Multiple-beam interferometr

The absolute measurement of the distance, D, between the surfaces is central to the SFA teclmique. In interferometric SFAs, it is realized tluough an optical method called multiple beam interferometry (MBI), which has been described by Tolansky [47]. 

The force, FC(D), is determined by virtue of multiple beam interferometry. The piezoelectric crystal is expanded or contracted by a known amount AD and subsequently the actual distance by which the surfaces move is determined interferometrically. Any difference between the expected distance of motion and the actual distance that the surfaces moved with respect to each other, AD0, when multiplied by the spring constant, k, gives, according to Hooke s law, the difference between the forces at the initial and the final separations, AFc ... 

Fig. 13.15), so as to resemble the surface of a latex particle. The two rubber cylinders, A and B, were mounted vertically, one above the other. The upper cylinder was held fixed during the experiment whereas the position of the lower cylinder could be moved up and down by a micrometer drive unit (D). Moving the cylinder A up towards B, after polymer had been adsorbed onto the poly(methyl methacrylate) surfaces, distorted the soft, elastic surfaces in the centi region, resulting in flat discs in the zone of interaction. The distance of separation between the surfaces was measured by multiple beam interferometry. The applied pressure caused an equilibrium contact area to be formed, the magnitude of which could also be determined interferometrically. The relationship between contact area and the equilibrium pressure was established by direct calibration. In this fashion, the equilibrium pressure could be measured for a given distance of separation. 

The interference process in this collinear approach is, however, different from the interference realized by mixing the local oscillator and the CARS field on a beam splitter. Interference takes place in the sample, which, in the presence of multiple frequencies, mediates the transfer of energy between the beams that participate in the nonlinear process. The local oscillator mixes with the anti-Stokes polarization in the focal volume, and is thus coherently coupled with the pump and Stokes beams in the sample through the third-order polarization of the material. In other words, the material s polarization, and its ability to radiate, is directly controlled in this collinear interferometric scheme. Under these conditions, energy from the local oscillator may flow to the pump and Stokes fields, and vice versa. For instance, when the local oscillator field is rout of phase with the pump/Stokes-induced anti-Stokes polarization in the focal interaction volume, complete depletion of the local oscillator may occur. The energy of the local oscillator field is not redistributed in terms... 

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