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Scientific Report 2007-2009
Astronomy & Astrophysics
A15. Kinetic Inductance Detectors
for Measurements of the Cosmic Microwave Background
Cosmic Microwave Background (CMB) observations
are currently limited by background radiation noise, even
for space-borne measurements. In this situation, the
only way to improve the efficiency of CMB measurements
is to boost the mapping speed of the experiment, using
arrays of microwave detectors.
The Microwave Kinetic Inductance Detectors
(MKIDs) are superconducting detectors providing
detection of low energy photons (in the meV range)
which can break Cooper pairs in a superconducting film,
changing its surface impedance, and in particular the
kinetic inductance L k . This can be measured by letting
the kinetic inductance be part of a superconducting
resonator, which can have very high merit factor Q (up
to ≃ 10 6 ), and thus be very sensitive to the variations of
its components. Furthermore, the high Q makes MKIDs
intrinsically multiplexable in the frequency domain:
in a 1 GHz bandwidth it is possible to accommodate
≃ 10 3 ÷ 10 4 detectors, biased at different frequencies,
all read simultaneously using a single coax cable, so
that they can be easily implemented into large format
arrays.
Aluminum film sputtered on a 400µm Silicon substrate.
We have setup a facility for test and optimization of
these devices. It is composed of a 0.3K cryogenic system
(pulse-tube cooler plus 3 He refrigerator), including two
low thermal conductivity coaxial cables to bias the array.
The facility includes a vector analyzer, frequency synthesizer,
microwave sources (Gunn oscillators and antennas)
and filter chains.
Figure 2: Resonance data (S 12 in dB) versus temperature
for one of our LEKID chips.
Figure 1: Picture of a 81 pixel array of lumped elements
kinetic inductance detectors, built by the RIC-INFN collaboration
and optimized for 140 GHz photons.
CMB photons with ν > 90 GHz have enough energy
to break Cooper pairs in Aluminum. We have thus focused
in the last 4 years on the development of aluminum
MKIDs [1]. Our resonators are distributed λ/2
ones; however their design follows an approach typical of
lumped elements resonators (LEKID), varying the geometry
of the circuit components in order for the resonator
to match the impedance of free space. The resonator
thus acts as a free absorber essentially on its whole area,
without the need of antennas or quasi-particles traps.
This makes the detectors easy to fabricate and to optimize
for the specific experimental needs. We have optimized
the geometry of the resonators with extensive use
of 2-D and 3-D electromagnetic simulations.
Our detector chips have been made at the Bruno
Kessler Foundation in Trento, and consist of a 40nm
A thorough electrical characterization, also useful for
calibration, can be achieved by making temperature
sweeps and measuring the resulting variation in the
amplitude and phase of the transmitted signal. The
temperature increase induces an excess of quasiparticles
N qp in the material, from which we can estimate the
responsivity in terms of deg/N qp . To get optical data,
we used a chopper alternating 300K and 77K blackbody
sources, filling the field of view of the detector. A series
of mesh-filters is placed on the windows on the cryostat
shields at different temperatures. These remove high
frequency radiation and define the transmission band,
which in our case ranges from 100 to 185 GHz. We
have measured typical optical NEPs ∼ 2 · 10 −16 W/ √ Hz
(1 ÷ 10Hz range). These detectors are already suitable
for ground-based astrophysical measurements, where
they are limited by the noise of the radiative background.
Devices suitable for space-borne missions are
currently under development.
References
1. M. Calvo et al., Mem. S. A. It. 79, 953 (2008).
Authors
M. Calvo, A. Cruciani, P. de Bernardis, C. Giordano, S.
Masi
Sapienza Università di Roma 162 Dipartimento di Fisica