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Indus-2 |
ISUD |
MAASCD |
ACEFD
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Research programs |
Study
of x-ray multilayers.
Optical
studies of different materials in the soft x-ray region
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| Study of x-ray multilayers |
Figure 1: Schematic of x-ray multilayers where incident wave field gets
reflected at successive interfaces. |
Surfaces and interfaces study
X-ray multilayers consist of alternating layers of two different
materials. The two materials of highest
possible refractive index contrast are chosen in order to obtain substantial
reflectivity from such artificially grown structures. The performance of multilayer optics is very sensitive to
structural properties and chemical stability of constituent elements. The role of interfaces is very crucial and if
these are not well defined, the performance of multilayer optics will deviate
from designed value. Schematic of such
multilayer device is illustrated in Figure –1.
The real multilayers do not have sharp interfaces at the boundary of
two materials. Due to variation in
kinetics and other influencing parameters, the profiles at the interfaces are
very different. The variations are occurred
due to interdiffusion, compound formation, roughness replication etc. An illustrative representation of real
multilayer is given in Figure-2. It is
shown that the sharp variation of refractive index at the interfaces may take
some complex shapes, which ultimately lead to destroy the optical
performances. |
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The interfacial roughnesses may have different growth styles. Due to growth correlations between the
successive layers, the roughnesses of different interfaces may be of
correlative type. The correlated and
uncorrelated types of roughnesses are shown in Figure-3. |
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X-ray reflectivity is a
non-destructive method to investigate the buried interfaces. The technique is able to derive all types of
structural information. Nowadays the
multilayer optics has found wide applications
in the third generation synchrotron beamline, where the high thermal load
generated by intense synchrotron beam may generate structural deformation,
therefore the structural properties of x-ray multilayers are need to be
investigated at elevated temperatures apart from their room temperature
characterization. In light of that various
material combinations have been studied in detail, still search is going on to
generate multilayer optics of ultra short period with having sharp interfacial
profiles and more stable structure. |
| Optical behavior in soft x-ray region |
For better prediction of optical
performances of any optical devices, the knowledge of optical constants is
indispensable. In x-ray region the
refractive index is defined as  , where
d is decrement index and
b is absorption index. These parameters are known as optical
constants. In general, the absorption
part
b
is derived from transmission measurements and thereafter the real part
d is
obtained by applying the integral transform method (Kramers-Kroning
method). However, this method requires
the complete knowledge of
b value over full spectral region in order to obtain a
d
value at a particular wavelength. In
this method the interpolation technique is used to generate the values of
b at
the intermittent energy region where absorption measurements are not
available. Since x-ray absorption
spectra consist of various fine features particularly near the absorption
edges, therefore interpolation may lead to smear out those fine features. Such techniques will affect the calculation
of other counterpart. Most of data
available in literature are based on similar method. X-ray reflectivity technique offers a unique opportunity to
derive both parameters simultaneously using the Fresnel reflectivity
formula. There is vast scope to
generate a reliable optical data in the soft x-ray region, particularly for the
materials exhibiting good x-ray optical response. |
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