Cryostat optics, optical path

This interferometric dilatometer consists of a rather simple and small Michelson interferometer, in which the two arms are parallel, and of a 4He cryostat, in which the sample to be measured is hold. The sample is cooled to 4 K, and data are taken during the warm up of the cryostat. The optical path difference between the two arms depends on the sample length hence a variation of the sample length determines an interference signal. The Michelson interferometer consists of a He-Ne stabilized laser (A = 0.6328 xm), two cube corner prisms, a beam splitter, three mirrors and a silicon photodiode detector placed in the focal plane of a 25 mm focal length biconvex lens (see Fig. 13.1). 

Preparation of R. capsulatus Dll chromatophores devoid of antenna pigments and of Oa reduced reaction centers of R. sphaeroides strain R-26 was described before (Vos et al., 1992). The samples were diluted in glycerol (60% vol/vol) to an optical density of 0.5 at 870 nm in a cuvette with an optical path of 1 mm and were cooled in the dark to lOK in a convection cryostat. 

Recently, first experimental results were published which have been obtained with a Michelson interferometer especially designed by J. Cast and L. Genzel > for reflection studies on small solid samples by means of asymmetric or amplitude Fourier spectroscopy. The main advantage of the optical layout is that sample and reference mirror are located at focal points which do not take part in the motion to produce the path diflerence in the interferometer. Therefore, these foci can be placed inside a cryostat that allows the sample to be cooled. Another recent development in this field is concerned with the difficulty that, when studying the reflectivity of solids, the determination of the phase depends strongly on the exact positioning of the sample mirror instead of the background mirror. This is... 

This setup has the following advantages (i) It is insensitive to vibrations, which occur in the cryostat (ii) the full fight path passes through suprasil and is not affected by immersion in liquid helium (iii) Suprasil has a small expansion coefficient, which does not change the optical properties of the objective at low and high temperatures and (iv) the shrinking of the optical components leaves the ratios of all dimensions invariant and does not alter its functionality. 

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