The next generation of Cosmic Microwave Background (CMB) missions require sensitive detectors to probe the processes that shaped the early Universe. The 50-90GHz frequency range is of particular interest, both for the measurement of the weak B-mode polarization patterns from primordial gravitational waves and for detection of spectral distortions in the CMB.
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.

The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.

The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.

Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.

We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.

Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.
... Read more

Two-dimensional materials have attracted scientific interest due to their exceptional chemical, optical, electronic, and mechanical properties. In particular, their Young’s modulus plays a crucial role in applications such as sensors, flexible electronics, and composite reinforcement. However, material defects, which are inevitable during fabrication and device operation, can significantly impact mechanical properties. The influence of defects on the Young’s modulus remains a topic of debate, with contradictory experimental and theoretical findings presented in literature. The most widely used method for measuring Young’s modulus is atomic force microscopy (AFM) nanoindentation. However, it suffers from tip-sample interactions, high stress concentrations, and significant variability in reported values. To address these limitations, this thesis presents a novel, all-optical method for measuring the Young’s modulus of monolayer membranes, eliminating physical contact with the sample and requiring no preliminary material assumptions. The proposed method provides Young’s modulus values by measuring nonlinear membrane dynamics together with higher harmonics and Brownian motion. Although the results exceed values reported in literature, it is noteworthy that this method yielded results for the modulus, despite inherent challenges associated with the measurement of 2D material monolayers. Various potential error sources have been identified and discussed, including measurement sensitivity, mode shape variations, and contamination effects. Recommendations for future research are provided to refine the approach and improve accuracy. With the right experimental advancements, this contactless optical technique could serve as a viable alternative to AFM nanoindentation, offering a non-invasive way to study the mechanical properties of 2D materials with greater precision and reproducibility. ... Read more

In this research a study of the response of a simply supported microbeam subject to an electric actuation is presented. A perturbation method called the method of multiple scales is explained and used to solve our problem. A model concerning the mid-plane stretching and an electric force with a direct and alternating current component is formulated. The method of multiple scales is used to construct a solution that is valid for a long time after the initial conditions. The effect of the frequency of the alternating current was studied by performing a stability analysis. The results show that for frequencies close to the eigenfrequency of the homogeneous problem, there is no stable equilibrium and resonance occurs. Furthermore, a start was made to study the effect of the damping coefficient. The results show that a smaller damping will always lead to resonance on a very small time scale. The results also indicate larger oscillations and an small increase of the importance of the non-linear terms for smaller damping. All results are validated by comparing them with a numeric solution and show excellent agreement. ... Read more

A technique that can be used in the study of the magnetic properties of ferritin is electron paramagnetic resonance (EPR), which is sensitive to the magnetic moments of unpaired electrons.

The magnetic properties of human liver ferritin were studied using 9 GHz EPR, with which temperature dependent spectra were acquired in the following range: 5-150 K. A novel approach, typically used in magnetic resonance spectroscopy, was employed to pre-process the baseline-corrected spectra and remove features for which it was unlikely to originate from the ferritin core. This allowed for the isolation and preservation of the general lineshape of the signal belonging to the ferritin core.
The spectra were further analysed using both a phenomenological approach and by employing the Spin Hamiltonian.
The phenomenological analysis showed that in the 20-70 K temperature range, the amplitude peak-to-peak of the ferritin-core signal decreases linearly with decreasing temperature, while its lineshape changes from Lorentzian to Gaussian between 150 and 70 K. The blocking temperature was suggested to occur between 10 and 20 K, where the signal amplitude was lost.
Whether and how the lineshape of the ferritin core signal shifts or broadens below 70 K could not be determined due to a six-line signal contamination, most likely caused by manganese impurities, obscuring the ferritin-core signal in the field range where its resonance field is positioned.
In order to study the magnetic properties of the ferritin core, and therefore gain insight on the magnetic and electronic structure of the ferritin core, a Spin Hamiltonian approach was employed. The simplified Giant Spin Hamiltonian model featured a single spin system with total spin S = 10. This analysis suggests that the ferritin-core signal is centred at g’=2.0154, hinting at a core composition of magnetite or maghemite, rather than of ferrihydrite.

An almost fully automated procedure to pre-process a set of human liver ferritin EPR spectra obtained at different temperatures is described in this thesis.
... Read more

Gas permeation through graphene membranes has received considerable attention for water purification and molecular sieving applications. However, characterization of the permeation has been limited to long timescales of minutes. This thesis shows a method for measuring gas permeation through porous graphene membranes at the microsecond timescale. Suspended porous graphene membranes, with an average pore size of 14 nm and a single 400 nm pore, are brought into sinusoidal motion by optothermal actuation. By monitoring the frequency dependent phase delay between actuation signal and mechanical motion, the gas dependent permeation time of the porous membrane is determined. The permeation time constant is demonstrated to be proportional to the square root of the molecular mass, indicating an effusion dominated permeation mechanism. The determination of permeation at timescales below 1 ms using a femtoliter gas cavity opens up opportunities for novel nanoscale porous graphene based gas sensors, with very fast response times. ... Read more