The experiments themselves are carried out in this vacuum chamber. When
the laser light hits the membrane, some of the light is reflected and
some is absorbed and leads to a small heating of the membrane. The
reflected light is reflected back again via a mirror in the experiment
so that the light flies back and forth in this space and forms optical
resonator (cavity). Changing the distance between the membrane and the
mirror leads to a complex and fascinating interplay between the movement
of the membrane, the properties of the semiconductor and the optical
resonances and you can control the system so as to cool the temperature
of the membrane fluctuations.
"In experiments, we have succeeded in achieving a new and efficient cooling
of a solid material by using lasers. We have produced a semiconductor
membrane with a thickness of 160 nanometers and an unprecedented surface
area of 1 by 1 millimeter. In the experiments, we let the membrane
interact with the laser light
in such a way that its mechanical movements affected the light that hit
it. We carefully examined the physics and discovered that a certain
oscillation mode of the membrane cooled from room temperature down to
minus 269 degrees C, which was a result of the complex and fascinating
interplay between the movement of the membrane, the properties of the
semiconductor and the optical resonances," explains Koji Usami,
associate professor at Quantop at the Niels Bohr Institute.
From gas to solid
Laser cooling of atoms has been practiced for several years in experiments in the quantum
optical laboratories of the Quantop research group at the Niels Bohr
Institute. Here researchers have cooled gas clouds of cesium atoms down
to near absolute zero, minus 273 degrees C, using focused lasers and
have created entanglement between two atomic systems. The atomic spin
becomes entangled and the two gas clouds have a kind of link, which is
due to quantum mechanics. Using quantum optical techniques, they have
measured the quantum fluctuations of the atomic spin.
"For some time we have wanted to examine how far you can extend the
limits of quantum mechanics – does it also apply to macroscopic
materials? It would mean entirely new possibilities for what is called
optomechanics, which is the interaction between optical radiation, i.e.
light, and a mechanical motion," explains Professor Eugene Polzik, head
of the Center of Excellence Quantop at the Niels Bohr Institute at the
University of Copenhagen.
But they had to find the right material to work with.
Koji Usami shows the holder with the semiconductor nanomembrane. The
holder measures about one cm for each link, while the nanomembrane
itself has a surface area of 1 times 1 millimeter and a thickness of 160
nanometers.
In 2009, Peter Lodahl (who is today a professor and head of the
Quantum Photonic research group at the Niels Bohr Institute) gave a
lecture at the Niels Bohr Institute, where he showed a special photonic
crystal membrane that was made of the semiconducting material gallium
arsenide (GaAs). Eugene Polzik immediately thought that this
nanomembrane had many advantageous electronic and optical properties and
he suggested to Peter Lodahl's group that they use this kind of
membrane for experiments with optomechanics. But this required quite
specific dimensions and after a year of trying they managed to make a
suitable one.
"We managed to produce a nanomembrane that is only 160 nanometers
thick and with an area of more than 1 square millimetre. The size is
enormous, which no one thought it was possible to produce," explains
Assistant Professor Søren Stobbe, who also works at the Niels Bohr
Institute.
The experiments are carried out by Koji Usami here in the Quantop
laboratories at the Niels Bohr Institute. The laser light that hits the
semiconducting nanomembrane is controlled with a forest of mirrors.
Basis for new research
Now a foundation had been created for being able to reconcile quantum
mechanics with macroscopic materials to explore the optomechanical
effects.
Koji Usami explains that in the experiment they shine the laser light
onto the nanomembrane in a vacuum chamber. When the laser light hits
the semiconductor membrane, some of the light is reflected and the light
is reflected back again via a mirror in the experiment so that the
light flies back and forth in this space and forms an optical resonator.
Some of the light is absorbed by the membrane and releases free
electrons. The electrons decay and thereby heat the membrane and this
gives a thermal expansion. In this way the distance between the membrane
and the mirror is constantly changed in the form of a fluctuation.
"Changing the distance between the membrane and the mirror leads to a
complex and fascinating interplay between the movement of the membrane,
the properties of the semiconductor and the optical resonances and you
can control the system so as to cool the temperature of the membrane
fluctuations. This is a new optomechanical mechanism, which is central
to the new discovery. The paradox is that even though the membrane as a
whole is getting a little bit warmer, the membrane is cooled at a
certain oscillation and the cooling can be controlled with laser light.
So it is cooling by warming! We managed to cool the membrane fluctuations to minus 269 degrees C", Koji Usami explains.
"The potential of optomechanics could, for example, pave the way for
cooling components in quantum computers. Efficient cooling of mechanical
fluctuations of semiconducting nanomembranes by means of light could
also lead to the development of new sensors for electric current and
mechanical forces. Such cooling in some cases could replace expensive
cryogenic cooling, which is used today and could result in extremely
sensitive sensors that are only limited by quantum fluctuations," says
Professor Eugene Polzik.
Provided by University of Copenhagen
From physorg
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