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Reflection zone plates

The company commercially offers custom-designed reflection zone plates (RZP) and 2-dimensional variable line space (VLS) gratings based on total external reflection on lamellar diffraction structures.

The standard substrate size is 100 mm (L) x 30 mm (W) x 10 mm, (H) super-polished down to 0.2 nm, with slope errors below 0.1 arc sec rms.

Other sizes are possible upon request.

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Technology:

  • Energy range 30 eV – 2000 eV

  • Absolute efficiency up to 20% (30 eV – 600 eV) and up to 10% (600 eV – 2000 eV)

  • Experimentally confirmed energy resolving power up to 2000.

  • Minimal focal distance 70 mm

  • Laminar profile line density up to 5000 l/mm, profile depth (5 nm – 150 nm) ± 0.5 nm

  • 2D VLS gratings with 1000% or more period variation

  • Planar and curved substrates possible upon request.

 

Applications:

  • Aberration corrected gratings for Hettrick -Underwood spectrometers

  • Single RZP multi-channel spectrometer optics

  • Special optics for fs spectroscopy

  • Time-delay compensating monochromators

Technology

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A Reflection Zone Plate (RZP), a two-dimensional laminar grating of variable line spacing located at a distance R1 from a source, is capable of imaging the source by diffraction onto a certain distance R2 along the optical axis, acting as both a dispersive and focusing optical element.

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Figure 1. Two kinds of reflection zone plates on spherical substrates: a) elliptical zone plates with point focus, b) astigmatic zone plates focused in line segment.

The divergent beam emitted by the source is dispersed and focused on a 2-dimensional pixel detector by the reflection zone plate (RZP) (a). The RZP has a special lamellar groove profile which provides better high order suppression of contaminating materials such as carbon and oxygen. The optical geometry provides compensation of aberrations and maximizes the energy range of high-resolution flat field spectral registration. In the case of astigmatic RZP the diffraction focusing provides highly defined linear focus on a detector (b). [1]

RZP technology.

The fabrication of 2D RZPs for the hard X-ray regime is a technological challenge. The minimal zone width of our X-ray RZPs amounts to ~ 50 nm. Hence, modern nano-structuring procedures such as direct electron beam lithography are necessary for their realization. Software for the calculation of the zone geometry with high resolution was developed, in order to generate data sets for the electron beam and laser scan lithography. [2]

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Figure 2. Flux diagram of the fabrication process of an RZP by electron beam lithography (left), including RIE (middle) and reflecting layer coating (right).

 

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The efficiency of the RZPs is maximized by a proper depth of profile. A special technology called “vector-scan ion etching technique” is used for fabrication [3]. Several RZPs can be fabricated on the same substrate with individual depth of profile with a depth variation from 2 nm to 100 nm in two dimensions. In addition, each RZP can be coated individually with carbon, nickel or gold layer providing extraordinary reflection quality of the grating structure.

Figure 3a  shows the absolute efficiency of 3 RZPs fabricated on super polished silicon substrate with radius of 3.8 m, coated with 40 nm carbon on RZP1 & 2 and 40 nm nickel on RZP 3. Figure 3b shows the absolute efficiency of 3 RZPs on a substrate with a radius of 55 m coated with gold, t is the lamellar profile depth. The measurements were done at the BESSY II Optical Beamline in Berlin. The dips near 277 eV and 525 eV refer to the inevitable contamination of the Ni coating with a natural layer composed of carbon (C) and oxygen (O).

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Figure 3a. Absolute efficiency of 3 RZPs fabricated on one substrate for spectroscopy of ultra-light elements in the energy range of 30-450 eV.

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Figure 3b. Absolute efficiency of 3 RZPs fabricated on one substrate for spectroscopy with a Laser Produced Plasma source in the energy range of 150-1400 eV.

Applications

New types of dispersive optical elements, namely reflection zone plates on spherical substrates as an innovative class of 2D VLS gratings with optimized performance, are implemented in the optical design for extreme ultraviolet (XUV) and soft X-ray instrumentation.

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Highly Efficient Soft X-ray Spectrometer for Transient Absorption Spectroscopy with Broadband Table-Top High Harmonic Sources

 

To maximize the transmitted flux most XAS experiments with table-top HHG sources rely on a compact flat-field grazing incidence spectrometer equipped with reflection zone plates (RZP). The instrument is based on the Hettrick-Underwood optical scheme [4] and consists of a grazing incidence spherical mirror for focusing and two high transmission efficiency RZPs with an average line density of 700 l/mm and 1250 l/mm for dispersion. A resolving power of 890 at the central photon energy of 410 eV and a RZP transmission efficiency of 12% were demonstrated. The substantially improved performance in transmission efficiency and resolution is illustrated with solution phase measurements at the calcium L2, L3- and nitrogen K-edges, cf. Figures 4 and 5.

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Figure 4. Schematic of the experimental setup. a) Top and side views of the soft X-ray experimental setup, including the HHG source, the sample and the soft X-ray spectrometer. b) Optical design of the RZP-based soft X-ray spectrometer. In this configuration the CCD detector covers the spectral range from 250 eV to 600 eV. c) Two reflection zone plates on a plane substrate.

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Figure 5. a) Soft x-ray absorption spectrum of a 750mM aqueous solution of calcium nitrate (black line), measured with an HHG source using the newly developed RZP spectrometer. The correspond-ding soft x-ray absorption spectra at the Ca L-edge and N K edge obtained after subtracting the water contribution are displayed in panels b) and c), respectively. For comparison, the N K-edge absorption spectra of sodium nitrate (NaNO3) in solution recorded at the BESSY II synchrotron facility (red line) and in our previous experiments with a VLS spectrometer (blue line) are also shown.

Flat Field Soft X-ray Spectrometry with Reflection Zone Plates on a Curved Substrate

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The spectrometer was built for the energy range of 150-750 eV using reflection zone plates (RZPs) on a spherical substrate with a radius 29 m [5]. With a micro-fluorescence source high-resolution flat field spectra within ±50% around the design energies were measured at an interval of 150 eV–750 eV, using only two RZPs: the first RZP, with its design energy of 277 eV, covered the band of 150–370 eV, and the second RZP, with a design energy of 459 eV, covered the band of 350–750 eV, where the upper boundary of this energy range was defined by the Ni coating of the RZPs. The absolute quantum efficiency of the spectrometer, including the optical element and the detector, was, on average, above 10%, and reached 20% at the design energies of the RZPs. The resolving power E/∆E exceeded 600 for energies E inside the core range of 200–550 eV, cf. Figures 6 and 7.

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Figure 6. Optical layout of the spectrometer. The spherical substrate (red rectangle) carried two RZPs in a parallel layout, and one of them was used to select the desired energy range (left). Reflection zone plate (right)

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Figure 7a. Measured sample spectrum of stainless steel.

Figure 7b. Measured Lη, Ll, and N Kα spectra of Ti and TiN.

Ultrafast NEXAFS spectroscopy in the lab using laser-based sources and advanced X-ray optics

 

Laboratory based laser driven short pulse X-ray sources like laser produced plasmas (LPP) and high harmonic generation (HHG) exhibit a great potential for spectroscopy in the soft X-ray range. These sources are complementary to large scale facilities like synchrotrons or free electron lasers. For applications of LPP or HHG sources for time-resolved X-ray absorption spectroscopy in the water window or beyond a high photon flux is crucial. The available photon flux strongly depends on energy, pulse duration and repetition rate of the pump laser. Depending on the experimental needs in time resolved experiments pulse durations of the X-ray pulse ranging from nanoseconds to sub-femtoseconds are required. Our RZPs were applied at a highly brilliant LPP source emitting soft X-rays in the photon energy range between 50 and 1500 eV based on CPA and thin disk laser technology as well as the high average power thin disk laser based OPCPA system for high photon flux HHG. The application of reflection zone plates on spherical substrates is leading to the design of a new generation of instrumentation that promises a remarkable high resolution over a wide spectral range providing an ideal and highly efficient diffractive instrument for time-resolved NEXAFS experiments in the lab.

The NOB GmbH technology provides up to four different RZPs on the same substrate, each with the same resolving power, together covering the energy range (150 – 700) eV with a HHG source and (210 – 1400) eV with a LPP source [6]. The spectrometers provide a single-shot online monitoring of the initial spectra of the source, an energy resolving power E/∆E above 1000 and a peak efficiency up to 25%. As a detector typically an X-ray CCD or a sCMOS camera are used.

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Figure 8. Optical layout of the spectrometer. The spherical substrate (red rectangle) carries four RZPs in a parallel layout, the bottom channel is used as a reference (left). Reflection zone plate with four RZPs (right).

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Figure 9. Calculated RZP efficiencies, assuming Au coating with a 2 nm thick CO contamination layer on the top of Au.

Wavelength-dispersive Spectrometers for Soft X-rays

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Even though ultralight elements (Z = 3, 4, 5) are quite rare and only found in compounds, they are used in a number of important applications. With the advent of the semiconductor industry, Li and B were used as dopants. One of the most promising energy storage technologies are Li- polymer and Li-ion batteries. All these light elements are also used for scientific purposes, such as Be X-ray windows or as materials with very high hardness and melting point, like B4C, B-N-C or c-BN.

A scanning electron microscope (SEM) is a very versatile tool that allows to obtain different information on the sample. In X-ray emission spectroscopy, the sample is excited by an electron beam and the atoms relax by emitting X-ray photons with characteristic energies that depend on the type of the atom, its chemical environment and the transition that the electrons in the atom underwent during the process. This method requires a suitable instrument to detect and analyze the emitted photons.

A novel parallel wavelength-dispersive X-ray spectrometer WDSX-300 is developed by NOB Nano Optics Berlin GmbH. It is based on a Fresnel reflection zone plate (RZP), which acts as a dispersive, reflective and focusing element at the same time.

The WDSX-300 has an efficiency which is one order of magnitude larger than that of existing grating spectrometers. In addition, our instrument features a high energy resolving power and angular acceptance. The optical system is based on a single optical element, a Fresnel reflection zone plate on a spherical substrate. The main reference concerning a possible optical layout solution is the RZP spectrometer as published in [7]. The WDSX-300 comprises an RZP covering the energy range 35 – 850 eV. The spectrometer is designed for the analysis of fine structures of the energy states of the binding electrons.

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Figure 10. Optical layout of the WDSX-300 (left) and RZP employed in the spectrometer (right).

The energy resolution was measured at the L- edge of pure aluminum metal, which is usually used to determine the resolution of spectrometers. The results of measurements are shown in figure 11.

The spectrometer can be customized to other energy ranges by changing the optical holder with other reflection zone plates.

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Figure 11. Al L spectrum, showing the Fermi edge (left). Metallic Li spectrum (right).

The spectral width of DE = 0.3 eV was measured between the intensity levels of 25% and 75% of the maximum at the Al Fermi edge.

With the proper adapter flange, WDSX-300 is compatible (has been tested) with ZEISS DSM 942, Zeiss EVO 40, JEOL 6400, JEOL 733 and CAMECA SX 100. For other instruments an appropriate adapter flange can be designed.

References

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[1] A. Erko, et al, Novel parallel vacuum ultra-violet/X-ray fluorescence spectrometer, Spectrochimica Acta Part 2012, B 67, 57–63

[2] A. Erko, et al., “Spectrometers and monochromators for femtosecond soft x-ray sources,” Proc. SPIE 11108, 111080J (2019).

[3] A. Borowski, Vector-Scan Ion Etching Technique for Production of 3-Dimensional VLS Grating Structures. In Proceedings of10th Fachtagung Prozessnahe Röntgenanalytik, Berlin, Germany, 28–29 November 2019.

[4] C. Kleine, et al., Highly Efficient Soft X-ray Spectrometer for Transient Absorption Spectroscopy with Broadband Table-Top High Harmonic Sources, Struct. Dyn. (2021), 8034302,

[5] J. Probst, et al., Flat Field Soft X-ray Spectrometry with Reflection Zone Plates on a Curved Substrate, Appl. Sci. (2020), 10, 7210

[6] H. Stiel, et all., "Ultrafast NEXAFS spectroscopy in the lab using laser-based sources and advanced X-ray optics, Proc. SPIE 11886, , 1188612 (2021);

[7] A. Hafner, et al., Reflection zone plate wavelength-dispersive spectrometer for ultra-light elements measurements, Optics Express, 23, (23), (2015), 29476

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