Cooling mirrors with light Jack Harris, Yale University, PHY 0555824 We have realized a new type of optomechanical coupling which allows us to greatly increase the effect of radiation pressure on micromechanical devices. We placed a 50 nm-thick, 1 mm square membrane inside a high- finesse Fabry-Perot cavity. By optimizing the membrane’s mechanical quality factor and the cavity’s finesse, we were able to laser cool the membrane a factor of 100 beyond what had been achieved previously, from 300 K to 7 mK. Measurements of the Brownian motion of a 1 mm x 1 mm x 50 nm silicon nitride membrane. The membrane is placed inside an optical cavity with a finesse of 15,000. As the laser illuminating the cavity is red-detuned slightly from resonance, the membrane experiences increased damping due to radiation pressure. The fits to each data set show the membrane’s mean-square displacement (which is a measure of its temperature) decreasing. The effective temperature extracted from each data set is also shown S x () (m 2 /Hz) 10 -26 10 -27 10 -28 10 -29 10 -30 10 -31 (kHz) 126 128 130 132 134 T eff = 2.34 K ± 0.13 K T eff = 253 mK ± 4.7 mK T eff = 80 mK ± 1.8 mK T eff = 13.3 mK ± 0.51 mK T eff = 6.82 mK ± 0.61 mK o pt i mi z in g la s er d e tu n i n g This “membrane in the middle” approach utilizes a dispersive coupling between the cavity and the membrane. Dispersive coupling is often used in atom/cavity experiments, but had not yet been realized in optomechanical systems. In addition to allowing for higher cavity finesse and mechanical quality factors, this new approach makes it possible to measure the square of the membrane’s displacement. Such a “displacement-squared” read-out will be crucial for measuring quantum jumps of a mechanical oscillator, a long standing goal in the field of quantum measurement.