Latest scientific news and
News from the Annual Report (in German only)

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“.

Single-photon detectors are a pivotal component in photonic quantum technologies. A precise and comprehensive calibration of the inherent detection efficiency is of utmost importance to ensure the proper evaluation of the performance in view of the specific technological application of interest, such as the protection against security breaches in quantum cryptographic solutions. Here we report on a systematic study on and comprehensive analysis of the estimation of the inherent detection efficiency of two commercial single-photon detectors based on single-photon avalanche diodes (SPADs) for various mean photon numbers and at high laser pulse repetition rates using different techniques. We observed an unexpected and significant drop in the inherent detection efficiency at detection rates of 10 % and higher relative to the maximum detection rate. It is demonstrated that for data analysis a statistical model for the detection rate conveniently can be used if no time-stamped data are available. We conclude that the full characterization of single-photon detectors used in critical applications should include the sensitivity of their inherent detection efficiency to high event rates.

Authors:
Sebastian M. F. Raupach; 4.5 Applied Radiometry
Ivo Pietro Degiovanni; Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy;
Istituto Nazionale di Fisica Nucleare, Sezione Torino, Via Giuria 1, 10125 Torino, Italy
Hristina Georgieva; 4.5 Applied Radiometry
Alice Meda; Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy
Helmuth Hofer; 4.5 Applied Radiometry
Marco Gramegna; Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy
Marco Genovese; Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Torino, Italy;
Istituto Nazionale di Fisica Nucleare, Sezione Torino, Via Giuria 1, 10125 Torino, Italy
Stefan Kück; 4 Optics
Marco López; 4.5 Applied Radiometry

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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.

Jan Spichtinger, a PhD student from PTB, won this year’s award for the best young scientist at the 12th High Level Expert Meeting on Asphere Metrology. The “Ultrapräzise Oberflächenbearbeitung” e.V. (Ultraprecise Surface Processing) Competence Center (CC UPOB) held this meeting virtually on 15-16 March 2022. The event was rounded off with the presentation of the Young Scientist Award (YSA) which honors young scientists in the fields of high-precision surface manufacturing and metrology. The meeting’s audience that was made up of experts from these fields selected the PTB doctoral student because of his work on the high-accuracy form measurement of moderately curved optical surfaces. He presented his research under the title of “Stitching interferometry for moderately curved freeform surfaces” and went on to introduce a new measurement method called SCaLA (stitching interferometry with cavity length and angle measurement). This method is an extension of conventional stitching interferometry. It is based on aligning an interferometer with a flat reference surface perpendicularly to a surface at each measurement position and then measuring the distance to it. In addition, tilt sensors are used to correct the angle errors of the motion system, and there is a global stitching algorithm which reconstructs the surface form. Real and virtual measurements were used to demonstrate exactly what this method can provide. In the future, PTB aims to use SCaLA on a form measuring system for moderately curved test specimens with diameters of up to 1.5 meters.

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.

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.