Sarfus

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Surface-enhanced ellipsometric contrast microscopy, commercially known as Sarfus^[1] is a nanodetection technique (an optical nanoscopy technique to make nanostructures visible) based on the association of an upright or inverted optical microscope in crossed polarization configuration and specific supporting plates called surfs on which the sample is deposited for observation.^[2]

Sarfus visualization relies on precise control of the reflection properties of polarized light on a surface, enhancing the axial sensitivity of an optical microscope by approximately two orders of magnitude without reducing its lateral resolution.^[2] This method increases the sensitivity of standard optical microscopes to a point that it becomes possible to directly visualize thin films (down to 0.3 micrometers) and isolated nano-objects in real-time, be it in air or in water.

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A 2006 study on polarized light coherence led to the development of new supports (the surfs) having contrast amplification properties for standard optical microscopy in cross polarizers mode.^[3] Made of optical layers on an opaque or transparent substrate, these supports do not modify the light polarization after reflection even if the numerical aperture of the incident source is significant. This property is modified when a sample is present on a surf; a non-null light component is then detected after the analyzer, rendering the sample visible.

The performance of these supports is evaluated by measuring the contrast (C) of the sample defined as: C = (I₁-I₀)/(I₀+I₁) where I₀ and I₁ represent the intensities reflected by the bare surf and by the analyzed sample on the surf, respectively. For a one nanometer-film thickness, the surfs display a contrast 200 times higher than on silicon wafer.

This high contrast increase allows the visualization with standard optical microscope of films with thicknesses down to 0.3 nm, as well as nano-objects (down to 2 nm diameter) and this, without any kind of sample labeling (neither fluorescence, nor radioactive marker). An illustration of the contrast enhance is given hereafter with the observation in optical microscopy between cross polarizers of a Langmuir-Blodgett structure on a silicon wafer and on a surf.

In addition to visualization, recent developments have allowed for measuring the thickness of analyzed samples. A colorimetric correspondence is carried out between a calibration standard made of nano-steps and the analyzed sample. Indeed, due to optical interference, a correlation exists between RGB (red, green, blue) parameters of the sample and its optical thickness. This leads to 3D-representation of the analyzed samples, the measurement of profile sections, roughness and other topological measurements.

Samples to be characterized are deposited by techniques such as dip-coating, spin coating, deposit pipette, or evaporation, on a surf instead of the traditional microscope slide. The support is then placed on the microscope stage.^[4]

Sarfus visualization can be integrated into existing analysis equipment, such as atomic force microscopy (AFM) and Raman spectroscopy, to add new functionalities such as optical imaging, thickness measurement, kinetics analysis, and sample pre-localization, which can save time and consumables (e.g., AFM tips).^[5]

• Biological films^[6][7]
• Biochip^[8]
• Phospholipids
• Soft lithography^[9]
• Cell adhesion

• Polymers films
• Langmuir-Blodgett films^[10]
• Liquid crystals
• Plasma treatment
• Self-assembled monolayers

• Carbon nanotubes
• Nanoparticles^[11]
• Nanowires
• Graphene^[12]
• DNA molecules

Optical microscopy has several advantages compared to the usual techniques of nanocharacterization. It is easy-to-use and directly visualizes the sample. The analysis in real-time allows kinetic studies (real-time crystallization, dewetting, etc.). The broad choice of magnification (2.5 to 100x) allows fields of view from several mm² to a few tens μm². Observations can be performed in controlled atmosphere and temperature.