Radiation Pressure Cooling of a Micromechanical Oscillator Using Dynamical Backaction

Cooling of a 58 MHz micromechanical resonator from room temperature to 11 K is demonstrated using cavity enhanced radiation pressure. Detuned pumping of an optical resonance allows enhancement of the blueshifted motional sideband (caused by the oscillator’s Brownian motion) with respect to the redshifted sideband leading to cooling of the mechanical oscillator mode. The reported cooling mechanism is a manifestation of the effect of radiation pressure induced dynamical backaction. These results constitute an important step towards achieving ground state cooling of a mechanical oscillator.

Figure 1

Main panel: A scanning electron micrograph of a toroid microcavity on a chip. Inset: Finite-element simulation showing the stress (color coded) and strain of the 57.8-MHz breathing mode of the device. The strain is exaggerated for clarity. Note that radiation pressure exerts a radial force owing to total internal reflection of the confined light. (b) A Fabry-Pérot equivalent diagram of the experiments.

Figure 2

Main figure shows the normalized, measured noise spectra around the mechanical resonance frequency for $\Delta \omega \tau \approx -0.5$ and varying power (0.25, 0.75, 1.25, and 1.75 mW). The effective temperatures were inferred using mechanical damping, with the lowest attained temperature being 11 K. (b) Increase in the linewidth (damping) of the 57.8-MHz mode as a function of launched power, exhibiting the expected linear behavior [cf. Eqs. (1)]. (c) Physical origin of the observed cooling mechanism due to the asymmetry in the motional sidebands.

Figure 3

Mechanical linewidth (a) and mechanical resonance frequency (b) of the 57.8-MHz radial breathing mode as a function of (normalized) detuning when the laser is tuned over a 113 MHz wide optical cavity resonance (corresponding to ${\omega }_m\tau =0.5$). The optical power was $P=1.0\mathrm{mW}$. The dashed region denotes occurrence of the parametric oscillation instability. For $\Delta \omega <0$ the 58-MHz resonance is being cooled. Plots (c),(d) show the characteristics (${\omega }_m/2\pi$ and linewidth) of the same mechanical mode, but for scanning over a narrower (50 MHz linewidth) optical resonance (corresponding to ${\omega }_m\tau \approx 1.1)$ with $P=0.13\mathrm{mW}$. Solid lines are theoretical predictions based on Eqs. (1, 2, 3). The only free parameter was the effective mass, which agreed well with the simulated value.

Figure 4

Main panel: The frequency response from 0–200 MHz. The plateau occurring between 1–200 MHz is due to the (instantaneous) Kerr nonlinearity of ${\mathrm{SiO}}_2$ (dotted line). The cutoff at 200 MHz is due to both detector and cavity bandwidth. Inset: magnification of main panel in the vicinity of radiation pressure response which shows the interference of the Kerr nonlinearity and the radiation pressure-driven micromechanical resonator (which, on resonance, is $\pi /2$ out of phase with the modulating pump and the instantaneous Kerr nonlinearity). From the fits (solid lines) it can be inferred that the radiation pressure response is ca. $\times 260$ larger than the Kerr response.