Adrian Jonas, Thomas Meurer, Birgit Kanngießer, Ioanna
Mantouvalou
Note: Reflection zone plates as highly resolving
broadband optics for soft X-ray laboratory
spectrometers
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Jonas, A., Meurer, T., Kanngießer, B., Mantouvalou, I. (2018). Note: Reflection zone plates as highly resolving
broadband optics for soft X-ray laboratory spectrometers. In Review of Scientific Instruments (Vol. 89, Issue 2,
p. 026108). AIP Publishing.
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© 2018 Author(s).
Note: Reflection zone plates as highly
resolving broadband optics for soft X-ray
laboratory spectrometers
Cite as: Rev. Sci. Instrum. 89, 026108 (2018); https://doi.org/10.1063/1.5018910
Submitted: 11 December 2017 • Accepted: 08 February 2018 • Published Online: 26 February 2018
A. Jonas, T. Meurer, B. Kanngießer, et al.
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REVIEW OF SCIENTIFIC INSTRUMENTS 89, 026108 (2018)
Note: Reflection zone plates as highly resolving broadband optics
for soft X-ray laboratory spectrometers
A. Jonas, T. Meurer, B. Kanngießer, and I. Mantouvalou
TU Berlin, Analytical X-Ray Physics, D-10587 Berlin, Germany
(Received 11 December 2017; accepted 8 February 2018; published online 26 February 2018)
The resolving power and relative efficiency of two off-axis reflection zone plates (RZPs) in the soft
X-ray range between 1 nm and 5 nm were investigated. RZPs focus only a very narrow bandwidth
around the design wavelength. By misaligning the RZP, the focused wavelength can be tuned through
a much wider spectral range. Using a laser-produced plasma source, we demonstrate that a single
RZP can be efficiently used for spectroscopy at arbitrary wavelengths in the investigated soft X-ray
range. Published by AIP Publishing. https://doi.org/10.1063/1.5018910
Soft X-ray spectroscopy in the range between 1 nm and
5 nm (1200 eV–200 eV) is still most commonly performed at
large scale facilities. This is due to the lack of efficient labora-
tory sources for the soft X-ray region. To meet the increasing
demand for spectroscopic techniques such as near edge X-
ray absorption fine structure (NEXAFS) spectroscopy, efforts
are conducted to design efficient experimental setups in the
laboratory.
One possibility is to use highly brilliant laser-produced
plasma (lpp) sources which deliver isotropic broadband radi-
ation.1,2For the diagnostics of such sources but also for the
envisioned spectroscopic techniques, adapted spectrometers
are needed with both high resolving power (RP) and broadband
efficiency. In most published work, the dispersive element used
for spectroscopy is either a transmission grating3,4or a con-
cave [variable line-spaced (VLS)] grating.5,6In recent years,
reflection zone plates (RZPs) have been shown to enable effi-
cient soft X-ray spectroscopy with laboratory sources such as
single shot NEXAFS7due to their high reflectivity and large
numerical aperture. These two-dimensional grating structures
can be used for focusing,8imaging,9and dispersing10 of
X-radiation. Up until now, though, specially designed RZPs
for specific energy regions are utilized7due to a fast degen-
eration of RP caused by chromatic aberration, necessitating
many high-priced optics if several absorption edges are of
interest.
We report that by misaligning the setup geometry, the
usable wavelength range can be varied between 1 nm and
5 nm without notable loss in efficiency or RP. The full wave-
length range can be covered by one single RZP structure in
sequential measurements. To illustrate this behavior, emission
spectra of tungsten and copper obtained with a lpp source
using two different RZP structures are shown with different
setup geometries. The setup utilizes a laboratory lpp source
described in Ref. 11, where a 1 ns, 100 Hz Yb:YAG thin disk
laser (1030 nm) is used to produce soft X-rays in the range
of 1 nm–5 nm with a variable emission spectrum.12 The high
vacuum spectrometer consists of the RZP structures, a CCD
camera (GE 2048 512 X-ray, Greateyes GmbH), and a 200 nm
Al filter which eliminates visible and IR light. The two RZP
structures which were designed for the investigation of the
C and N K edge at 4.4 nm and 3 nm, respectively, with an
object distance F1of 1500 mm and an image distance F2of
2500 mm are written on the same substrate and are thus
exchangeable by a translation of the optic. They can be addi-
tionally rotated in two directions and the CCD camera can be
aligned in 3D.
The RZP structure focusses only on radiations with the
design wavelength in the CCD plane. The image of other
wavelengths is broadened in the vertical as well as the hor-
izontal direction, resulting in a cross shaped pattern on the
CCD; see Fig. 1. This allows long image distances F2with-
out losing detectable solid angle. Because only the outer part
(off-axis) of the actual RZP structure is used, the grating
period d varies only slightly. Therefore, the conventional grat-
ing formula can be used in good approximation. The grating
period in the middle of the RZP structure is 3.054 µm for C
and 2.115 µm for N resulting in a linear dispersion dx/dλof
13 mm/nm and 19 mm/nm, respectively. With a 25.4 mm long
CCD chip, a wavelength range of approximately 1.5 nm can
be detected. For the measurement of the full lpp spectrum
between 1 nm and 5 nm, the CCD must be shifted several cen-
timeters along the dispersion axis. Using the grating formula
and geometric considerations, the RP can be expressed by
λ
∆λ
=
F2λ
Sd sin( β).
Here, S stands for the effective source size and βis the diffrac-
tion angle. With F2= 2500 mm and S = 50 µm (FWHM),
λ/∆λ of about 1100 at the design wavelength λfor both RZP
structures is expected. The RP peaks at the design wavelength
and degenerates linearly in x and y to both sides, which is
equivalent to 1/λfor small dλ. By changing either F2or β,
the focused wavelength in the detector plane can be changed.
This ultimately tunes the wavelength of maximum RP, so other
wavelengths can be measured by misaligning the experimental
setup. Changing F2is not advisable because the misalign-
ment for a notable change of focused wavelength is in the
order of a couple of meters. The diffraction angle β, on the
other hand, only has to be changed a few mrad for a focus
wavelength change of 0.1 nm. By introducing an aperture,
the depth of the field can be increased, resulting in a slower
degeneration of the RP. This can be accomplished with a
knife edge which is placed in the middle of the RZP structure
0034-6748/2018/89(2)/026108/3/$30.00 89, 026108-1 Published by AIP Publishing.
026108-2 Jonas et al. Rev. Sci. Instrum. 89, 026108 (2018)
FIG. 1 . Schematic view of the spectrometer principle. Different wavelengths
are focused at different positions on the optical axis.
perpendicular to the grooves. Assuming a 0.25 mm distance
between the knife edge and the RZP structure, the aperture
is reduced by a factor of 10 with an accompanying tenfold
reduction of intensity. Due to the intrinsic RZP structure, the
number of illuminated grooves is not reduced, therefore pre-
serving the RP. The calculated average efficiency for the used
optics is 17.4% for the C structure and 17.1% for the N struc-
ture. It is dependent of the line density, angle of incidence,
the silicon wafer surface properties, and the grating profile
depth.5
To acquire the spectrum from the CCD image, the back-
ground is subtracted and the counts in each pixel row are
summed up. This simple summarization is well justified due to
the negligible curvature of the equi-energy lines near die hor-
izontal focus. For the determination of the relative efficiency,
the whole plasma emission range from 1 nm to 5 nm was mea-
sured by moving the detector along the direction of dispersion.
The angle of the RZP was optimized to focus 2.5 nm vertically
in the middle of the CCD. Because of the limited size of the
CCD chip size, the CCD had to be shifted three times. The aper-
ture was decreased by a factor of ten to increase the RP out
of focus. Despite the closed aperture, the measuring time with
120 mJ of laser energy was between 0.5 s and 10 s, depending
on the distance to the horizontal focus. The wavelength axis
was calibrated using the NIST database for the atomic copper
lines. Measurements were performed with both RZP structures
(C, N) successively and copper as well as tungsten as the tar-
get material; see Fig. 2. As the setup is not fully calibrated,
only the relative efficiency can be discussed. For comparison,
a spectrum that has been collected with a VLS spectrometer
(description see Ref. 7) is shown.
The y-axis is normalized to the acquisition time and the
NA of the RZP structure. No correction was performed con-
cerning the filter and the CCD as they were used for all mea-
surements. The shape of the spectra is similar for the reference
spectrum and the spectrum collected with the C structure. The
efficiency of the N structure decreases to smaller wavelengths,
which is contrary to a possible efficiency decrease with dis-
tance to the design wavelength. This behavior is presumed to
originate from diffraction effects caused by the profile depth
or the quality of the structure.
To determine the RP, isolated lines in the Cu spectrum
can be used by calculating the FWHM of the respective lines.
The hereby obtained values are the lower limit for the RP
because the natural linewidth is neglected. The achieved RP
FIG. 2. Tungsten (top) and copper (bottom) plasma emission spectrum
obtained with the N (blue) and C (black) RZP structures and a VLS spec-
trometer (grey). The VLS grating has a C containing contamination layer
visible in a C K-edge feature in the spectrum. The RZP structures show O
contamination.
while peaking at the design wavelength and degrading to both
higher and lower wavelengths is for all wavelengths better
than λ/∆λ = 140, comparable to the lower performance of
commercially available VLS gratings.
For the determination of the optimally achievable RP,
several measurements with different RZP angles and open
aperture were conducted. Figure 3presents an example on how
the spectrum changes with different RZP angles and empha-
sizes the small depth of the field with open aperture. The copper
spectrum measured with the N-RZP is shown. The spectrum
consists of Cu relaxation lines from up to 20-times ionized Cu.
The focus wavelength clearly moves between the two measure-
ments. The change is most notably for the Cu XX line doublet
at 1.257 nm and 1.2827 nm belonging to 2p-3s transitions.
The determination of the RP for selected isolated atomic
lines over the whole spectral range is shown with the help of ten
single overlapping measurements (depicted with different line
FIG. 3. Copper spectrum around 1.2 nm measured with the N-RZP at two
different RZP angles. The full aperture was used and the RP for four selected
lines is depicted.
026108-3 Jonas et al. Rev. Sci. Instrum. 89, 026108 (2018)
FIG. 4. RP for selected isolated plasma lines at different setup geometries
measured with the C structure.
FIG. 5. RP for selected isolated plasma lines at different setup geometries
measured with the N structure.
style) in Fig. 4(C structure) and Fig. 5(N structure). For each
measurement, the fast degeneration of the RP is evident. The
highest determined RP for each measurement is marked with a
pentagram. Please note that there are no atomic lines between
1.3 nm and 2.4 nm available in the spectrum. The maximum
measured RP for both RZP structures degenerates from around
1100 at 4.7 nm to 600 at 1.05 nm. These values must be taken
as the lower limit, as for some of the measurements, suitable
isolated lines are missing; see Fig. 1. The N-RZP shows a
higher overall performance. There seems to be no correlation
between RP and RZP structure design energy.
The usability of a single RZP structure as a broadband
dispersive element for soft X-ray spectroscopy was demon-
strated. By misaligning the structure and collecting the spectra
sequentially, the range between 1 nm and 5 nm can be covered
with a high efficiency (>10%) and RP (>600). Compared to
widely used VLS gratings, an improvement of a factor of 2–4
was achieved. As RZP structure sizes are limited to lines/mm
depending on the manufacturing technique, and this is different
for different design wavelengths, the use of one RZP struc-
ture designed for center wavelength of the source emission
might be advantageous for efficient spectroscopy at arbitrary
wavelengths in the investigated soft X-ray range.
The authors thank H. Stiel, H. L¨
ochel, and A. Erko for
fruitful discussions. The work was conducted in the framework
of DFG Project No. 313838950.
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