Allison B. Churnside

Upper limits to submillimetre-range forces from extra space-time dimensions.

Results of an experimental search for new macroscopic forces with Yukawa range between 5 and 500 microns are presented. The experiment uses 1 kHz mechanical oscillators as test masses with a stiff conducting shield between them to suppress backgrounds. No signal is observed above the instrumental thermal noise after 22 hours of integration time. These results provide the strongest limits to date between 10 and 100 microns, improve on previous limits by as much as three orders of magnitude, and rule out half of the remaining parameter space for predictions of string-inspired models with low-energy supersymmetry breaking. New forces of four times gravitational strength or greater are excluded at the 95% confidence level for interaction ranges between 200 and 500 microns.String theory is the most promising approach to the long-sought unified description of the four forces of nature and the elementary particles, but direct evidence supporting it is lacking. The theory requires six extra spatial dimensions beyond the three that we observe; it is usually supposed that these extra dimensions are curled up into small spaces. This ‘compactification’ induces ‘moduli’ fields, which describe the size and shape of the compact dimensions at each point in space-time. These moduli fields generate forces with strengths comparable to gravity, which according to some recent predictions might be detected on length scales of about 100 µm. Here we report a search for gravitational-strength forces using planar oscillators separated by a gap of 108 µm. No new forces are observed, ruling out a substantial portion of the previously allowed parameter space for the strange and gluon moduli forces, and setting a new upper limit on the range of the string dilaton and radion forces.

Ultrastable Atomic Force Microscopy : Atomic-Scale Stability and Registration in Ambient Conditions

Instrumental drift in atomic force microscopy (AFM) remains a critical, largely unaddressed issue that limits tip-sample stability, registration, and the signal-to-noise ratio during imaging. By scattering a laser off the apex of a commercial AFM tip, we locally measured and thereby actively controlled its three-dimensional position above a sample surface to <40 pm (Deltaf = 0.01-10 Hz) in air at room temperature. With this enhanced stability, we overcame the traditional need to scan rapidly while imaging and achieved a 5-fold increase in the image signal-to-noise ratio. Finally, we demonstrated atomic-scale ( approximately 100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images on transparent substrates. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.

Routine and Timely Sub-picoNewton Force Stability and Precision for Biological Applications of Atomic Force Microscopy

Force drift is a significant, yet unresolved, problem in atomic force microscopy (AFM). We show that the primary source of force drift for a popular class of cantilevers is their gold coating, even though they are coated on both sides to minimize drift. Drift of the zero-force position of the cantilever was reduced from 900 nm for gold-coated cantilevers to 70 nm (N = 10; rms) for uncoated cantilevers over the first 2 h after wetting the tip; a majority of these uncoated cantilevers (60%) showed significantly less drift (12 nm, rms). Removing the gold also led to ∼10-fold reduction in reflected light, yet short-term (0.1-10 s) force precision improved. Moreover, improved force precision did not require extended settling; most of the cantilevers tested (9 out of 15) achieved sub-pN force precision (0.54 ± 0.02 pN) over a broad bandwidth (0.01-10 Hz) just 30 min after loading. Finally, this precision was maintained while stretching DNA. Hence, removing gold enables both routine and timely access to sub-pN force precision in liquid over extended periods (100 s). We expect that many current and future applications of AFM can immediately benefit from these improvements in force stability and precision.

Atomic force microscopy with sub-picoNewton force stability for biological applications

Atomic force microscopy (AFM) is widely used in the biological sciences. Despite 25 years of technical developments, two popular modes of bioAFM, imaging and single molecule force spectroscopy, remain hindered by relatively poor force precision and stability. Recently, we achieved both sub-pN force precision and stability under biologically useful conditions (in liquid at room temperature). Importantly, this sub-pN level of performance is routinely accessible using a commercial cantilever on a commercial instrument. The two critical results are that (i) force precision and stability were limited by the gold coating on the cantilevers, and (ii) smaller yet stiffer cantilevers did not lead to better force precision on time scales longer than 25 ms. These new findings complement our previous work that addressed tip-sample stability. In this review, we detail the methods needed to achieve this sub-pN force stability and demonstrate improvements in force spectroscopy and imaging when using uncoated cantilevers. With this improved cantilever performance, the widespread use of nonspecific biomolecular attachments becomes a limiting factor in high-precision studies. Thus, we conclude by briefly reviewing site-specific covalent-immobilization protocols for linking a biomolecule to the substrate and to the AFM tip.

Ultrastable atomic force microscopy: Improved force and positional stability

Atomic force microscopy (AFM) is an exciting technique for biophysical studies of single molecules, but its usefulness is limited by instrumental drift. We dramatically reduced positional drift by adding two lasers to track and thereby actively stabilize the tip and the surface. These lasers also enabled label‐free optical images that were spatially aligned to the tip position. Finally, sub‐pN force stability over 100 s was achieved by removing the gold coating from soft cantilevers. These enhancements to AFM instrumentation can immediately benefit research in biophysics and nanoscience.

Label-free optical imaging of membrane patches for atomic force microscopy

In atomic force microscopy (AFM), finding sparsely distributed regions of interest can be difficult and time-consuming. Typically, the tip is scanned until the desired object is located. This process can mechanically or chemically degrade the tip, as well as damage fragile biological samples. Protein assemblies can be detected using the back-scattered light from a focused laser beam. We previously used back-scattered light from a pair of laser foci to stabilize an AFM. In the present work, we integrate these techniques to optically image patches of purple membranes prior to AFM investigation. These rapidly acquired optical images were aligned to the subsequent AFM images to ~40 nm, since the tip position was aligned to the optical axis of the imaging laser. Thus, this label-free imaging efficiently locates sparsely distributed protein assemblies for subsequent AFM study while simultaneously minimizing degradation of the tip and the sample.

Improved performance of an ultrastable measurement platform using a field-programmable gate array for real-time deterministic control

Many precision measurement techniques (e.g. scanning probe microscopy, optical tweezers) are limited by sample drift. This is particularly true at room temperature in air or in liquid. Previously, we developed a general solution for sample control in three dimensions (3D) by first measuring the position of the sample and then using this position in a feedback loop to move a piezo-electric stage accordingly (Carter et al., Optics Express, 2007). In that work, feedback was performed using a software-based data acquisition program with limited bandwidth (≤ 100 Hz). By implementing feedback through a field programmable gate array (FPGA), we achieved real-time, deterministic control and increased the feedback rate to 500 Hz - half the resonance frequency of the piezo-electric stage in the feedback loop. This better control led to a three-fold improvement in lateral stability to 10 pm (Δf = 0.01-10 Hz). Furthermore, we exploited the rapid signal processing of FPGA to achieve fast stepping rates coupled with highly accurate and orthogonal scanning.

Independent measurements of force and position in atomic force microscopy

Historically, precise vertical control of an atomic force microscope (AFM) tip while it is disengaged from the surface has been an unsolved problem. By separately scattering a pair of lasers off the tip and a fiducial mark in the sample, we locally measured and thereby actively controlled tip and sample position in three dimensions, achieving atomic-scale (0.1 nm) precision at ambient conditions. We also measured cantilever deflection (force) using the standard optical-lever- arm geometry. Both detection techniques were used to determine the vertical location of the surface (z = 0) relative to the AFM tip assembly. The difference in these vertical determinations was 0.0 ± 0.3 nm (mean ± S.D.; N = 86). This agreement allowed us to establish an optically based reference frame to measure the vertical position of the tip relative to the surface. This reference frame is insensitive to long-term mechanical drift of the AFM assembly and complementary to the cantilever deflection sensing, which measures force. We expect this dual z-detection to be useful in a broad array of applications that demand precise tip-sample control, including tip-based nanofabrication and single-molecule force spectroscopy.

A Precision Force Microscope for Biophysics

Mechanical drift between an atomic force microscope (AFM) tip and sample is a longstanding problem that limits tip-sample stability, registration, and the signal-to-noise ratio during imaging. We demonstrate a robust solution to drift that enables novel precision measurements, especially of biological macromolecules in physiologically relevant conditions. Our strategy – inspired by precision optical trapping microscopy – is to actively stabilize both the tip and the sample using locally generated optical signals. In particular, we scatter a laser off the apex of commercial AFM tips and use the scattered light to locally measure and thereby actively control the tip’s three-dimensional position above a sample surface with atomic precision in ambient conditions. With this enhanced stability, we overcome the traditional need to scan rapidly while imaging and achieve a fivefold increase in the image signal-to-noise ratio. Finally, we demonstrate atomic-scale (∼100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.

Optical trapping meets atomic force microscopy: a precision force microscope for biophysics

Mechanical drift between an atomic force microscope (AFM) tip and sample is a longstanding problem that limits tipsample stability, registration, and the signal-to-noise ratio during imaging. We demonstrate a robust solution to drift that enables novel precision measurements, especially of biological macromolecules in physiologically relevant conditions. Our strategy - inspired by precision optical trapping microscopy - is to actively stabilize both the tip and the sample using locally generated optical signals. In particular, we scatter a laser off the apex of commercial AFM tips and use the scattered light to locally measure and thereby actively control the tips three-dimensional position above a sample surface with atomic precision in ambient conditions. With this enhanced stability, we overcome the traditional need to scan rapidly while imaging and achieve a 5-fold increase in the image signal-to-noise ratio. Finally, we demonstrate atomic-scale (~ 100 pm) tip-sample stability and registration over tens of minutes with a series of AFM images. The stabilization technique requires low laser power (<1 mW), imparts a minimal perturbation upon the cantilever, and is independent of the tip-sample interaction. This work extends atomic-scale tip-sample control, previously restricted to cryogenic temperatures and ultrahigh vacuum, to a wide range of perturbative operating environments.