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Self-cooling of a micromirror by radiation pressure



Cooling of mechanical resonators is currently a popular topic in many fields of physics including ultra-high precision measurements1, detection of gravitational waves2,3 and the study of the transition between classical and quantum behaviour of a mechanical system4,5,6. Here we report the observation of self-cooling of a micromirror by radiation pressure inside a high-finesse optical cavity. In essence, changes in intensity in a detuned cavity, as caused by the thermal vibration of the mirror, provide the mechanism for entropy flow from the mirror’s oscillatory motion to the low-entropy cavity field2. The crucial coupling between radiation and mechanical motion was made possible by producing free-standing micromirrors of low mass (m ≈ 400 ng), high reflectance (more than 99.6%) and high mechanical quality (Q ≈ 10,000). We observe cooling of the mechanical oscillator by a factor of more than 30; that is, from room temperature to below 10 K. In addition to purely photothermal effects7 we identify radiation pressure as a relevant mechanism responsible for the cooling. In contrast with earlier experiments, our technique does not need any active feedback8,9,10. We expect that improvements of our method will permit cooling ratios beyond 1,000 and will thus possibly enable cooling all the way down to the quantum mechanical ground state of the micromirror.

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We thank C. Brukner, S. Gröblacher, J. Kofler, T. Jennewein, M. S. Kim, A. Vandaley and D. Vitali for discussion. We acknowledge financial support by the Austrian Science Fund (FWF), by the City of Vienna, by the Austrian NANO Initiative (MNA) and by the Foundational Questions Institute (FQXi).

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Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.

Correspondence to M. Aspelmeyer.

Supplementary information

  1. Supplementary Notes

    This file contains a detailed characterization of the micro-mechanical oscillator and of the optical cavity, together with a description of the methods used. (PDF 443 kb)

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Further reading

Figure 1: Sketch of the experimental setup.
Figure 2: Power spectrum of the mechanical mode at two different relative detuning levels Δ of the cavity for an input power of 2 mW.
Figure 4: Self-cooling of the mechanical resonator.
Figure 3: Radiation-pressure-induced damping of mirror dynamics.


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