Thermal properties in mesoscopics: physics and ... - ResearchGate

that the performance improvement is maximum when the
temperature of the heat bath of the detector is lowered
(Anghel et al., 2001). Modest performance increase is
possible with direct electronic cooling, such as is the case
in many of the SINIS bolometer experiments.
In addition to improved noise performance, direct coupling
of a SINIS cooler to a thermal detector could allow
for increasing the dynamic range of for example bolometers
based on transition edge sensors (TES): A SINIS
cooler can be used to draw a constant power from a TES
that would otherwise saturate due to an optical load.
B. Bolometers: Continuous excitation
Thermal detectors that are used to detect variations
in the incident flux of photons or particles are called
bolometers (from the Greek word bole - beam). This
condition is generally met when the mean time between
incident quanta of energy that arrive at the detector is
much shorter than the recovery time of the bolometer
3 . The bolometric operating principle is very simple:
Change ∆ ˙ Qopt in the incident optical power creates a
change in the temperature of a thermally isolated element
by ∆T = ∆ ˙ Qopt/Gth. A sensitive thermometer is used
to measure this temperature change. The recovery time
τ0 is determined by the heat capacity C of the bolometer,
and the thermal conductance Gth with τ0 = C/Gth,
analogously to an electrical RC circuit.
Bolometric detectors remain popular today, 125 years
after their first introduction. Probably the most important
advantage of thermal detectors is their versatility:
Bolometers can detect radiation from α -particles to radio
waves, their dynamic range can be easily adapted for
a variety of signal or background levels. As an extreme
example, bolometers have been used to detect infrared radiation
from nuclear fireballs (Stubbs and Phillips, 1960),
and the cosmic microwave background. (Lamarre et al.,
2003).
In the early days, bolometers typically utilized handcrafted
construction (dental floss, cigarette paper and
balsa wood are examples of typical materials used in the
construction) (Davis et al., 1964). If detecting electromagnetic
waves, typically the absorber consisted of a
metal with a suitable thickness yielding a square resistance
of 377 Ω, i.e. matching the impedance of the vacuum.
Further improvements on matching were achieved
by placing the bolometer in a resonant cavity. One major
setback with bolometers in their early days was unavoidably
slow speed, caused by the large heat capacity
resulting from the macroscopic size of the components
used. The dawn of modern microfabrication techniques
has all but eliminated this shortcoming, with bolometers
of high sensitivity achieving time constants as short as a
3 The opposite (calorimetric) limit will be discussed in section IV.C
25
few hundred nanoseconds.
Today, the most common type of cryogenic resistive
bolometers utilize transition edge sensors for the thermometry.
In a TES bolometer, a superconducting film
with a critical temperature Tc is biased within its superconductor
- normal metal transition where small changes
in the film temperature result to changes in the current
through the device (or the voltage across the film). In
most cases, the TES consists of two or more sandwiched
superconductor-normal metal layers. The relative thicknesses
of the S and N layers are used to tune the transition
temperature to a desirable value by the proximity
effect.
Transition-edge sensors are by no means a novel type of
a thermal detector, as first suggestions for their use came
out already in the late thirties (Andrews, 1938; Goetz,
1939), and first experimental results by 1941 (Andrews,
1941). Two principal problems prohibited the wide use
of this type of thermal detectors for some fifty years:
Typically the normal-state resistance of the superconducting
films was too low in order to obtain adequate
noise matching with field effect transistor (FET) preamplifiers,
and the lack of good transimpedance amplifiers
usually required the films to be current biased with a voltage
readout. This introduced a requirement to tune the
heat bath temperature very accurately within the narrow
range of temperatures in the superconducting transition.
This also made the devices exceedingly sensitive to small
variations in the bath temperature, introducing stringent
requirements for the heat bath stability.
These limitations can be overcome by the use of an
external negative feedback circuit that maintains the
film within its transition temperature and above the
bath temperature. Such an approach was adopted by
Clarke et al. (1977), who were able to demonstrate
NEP=1.7 · 10 −15 W/ √ Hz at an operating temperature
of 1.27 K, using a transformer-coupled FET as the voltage
readout. Introducing a negative feedback has similar
advantages as in the case of operational amplifiers: linearity
is improved, sensitivity to internal parameters of
the amplifier (or bolometer) is reduced, and the speed is
increased. Interestingly, the use of an external negative
feedback in conjunction with superconducting transition
edge sensors never found widespread use, possibly due
to the (slightly) more complicated read-out architecture,
and the need for a matching transformer.
The breakthrough of TESs came in 1995, when superconducting
quantum interference device (SQUID) ammeters,
which are inherently well suited in matching to low
load impedances, were introduced as the readout devices
for TESs (Irwin, 1995; Irwin et al., 1995a). This allowed
for the use of voltage biasing, which introduces strong
negative electrothermal feedback (ETF) that causes the
thermally isolated film to self-regulate within its superconducting
transition. The local nature of the ETF
makes the operation of these detectors very simple as
no external regulation is necessary. As with an external
negative feedback, an important advantage of the voltage