Optical piston

An interesting phenomenon based on collisions between excited atoms and ground-state atoms is the macroscopic diffusion of optically pumped atoms realized in the optical piston [1014]. It is caused by the difference in cross sections for velocity-changing collisions involving excited atoms A or ground-state atoms A, respectively, and results in a spatial separation between optically pumped and unpumped atoms, which can be therefore used for isotope separation [1015]. 

We have above discussed certain mechanical actions on atoms induced by laser light. Using resonance-radiation pressure atomic beams can also be focused and manipulated in interesting ways [9.233,234]. Several other interesting mechanical effects exist and have been explored. Studied phenomena include light diffusive pulling [9.235], light-induced drift [9.236] and the optical piston [9.237]. 

G. Nienhuis Theory of light-induced drift and the optical piston. Phys. Rev. A 31, 1636 (1985)... 

The single-cylinder engine with optical access (a) cross section showing quartz windows and the extended piston construction view of the engine during testing. (Courtesy of Dec, J.)... 

The electrochemical cell used in our laboratory has been fully described elsewhere (5). The cell body is made of chemically inert Kel-F and the electrode is mounted on a piston so that its surface can be pushed to the optical window, to a spacing of the order of 1-3 microns, in order to minimize the signal from the bulk electrolyte. For Raman scattering spectroscopy the window is of flat fused quartz, and the exciting laser beam is incident at about 60°. The scattered light is collected off-normal, but the geometry is not critical for SERS due to the high sensitivity. Details on the SERS measurements in our laboratory have been reported previously (6,7). 

Fig. 61. Schematics of pressure-induced and applied-potential-induced BLM deformations. Application of hydrostatic pressure (by lowering a piston into the aqueous solution bathing the cis side of the BLM) displaces the BLM from position 1 to position 2. The displacement involves both translational (lateral) motion (Ft) and curvature increase (Fc). As indicated, deformation of the BLM is accompanied by a change in its torus (Plateau-Gibbs border). 2R and 2Rm represent the diameters of the aperture of the pinhole in the Tefzel film and that of the membrane (excluding the torus). The object laser beam, incident upon the trans side of the BLM and reflected by it at 45° at a shortened wavelength produces concentric optical interference fringes with the reference laser beam. Ag/AgCl electrodes, placed in the cis and trans sides of the BLM, allow for continuous electrical measurements [413]...

K. The movement of piston is communicated by means of the rod V, to an optical arrange-... 

Measurements of sound velocity at ultrasonic frequencies are usually made by an acoustic interferometer. An example of this apparatus11 is shown in Fig. 2. An optically flat piezo-quartz crystal is set into oscillation by an appropriate electrical circuit, which is coupled to an accurate means of measuring electrical power consumption. A reflector, consisting of a bronze piston with an optically flat head parallel to the oscillating face of the quartz, is moved slowly towards or away from the quartz by a micrometer screw. The electrical power consumption shows successive fluctuations as the distance between quartz and reflector varies between positions of resonance and non-resonance of the gas column. Measurement of the distance between resonance positions gives a value for A/2, and if /... 

The FundaLux system, for instance, aspirates a liquid aliquot with a Teflon piston into an external glass cell, allows a (selectable) time (typically 2 min) to degas, measures transmission in comparison to an air blank, and releases the aliquot back to the reactor an interesting feature - specific to this instrument -is the repetitive cleaning of the optical window by the moving Teflon piston. Some problems with infections have been communicated with this device since the measuring cell is external to the bioreactor and the sensor is probably insufficiently sterilized in situ. 

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