent
through the resonator. The change in the surface impedance of the film following a photon
absorption event pushes the resonance to lower frequency and changes its amplitude. If the
detector (resonator) is excited with a constant on-resonance microwave signal, the energy
of the absorbed photon can be determined by measuring the degree of phase and amplitude
shift. Right: The top panel shows the results the equivalent circuit of multiplexed MKIDs,
and the bottom panel shows microwave transmission data from actual MKIDs with very
accurate frequency spacing.
the supercurrent requires extracting the kinetic energy stored in the supercurrent, which yields
an extra inductance. This change can be accurately measured by placing this superconduct-
ing inductor in a lithographed resonator. A microwave probe signal is tuned near the resonant
frequency of the resonator, and any photons which are absorbed in the inductor will imprint
their signature as changes in phase and amplitude of the probe signal. Since the quality factor
Q of the resonators is high and their microwave transmission off resonance is nearly perfect,
multiplexing can be accomplished by tuning each pixel to a different resonant frequency with
lithography during device fabrication. This is accomplished by changing the total length of the
inductor with a “trombone section”, resulting in a lower inductance and therefore a higher res-
onant frequency. A comb of probe signals can be sent into the device, and room temperature
electronics can recover the changes in amplitude and phase without significant cross talk ,
as shown in Figure 1.
MKIDs are extremely versatile, as most resonators with a superconductor as the inductor will
function as a MKID. We have decided to pursue a lumped element resonator design , shown
in Figure 2. The resonator itself consists of a 20 nm thick sub-stoichiometric titanium nitride
(TiNx) film , with the nitrogen content tuned with x < 1 such that the superconducting
transition temperature Tcis about 800 mK. Due to the long penetration depth of these films
(∼1000 nm) the surface inductance is an extremely high 90 pH/square, allowing a very compact
resonator fitting in a 100×100 µm square. Due to bandwidth limitations of our electronics we
use two feedlines to read out the array, each serving 512 resonators. The resonators are designed
to be separated by 2 MHz within a 4–5 GHz band.
To avoid crosstalk between pixels the inductors are made with a double meander design that
Fig. 2. Left: A photograph of the 1024 pixel OLE MKID array with microlenses mounted
into a microwave package. The greyscale insets are scanning electron microscope (SEM)
images of the array to show the pixel design. The pixels are on a 100 µm pitch, with
slot widths inside the resonator of 0.5 µm. Right: A SEM of a OLE MKID pixel. The
microwave feedline runs down the middle, with ground straps shorting the finite ground
planes together. An L-shaped piece of niobium is connected to the center strip and enables
strong coupling of the resonator to the feedline. The resonant frequency is adjusted by
changing the length of a “trombone section”. The tapering is visible as the slow increase in
leg width with increasing distance from the feedline.
allows the electric field from the charge in each meander leg to be precisely cancelled by the
adjacent leg . The array is designed so that resonators close together in resonant frequency
are physically far apart. To improve the quantum efficiency of the device a 100 µm pitch cir-
cular microlens array is used to focus the incoming light on the inductor, since photons hitting
the capacitor or wiring will not be detected or will appear as photon events with an energy sig-
nificantly below their true energy. The circular microlenses used in these measurements limits
the effective fill factor to 67%. An improved lens with square lens elements could increase the
fill factor above 95%.
In order to achieve high energy resolution, the OLE MKID must have a uniform response
to photons hitting anywhere inside the spot produced by the microlens, which is expected to
have a wavelength dependent diameter of around 15 µm. The responsivity of an OLE MKID
depends on the current density in the meander leg at the location the photon is absorbed. Since
the capacitance of our resonator is small the current density changes by nearly a factor of two
over the length of the inductor. Diffusion does not even out the quasiparticle distribution since
the quasiparticle diffusion length in TiN is expected to be short (on the order of 10 µm). In
order to normalize the response we taper the width of each leg to give a uniform current density
in the last eight legs (the microlens target) based on electromagnetic simulations of the current
density using the SONNET software package, as shown in Figure 3. The widths of the legs vary
from 2.5–4 µm, while the slots are 0.5 µm.
Early prototypes used coplanar waveguide (CPW) or coplanar slotline (CPS) feedlines to
frequency, presumably due to undesired modes being excited on the feedline at discontinuities
CPW (FCPW) feedline with regular straps that connect the ground planes together to suppress