Cooling semiconductor by laser
31 January 2012
It is not difficult to associate lasers with heat. Laser cutting instruments are widely used in industry for precision cutting of a variety of materials...
These lasers are also starting to become available for use in other areas such as at home and in schools. Now researchers at the Niels Bohr Institute have discovered a way to use lasers to cool down material, specifically a semiconductor membrane. With electronics shrinking leading to temperature problems, it is not hard to see that this could lead to important developments for the electronics industry.
The researchers cooled the semiconductor material by combining quantum physics and nano physics. By examining the physics, the researchers discovered that a certain oscillation mode of the membrane cooled it from room temperature down to -269°C.
The membrane was formed of gallium arsenide (GaAs) with very specific dimensions, a thickness of 160 nanometers and a surface area of 1 by 1 millimetre. It took a year to produce the nanomembrane because of the precise size, which was comparatively enormous. Gallium arsenide was chosen because the material had many advantageous electronic and optical properties.
In the experiments, the membrane interacted with the laser light in such a way that its mechanical movements affected the light that hit it. The laser light was shone 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 resonance and the system can be controlled 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! Researchers managed to cool the membrane fluctuations to -269°C.
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