We use coherent soft x rays for optics experiments. We continue to invent and develop new forms of x-ray microscopy and holography, explore the principles behind them, and collaborate with experts in biology, materials science and other fields in applying them to important problems. Much of our effort is directed towards using Fresnel zone plates to produce the smallest focused spot of electromagnetic waves of any wavelength, and exploiting that microfocused beam for studies of biological and materials science specimens. What's meant by all that? The whole story follows; you can also click on one of the words listed to jump ahead to that part of the story:
By soft x rays we mean x rays with an energy of about 100 to 1000 eV. These energies are well-matched to K shell absorption edges of low Z atoms like carbon and oxygen, or L shell edges of atoms like calcium. As a result, we have the ability to make quantitative elemental and chemical state maps of major constituents. The wavelength of these x-rays is in the 1 to 10 nm range (as opposed to 350-700 nm for visible light), offering the potential for very high resolution imaging. Our experiments are centered at the National Synchrotron Light Source, which is located at Brookhaven National Laboratory (about 25 km from Stony Brook). We have an undulator beamline which is able to deliver 108 photons/sec of coherent x rays to our experiments. This coherent flux makes possible experiments which were not possible a decade ago, including x-ray holography at 40 nm resolution.
With soft x rays, we have to use diffraction rather than refraction for focussing. A Fresnel zone plate consists of concentric rings that are finely spaced at the outside (producing a large diffraction angle) and coarsely spaced at the center (producing a small diffraction angle) so as to produce a point focus. The resolution is determined by how finely one can make the outermost rings, and how accurately one can place them over the zone plate diameter. Our zone plates have finest rings of 30-50 nm width on a diameter of 80-90 micrometers; these nanostructures are fabricated using electron beam lithography. We have used zone plates made in a collaborative arrangement between Lawrence Berkeley National Laboratory and IBM, and more recently we have fabricated our own in the laboratory of Don Tennant, a collaborator at ATT Bell Laboratories in Holmdel, New Jersey. With these optics, we are able to generate 35-55 nm Rayleigh resolution focal spots, which is the smallest focused spot of electromagnetic waves of any wavelength.
By operating at photon energies between the K shell absorption edges of carbon and oxygen, we are able to image organic material in water at high contrast. This offers unique capabilities for biological microscopy which complement optical and electron microscopes: the resolution is higher than that of conventional and confocal light microscopes, the image offers quantitative information about the specimen, and the penetration and intrinsic contrast of soft x rays means we can look at whole, unsectioned cells of many types. We are in the process of commissioning a version of our microscope which can image frozen hydrated specimens, and to use it for elemental and chemical state mapping and nanotomography. See also: photo gallery of some recent biological projects; some recent publications.
We collaborate with Prof. Harald Ade, NCSU, on the development and utilization of the Scanning PhotoEmission Microscope, SPEM. Again, a zone plate creates a finely focused probe of soft x-rays, which in this case explores the surface of a semiconductor microstructure, a catalyst, a defective computer disk, or a heterogeneous polymer. Photoelectrons emitted from this small area are analyzed to determine the elements present, and their chemical state. The distribution of a given species can be mapped by selecting a fixed photoelectron energy while scanning the specimen.
For more information, including the microscope and beamline instruction manual, a version of the Henke x-ray database, some JEOL .j51 file utilities, and so on, click here to get to our home page.
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This page updated Jan. 3, 1996 by Chris Jacobsen.