A Floating Probe Force Microscope With Sub-Femtonewton Resolution

Abstract

Forces at the femtonewton level and below are poised to be highly relevant in a number of scientific disciplines– including studies in optics and photonics, molecular biology, and even microfluidics. It is highly practical, if not necessary, to perform these force measurements in liquid. But due largely to increased thermal noise, conventional force spectroscopy methods, such as atomic force microscopes (AFM’s), underperform in a liquid environment. The most advanced AFM’s, for instance, are limited to a resolution of around a piconewton. On the other hand, a new class of force microscopes utilizing a floating, optically trapped probe, have been proposed. These instruments, including the photonic force microscope (PFM), promise lower thermal noise thresholds, due to a smaller profile probe. But thusfar, due to fundamental flaws in design and methodology, they have been unable to produce thermally limited measurements. This document summarizes the design and development of a highly sensitive force microscope, based on such a floating probe, which is able to measure at the thermal limit. Thus, our force microscope’s sensitivity exceeds that of the best current techniques in fluid by two orders of magnitude, enabling resolution of forces below one femtonewton (fN) in 100 seconds of measurement. The first two chapters serve as a general introduction to the fundamental physical phenomena which form the basis of our work. Next, some formalism is introduced and a background is given on force microscopy and existing techniques. The contents of three published papers populate Chapters 5-7, and finally, an operation manual for the instrument in its current form is provided in the Appendix.