Optical feedback imaging

Fig. 4. (A) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under low surface pressure. (B) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure. (C) Near-field fluorescence image of a DPPC monolayer at the air-sucrose solution interface under high surface pressure, collected using the optical feedback approach. Reproduced with permission from Ref. [19]. Copyright 1999 Blackwell Publishing.

An imaging high-pressure detector can be envisioned from an array of vertically cylindrical ionization chambers, with spatial resolution set by each tube diameter. It may further be possible to segment the collection anode, to derive an azimuthal co-ordinate within each detector and to use signal risetime to get a radial co-ordinate. The precision of such techniques, and the low-energy performance of such detectors is critically dependent upon the preamplifier noise. It may be possible to achieve around 50 electrons rms with modern (optical feedback, or no feedback) amplifiers resulting in an energy resolution of a few percent at 100 keV. 

Fig. 2. (A) Optical feedback system. For transmitted light imaging using a video camera (VC), the sample circuit (SM) was illuminated from beneath with a surface light source (LS). The recorded image was processed using a PC to update the monochrome image for illumination with a projector (PJ). (B) Optical feedback rule active state Xi t) = 1 triggers light illumination + At) = yi+i t + At) = 1 (white light...

Shiku, H., J. R. Krogmeier and R. C. Dunn (1999). "Noncontact near-field scanning optical microscopy imaging using an interferometric optical feedback mechanism." Langmuir 15(6) 2162-2168. 

The optical system comprises a laser, which is reflected by a mirror mounted on the back of the cantilever to another mirror that sends the reflected beam to an array detector. The position of the beam translates in the position of the cantilever in the vertical direction, whereas the lateral position in xy coordinates is inferred from the movement of the xy table. Essentially, AFM uses a feedback system to measure and regulate the force applied on the scanned sample, which allows the acquisition of images using very low forces. 

AFM, invented in 1986 [25], has been optimized as a surface imaging tool for a variety of materials from metals to insulators by scanning the surface with a thin metallic cantilever, with a sharp tip on the end, coming to a point with a radius on the order of 10 nm. As the tip is scanned across a surface, the tip-stage position is controlled in three dimensions by a set of oriented piezo crystals (see Figure 4.2). An optical lever comprised of a laser in conjunction with a photodiode is used to measure cantilever deflection in a closed loop feedback system (based on amount of deflection of the cantilever in the simplest mode of operation). As the tip rasters across the surface, a three-dimensional topography map of the surface is created. 

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