Optical Cooling Applied to Microlever
Daniel S. Burgess
A team of physicists at Ludwig Maximilians Universität in Munich, Germany, has employed a laser technique to damp the amplitude of Brownian motion in a micromechanical cantilever, cooling it from room temperature to 18 K. Theoretically, the approach may enable cooling to effective temperatures in the submillikelvin range and even to the quantum limit.
The demonstration follows on earlier inquiries into the quieting of thermal noise in a mirror using radiation pressure. The gold-coated silicon microlever and the tip of a similarly gold-coated optical fiber, through which the laser radiation is injected, form a resonant cavity with a nominal separation of 34 µm.
Because the pressure on the cantilever is proportional to the intensity of the radiation in the cavity and is at a maximum when the HeNe laser's 633-nm wavelength matches a cavity resonance, the net effect is to quench the motion of the 223 × 22 × 0.46-µm microlever. Increasing the laser power to 130 µW reduced the amplitude of Brownian motion from thermal noise by nearly a factor of 100.
The physicists suggest that a setup precooled to 100 mK and employing a 1 × 1 × 1-µm mechanical resonator, a 20-cm cavity and 2 µW of 1-µm laser radiation might cool the resonator to the quantum limit, which could enable researchers to investigate the superposition of macroscopic ensembles of atoms. It remains to be seen, however, whether residual optical absorption will lead to heating of the vibrational mode before this limit is reached.
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