Johannes Hoffmann

Comparison of electromagnetic field solvers for the 3D analysis of plasmonic nano antennas

Plasmonic nano antennas are highly attractive at optical frequencies due to their strong resonances - even when their size is smaller than the wavelength - and because of their potential of extreme field enhancement. Such antennas may be applied for sensing of biological nano particles as well as for single molecule detection. Because of considerable material losses and strong dispersion of metals at optical frequencies, the numerical analysis of plasmonic antennas is very demanding. An additional difficulty is caused when very narrow gaps between nano particles are utilized for increasing the field enhancement. In this paper we discuss the main difficulties of time domain solvers, namely FDTD and FVTD and we compare various frequency domain solvers, namely the commercial FEM packages JCMsuite, Comsol, HFSS, and Microwave Studio with the semi-analytic MMP code that may be used as a reference due to its fast convergence and high accuracy. The current version of this paper has had a correction made to it at the request of the author. Please see the linked Errata for further details.

VNA Tools II: S-parameter uncertainty calculation

This paper describes a software, METAS VNA Tools II, which is designed to compute uncertainties of coaxial S-parameter measurements. A bottom-up concept is used. Thus basic influence quantities are propagated through the calibration of the vector network analyzer to the S-parameters of a device under test. METAS UncLib is used for the linear propagation of uncertainties. The result is not only an uncertainty region but a list of uncertainty contributions with correlations. Thus the uncertainties can be propagated into eventual post-processing steps. In the present paper the concept has been verified by computing uncertainties of a calibration with a traditional quick short open load thru algorithm and an algorithm which involves optimization. The observed differences between the resulting uncertainties are lower than 0.3 percent, which can be explained by numerical inaccuracies.

A calibration algorithm for nearfield scanning microwave microscopes

This paper presents a new algorithm for the calibration of nearfield scanning microwave microscopes. By adopting techniques known from vector network analyzer calibration, a nearfield scanning microwave microscope can be calibrated at a specific microwave frequency with three standards. The advantages compared to existing calibration methods are that the calibration is valid for all possible samples and that the measurements require less time than other algorithms.

Metas.UncLib—a measurement uncertainty calculator for advanced problems

Metas.UncLib is a software library that facilitates the linear propagation of uncertainties through a measurement model. It is able to handle complex-valued and multivariate quantities and supports higher mathematics. It is therefore able to deal with advanced metrological problems that require, e.g., matrix manipulations. The software is optimized for short computation times and low memory use.

Pin gap investigations for the 1.85mm coaxial connector

This paper presents a detailed investigation of the influence of pin gap size on the S-parameters of the 1.85 mm connector. In contrast to earlier publications connector geometry is simulated with all chamfers, gaps and contact fingers. Simulation results are verified by cross-checking between finite element frequency domain and finite difference time domain methods. Based on reliable simulation results, a very fast tool was developed to calculate S-parameters for a given connector geometry. This was done using database and interpolation techniques. The most important result is that very small pin gaps in conjunction with large chamfers have a drastic impact on connector S-parameters for frequencies above 50 GHz.

Measuring low loss dielectric substrates with scanning probe microscopes

This letter presents an algorithm for measuring the relative permittivity of thick dielectric substrates with scanning probe microscopy. Our technique does not rely on a specific type of microscopy setup and does not require expensive numerical field simulations. To demonstrate the versatility of our method, we perform measurements at high frequencies (18 GHz) with a scanning microwave microscope and at low frequencies (2 kHz) with electrostatic force microscopy. In our experiments, we study dielectric materials with epsilon values ranging from 4 (SiO2) to 300 (SrTiO3). For low epsilon values, the accuracy of the algorithm is better than 2% for tips with less than 80 nm tip radius.

Propagation Constant of a Coaxial Transmission Line With Rough Surfaces

This paper describes the accurate calculation of the propagation constant of a precision coaxial transmission line with rough plated conductors. The results are compared with previous publications and with traceable measurements of 2.4- and 1.85-mm air lines. In contrast to all tested existing procedures the present algorithms results agree simultaneously with the real and imaginary part of the measured propagation constant. In the new algorithm, the effects of roughness and plating are represented by perturbed material parameters of a smooth coaxial line problem. A highly accurate multiple multipole field solver is used for the computation of the perturbed material parameters. The differences between the predicted and the measured propagation constant of 1.85-mm air lines are less than 0.01% at 67 GHz. Such high-precision computations are needed for the specification of offset short standards for vector network analyzer calibration.

S-parameters of slotted and slotless coaxial connectors

Accurate calibrations of vector network analyzers require that coaxial connectors are considered in the definition of calibration standards. In this paper the S-parameters of 2.4 mm and 1.85 mm connectors are computed using 3D field simulations. It is found that male pin diameter variations have lower impact on slotless 2.4 mm connectors than on slotted 1.85 mm connectors. Performing calibrations with and without taking into account the true S-parameters of the connectors results in a typical difference |ΔS11| = 0.01…0.02 for female 2.4 mm devices under test.

Comparison of 1.85mm line reflect line and offset short calibration

This paper shows how the S-parameters in 1.85 mm coaxial line systems can be defined in a traceable way. It is found that a very fail-proof verification method is to compare two different and independent calibration techniques. Here a line reflect line method is compared to an offset short calibration. Measurements of a load, which were corrected with both calibrations, showed an agreement which was better than 0.003 in linear S-parameters up to 67GHz. Key factors for achieving this accuracy are the taking into account of connector effects and the setting of pin gaps to defined values with dielectric rings.

Extended S-parameters for imperfect test ports

Reflection and transmission of microwaves in coaxial devices are usually described by S-parameters. The current definition of S-parameters requires that the reference plane is in a section of ideal wave guide. Due to this, tremendous effort is necessary to facilitate the dissemination of standards, for the comparison of measurement values and for cascading devices. These processes can be simplified by extending the definition of S-parameters to reference planes in sections of non-ideal wave guide, e.g. in connectors. Extended S-parameters can be approximated with conventional simulation programs. Practical experiments show that extended S-parameters can be compared and cascaded without effort.