In 1911, Heike Kamerlingh Onnes and his research team measured a steep drop to zero in the resistance of mercury when cooling it below4.2 K: superconductivity was discovered. Forty-six years later, the microscopic theory to describe superconductivity was formulated. It predicts that disorder (the increase of electron scattering due to impurities, defects or other causes), to first order, does not influence the superconducting state. From an application perspective, disordered superconductors are interesting as they have a high kinetic inductance: they strongly - and non-linearly - oppose the change in current due to the inertia of the superconducting charge carriers, which are paired electrons, or Cooper pairs. Therefore, disordered superconductors can be used in superconducting circuits as highly inductive, nonlinear elements to realize, for example, quantum bits, quantum-limited amplifiers and single-photon detectors. In addition, the high normal state resistance of disordered superconductors allows an efficient, broadband absorption of light, which enables single photon counting, energy resolving microwave kinetic inductance detectors (MKIDs) for visible and near-infrared light.
These applications are, however, hindered by an unexplained microwave loss and, for MKID applications, an enhanced decay rate of the elementary excitations in the superconductor. Both these effects become stronger with increasing disorder. The elementary excitations are called quasiparticles and are essentially broken Cooper pairs. Their density decays as they recombine pair-wise into Cooper pairs. Since quasiparticles also induce microwave loss, these observations point towards an effect of disorder on the quasiparticle dynamics. It is however unclear how disorder affects the quasiparticle dynamics exactly.....

The search for extraterrestrial life is currently one of the main topics of astronomical research. One of the methods for this is to look for the spectral lines of molecules needed for life, called biosignatures, in the atmosphere. The Large Interferometer for Exoplanets (LIFE) mission will explore the mid-IR in the search for biosignatures. This mission is a spectrometer that uses nulling interferometry to cancel the signal from the nearby star. A spectrometer then splits the remaining planet signal to find biosignatures. The wavelength band of LIFE is 4-18.5 µm. LIFE places a strict requirement on the dark current: ID ≤ 10−2 counts/sec. Kinetic Inductance Detectors (KIDs) are a type of superconducting detectors that are single-photon counting, with inherent energy-resolving power, for wavelengths down to 25 µm. KIDs do not have a dark current and, therefore, meet the detector requirements for LIFE. This absence of dark current makes KIDs ideal for other low-light level applications in the mid-IR.

In this work, we present the design of lens-absorber coupled single-photon counting KIDs for two wavelengths within the band of LIFE: 10 µm and 18.5 µm. The absorber consists of two meandering 23 nm thick Al lines (ρ = 2.4 × 10−8 Ω m) with widths of 250 nm and 200 nm for 18.5 µm and 10 µm, respectively. The absorber is simulated using the Floquet theorem in CST. The absorber is divided into unit cells simulated with periodic boundary conditions. These unit cells must be smaller than 0.8λd to avoid crosstalk between absorbers. The simulation is performed for all angles from broadside incidence up to the subtended angle of the lens. These simulations show absorption efficiencies for the TE and TM fundamental Floquet modes of up to 81% and 65% for the 18.5 µm design and the 10 µm design, respectively. The absorber acts dominantly as an inductor and is coupled to a coplanar waveguide (CPW) to tune the resonance frequency of the KIDs.

We measured one of the designed chips: a chip with 18.5 µm lens-absorber coupled KIDs on a substrate. We performed power-integrating measurements to estimate the optical coupling of these devices. For these KIDs, we expect a total optical coupling of 1.77. This coupling is calculated with respect to a single mode and 1 polarization. Our absorber can couple to 2 polarizations and multiple modes, resulting in a coupling of larger than 1. The measurements show a reasonable match between the expected and measured optical coupling.

We also performed single-photon counting experiments. The KIDs showed a low resolving power, making distinguishing the source’s photons from noise and cosmic rays impossible. Phonon losses due to the substrate were assumed to cause this. Therefore, another chip was measured with 25 µm lens-absorber coupled KIDs on a membrane. The two chips share identical design and fabrication processes, differing only in foundation: one is built on a membrane, the other on a substrate. The KIDs on the membrane showed a relatively high resolving power of approximately 4. This confirmed the hypothesis that phonon losses were the cause of the low resolving power for the KIDs on a substrate.

Future work should focus on redesigning the absorbers and KIDs for use on a membrane and adding a quarter-wavelength backing reflector and matching layer to increase the optical efficiencies of the devices.

Single-photon counting MKIDs (microwave kinetic inductance detectors) are light detectors that use
the change in the electric properties of a superconductor due to electron excitations to detect individual
photons. Novel MKIDs are limited by an uncertainty in the down-conversion process from a photon to
a number of excited electrons, called the Fano factor. These Fano-limited MKIDs could be improved if
a method is found to lower the Fano factor, further increasing the resolving power of MKIDs, and pos-
sibly other detectors that rely on pair-breaking. In this report, a Monte Carlo simulation is constructed
to simulate a large number of individual photons incident on a superconductor. The data from the sim-
ulation is then used to calculate the Fano factor. Simulations are performed to test the dependence of
the Fano factor on both the choice of superconducting material (Sn, Nb and Al), and the energy of the
photon incident on the superconductor. F = 0.208 ± 0.003 is found independent of material and photon

energy. The dependency of the Fano factor on the phonon spectrum of the superconductor is tested
and the possible implementation of phonon spectrum tuning in order to improve the Fano factor are
discussed. We find that F can be reduced to F = 0.151 ± 0.003 by implementing a phonon band gap

from 0-2∆. The resolving power of an MKID then improves with up to 24%. To realise this band gap,
holes could be introduced in the superconductor, creating a pattern with a period of approximately 80
nm. The smallest feature size in this pattern would be approximately 40 nm, which is achievable with
current technology. It is unlikely that a perfect 0-2∆ band gap can be created and the real improvement
of F by implementation of a phonon band gap would probably be lower than 24%. We conclude that
improvement of the resolving power of an MKID via the Fano factor is possible in theory, but further
research is required to test the viability of such a band gap and calculate the specific shape and size
of these holes.

Kinetic inductance detectors (KIDs) are superconducting resonators whose resonance condition strongly depends on the properties of a thin superconducting film. Below the critical temperature of the superconducting film, most of the electrons have paired up into Cooper pairs, which give rise to a kinetic inductance. The remaining excitations are a mix of electrons and holes, which can be described as quasiparticles and lead to microwave loss. The resonator’s resonance frequency then depends on the Cooper pair density, while the resonator’s internal loss depends on the quasiparticle density. When the resonator is exposed to a photon flux of sufficient energy to break Cooper pairs, either by direct absorption or through an antenna, excess quasiparticles are created. Due to the change in Cooper pairs and quasiparticle densities, the resonator shifts to a lower resonance frequency while the internal losses increase. We can measure this change using a homodyne microwave readout scheme. This thesis describes my work of the past four to five years on hybrid lumped element kinetic inductance detectors based on high resistivity disordered superconductors. The thesis can be divided into four parts: A theoretical and experimental background, the energy resolution of hybrid lumped element KIDs, improving the quantum efficiency of KIDs based on high resistivity superconductors with antireflection coatings and optical stacks, and reducing the pixel pitch of KIDs with parallel plate capacitors...

Microwave Kinetic Inductance Detectors (MKIDs) are remarkable photon detectors, that have single photon detection and energy resolving capabilities in the near-infra red and higher frequency range. At lower frequencies, MKIDs are excellent radiation detectors as well, because of their high sensitivity and natural multiplexing capabilities, which enable large scale detector arrays.In the (near) optical regime, MKIDs can use their single photon energy resolving capabilities for direct exoplanet detection, in missions like Hab-Ex or LUVOIR. This would enable atmospheric characterization, potentially finding habitual exoplanets. However, two aspects of the MKID still need improvement for this application: the photon absorption efficiency must go from 30% to 50% and the resolving power must go from 8 to 100, both for wavelengths of 1 µm. In this thesis, we study the single photon response and generation-recombination (GR) noise in optical NbTiN-Al hybrid MKIDs, to find knowledge gaps and opportunities to improve detector performance. For the single photon response, we set up a model starting from the Rothwarf-Taylor and Mattis-Bardeen equations, including the pair-breaking efficiency as only fit parameter. Both at high (220 mK and 250 mK) and low (120 mK) temperatures, the model predicted the single photon response of 4 different wavelengths correctly, with a fitted pair-breaking efficiency close to expected values (30%-70%), but somewhat high, which might be due to a non-thermal distribution of quasiparticles caused by read power. In particular, the effect of phonon trapping was captured by the model, which was verified by considering MKIDs on substrate and membrane.At lower temperatures (120 mK), a second exponential decay in both the amplitude and phase single photon pulse tail was observed, which is not explained by the model. This second decay was faster for higher read powers. For the GR noise, also an unexpected feature was observed: the GR noise level dropped exponentially, when lowering temperature (<250 mK), for the amplitude, phase and cross power spectral densities (PSDs). High read powers mask this effect, due to the creation of excess quasiparticles. The noise drop was present in all analysed MKIDs, except for an MKID with an 150 nm (instead of 50 nm) thick Al film, which might be due to read power induced excess quasiparticles. When assuming the GR noise to be Poissonian, we show that the behaviour could be explained by a process which limits the quasiparticle lifetime, while keeping the quasiparticle density thermal.We hypothesize the cause of both of these unpredicted measurements to be quasiparticle trapping, which is the localisation of quasiparticles. This process is known to degrade superconducting tunnel junction detectors and limit quasiparticle lifetimes in MKIDs. A secondary ion-mass spectroscopy (SIMS) analysis showed Fe contamination on the substrate-Al film interface, which is thought to be the quasiparticle trapping cause in our systems, consistent with the observation that increasing the film thickness diminishes the GR noise drop. To our knowledge, quasiparticle trapping has not be studied in steady state experiments, such as GR noise measurements.Different models including quasiparticle trapping have been set up and can predict the amplitude PSDs, when assuming that trapped quasiparticles cannot dissipate microwave power. However, comparison of the fitted model parameters with the trapping and detrapping rates calculated by Kozorezov et al., showed a orders of magnitude difference. Combining this with the fact that amplitude, phase and cross PSDs show similar behaviour, implying that they relate to Cooper-pair fluctuations, led us to conclude that the trapping process cannot be the cause of the observed behaviour. On-trap recombination most likely plays a role in the GR noise drop, but models including this process involve cyclic transitions and non-equilibrium steady states, greatly complicating the calculations. The second exponential decay could not be described by the trapping models, but also here on-trap recombination and Cooper-pair fluctuations must be considered.A qualitative analysis on the MKID detector performance (both single photon and radiation power integration) showed that quasiparticle trapping effects can increase the energy resolving power and noise equivalent power at low temperatures, when this effect replaces the usual lifetime saturation due to excess quasiparticles.