The experiments are carried out in the Quantop laboratories at the Niels Bohr Institute. The laser light that hits the semiconducting nanomembrane is controlled with a forest of mirrors. Image: Ola J. Joensen |
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, light-emitting diodes (LEDs), and many other
electronics, and the efficient cooling of components is important for future
quantum computers and ultrasensitive sensors. The cooling method works quite
paradoxically by heating the material. Using lasers, researchers cooled
membrane fluctuations to -269 C. The results are published in 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 nm and an
unprecedented surface area of 1 by 1 mm. 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 -269 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, -273 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.
“We managed to produce a nanomembrane that
is only 160 nm thick and with an area of more than 1 square millimeter. 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 -269 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.