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 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 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.
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
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 (Δ<i>f</i> = 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.