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PLANAR scanning probe microscopy
the sample which create a far-field interference pattern that sensitively depends on the gap distance [ 3 ].
For sensor-sample gaps smaller than around 100 nm, the absolute value of the sensor-sample gap can also be measured by measurements based on optical polarisation, reminiscent of optical ellipsometry [ 2 ]. The sensor-air-sample multilayer is a dielectric multilayer system, similar to spin-coated photoresist films in nanofabrication, where ellipsometry is equally a workhorse technique to measure sub-wavelength film thickness. A suitable polarisation-sensitive imaging scheme is Brewster angle microscopy, where a polarized beam of parallel light is totally internally reflected off the sensor surface. If the gap is sufficiently small, the light can tunnel across it and instead reflect off the sample surface, giving rise to an altered polarisation. Crucially, this process can be modelled analytically, so that the gap can be quantitatively inferred from the polarisation rotation, similar to a film thickness measurement in ellipsometry. Notably, this measurement does not require to move the sensor and sample into contact to calibrate the zero-distance point, in contrast e. g. to“ lift-mode” AFM. The sensor can thus be flown at a controlled height without ever physically touching the sample, enabling work with fragile sensors.
THE PAST – THE SURFACE FORCE APPARATUS AND EARLY PRECURSORS Approaching two planar surfaces into nanoscale proximity under optically controlled feedback has been explored in different contexts in earlier decades. One notable example is the“ surface
Figure 3. The tilt and the gap distance between the sensor and the sample can be measured by various optical schemes. θ c critical angle.
force apparatus” extensively used in nano-tribology. In this setup, two cylinders are approached into nanometer-scale proximity of each other under interferometric optical distance control to study forces between them. The use of cylinders greatly simplifies the design, because it ensures that the surfaces are parallel to each other at the contact point even without control of the tilt angle. Interestingly, even a scanning probe microscope with interferometric distance feedback has been demonstrated in the nineties to perform optical near-field microscopy [ 4 ]. Similar to the cylinder trick employed in the surface force apparatus, the sensor here was not planar but a hemispherical surface so that no tilt alignment was needed. However, the technique appears to have fallen out of fashion and to have been superseded by tip-based positioning schemes like shear-force feedback.
THE PRESENT – NANO- FLUIDICS, NEAR-FIELD OPTICS AND MAGNETOMETRY The idea of aligning two planar surfaces parallel to each other and approaching them into nanoscale proximity has re-appeared in very different fields of research in the past decade, demonstrating its versatility and timeliness. Extensive work has been performed in the field of near-field optics, where it has been employed to position plasmonic antennas in plasmonic nano-lithography and study radiative heat transfer in nanoscale gaps [ 3 ], which is orders of magnitude faster than expected from blackbody theory for reasons still under discussion. The technique made an independent appearance in nano-fluidics where the planar setup is known as the“ nanofluidic confinement apparatus” [ 1 ]. Here, the planar surfaces themselves are less of an interest than the fluid confined between them, which can be confined to a channel of constant and tuneable height by controlling the tilt and the distance of two planar plates. This approach has been employed to study the diffusion of nanoparticles in“ Brownian ratchets”, tooth-shaped surfaces where an alternating driving force together with random diffusion is rectified into a directed motion of the nanoparticles [ 5 ].
Finally, planar scanning probe microscopy has been introduced to the field of NV centers in diamond [ 2 ], where both defect-based near-field microscopy [ 2 ] and magnetometry [ 6 ]
Figure 4. Future possible sensors. a) arrays of NV centers. b) superconducting microcavities. c) plasmonic nanoparticles which can be operated as scanning nanogap cavities( right picture).
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