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The Berkeley bolometer group has successfully demonstrated a Antenna-Coupled, TES
Bolometer. This is a conerstone technological achievement for the PolarBear project,
and is the first step in achieving large imaging arrays that are required for the
next generation of mm-wave experiments. Just as CCD's revolutoinized optical
astronomy during the past few decades, bolometer arrays have similar potential
in the longer wavelength regime.
Antenna
The antenna used in our detector is a double slot dipole. The antenna is
used in conjunction with a silicon hyperhemispherical lens. The detector
chip sits directly on the lens. This antenna/lens combination has been
used extensively at these frequencies and can couple efficiently to
typical telescope optics. The antenna is also linearly polarized and a
dual linearly polarized version can be used. This is convenient in our
application, since the polarization of the incident light is what we want
to measure.
Superconducting Microstrip
The antenna is connected to a transmission line, which is used to bring
the incoming optical power to the detector. Conventional transmission
line materials would be very high loss at these frequencies, which would
be unacceptable for an experiment such as this where we are already
looking for an extremely weak signal. However, superconducting microstrip
is a very low loss transmission line that will work well in our
application. A convenient choice of materials is niobium, which has the
highest superconducting temperature of all the elements. Niobium
microstrip was originally investigated decades ago for use as
interconnects in ultra high speed superconducting digital circuits. The
loss in niobium microstrip is quite small for frequencies up to about 600
to 700 GHz, which makes it well suited for our application.
Band defining microstrip filters
An advantage of the use of microstrip to connect the antenna to the
bolometer is that band defining microstrip filters can be integrated into
the transmission line. In a conventional millimeter wave receiver, band
defining filters are metal mesh off-chip optical filters. If several
bands are required, several of these off-chip filters must be used. In
our detectors, the filters are integrated on the chip and different pixels
can easily have different frequency sensitivities. Increasing the level
of integration will ease the scaling of current receivers to higher pixel
counts.
Bolometers
The bolometers are composed of a terminating resistor and a
superconducting Transition Edge Sensor (TES) located on a leg isolated
silicon nitride substrate. The incoming power on the superconducting
microstrip is dissipated in the load resistor as heat, and the change in
temperature is measured by the TES. In order to achieve the sensitivity
we need, the bolometer must be thermally isolated by the silicon nitride
legs and the bath temperature must be below 300 mK. This reduces the
detector noise to below that in the incident light.
TESs have many advantages over conventional semiconducting bolometers.
Probably the most important one in our application is that the TES readout
electronics can be multiplexed, so that the signal for more than one pixel
can be brought out on each pair of wires. As the pixel count of bolometerBolometers
The bolometers are composed of a terminating resistor and a
superconducting Transition Edge Sensor (TES) located on a leg isolated
silicon nitride substrate. The incoming power on the superconducting
microstrip is dissipated in the load resistor as heat, and the change in
temperature is measured by the TES. In order to achieve the sensitivity
we need, the bolometer must be thermally isolated by the silicon nitride
legs and the bath temperature must be below 300 mK. This reduces the
detector noise to below that in the incident light.
TESs have many advantages over conventional semiconducting bolometers.
Probably the most important one in our application is that the TES readout
electronics can be multiplexed, so that the signal for more than one pixel
can be brought out on each pair of wires. As the pixel count of bolometer
arrays grows this becomes increasingly important. See the ^ÓMultiplexer^Ô
section for details on our work on TES readout multiplexers.
arrays grows this becomes increasingly important. See the ^ÓMultiplexer^Ô
section for details on our work on TES readout multiplexers.
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