Physicists in Germany have used a laser to cool a micron-sized cantilever from room temperature to 18 Kelvin, and it may be possible to reach sub-millikelvin temperatures with the same technique in the future. Constanze Höhberger Metzger and Khaled Karrai of the Ludwig-Maximilians University in Munich exploited a photothermal force generated inside a cavity formed by the cantilever and a gold-coated optical fibre (Nature 432 1002).
Microlevers are used in a variety of devices, including various atomic force and magnetic-resonance force microscopes. Cooling the microlevers in these devices improves their sensitivity. Moreover, if the microlevers could be cooled to sub-millikelvin temperatures, they could be used to perform a range of fundamental tests of quantum theory with macroscopic objects.
The mirrors in the Munich experiment are separated by about 34 microns (see figure). The first mirror is made by coating the cantilever – which is 223 microns long, 22 microns wide and 0.46 microns thick – with a thin film of gold. The gold-coated optical fibre that acts as the second mirror also transports the laser radiation, which has a wavelength of 633 nanometres, into the cavity.
The force exerted by the laser on the cantilever is proportional to the intensity of the light inside the cavity, and is at its strongest when the laser and the cavity are in resonance. The force exerted by the laser can, under the right conditions, reduce the Brownian motion of the cantilever by a factor of 100.
Höhberger Mezger and Karrai were able to determine the temperature of the cantilever by analysing the thermal noise spectrum of the radiation that escapes from the cavity through the optical fibre. By changing the geometry of the lever, the size of the cavity and the materials used in the device it may be possible to reduce the thermal vibrations to a minimum so that only quantum fluctuations remain.
