A silicon nitride photonic crystal cavity is positioned close to a levitated particle, modifying the tweezer trap into a focused standing wave trap at a fixed distance from the cavity surface of 300 nm. High evanescent cavity field gradient at this position enables high near-field optomechanical coupling to the motion of the levitated nanoparticle. At the same time, the single sided photonic crystal cavity couples evanescently to an HF tapered fiber with an efficiency of more than 30%. Optomechanical coupling of 9kHz together with the high detection efficiency enable measurement of the motion of the levitated particle with a resolution of less than 4 pm/Hz1/2 with less than 1000 intra-cavity photons, corresponding to a displacement sensitivity per photon that is more than 100 times better than what is measured with far-field detection schemes. This will be crucial to achieve Heisenberg limited measurements when operating in UHV, where decoherence of the particle motion is dominated by photon recoil. In this regime, near-filed tailoring of the optical potential and strong optomechanical interaction will enable study of nonlinear quantum dynamics of large objects and measurement of short-range fluctuation forces. In addition, by fine control of the particle position with respect to the photonic crystal, we are able to map the 3D cavity field gradient with high resolution.
Near-field coupling of a levitated nanoparticle to a photonic crystal cavity
L. Magrini, R. A. Norte, R. Riedinger, I. Marinković, D. Grass, U. Delić, S. Gröblacher, S. Hong, and M. Aspelmeyer
Optica Vol. 5, Issue 12, pp. 1597-1602 (2018)
