Workshop 1

Tuesday, May 2
Bldg. 200, Auditorium
9:00 - 12:15
1:30 - ~4:30

Toward 1-nanometer X-ray Beams

Organizers:
Paul Evans, University of Wisconsin-Madison
Jörg Maser, Argonne National Laboratory

To agenda >

Overview

This workshop features discussions of the ultimate spatial resolutions that can be reached by x-ray focusing, as well as presentations of the state of the art and potential intrinsic limits of optical fabrication technologies. An informal session following the invited talks will allow attendees to provide thoughts and feedback and present directions for the rapidly evolving field of focused x-ray beams. We invite participants to prepare two slides with their thoughts about technical and fabrication approaches suitable to approaching ultimate x-ray resolution.

Details

Since the nature of x-rays was first understood more than one hundred years ago, it has been realized that they would be an ideal tool for microscopy. The availability of x-ray optics that provide the ultimate spatial resolution would open up new frontiers in x-ray studies from nanoscale imaging of objects buried deep inside a specimen to coherent manipulation of nanoparticles. Spatial resolutions of x-ray probes could in principle approach the wavelength of hard x-rays (well below 1 nm) and provide information based on atomic and electronic structure, chemical composition, and magnetism. Such unique capabilities would impact many disciplines across the biological and physical sciences.

The resolving power for any optical element is limited simply by its numerical aperture and the wavelength of the radiation. Current hard x-ray optics have very small numerical apertures (<0.001), and their resolving power is far from the ultimate limit. To achieve large numerical apertures fundamentally requires deflecting x-rays through large angles. Since hard x-rays with wavelengths of order 1 Å interact only weakly with matter and are not affected by electric or magnetic fields it has been a challenge to produce optics that produce sufficiently large deflections.

The three basic approaches to achieving x-ray optics with high numerical apertures have been refraction, reflection, and diffraction. These approaches are complementary in their capabilities. While it is not yet clear which approach will produce the ultimate resolution, they share many of the same challenges for fabrication and at the smallest spot sizes, the already artificial distinctions between these techniques are blurred. It is even possible that the ultimate optics could employ compound optics incorporating combinations of the three approaches.

Today, the highest resolutions are close to 10 nm in the soft x-ray range using Fresnel zone plate optics and better than 50 nm in the hard x-ray range using approaches based both on reflective and on diffractive optics. With recent advances in state-of-the-art fabrication techniques, hard x-ray optics with a focal spot below 10 nm seem feasible, and a push toward exploring true nanometer focusing is already beginning. As with all other microscopy techniques, including electron and optical microscopes, x-ray microscopes require highly brilliant sources in addition to large numerical aperture optics. Bright sources of hard x-rays are already available at third-generation light sources including the Advanced Photon Source the upcoming NSLS II and are planned for electron lasers such as the DOE’s Linac Coherent Light Source.

It is now possible to articulate a vision for what the ultimate x-ray focusing optics should look like. This vision will most likely involve several types of optical devices. For nanocrystalline materials structural studies may require multi-wavelength or "white" beams of x-rays and hence mirror optics. Elemental mapping, for example of trace metals in biological cells, will require single-wavelength excitation at maximum resolution and hence diffractive optics. High-resolution diffraction and coherent imaging require beams with well-defined incident wave fronts, but could in principle make use of any one of several focusing techniques.

Agenda confirmed as of April 12, 2006