Kinetic inductive electromechanical transduction for atomic force microscopy

by Ermes Scarano (Tillämpad fysik, Quantum and Nanostructure Physics)

Friday 14 Jun 2024, 09:00 → 12:00 Europe/Stockholm

FA32 (AlbaNova Main Building)

Abstract

The Atomic Force Microscope (AFM) is considered one of the most powerful tools in surface science thanks to its ability to sense forces at the nanoscale and image surfaces with high lateral resolution. The AFM employs a microcantilever with a sharp tip as a force transducer operated in close proximity to a surface. Nanoscale force sensing in AFM is achieved by measuring the motion of the cantilever under the influence of the localized tip-surface interaction. Cavity optomechanics provides a framework to measure cantilever motion at the fundamental limit of sensitivity. This thesis applies the principles of cavity optomechanics to realize an integrated force sensor fulfilling the requirements of low-temperature AFM applications, with the goal of enhancing the sensitivity and speed of imaging. The optical cavity is replaced by a superconducting microwave resonant circuit and the optomechanical detection principle is based on a novel type electromechanical coupling developed by our group. Compressive or tensile surface strain produced by the bending of the microcantilever, causes a change in the kinetic inductance of a superconducting meandering nanowire, thereby changing the resonant frequency of a high-Q microwave mode.We discuss the design and fabrication of these AFM force sensors, including the deposition of sharp, conducting tips. The compact, integrated microwave resonant circuit is realized in a fully coplanar layout from a single superconducting NbTiN thin film deposited on a SiN layer on a Si substrate. The microcantilever beam is formed from the SiN layer which is released from the Si substrate.

We present experimental data characterizing the properties of the microwave resonator, the cantilever's flexural eigenmode, and the interaction between the two through the kinetic-inductive electromechanical coupling. We use different techniques to detect motion in a manner suitable for the typical modes of operation in traditional AFM, as well as additional methods specific to the electromechanical detection. We also integrated the force sensors into a prototype low temperature AFM scanning system built inside a dilution refrigerator. With this prototype we demonstrate the detection of tip-surface force gradients, and we show our initial attempts at imaging with electromechanical detection of motion.