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Annual Report 2021

Division 7
Cover photo: Gold-plated surface structure for the most diffuse infrared reflection possible.
(Elena Kononogova)

The year 2021 was difficult for Division 7 not only because of the pandemic, but also due to departures and staff absences due to illness. Nevertheless, it was very successful due to a strong commitment and good cooperation at all levels. A number of new colleagues were recruited, third-party funding remained stable at a very high level, publications, doctorates and also calibration revenues increased again. Steffen Rudtsch, Head of Department 7.4, was elected as the new Chairman of the EURAMET Technical Committee for Thermometry (TC-T), Mathias Richter, Head of Division, was elected to the boards of the Initiativgemeinschaft Außeruniversitärer Forschungseinrichtungen in Adlershof e.V. (IGAFA) and the Physical Society of Berlin (PGzB) as well as to the council of the German Physical Society (DPG). The department's web presence has been redesigned with a dedicated page for metrology with synchrotron radiation of the departments 7.1 and 7.2 at the electron storage rings Metrology Light Source (MLS) and BESSY II in Berlin-Adlershof.

Figure 1: The participants of the 314th PTB Seminar "VUV and EUV Metrology" on October 19-20, 2021 at the Berlin Institute of PTB.

In the field of EUV metrology with synchrotron radiation of Department 7.1, the workflows as well as the beam operation of PTB's own MLS were adapted to the restrictions caused by the corona pandemic in such a way that services, in particular for industrial partners, could be fully provided. This also became clear at the now 6th seminar on VUV and EUV metrology, which took place in a hybrid format from October 19-20, 2021, in the Hermann-von-Helmholtz Building at the Berlin-Charlottenburg site (Fig. 1). In addition to major advances in EUV lithography (EUVL) for the semiconductor industry, the program again included numerous interesting contributions, for example from the field of optical technologies or on accelerator-based photon sources.

One contribution referred to a current project of the extremely fruitful cooperation of PTB with the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in the field of operation and further development of the MLS, which has lasted for decades. In collaboration with Tsinghua University in Beijing, the effect of Steady State Micro Bunching (SSMB) on the first undulator harmonic at the excitation wavelength of 1064 nm was demonstrated for the first time. The development of SSMB future technology for novel photon sources in the VUV and EUV spectral range will also be highlighted in the Conceptual Design Report (CDR) for the successor facilities of MLS and BESSY II in Berlin-Adlershof, which is currently being prepared by HZB in close coordination with PTB. For the coordination of PTB's participation in this future project (Berlin Photon Factory) as well as of PTB's PhD program, a position could be established in Department 7 and filled by Dr. Cornelia Streeck.

In addition to the further development of accelerator technology, such as a current work on laser wakefields in plasmas, the operation of MLS and BESSY/nbsp;II for metrology with synchrotron radiation is at the center of the PTB-HZB cooperation. Recent scientific successes have been achieved in the entire spectral range from THz to X-rays. In the infrared range, a novel contrast mechanism has been developed at MLS for the measurement of antiferromagnetic order by scanning probe microscopy, in which locally induced temperature gradients are detected via the magneto-Seebeck effect. Also at MLS, the method of VUV photoemission tomography has been successfully applied in cooperation with Forschungszentrum Jülich and Karl-Franzens-Universität Graz for imaging orbital structures and detecting chemical reactions of different molecules on surfaces.

A new setup has been put into operation at the four-crystal monochromator beamline of Department 7.2 at BESSY II, which allows X-ray scattering from nanomaterials in an external magnetic field. This allows the alignment and formation of superstructures or magnetically sensitive microorganisms to be studied under the influence of magnetic fields. Furthermore, in collaboration with HZB, the University of Hamburg and the Humboldt University of Berlin, anomalous small-angle X-ray scattering (ASAXS) and near-edge spectroscopy (XANES) made it possible for the first time to directly observe the physisorption of xenon atoms in a nanoporous silicon memory structure.

Figure 2: Reconstruction of a periodic Si3N4 nanostructure using machine learning and grazing incidence X-ray fluorescence spectroscopy (GIXRF).

In the soft X-ray range, anisotropy in the optical constants of quartz crystals was detected at BESSY II, which is of great importance for the development of new optical components. Also, machine learning was used in cooperation with departments 7.1, 7.2 and 8.4 to reconstruct periodic Si3N4 nanostructures for the semiconductor industry in an element-selective way from a combined use of X-ray scattering and X-ray fluorescence (Fig. 2).

Figure 3: Operando measuring cell for X-ray spectrometric investigation on lithium-sulphur batteries for the cathode side (left) and anode side (right).

In the priority program "Polymer-based Batteries" of the German Research Foundation (DFG), novel battery cells were successfully investigated by electrochemical characterizations and impedance spectroscopy for pre-selection for subsequent operando measurements with X-ray analytical methods within the framework of a master thesis in Department 7.2. The aging mechanisms in lithium-sulphur batteries during cycling as well as the rearrangement of transition metals from the cathode to the anode of lithium-ion batteries have already been the subject of extensive measurements at BESSY II with quantitative X-ray spectroscopy (Fig. 3).

Figure 4: Laser Doppler Velocimetry (LDV) for measuring flow profiles.

Developments for the Energiewende (battery research, photovoltaics) play an increasing role not only in the quantitative investigation of complex materials (Advanced Materials) with synchrotron radiation in Departments 7.1 and 7.2 but also in the other areas of Division 7. With their core competence in thermometry, Charlottenburg's Departments 7.3, 7.4 and 7.5 contribute to research and development work in the cross-cutting topics of energy, environment, and climate. The efficiency of solar thermal systems and of power plants of all kinds is the focus of Department 7.5 through accurate and modern methods of flow and heat measurement, for example via the method of laser Doppler velocimetry (LDV, Fig. 4). At an online workshop held in March 2021 together with EMATEM, representatives of industrial associations, manufacturers of measuring instruments, calibration offices, testing laboratories and research institutes from among the more than 60 participants encouraged PTB to solve the serious problem of non-traceable measurements of this type at higher temperatures (> 90 °C) for certifications and industrial users by means of a new standard.

Figure 5: Combination of dielectric constant gas thermometer and Burnett expansion unit for measuring the thermophysical properties of gases.
Figure 6: Gas modulation refractometry (GAMOR) for fast optical gas pressure measurements.

The real gas properties of pure hydrogen as an energy carrier are determined experimentally with the highest precision in Department 7.4 within the framework of the EMPIR project MetHyInfra. In addition, the H2MIXPROP project was started in cooperation with the Helmut Schmidt University Hamburg, in which the properties of hydrogen mixtures (at PTB hydrogen and methane) are investigated both experimentally and theoretically with unprecedented accuracy. In both projects, an innovative combination of dielectric constant gas thermometry and expansion methods is used (Fig. 5). This method was originally developed and used to investigate the thermophysical properties of neon and argon at the highest level.

The work on argon and the development of the methodology were carried out within the framework of a doctorate, funded by the two EMPIR projects Real-K and Quantum Pascal. The results are used in particular in the field of gas metrology, such as primary gas thermometry, or in capacitive and optical pressure standards for vacuum metrology in Department 7.5. In this context, it was also possible to prove in the Quantum Pascal project by means of simulations and experimental validation that with gas modulation refractometry (GAMOR) the thermodynamic influence of the gas expansion is less than one millikelvin after only a few seconds (Fig. 6). GAMOR systems are thus ready for measurement about a hundred times faster than conventional photonic pressure standards.

Figure 7: Walther Meissner Building - PTB's Centre for Quantum and Cryotechnology in Berlin.

The cooperating working groups 7.43 "Cryo- and Primary Thermometry", 7.55 "Photonic Pressure Measurement" as well as 7.44 "Cryogenic Technology and Themometry" will relocate their work to the Walther Meißner Building (WMB) in the course of the next few months, a new research building on the Berlin-Charlottenburg campus with highly specialised laboratory, measuring and clean rooms on a floor space of 2,325 sqm , which was handed over to PTB at the end of 2021 (Fig. 7). The official inauguration of this centre for quantum and cryotechnology has been scheduled for April 2022. Its use will focus on the development, production and application of superconducting quantum interference devices (SQUIDs) for measuring the smallest magnetic fields and currents at low temperatures, in particular also for noise thermometry. Department 7.6 "Cryosensors" will move completely from the Warburg- Building to the WMB for this purpose, together with its two industrial partners Magnicon GmbH and Entropy GmbH.

Figure 8: Planar fine-pitch coils made of niobium structured at the cleanroom centre in Braunschweig (Department 2.4).
Figure 9: Technology development in the Walther Meissner Building.

Planar fine-pitch coils made of niobium (Fig. 8) and Josephson junctions form the basis for next-generation SQUID sensors. For the determination of the characteristic parameters, i.e. critical current and normal line resistance of Josephson junctions, a standard was developed with the participation of Department 7.6 in Working Group 14 of Technical Committee 90 "Superconductivity" of the International Electrotechnical Commission. It describes a new measurement and evaluation method and thus forms an appropriate and consensual technical basis for manufacturers and users for the characterization of Josephson junctions.

In the field of development and application of Metallic Magnetic Calorimeters (MMCs), Department 7.6 has been cooperating with Department 6.1 for many years. In November 2021, the EMPIR project MetroMMC, coordinated by PTB, was successfully completed. The essential content was measurung energy spectra of radionuclides decaying by electron capture with the partner CEA-LIST. The PrimA-LTD project (Towards new primary activity measurement standardization methods based on low-temperature detectors), which started in June 2021, follows on directly with the aim of combining MMC-based decay spectrometry with direct activity determination.

In the near future, an application laboratory for quantum technology applications in the millikelvin range is to be established at the WMB, combined with the expansion of the existing networks with the TU Berlin, the HU Berlin, the Berlin Charité, the University of Heidelberg, the Royal Holloway University, the NIST, the AIST, the companies Magnicon and Entropy as well as the PTB divisions 2, 4, 6, 7, 8 and 9 for a wide variety of applications (Fig. 9). In the field of quantum radiometry, Departments 7.6 and 7.3, together with the TU Berlin, are currently developing a special high-sensitivity cryogenic substitution radiometer as an electrically calibrated primary standard for very low photon fluxes and for the investigation of single-photon detectors in the visible and near-infrared spectral range.

By calibrating against an existing cryogebic radiometer, the quantum efficiency of special "induced junction" silicon photodiodes was determined in Department 7.3. This improved the modelling of silicon photodiodes within the EMPIR project chipS-CALe (Selfcalibrating photodiodes for the radiometric linkage to fundamental constants). In the same project, the simultaneous, direct measurement of the reflectance, transmittance, and emissivity of the thin semi-transparent silicon wafer layer structures of a Predictable Quantum Efficient Detector (PQED) was also achieved.

Figure 10: THz reflection measuring head for quality assurance of the coating of textiles.
(red: distance measuring system, yellow: optical axis of the THz beam path, blue: mirror for collimation and focussing of the THz beam, green: xy adjustment device for THz transmitter and THz receiver)

Department 7.3 determined material parameters of laminated textiles in the THz spectral range for the now completed R&D cooperation project TeraMeTex within the framework of the Central Innovation Programme for SMEs (ZIM) and developed a novel THz reflection measuring head in the process (Fig. 10). The cooperating German institutes for textile and fibre research were thus able to prove that a fast THz measuring system of the industrial partner can measure the layer thickness, freedom from bubbles and adhesion between textile and coating inline without contact.

In addition to cryogenic substitution radiometers as primary detector standards, Department 7.3 also uses various black body radiators as primary source standards whose radiation can be calculated according to Planck's radiation law. In the infrared spectral range, the scale for spectral responsivity in the MIR range, which is currently being developed, was recently successfully validated by calibrating pyroelectric detectors with both primary methods.

Figure 11: Fibre-coupled integrating sphere as transfer standard with variable radiance for Meteosat Third Generation in clean room cabin at radiance measurement station

The department also develops special reference black body radiation sources and methods for remote-sensing for weather and climate research. For example, a series of emissivity measurements were carried out to determine the geometry and coating for the cavity radiator of the GLORIA stratospheric balloon mission. By means of a portable, dedicated transfer standard of variable radiance based on an integrating sphere (Fig. 11) for eight spectral channels in the wavelength range from 384 nm to 2324 nm, the radiometric traceability of the Flexible Combined Imager (FCI), which is part of the imaging instrumentation of the Meteosat Third Generation (MTG) weather satellite programme, to the international system of units could also be ensured by comparison with the PTB primary standard for spectral radiance.