Researchers at the Niels Bohr Institute have combined two worlds –
quantum physics and nano physics, and this has led to the discovery of a
new method for laser cooling semiconductor membranes. Semiconductors
are vital components in solar cells, LEDs and many other electronics,
and the efficient cooling of components is important for future quantum
computers and ultrasensitive sensors. The new cooling method works quite
paradoxically by heating the material! Using lasers, researchers cooled
membrane fluctuations to minus 269 degrees C. The results are published
in the scientific journal, Nature Physics.
"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.
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IMAGE:
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... |
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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.
Lucky coincidence
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.
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IMAGE:
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... |
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"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.
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.
For more information:
Koji Usami, Associate Professor, Quantop, Niels Bohr Institute at the University of Copenhagen, 45-3532-5268, 45-2829-7487, usami@nbi.dk
Eugene Polzik, Professor, Head of Quantop, Niels Bohr Institute at the University of Copenhagen, 45-3532-5424, 45-2338-2045, polzik@nbi.dk
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