Non-tilting out-of-plane mode high-Q mechanical silicon oscillator as a moving cavity mirror

Abstract

A very sensitive optomechanical sensor to detect femtometer-level displacements in the position of one of the cavity mirrors in a Fabry–Pérot interferometer was constructed and characterized. We use a high-reflectivity coated, non-tilting out-of-plane mode high-Q mechanical silicon oscillator as a rear mirror in the Fabry–Pérot interferometer. The benefit of our novel oscillator, if compared to traditional torsional, flexural and many bulk acoustic mode oscillators, is that the action of weak forces is observed to cause only pure linear translation of the moving mirror without any tilting or deformation of the mirror surface. This non-tilting behavior allows, in principle, more precise optical mode stabilization, use of very short optical cavities and studies of short-range interactions between parallel surfaces. The resonance frequency and Q value of the high-reflectivity coated silicon oscillator are f₀=27.5 kHz and Q=19000 at low pressure (p=0.1 mbar) and at room temperature. The finesse of the optical cavity is $\mathcal{F}=2600$. The sensitivity of the displacement measurement is Δx_min=5 fm with a 1 Hz bandwidth. The prospects of reaching the standard quantum limit in an interferometric displacement measurement using a macroscopic oscillator are discussed.