We wish to describe the construction of a hybrid glassmetal liquid helium dewar for low temperature optical studies. This dewar has been successfully used in the experimental study of second harmonic generation (SHG) in InSb using a CO 2 laser as the source of coherent radiation. 1 For this experiment an optical system was required which was transparent to radiation near 5'3/2 and 10-6/2. This precluded the use of a standard all-glass dewar with glass windows, Siffce glass is not transparent at these wavelengths.
Commercially available dewars are usually made completely out of metal or almost completely of glass. They are usually custom adaptations of a standard dewar assembly. We find that it is considerably less expensive as well as time-saving to make those parts of the dewar that are not in contact with liquid helium out of one piece of glass, and then to have a removable metal container (can) for the liquid helium and the sample chamber. The outer windows are fastened directly to glass flanges with O-ring seals, and the metal can is also joined to the glass by an O-ring seal. These seals always remain close to room temperature so that there is no problem of freezing of the O-rings.
The hybrid dewar is shown in Figure , and we shall now point out some of the interesting features and advantages.
First note that the outer three walls are made of pyrex and may be easily silvered to minimize heating due to radiation. Figure is a cross-section taken through the window apertures. Note that the holes through the walls of the liquid N 2 vessel are made only large enough to provide a suitable light aperture and to provide mechanical stability consistent with the dewar size and weight. If, in order to reduce incoming radiation, one desires an aperture smaller than that consistent with mechanical stability requirements, aluminum foil with the appropriate-sized hole may be mounted in these apertures.
Next we note that the window flanges may be rather short, since it is not necessary to make graded seals or glass to metal seals requiring an additional 6 cm for each window flange. Keeping these flanges short eases the task of maximizing the aperture of the optical system.
The top flange on the pyrex part of the dewar is also suitable as an O-ring surface. So far, we have used only glass because of inexpensive, easy, quick fabrication, because it is cleaner than metal and therefore pumps'down more quickly, and because it is easy to make vacuum tight with small likelihood of leaks developing. But now the inner section is best fabricated completely out of metal which may be machined to exactly match the glass in size. in addition, it is unwise to have graded seals which are repeatedly subject to wide temperature cycling, since these seals usually have built-in strains and are in danger of eventually cracking. Thus glass is a bad choice for any part of the inner section where one must also have some metal joined to it.
The completely metal inner can be prepared with a flange that has an O-ring groove for positioning the upper