Cavity optomechanics for sensing force gradients in atomic force microscopy

by Elisabet Arvidsson (Applied Physics, KTH)

Friday 13 Dec 2024, 09:00 → 12:00 Europe/Stockholm

FD41 (AlbaNova Main Building)

Opponent: Professor Albert Schliesser, Niels Bohr Institute, University of Copenhagen, Denmark

Supervisor: Professor David B. Haviland, Tillämpad fysik

Abstract

In this thesis we investigate force gradient sensing based on the principles of cavity optomechanics, specifically in the context of atomic force microscopy (AFM). Tip-surface forces perturb the motion of a cantilever and the cantilever's change of motion is detected with an integrated compact low-noise motion sensor based on cavity optomechanics.  

To this end we design and fabricate probes on a wafer scale. The probes consist of a cantilever in the shape of a triangular Si-N cantilever released from a substrate of Si. We fabricate low-frequency, 700 kHz, cantilevers as well as high-frequency, 5 MHz, cantilevers allowing for operation in the resolved sideband regime. At the base of the cantilever we fabricate a superconducting microwave resonant circuit (cavity) patterned from a thin film of Nb-Ti-N. The microwave circuit has a resonance frequency of 4-5 GHz. The coupling between the cantilever and the microwave circuit is based on strain modulation of the kinetic inductance of a meandering nanowire placed at the base of the cantilever. The mechanical motion can thus be detected through the modulation of the cavity resonance frequency. 

We characterise the mechanical mode as well as the microwave circuit and demonstrate the novel strain dependent coupling. We investigate the losses of the microwave circuit and find that in the temperature range 1.7-6 K the losses are not dominated by thermal-equilibrium quasi-particles. We also explore the possibility of using the nonlinear current dependence of the kinetic inductance as means to parametrically amplify the motional sidebands produced by the optomechanical interaction. Finally, with a prototype scanner assembly for low-temperature AFM we detect force gradients and image a surface implementing a two-tone pumping scheme for the microwave circuit while actuating the mechanical resonator.