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Introduction Periodic Multilayers Normal-Incidence Multilayer Optics Depth-Graded Multilayers Grazing-Incidence Multilayer Optics Aperiodic and Laterally-Graded
Multilayers Facilities
At X-ray energies above ~1 keV, multilayer coatings operate efficiently only at
shallow graze angles (although these angles are nevertheless several times
larger than the critical angle for total external reflection of the best
single-layer materials). While periodic X-ray
multilayer coatings can provide high reflectance over a narrow spectral
band even in the hard X-ray, for many applications, such as X-ray astronomy,
broad spectral response is required.
A
“depth-graded multilayer” coating (DGML), illustrated conceptually in the
figure at right, is a film stack containing a range of layer thicknesses so as
to achieve high reflectance over a wide band of X-ray energies. One type of
depth-graded multilayer uses a power-law distribution of layer thicknesses, as
shown in Figure
1, for
example. The theoretical response of a DGML comprising Si/W bilayers is shown
in Figure
2, relative
to the reflectance of single-layer Pt and Ir coatings.
Figure 1. Left: layer thickness vs. layer
index (counting from the top surface), for a Si/W depth-graded multilayer
containing 300 bilayers with a power-law
distribution of layer thicknesses so as to achieve broad spectral response.
Right: a TEM image of the top few bilayers of a DGML coating similar to the
design shown at left.
Figure 2. Theoretical grazing-incidence X-ray
reflectance of a Si/W DGML, relative to Ir and Pt single-layer coatings.
The
W-based DGML coating shown in Figure 2 is
efficient up to the W K-edge near 70 keV. Other material combinations can be
used for efficient operation at even higher energies, as illustrated in the next
figure.
Figure 3. Theoretical grazing-incidence X-ray reflectance of W-, Pt-, and Co-based DGML coatings.
Introduction Periodic Multilayers Normal-Incidence Multilayer Optics Depth-Graded Multilayers Grazing-Incidence Multilayer Optics Aperiodic and Laterally-Graded
Multilayers Facilities
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