Schematics
of the device, which consists of a graphene Josephson junction, which is
integrated into a microwave circuit. Courtesy: ICFO.
Bolometers
are devices that measure the power of incident electromagnetic radiation thru
the heating of materials, which exhibit a temperature-electric resistance
dependence. These instruments are among the most sensitive detectors so far
used for infrared radiation detection and are key tools for applications that
range from advanced thermal imaging, night vision, infrared spectroscopy to
observational astronomy, to name a few.
Even
though they have proven to be excellent sensors for this specific range of
radiation, the challenge lies in attaining high sensitivity, fast response time
and strong light absorption, which not always are accomplished all together.
Many studies have been conducted to obtain these higher-sensitivity bolometers
by searching to reduce the size of the detector and thus increase the thermal
response, and in doing so, they have found that graphene seems to be an
excellent candidate for this.
If we
focus on the infrared range, several experiments have demonstrated that if you
take a sheet of graphene and place it in between two layers of superconducting
material to create a Josephson junction, you can obtain a single photon
detector device. At low temperatures, and in the absence of photons, a
superconducting current flows through the device. When a single infrared photon
passes through the detector, the heat it generates is enough to warm up the
graphene, which alters the Josephson junction such that no superconducting
current can flow. So you can actually detect the photons that are passing
through the device by measuring the current. This can be done basically because
graphene has an almost negligible electronic heat capacity. This means that,
contrary to materials that retain heat like water, in the case of graphene a
single low-energy photon can heat the detector enough to block the
superconducting current, and then dissipate quickly, allowing the detector to
rapidly reset, and thus achieving very fast time responses and high
sensitivities.
Trying to
take a step further and move to higher wavelengths, in a recent study published
in Nature, a team of scientists which includes ICFO researcher Dmitri Efetov,
together with colleagues from Harvard University, Raytheon BBN Technologies,
MIT, and the National Institute for Material Sciences, has been able to develop
a graphene-based bolometer that can detect microwave photons at extremely high
sensitivities and with fast time responses.
Just like
with the infrared range, the team took a sheet of graphene and placed it in
between two layers of superconducting material to create a Josephson junction.
This time, they went an entirely new route and attached a microwave resonator
to generate the microwave photons and by passing these photons through the
device, were able to reach an unprecedented detection levels. In particular,
they were able to detect single photons with a much lower energy resolution,
equivalent to that of a single 32 Ghz photon, and achieve detection readouts
100.000 times faster than the fastest nanowire bolometers constructed so far.
The
results achieved in this study mean a major breakthrough in the field of
bolometers. Not only has graphene proven to be an ideal material for infrared
sensing and imaging, but it has also proven to span to higher wavelengths,
reaching the microwave, where it has also shown to attain extremely high
sensitivities and ultra-fast read out times.
As Prof.
at ICFO Dmitri Efetov comments "such achievements were thought impossible
with traditional materials, and graphene did the trick again. This open
entirely new avenues for quantum sensors for quantum computation and quantum
communication."