High Finesse Opto-Mechanical Cavity with a Movable Thirty-Micron-Size Mirror

We report on the demonstration of a high finesse micro-optomechanical system and identify potential applications ranging from optical cooling to weak force detection to massive quantum superpositions. The system consists of a high quality $30\mu \mathrm{m}$ diameter flat dielectric mirror cut from a larger substrate with a focused ion beam and attached to an atomic force microscope cantilever. Cavity ring-down measurements performed on a 25 mm long Fabry-Pérot cavity with the $30\mu \mathrm{m}$ mirror at one end show an optical finesse of 2100. Numerical calculations show that the finesse is not diffraction limited and that orders of magnitude higher finesse should be possible. A mechanical quality factor of more than ${10}^5$ at pressures below ${10}^{-3}\mathrm{mbar}$ is demonstrated for the cantilever with a mirror attached.

Figure 1

Scanning electron microscopy images of a $15\mu \mathrm{m}$ prototype mirror during the cutting process (a), (b) and after attachment to a cantilever (c). Small scratches are visible on the mirror in (c) from the attachment process. The fabrication procedure was subsequently adjusted to prevent this.

Figure 2

Diagram of the experimental setup. A 780 nm TLD is used for frequency scanned measurements. A 633 nm helium neon laser is used for alignment. The light from either laser is then passed through a spatial filter (A) and collimated with a lens (B). The lens is chosen to match the cavity mode. For the ring-down measurements a 780 nm 200 fs pulsed laser is coupled in via a fiber (C). A periscope (D) aligns light to the cavity. The large mirror and an incoupling lens (E) are mounted on a motorized stage allowing control of tip/tilt as well as the overall length of the cavity in vacuo. The cantilever or small mirror (F) are mounted on a Gimbal mount which is prealigned outside of the vacuum chamber. A fraction of the light leaving the cavity is used for imaging on a CCD, while the remainder is sent either to a PMT or APD.

Figure 3

Fabry-Pérot scan (peaks inset). Higher order modes are visible; adjustment of the incoupling reveals that the cavity supports several more. (a) The Lorentzian peak has FHWM $5.9\pm 0.2\mathrm{MHz}$, resulting in a finesse (limited by the laser linewidth) of $1020\pm 50$. (b) If the laser is scanned at a lower rate, thermal vibrations of the cantilever become visible.

Figure 4

Cavity ring-down measurement. A laser pulse enters the cavity at time 0. The scattered light is bright enough to saturate the APD, resulting in a 50 ns dead time. The light intensity is low enough after the recovery that saturation effects can be ignored. A fit of the data from 100–2000 ns demonstrates a finesse of $2100\pm 50$. The slightly faster decay at 50 ns is due to light leaking from higher order modes.