Opto-mechanical probe for high speed AFM microscopy

In the field of microscopy, the atomic force microscope (AFM) invented in 1986 was brought little, but nonetheless impressive, incremental developments since then. This instrument’s performances, and in particular imaging speed, are mainly limited by its cantilever-type force probe whose resonance frequency peaks at a few MHz. This thesis work presents a new concept of AFM probe, an optomechanical (OM) one, and custom instrument’s components to exploit its performances. Indeed, the 100+ MHz vibrating OM probes tested in this manuscript demonstrate an exquisite thermomechanical limit of detection of 4.5x1E-17 m/√Hz at room temperature, lower than any other AFM probe detection, and an instrument-limited 10 pm vibration amplitude. This OM probe consists of a suspended silicon ring with a 10 µm radius, acting as a mechanical resonator and a whispering-gallery-mode optical resonator. The two are intimately coupled by the ring shape: when the ring vibrates in a breathing mode, the optical cavity length varies and so does its resonance wavelength around its central value 1.55 µm. Characterization of numerous OM probes with different designs are investigated to find optimal designs, that is a 100 nm to 200 nm evanescent-coupling-gap and spokes width below 100 nm. Through deep characterization, acute phenomenon is also highlighted as the super-mode. Two alternatives to put the probe in vibration are compared: capacitive and optical. Stability and noise study of the probe help identify an additional noise source in optical actuation, that seem to be related to the optical background signal. Each developed component of the AFM instrument is detailed from piezoelectric scanner to data acquisition and processing. Because of a fabrication technological lock, the tip of the OM probe could not approach any surface as it did not protrude from the substrate on which the probe is made. A conventional AFM lever is therefore used to interact mechanically with the AFM probe. The instrument’s bandwidth is then characterized in operation, demonstrating a state-of-the-art 100 kHz feedback-loop bandwidth. Finally, a first pseudo-image is achieved with such probes, demonstrating the whole instrument operation.