Visualization

Contrast enhancement

Sarfus visualization results from a significant increase of an optical microscope sensitivity.
To improve this sensitivity, this technique enhances the contrast of the object compared to the background.
The contrast (C) is defined as :




where I₁ is defined as the object intensity and I₀, the background intensity

The selected way to increase the contrast is to minimize the background intensity I₀.

An upright material optical microscope is used. Specific sample supporting plates – the Surfs – are designed for not modified the polarization state of the light after reflection on them. Thus the use of a crossed analyzer allows cutting off the reflected light (I₀ = 0). Infinitesimal quantity of matter on the surface of the Surf changes this property (i.e. modifies the polarization state) and reveals the sample.

The figure shows the contrast difference versus the sample thickness between a sample (refractive index n = 1.5) deposited on a Surf and on a silicon substrate.

For a sample (i.e. ultra-thin film) of 10nm thickness, the contrast on a Standard Surf is x25 better than on Si Wafer and is x160 better for 1nm thickness.

The images below illustrate this contrast enhancement. Successive 6nm-steps have been deposited on a Si Wafer (on the left) and on a Standard Surf (on the right)



	Optical pathways through a reflected microscope

	The white light beam (10) passes through a linear polarizer (9) to produce a linear polarized light that is reflected by the Surf (1) which supports the sample (4). Only the sample induces a change of the polarization state. The reflected beams pass through an analyzer (12) crossed with the polarizer : the light coming from the Surf is stopped whereas the light coming from the sample passes through. Very high contrast images are obtained and allow the direct observation of nanometric thickness films or nanometric diameter objects. Sarfus gives also impressive results in reflected light DIC (8) Microscopy. 1- Optical polarizing microscope 2- Surf 3- Incident medium 4- Nanometric sample 5- Δθ₀ 6- θ₀ 7- Objective 8- Differential Interference contrast device 9- Polarizer 10- Non-polarized white light beam 11- Mirror 12- Crossed polarizer = analyzer

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