Abstract:
A sample holder apparatus for a top-loading cryostat insert based on a cage assembly system which is mounted onto a vacuum flange. The vacuum flange has a group of blind bores on the vacuum side of the flange, in which are mounted hollow cage assembly rods, and a further corresponding group of four blind bores on the air side of the flange for receiving further cage assembly rods. A cage assembly system can thus be provided on the vacuum side of the flange and, if desired, extended through the vacuum flange onto the air side with optical communication between the vacuum and air sides taking place through a window at the top of the flange. Feed-throughs on the flange provide for access of optical fibers and electrical leads. The design provides a sample holder which is readily adaptable and reconfigurable to perform a variety of optical experiments in a top-loading cryostat. Examples of near-field scanning microscopy (NSOM) and confocal microscopy are described.

Description:
FIELD OF THE INVENTION 
     The invention relates to a sample holder for a cryostat insert, more especially but not exclusively to a sample holder for a top-loading cryostat insert. 
     BACKGROUND ART 
     Conventionally sample holders for cryostat inserts are constructed by brazing and/or welding tubes, plates and disc-shaped heat shields onto a vacuum flange. One type of cryostat in widespread use is the top-loading type in which the sample holder fits into a cryostat insert tube having a flange at one end for mating with a corresponding flange of the sample holder and being closed at the other end. When the insert tube and sample holder are fitted together to form the insert, the space inside the insert can be evacuated and the insert loaded into the top side of a top-loading cryostat. Cryostats of this kind are designed to receive an insert of a fixed diameter, for example one-and-a-half inches, two inches and 50 mm. For magnet cryostats the diameter is usually dictated by the superconducting magnet coil dimensions, i.e. the inner coil diameter for a conventional vertical bore magnet coil alignment. 
     Top-loading cryostats with inserts of this kind are generally not considered to be ideally suited to perform experimentally demanding optical measurements. However, it is common to perform experimentally simple optical measurements, such as photoluminescence, in top-loading cryostats with the aid of optical fibers. The optical signals are conveyed to and from the sample via an optical fiber which extends into the insert via a vacuum-tight feed-through on the sample holder flange and to the close proximity of the sample, which is mounted near the base of the sample holder. For optical measurements of greater experimental complexity, optical fiber based excitation and signal collection is often not convenient in which case cryostats with side windows are generally favored so that free-space optics may be used. In the case of magnet cryostats, a split-coil magnet alignment is required if side window access is desired. Split-coil magnet cryostats are several times costlier than equivalent vertical-coil magnet cryostats. In a vertical-coil magnet cryostat a single base window is often provided for limited external optical access. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a sample holder apparatus based on a conventional cage assembly optical rail system which extends at least on the vacuum-side of a sample holder vacuum flange. The cage assembly system preferably extends also on the air-side of the vacuum flange to form a contiguous cage assembly system extending on both sides of the vacuum flange which is provided with a window for optical communication through the vacuum flange. A cage assembly based sample holder complete with free-space optical components can be sleeved into a cryostat insert tube thus to allow a wide range of optical measurements to be performed at low temperature. 
     In the preferred embodiment, the sample holder flange is based on a standard vacuum flange into which has been bored on the vacuum side a group of four blind bores conforming to the square grid of a conventional cage assembly system and for receiving cage assembly rods. Rod holders may be implemented in many different ways other than blind bores, for example as sleeves extending from the main body of the vacuum flange. Preferably, a corresponding group of blind bores, conforming to the same square grid, is provided on the air-side of the flange for receiving further cage assembly rods, the bores on the air-side and vacuum-side of the flange being arranged in co-axial pairs with one of each pair on either side of the flange. 
     In the preferred embodiment, the rods used on the vacuum side are thin-walled stainless steel tubes, rather than the solid rods of conventional cage assembly system, thus providing a much lower thermal mass than solid rods. The interior of the tubes can also provide shielded routing channels for electrical leads or optical fibers which may pass into the tube interior via side holes in the tube wall. Conventional square cage plates are machined down at their corners to fit within an insert tube of inner diameter 49.6 mm so as to conform to an arcuate profile of a single circle, the center of which lies on or close to the main optical axis of the cage assembly system. 
     By basing the sample holder on a cage assembly system, a sample holder having the flexibility of a cage assembly system can be provided. Optical components such as lenses, irises, filters and polarizers can be moved, added and removed at will. Double cage plates can be incorporated to allow the cage assembly to be split up into detachable modules. For example, the lowest module, which may be a sample mounting module, can be detached and later reattached and realigned. A module, such as an optics module, may then be positioned above the sample mounting module. The optics module may be exchanged for another optics module for performing a different kind of optical measurement. If the cage assembly system is extended out onto the air-side of the flange, a camera such as a CCD camera can be mounted on a cage plate so as to view into the insert tube through the window provided in the vacuum flange. Other optical detector devices could also be mounted on the air-side, e.g. photomultipliers, CCD array detectors, multichannel plates and so forth. 
     Cage assembly systems are well known for bench-top optical arrangements. 
     One commercially available system is from the U.S. company, Thor Labs, Inc., New Jersey. This system is based on a square grid of four parallel rods of diameter 6 mm, the rod axes lying on a square of 30 mm side length. Along the rods are mounted cage plates having a corresponding square grid of four bores through which the rods can pass. The bores are arranged in respective corner regions of the plates which have outer dimensions of 40.6 mm square. The plates may have threaded holes of a standard diameter of 1.035 inches, i.e. approximately 25 mm, and standard threads such as RMS mount threads and C-mount optics threads. Optical components can then be mounted in these threaded holes. 
     Another commercially available cage assembly system, which is somewhat smaller, is from the German company Spindler &amp; Hoyer and has the trade name “Mikrobank”and is based on a square grid of 16 mm side length. The exterior side lengths of the cage plates are 25 mm and the threaded through holes are 16 mm in diameter for receiving 15 mm diameter optical components. The diameter of the rods is 4 mm. 
     A third commercially available cage assembly system is from the U.S. company, AF Optical Company of Irvine, Calif. and has the trade name “MICROPTIC”. This system is based on annular cage plates which are 10 mm thick and have an outer diameter of 49.25 mm and an inner diameter of either 25 mm or 30 mm. Each cage plate is provided with four holes based on a square grid dimensioned to receive rods of 6 mm diameter. 
     Cage assembly systems based on polygonal grids other than square ones could also be used, for example triangular, rectangular, pentagonal or hexagonal grids. Moreover, the number of rods provided need not be equal to the side number of the polygon. For example, with the square grid three rods can be used instead of four. This provides open access to one side of the cage assembly so that optical components can be inserted into and removed from a cage plate laterally with ease. For example, a polarizer may be inserted into a cage plate and then removed when no longer required. The cage plate can be left for possible future use. In addition a system using three rods is perfectly defined, being neither overdefined, i.e. hyperstatic, nor underdefined, i.e. hypostatic. 
     Further aspects of the invention are exemplified by the attached claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect the invention is described in the following by way of example with reference to the drawings in which: 
     FIG. 1 is a sectional side view of a vacuum flange part of a sample holder according to a first embodiment of the invention; 
     FIG. 2 is a sectional side view of a part of the sample holder of the first embodiment arranged on the air side of the vacuum flange part of FIG. 1; 
     FIG. 3 is a sectional side view of a part of the sample holder of the first embodiment arranged on the vacuum side of the vacuum flange of FIG. 1, including an optics module and a sample mounting module; 
     FIG. 4 is a plan view of a cage plate of the first embodiment; 
     FIG. 5 is a sectional side view of a cryostat insert tube for receiving the sample holder of the first embodiment; and 
     FIG. 6 is a sectional side view of an optics module of a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows in sectional side view a vacuum flange  10  of a sample holder apparatus according to a first embodiment of the invention. The vacuum flange  10  has a group of four rod holders, in the form of blind bores  13 , on the vacuum side of the flange  10 , the bores being arranged to define a square grid. The vacuum flange  10  has a further group of four rod holders, also in the form of blind bores  8 , on the air side of the flange arranged to define a further square grid, the respective squares having a common center point in plan view, defining the principal optical axis of the sample holder. 
     The blind bores  8  and  13  of the two groups of blind bores are arranged in pairs with one of each pair on either side of the flange. The blind bores of each pair of blind bores are aligned to hold respective rods coaxially on either side of the flange  10 . 
     A group of three or four rods  12  are seated in respective ones of the blind bores  13  to extend parallel to each other away from the flange  10  conforming to the square grid and forming the cage of the cage assembly optical rail system. 
     An optical component mounting plate  16 , a so-called cage plate, also known as a slide plate, is screwed to the underside of the vacuum flange  10  (screws not shown). Further cage plates can be provided as desired and can be mounted guided on the rods  12 . 
     The lower part of the flange  10  has a flared portion  11  for fitting together with an insert tube to form a vacuum-tight seal via an O-ring  15  supported by an aluminum ring  14 , the ring being received in a recessed portion  17  in the underside of the flange  10 . 
     In the upper side of the vacuum flange  10  there is a cage plate  9  screwed onto the vacuum flange  10  (screws not shown) to form a vacuum tight seal via a ring shaped channel  7  and an O-ring  6  located by the channel  7 . The cage plate  9  has a central threaded through hole in which is placed a window  3  held by a pair of threaded rings  4  and  5 , a vacuum-tight seal being formed by cementing around the rings  4  and  5  with adhesive bonding material such as epoxy resin. 
     The vacuum flange is provided with a vacuum-tight feed-through  1  for electrical leads and a further vacuum-tight feed-through  2  for optical fibers, these feed-throughs being shown schematically with dashed lines in FIG.  1 . 
     FIG. 2 shows an air-side module  20  of the cage assembly system which has three or four rods  22  which fit into the blind bores  8  of the flange  10  shown in FIG.  1 . The rods  22  extend parallel to each other away from the flange and parallel to the rods  12  on the vacuum side of the flange to form a contiguous cage assembly system extending through the vacuum flange  10 . The rods are thus arranged in coaxial pairs. This is a convenient arrangement, but any other arrangement providing for a contiguous optical axis would suffice. For example, the square rod grid on one side of the flange could be rotated by any angle relative to the square rod grid on the other side of the flange, e.g. by 45°. 
     The air-side module  20  is based around a CCD camera  27  mounted on a zoom holder  28  and lens tube  29  containing a lens  30  for focusing light onto the CCD camera chip. The lens tube  29  is threaded to a tiltable cage plate  26  of a tilt stage  21  comprising a further cage plate  24  which is secured to the rods  22 . Tilting is effected by a standard thumb screw  23  and hinge  25  arrangement. The camera  27  can thus be used to view along the optical axis through the window  3  and into the vacuum side of the flange. The zoom holder  28 , lens tube  29  and tilt stage  21  are all standard commercially available cage assembly system components. 
     FIG. 3 shows a lower part of the sample holder of FIGS. 1 and 2 comprising two detachable cage assembly modules  40  and  60 , based on sets of rods  42  and  62  respectively. The upper detachable cage assembly module  40  is more specifically referred to as the optics module  40 . The module  40  is attached at the ends of the rods  12  which extend from the vacuum flange  10  as described further above with reference to FIG.  1 . The module  60  is attached to the lower end of the module  40 . 
     The module  40  houses optical components of a near-field scanning optical microscope (NSOM) and is referred to in the following as the optics module. The module  60  houses a sample holder  67  mounted on an xyz positioning device  68  which is in turn mounted on a base plate  70  which is a further cage plate of the cage assembly system. 
     The optical components of the optics module  40  perform the functions of sample excitation, signal collection and external viewing of the sample and sample area by the camera  27  described above with reference to FIG.  2 . Sample excitation is performed via the single mode optical fiber  53  which terminates in a near-field aperture facing the sample holder  67 . The single mode optical fiber  53  has attached to it one leg of a quartz tuning fork  55  which is bonded by its base onto a printed circuit board (PCB)  58  which comprises signal preprocessing circuitry for preprocessing the AC signal received from the electrodes of the tuning fork  55 . The circuit board also has mounted thereon a light emitting diode (LED)  59  for illuminating a sample mounted on the sample holder  67  to allow viewing by the camera  27 . The LED  59  is mounted in the shadow of the PCB  58  to avoid saturating the camera  27 . The PCB  58  is attached to a mounting body  57  which is in turn attached to a cage plate  54 . A further cage plate is positioned below the cage plate  54  and comprises an ellipsoid mirror  56  for signal collection from around its lower focal point which is in the region of the tip of the single mode optical fiber  53 . A lens mount  47  in which is mounted an achromat lens  48  is positioned level with the upper focal point of the ellipsoid mirror  56 . The lens  48  has a hole through its optical axis in which is fitted a multimode fiber  50  inside an axially slidable ferrule  52 . The aperture of the fiber  50  is positioned at the upper focal point of the ellipsoid mirror  56  to collect sample signal from the sample area as shown by the dashed lines in FIG.  3 . The lens  48  is selected approximately to collimate light from the sample area as shown by the dot-dashed lines so as to pass light from the sample area onto the lens  30  of the air-side unit  20 . The lens mount  47  is fitted into the cage assembly system via a further cage plate  46  which together with another cage plate  45  forms a double cage plate  44  bridging the rods  12  attached to the vacuum flange  10  and the rods  42  of the module  40 . The module  40  can be detached from the vacuum flange  10  by releasing the double cage plate  44 . At the lower end of the module  40  a further double cage plate  64  bridges the rods  42  of the module  40  and the rods  62  of the further module  60 . The further double cage plate  64  comprises a cage plate  65  attached to the rods  42  and a cage plate  66  attached to the rods  62 . 
     The double cage plate arrangements  44  and  64  thus serve as rod group linkage mechanisms by which the respective optical axes of adjacent rod groups can be realigned, and which also provide for convenient detachment of adjacent rod group assemblies. 
     The above description of the optical and mechanical components of the modules  40  and  60  is given merely by way of example of the kind of cage assembly based system which may be constructed in the format of a sample holder for a tubular cryostat insert. Many other kinds of optical system which can be realized within the cage assembly format will be readily contemplated. 
     One variation is now described with reference to FIG.  6 . FIG. 6 shows a further optics module  80  for performing confocal microscopy which can replace the NSOM optics module  40  of FIG.  3 . The confocal optics module  80  comprises a set of rods (not shown), a cage plate  86  with an FC adapter  90  secured by clamping rings  91  &amp;  92 , and an FC coupler  82  with an associated optical fiber  98 . A lens tube  87  is attached to the cage plate  86  and houses an objective lens  89  and a focusing lens  88 . 
     To change the cage assembly system from that shown in FIGS. 1 to  3 , which is for near-field scanning microscopy, the sample mounting module  60  is detached, followed by the NSOM optics module  40 . The confocal optics module  80  is then attached to the flange rods  12  via the double cage plate  44  and the sample mounting module  60  reattached. In a few steps, the sample holder can thus be reconfigured to perform a different kind of microscopy. 
     Further variations include exchanging the objective lens  89  for a solid immersion lens to perform solid immersion microscopy and exchanging the objective lens  89  for a standard microscope objective mounted in the lens tube via an RMS adapter to perform conventional microscopy. Generally, the freedom provided by the cage assembly system format will allow all manner of optical systems to be incorporated into the sample holder which may not include any of the optical or mechanical components described in the examples shown in FIGS. 3 and 6. 
     In general, any two modules of the modular cage assembly are releasably interconnected by respective rod group linkage mechanisms in the form of double cage plates  44 ,  64  which are adjustable between a locked condition in which the respective adjacent groups of rods, e.g.  12  &amp;  42  and  42  &amp;  62  in the case of FIG. 3, are fixed in position relative to each other and an unlocked condition in which the respective adjacent groups of rods are displaceable relative to each other in a plane perpendicular to the optical axis around a position of axial alignment of the respective adjacent groups of rods. The double cage plates comprise first and second slidable cage plates  45  &amp;  46 , and  65  &amp;  66 , secured to end sections of the adjacent groups of rods respectively, the slidable cage plates being adjustable between a locked condition in which the slidable cage plates are fixed in position relative to each other and an unlocked condition in which the slidable cage plates are slidable relative to each other in the plane of the slide plates thereby to adjust the relative coaxial alignment between the adjacent groups of rods. 
     FIG. 4 shows in plan view the cage plate  54 . The cage plates  45 ,  46 ,  65  and  66  on the vacuum side of the flange  10  are similar. The cage plate has four bores  70  through which the rods may pass. A threaded through hole  74  of internal diameter Φ passes through the center of the cage plate and can be used for mounting components using threaded rings. The cage plate  54  is based on a standard commercially available cage plate which has had its four corners  71  (shown with dot-dashed lines) machined off to form four arcuate edge portions  72  lying on the arc of a single circle of diameter φ the center of which will lie on or near to the optical axis in the assembled system. The double cage plates referred to above are also standard commercially available items with the exception that the two cage plates have machined corners as described immediately above. When fitted into an insert tube, the arcuate portions  72  of the cage plates are in close proximity to the inner surface of the insert tube, but four gaps are formed between the inner surface of the insert tube and the straight portions of the outer periphery of the cage plates extending between the arcuate portions. These gaps can be used to provide space for feeding through electrical leads and optical fibers on the outside of the sample holder. In an alternative design, the outer periphery of the cage plates could be circular, or have one arcuate portion extending through the major part of a circle, i.e. through more than 180 degrees, and one chordal straight portion interconnecting the ends of the single arcuate portion. 
     FIG. 5 is a sectional side view of a cryostat insert tube  100  for receiving the above-described sample holders. The insert tube  100  has a conventional construction with a flange  111  at one end for mating with the flange  11  of the vacuum flange  10  shown in FIG.  1 . The insert tube  100  has a long tubular body section  112  terminating in a closed base partition. The tubular body section  112  has an inner diameter φ+δ of 50 mm, the outer diameter φ of the cage plates which are to be fitted into the insert tube  100  being slightly smaller so as to form a loose push fit into the tube so as to locate the sample holder laterally. The insert tube also comprises a standard evacuation valve and a standard pressure relief valve neither of which are shown. These valves are mounted in an upper portion of the tubular body section  112  towards the flange  111 .