NEAR-FIELD SCANNING OPTICAL MICROSCOPY OF BIOLOGICAL MEMBRANES - Life at the Nanoscale: Atomic Force Microscopy of Live Cells - page 188

Figure 9.3. Examples of aperture probes of different diameters after FIB treatment.
Left: 35 nm aperture; middle: 95 nm aperture; right: 530 nm aperture.
9.2.3 Shear-Force Feedback to Control Probe - Sample Distance
Since the near-ield intensity exiting the subwavelength aperture probe
decays exponentially with distance from the probe, for eficient excitation
it is essential to have accurate control of the probe-sample distance during
scanning. Several different techniques have been implemented so far to
monitor the vertical position of the probe tip. First NSOMs relied on electron-
tunnelling feedback, later extended to photon tunnelling in the photon
scanning tunnelling microscopy. Today, the majority of NSOMs utilize two
distinct feedback methods, which have analogous sensitivity and performance
and are similar to noncontact AFM. These techniques are called shear-force
feedback and tapping-mode feedback. This latter method implies the use
of bent tips, which are usually characterized by lower optical throughput,
technical dificulties in the fabrication and higher mechanical vibrations.
To date, the most commonly employed NSOM coniguration relies on shear-
force feedback based on the use of quartz tuning forks. 38 In this approach, the
NSOM tip is glued onto one of the arms of the tuning fork. The tuning fork-
probe system is oscillated at its resonance frequency in a lateral vibrational
mode (with a <1 nm amplitude). When in proximity to the sample, shear
forces dampen this motion and induce measurable changes in the oscillation
amplitude and phase. An electronic feedback system, controlling the probe-
sample distance directly through the piezo-electric scan stage, is subsequently
used to maintain a constant oscillation amplitude/phase during scanning.
In this way, a constant probe-sample distance of <10 nm is realized. The
feedback signal itself, as in AFM, is used to generate a topographic map of the
sample surface with comparable resolution and sensitivity as tapping-mode
AFM ( Fig. 9.4a ) . Of course, unique to NSOM is the fact that a corresponding
luorescence map is simultaneously generated.