Cavity nano-optomechanics inside a fiber-based micro-cavity

Abstract

The field of cavity optomechanics studies the interaction between electromagnetic radiation and a macroscopic mechanical mode enhanced by an optical cavity. The use of low-dimensional objects in the field of cavity optomechanics is limited by their low scattering cross section compared to the size of the optical cavity mode. Fiber-based Fabry-Pérot micro-cavities can feature tiny mode cross sections and still maintain a high finesse, boosting the light-matter interaction and thus enabling the sensitive detection of the displacement of minute objects. This work presents such a fiber-based microcavity that features a micrometer sized mode cross section in combination with an ultrahigh finesse. For a proof-of-principle demonstration of dynamical backaction in the system, we study stripes made out of a low-stress silicon nitride (SiN) membranes. We are able to position-tune the static optomechanical coupling to the stripe, reaching frequency pull parameters of up to G/2π = 3 GHz/nm. We also demonstrate the optical spring effect on the fundamental out-of-plane flexural mode of the stripe. The measurements match established theory and we obtain a single-photon coupling of g0/2π = 2.5 kHz. With the switch to stripes made out of high-stress silicon nitride (Si3N4), absorption in the stripe is rreduced and and the loaded finesse reaches values up to F = 195000. This is the largest finesse reported in loaded fiber cavities so far. The optomechanical coupling strength is reduced compared to the SiN sample due to a reduced refractive index. We demonstrate the optical spring effect and optical induced damping on two out-of-plane flexural modes of the stripe with a single-photon coupling of g0/2π = 575 Hz. Dynamical backaction cooling of the mechanical mode starting from room temperature down to around 12 K is shown. Towards the realization of dynamical backaction on vibrations of carbon nanotubes (CNTs), we study two sample geometries that allow to position the CNT inside the cavity mode. In a tuning-fork configuration, the presence of the CNT results in a increase of the cavity linewidth. CNTs grown between adjacent SiN stripes allow more flexibility in the sample positioning and alignment. For the first time, we map a static dispersive coupling and are able to position-tune the optomechanical coupling to the CNT with frequency pull parameters of up to G/2π=120 kHz/nm. At a sample position with g0 ≈10 kHz and a loaded cavity finesse of F = 95 000, we observe a peak in the cavity transmission spectrum that we attribute the a flexural vibrational mode of the CNT. We measure first hints of the optical spring effect and of optical induced damping on the vibrations of the CNT.