Aktuelle Forschungsnachrichten und Nachrichten aus dem Jahresbericht

New article in JOSA A:
Structured metasurfaces enable new possibilities for beam shaping and the miniaturisation of optical elements.
In the literature metasurfaces employing subwavelength structures are commonly related to anomalous reflection and refraction and associated with discontinuities of the wave vector at the interface.
In our article we employ classic diffraction theory to describe the far field diffraction pattern of such structured metasurfaces, define clear limitations for the applicability of generalized reflection and refraction laws and resolve some issues with misleading explanations.
The article supports the understanding of light propagation through subwavelength structured metasurfaces and contains important recommendations how to avoid misleading nomenclature.

Auf der 123. Jahrestagung der Deutschen Gesellschaft für angewandte Optik (DGaO) in Pforzheim wurde Jan Krüger, Doktorand an der PTB, mit dem Posterpreis ausgezeichnet. Der Titel des Posters lautete: „Untersuchung des Schwingungseinflusses auf gemessene Punktspreizfunktionen in der optischen Mikroskopie“.

Ein zentraler Baustein für optischen Quantentechnologien sind Einzelphotonendetektoren. Ihre genaue und umfassende Charakterisierung ist äußerst wichtig, um beispielsweise Sicherheitslücken in Systemen zur optischen Quantenschlüsselverteilung zu vermeiden. Bisher werden quantenradiometrische Untersuchungen typischerweise bei sehr geringen Ereignisraten durchgeführt, während für praktische Anwendungen oft möglichst hohe Zählraten wünschenswert sind. Dabei war bisher die implizite Annahme, dass die gemessene inhärente Detektionseffizienz des Detektors auch für diesen Fall unverändert gilt.
In einer systematischen Untersuchung der Detektionseffizienz zweier kommerzieller, halbleiterbasierter Einzelphotonendetektoren konnten wir unter Berücksichtigung der Photonenstatistik nun zeigen, dass bereits bei nur einem Zehntel der maximal möglichen Zählrate ein signifikanter Abfall der Detektionseffizienz zu beobachten ist.  
Insbesondere für sicherheitskritische Anwendungen sollte die Charakterisierung von Einzelphotonendetektoren daher zukünftig auch den Fall hoher Ereignisraten umfassen.

Solid immersion lenses (SILs) have the potential to enhance the single-photon emission properties of color centers in diamond, e.g., of germanium-vacancy (GeV-) centers, due to an increase of the photon extraction from the diamond. We present the successful generation of GeV-centers in bulk diamond, the fabrication of hemispherical SILs with GeV-centers using a focused ion beam (FIB) and the metrological characterization of the emission of the GeV-centers in SILs in terms of the spectral distribution, the single-photon purity, the saturation behavior, and the emitter’s excited state lifetime. We could verify a significant increase of the detector count rate for GeV-centers in SILs compared to those without a SIL. The singlephoton purity and detector count rate were measured for different excitation powers and were evaluated in a combined model. This allows the possibility to distinguish between the linear background emission and multiple GeV-centers in a SIL.

Dr. Richard Lange hat den diesjährigen SAMOP-Dissertationspreis der Deutschen Physikalischen Gesellschaft (DPG) für seine Doktorarbeit mit dem Titel „High-Precision Frequency Comparisons and Searches for New Physics with Yb+ Optical Clocks“ erhalten. Er setzte sich auf der DPG Frühjahrstagung 2022 in Erlangen gegen drei weitere Finalisten durch.

Im Rahmen des 12. High Level Expert Meeting Asphere Metrology des Kompetenzzentrums "Ultrapräzise Oberflächenbearbeitung" e.V. am 15./ 16. März 2022 hat der PTB-Doktorand Jan Spichtinger den Preis für den besten Nachwuchswissenschaftler gewonnen. Mit dem sog. Young Scientist Award (YSA) werden Nachwuchswissenschaftler in den Bereichen der hochgenauen Oberflächenfertigung und -metrologie ausgezeichnet. Das Fachpublikum wählte den PTB-Doktoranden für seine Arbeiten im Bereich der hochgenauen Formmessung an leicht gekrümmten optischen Oberflächen aus. Hr. Spichtinger präsentierte seine Forschungsarbeiten unter dem Titel „Stitching interferometry for moderately curved freeform surfaces“. Dabei wurde ein neues Messverfahren namens SCaLA (Stitching interferometry with cavity length and angle measurement) vorgestellt. Das Verfahren ist eine Erweiterung der herkömmlichen Stitching-Interferometrie. Es basiert darauf, dass ein Interferometer mit ebener Referenzfläche an jeder Messposition senkrecht zur Oberfläche ausgerichtet und der Abstand dazu gemessen wird. Zusätzlich werden mit Neigungssensoren die Führungsfehler des Bewegungssystems korrigiert. Mit Hilfe eines globalen Stitching-Algorithmus wird die Oberflächenform rekonstruiert. An realen und virtuellen Messungen wurden die Möglichkeiten dieser Methode aufgezeigt. Ziel der PTB ist, SCaLA zukünftig an einem PTB-Formmesssystem für schwachgekrümmte Prüflinge mit Durchmesser bis 1,5 Meter einzusetzen.

The tilted-wave interferometer (TWI) is a recent and promising technique for optically measuring aspheres and freeform surfaces and combines an elaborate experimental setup with sophisticated data analysis algorithms. There are, however, many parameters that influence its performance, and greater knowledge about the behavior of the TWI is needed before it can be established as a measurement standard. Virtual experiments are an appropriate tool for this purpose, and in this paper we present a digital twin of the TWI that was carefully designed for such experiments. The expensive numerical calculations involved combined with the existence of multiple influencing parameters limit the number of virtual experiments that are feasible, which poses a challenge to researchers. Experimental design is a statistical technique that allows virtual experiments to be planned such as to maximize information gain. We applied experimental design to virtual TWI experiments with the goal of identifying the main sources of uncertainty. The results from this work are presented here.

Single-photon sources have a variety of applications. One of these is quantum radiometry, which is reported on in this paper in the form of an overview, specifically of the current state of the art in the application of deterministic single photon sources to the calibration of single photon detectors. To optimize single-photon sources for this purpose, extensive research is currently carried out at the European National Metrology Institutes (NMIs), in collaboration with partners from universities. Single-photon sources of different types are currently under investigation, including sources based on defect centres in (nano-)diamonds, on molecules and on semiconductor quantum dots. We will present, summarise, and compare the current results obtained at European NMIs for single-photon sources in terms of photon flux, single-photon purity, and spectral power distribution as well as the results of single-photon detector calibrations carried out with this type of light sources.

Aspheres and freeform surfaces play an important role in today's optics industry. However, the measurement of such complex surfaces is still challenging even with state-of-the-art manufacturing technology, and there is an urgent need in industry for a non-contact, highly accurate reference measurement technique. To meet this demand, at PTB, a metrological reference system for the contact-free form measurement of aspheres and freeform surfaces is under development. The measurement system is based on a tilted-wave interferometer. Advances in computational capabilities have made it possible to solve the complex inverse problems associated with this measurement system and to develop sophisticated analysis procedures for reconstructing the surface under test from the measured interferogram data. In this paper, we will present the status of the tilted-wave interferometer-based measurement system at PTB, describe the analysis procedures we have designed and show initial measurement results. The benefit of the implementation presented here is that it allows insight to be gained into the performance of the measurement system and enables traceable measurements to be established with low uncertainty.

Comparing form measurement data for aspheres and freeform surfaces is an important tool for ensuring the quality and functionality of the devices used to take such measurements and may also allow the underlying measurement methods to be evaluated. However, comparing the highly accurate form measurements of such complex surfaces is a demanding task. It is difficult to analyze measurement results whose accuracies are in the range of several tens of nanometers root-mean-square, especially when comparing data with different, and anisotropic distributions of the 3D measurement points on the surface under test. In this paper, we investigate eight different 3D measurement point distributions that are typical of highly accurate measurement systems currently in use and demonstrate the effects of these distributions on the comparison results by using virtually generated data and applying different evaluation strategies. The results show that, for the examples investigated, the different 3D measurement point distributions can yield different levels of accuracy for the comparison. Furthermore, an improved evaluation procedure is proposed and recommendations on how to significantly reduce the influence of the different 3D measurement point distributions on the comparison result are given. A method of employing virtually generated test data is presented that may be generalized in order to further improve and validate future comparison methods.