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

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..... ... Read more

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. ... Read more

Microwave kinetic inductance detectors (MKIDs) are superconduct- ing detectors that are excellent candidates for astronomy in the far- infrared (FIR), roughly 100 GHz to 10 THz. Radiation in this part of the electromagnetic spectrum is particularly hard to detect compared to optical or near-infrared radiation. Furthermore, some sources in the FIR are so faint that the detectors are required to detect sin- gle photons to determine the incident photon rate. Recent MKIDs are highly sensitve and are capable of detecting single photons in the FIR, although detection of lower energy photons remains a challenge. Photons produce pulses in the output signal of the detector. As the pulse height is dependent on the photon energy, low energy photons are hard to distinguish from the noise. This thesis presents a system model that is used in estimating the photon rate. The system model describes signal relations and noise characteristics, so that it provides a foundation for developing statistical estimation and detection algorithms. Based on this model, various estimators are proposed, e.g. a generalized matched filter. This shows the utility of the system model in deriving solutions for estimation problems. This thesis represents a first step in advancing signal processing techniques for single photon detection in MKIDs designed for FIR astronomy. ... Read more

Deposited dielectrics with low loss at millimeter-submillimeter (mm-submm) wavelengths are beneficial for the development of superconducting integrated circuits (ICs) for astronomy, such as filter banks, on-chip Fourier-transform spectrometers, and kinetic inductance parametric amplifiers. Although it is possible to fabricate microstrip lines using crystalline Si extracted from a silicon-on-insulator wafer by a flip-bonding process, deposited dielectrics al- low for simpler and more flexible chip designs and fabrication routes. In the ∼10–100 THz frequency range, dielectric losses are typically dominated by infrared absorption due to vibrational modes, whereas in the microwave fre- quency band (∼1–10 GHz) and at sub-Kelvin temperatures the dielectric loss is typically dominated by absorption due to two-level systems (TLSs). How- ever, the origin of the mm-submm (∼0.1–1 THz) loss in deposited dielectrics was unknown. In this dissertation we show that the mm-submm loss in de- posited dielectrics can be explained by the absorption tail of vibrational modes which are located above 10 THz. Furthermore, we found that hydrogenated amorphous silicon carbide (a-SiC:H) has a very low mm-submm loss tangent of 1.3×10−4 at 350 GHz, which makes it a promising low-loss deposited dielectric for mm-submm superconducting integrated circuits.

Based on a literature study we identified a-SiC:H and hydrogenated amor- phous silicon (a-Si:H) as potentially promising low-loss dielectrics. In order to find the origin of the mm-submm loss, and to define which materials we investi- gated in this PhD project, we characterized the dielectrics’ material properties at room temperature prior to performing the cryogenic loss measurements. We deposited a-SiC:H at a substrate temperature Tsub of 400◦C using plasma- enhanced chemical vapor deposition (PECVD), and we deposited the a-Si:H films at Tsub of 100◦C, 250◦C, and 350◦C. We characterized the films’ material properties using Fourier-transform infrared spectroscopy (FTIR), Raman spec- troscopy and ellipsometry. For the a-Si:H we determined the hydrogen content and the microstructure parameter from the FTIR data, the bond-angle disorder from the Raman data, and the void volume fraction from the ellipsometry data. For both the a-Si:H and the a-SiC:H we determined the band gap and optical refractive index from the ellipsometry data, and the infrared refractive index from the FTIR data. From the Raman spectra we observed that the a-SiC:H and the a-Si:H films were amorphous. Furthermore, we performed electron diffraction spectroscopy to determine the Si to C ratio of the a-SiC:H. For the a-Si:H we found that the all the material properties depend monotonically on Tsub. Additionally, we measured the cryogenic microwave loss of the a-Si:H films, but we found no correlation between the microwave loss and Tsub.

No cryogenic mm-submm and microwave loss data was available for a- SiC:H. We measured the low-power and cryogenic microwave loss of the a- SiC:H and found that the microwave loss tangent (tanδ ∼ 10−5) is compa- rable to the loss of a-Si:H. Furthermore, we measured the mm-submm loss in the range of 270–385 GHz using an on-chip Fabry-Pérot experiment. The observed mm-submm losss value of 1.2 × 10−4 at 350 GHz was significantly lower than what was reported for a-Si:H, which previously exhibited the low- est reported microwave and mm-subm wave loss values among the deposited dielectrics which are commonly used in superconducting ICs. Furthermore, we found that the mm-submm loss of the a-SiC:H increases monotonically with frequency. This was surprising in the framework of TLSs and led us to the hypothesis that another loss mechanism than TLSs might be dominant at mm- submm wavelengths. In addition to the low losses, the a-SiC:H was found to be beneficial thanks to its very low stress, lack of blisters, and the possibility to fabricate a membrane from the a-SiC:H on a c-Si wafer.

To study the origin of the frequency dependent mm-submm loss in the a- SiC:H, we extended the on-chip Fabry-Pérot experiment to the 270–600 GHz range by making use of a wideband leaky antenna. Additionally, we measured the complex dielectric constant of the a-SiC:H in the 3–100 THz range using Fourier-transform spectroscopy (FTS). We modeled the FTS data using the Maxwell-Helmholtz-Drude (MHD) dispersion model and obtained the complex dielectric constant in the 3-100 THz range. Finally, we modeled the combined on-chip loss data from the Fabry-Pérot experiments and the FTS data by fitting the MHD dispersion model in the frequency range of 0.27–100 THz. Our model demonstrates that the mm-submm loss in the a-SiC:H above 200 GHz can be explained by the absorption tail of vibrational modes which are located above 10 THz. These results pave the way for a thorough understanding of the mm-submm loss in deposited dielectrics.

The low losses of the a-SiC:H allow for integrated superconducting spec- trometers with a large frequency bandwidth and relatively high resolving pow- ers without sacrificing too much optical efficiency. This has led to the application of the a-SiC:H in the DESHIMA 2.0 filter bank, which has seen first light in 2023 at the ASTE telescope in the Atacama Desert. ... Read more

Despite advances in astronomy, much of the universe remains hidden behind gas and dust clouds that absorb optical radiation, making it undetectable by conventional telescopes. Far-Infrared (FIR) radiation is capable of propagating through these clouds, and offers a solution for observing these obscured interstellar regions. However, the sensitivity of existing detector systems is insufficient to detect FIR radiation effectively. This thesis focuses on the development of highly sensitive detectors based on Microwave Kinetic Inductance Detectors (MKIDs) that meet the sensitivity requirement for use in future space missions, specifically the NASA led POEMM and PRIMA missions, covering a frequency range from approximately 1.5 THz to 12 THz.

While detectors have been developed previously covering the upper band of this frequency range at 6.98 THz and 12 THz, the lower band remains uncovered. Therefore, lens-absorber coupled MKIDs designed to operate at 1.5 THz are presented, with their performance analysed using models based on the Geometrical Optics-Fourier Optics (GOFO) technique in combination with a Floquet wave approach for periodic absorbers. Additionally, the design methodology of MKIDs to operate at specific readout frequencies is discussed. Absorber coupled MKIDs are limited to lower resonance frequencies with respect to comparable antenna based MKIDs, which offer advantages for space-based missions due to reduced power consumption, but do present challenges for readout systems operating at higher frequencies. In this thesis, two lens-absorber coupled MKID designs were developed and are currently being fabricated as candidates to experimentally demonstrate highly sensitive detector arrays operating at 1.5 THz.

The imaging spectrometers in the POEMM and PRIMA missions require a dispersive element to separate different wavelengths of the incoming radiation. The dispersive element envisioned for these missions, the Virtually Imaged Phased Array (VIPA), suffers from non-idealities, causing phase aberrations in the transmitted field, and part of the power transmitting towards unwanted propagation directions, which can degrade instrument performance. This thesis presents and analyses optimisation techniques to mitigate these effects, thereby enhancing the overall efficiency and degrees of freedom for designing the full instrument.

Furthermore, absorbers, unlike antenna-based devices, are multi-mode detectors. Understanding how absorbers couple to higher-order modes is crucial for optimising their performance. This thesis introduces and validates a model based on a novel experimental technique, Energy Absorption Interferometry (EAI), to extract a modal description of detectors. In the future steps related to the POEMM mission, the findings from this initial investigation into multi-mode absorbers will be used to gain insight and explore possible improvement routes in the coupling of these detectors to complex optical systems. ... Read more

In the mission of understanding the origins of life on earth and in the universe, astronomers are looking to exoplanets for signs of life. The spectra of light that has passed through an exoplanet's atmosphere is analysed for biomarkers. Until now most exoplanets have been found and characterised using the transit method. In the coming decade the aim is to look for planets using direct detection methods. In part for this purpose the European Extremely Large Telescope (EELT) is currently under construction and in future the Habitable World Observatory (HWO) is planned as a space-based telescope. To make full use of the EELT and the WHO extreme adaptive optics (XAO) systems using wave front sensors and deformable mirrors are needed. Contemporary CCD cameras utilizing semiconductor band gap energies are not sufficiently sensitive for wavefront sensing in direct exoplanet detection, let alone perform measurements to characterise their atmospheres. To this end different detectors are needed.

Kinetic inductance detectors (KIDs) are promising superconducting, energy resolving devices capable of single photon detection in the near-infrared and visible regimes. Another often cited advantage of KIDs is that, in theory, many detectors can easily be coupled to a single set of readout electronics. To make devices capable of producing images comparable to those achieved with CCDs we need a similar amount of pixels. The largest KID arrays to date are in the order of a thousand pixels on one readout line, whereas CCDs have in the order of millions of pixels. In this thesis some theoretical and practical limits on KID multiplexability while retaining high pixel yield are explored. ... Read more

Optical kinetic inductance detectors (OKIDs) are a type of superconducting single photon detector whose signal corresponds to the number of quasiparticles that are excited when the detector absorbs a photon. OKIDs whose absorptive element is made out of β-phase Tantalum (β-Ta), a disordered superconductor, exhibit faster than exponential (supra-exponential) decay of its pulse-shaped single photon response. Faster decay generally corresponds to a higher density of quasiparticles. These pulses cannot be explained by the equations typically used to describe how these quasiparticles decay in a process called recombination. One possible explanation is that the high degree of disorder of β-Ta causes the diffusion of quasiparticles through the material to be slow, which leads to a high local density of quasiparticles, causing the faster decay. The usual model for this recombination does not include this diffusion.
In this thesis, we model the OKID’s single photon pulse with a local model of the recombination process, that does include this diffusion. To this end, we analyze the downconversion process, which describes the generation of particles due to a photon absorption. This analysis therefore gives information on the initial conditions of the recombination process. Next, we study and solve this diffusion and recombination model.
Next, we test the response pulse predicted by this model against measurement data of the single photon pulses of a β-Ta OKID, for different photon energies. We show that the model must assume that an unfeasibly high number of quasiparticles is excited in order to reproduce the data. Furthermore, the pulses generated by the model exhibit a dependence on the absorbed photon energy, which is not observed in the data. Therefore we reason that the model must be missing some photon energy dependent effect. We reason that this energy dependence could lie in the downconversion process, or that it could lie in the responsivity of the OKID.
We also investigate the pulses of OKIDs that are designed with an absorber composed of multiple small elements of β-Ta, which constrain the quasiparticles in space. We measure the correspondence of the size of the β-Ta elements and the decay time of the response pulse. We show that the decay time increases for larger elements, until it saturates. This behavior is also predicted by our model. We therefore conclude that the diffusion indeed plays a significant role in the cause of supra-exponential decay. ... Read more

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 possibly 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 simulation 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. ... Read more

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... ... Read more

Magnetic Kinetic Inductance Detectors (MKIDs) are very good radiation detectors which are even capable of single photon detection in the near-infra red and higher frequency range. MKIDs are currently used to detect exoplanets and the goal is to also retrieve information of the atmosphere of exoplanets.
However, MKIDs don’t have the photon absorption efficiency and resolving power to do this yet.
In this thesis we look at the single photon pulses of a new superconducting material, beta phased tantalum (β-Ta), since this material shows promising properties for MKIDs. The single photon pulse shapes of this material are not yet fully understood. Therefore we will create models for the quasiparticle dynamics in β-ta to try and further our understanding of the single photon pulses in this material.
From the Rothwarf-Taylor equations we derive multiple models. These are then tested on the data. We first try the double exponential model which does not work. Then we look at the 1/t model and this model does seem to work better. We propose a different response time of the system. Fitting a new response time we get a very good fit to the single photon pulses. The main hypothesis is that there is an extra relaxation time for the quasiparticles as they need to distribute themselves throughout the superconductor. We see that the fitted response time is wavelength dependent which would support the hypothesis.
We conclude that the 1/t model with an adjusted response time explains the single photon pule shapes the best. ... Read more

Terahertz astronomy has been exceptionally unexplored until the last decades due to a technological gap, but exactly at these wavelengths the most distant galaxies appear very bright. Efficient instruments that are capable of spectrometry are essential in understanding the physics of these distant objects. Within the framework of this thesis, an efficient, moderate spectral resolution, on-chip filterbank spectrometer is presented. Several band-pass filter units were investigated and compared, being the directional band-pass filter the most robust and performing. A circuit model is constructed for each of these units and arrayed into a filterbank configuration resorting to microwave techniques. The performance of the filterbank with the circuit model, both with and without realistic losses and tolerances is modeled. A microstrip directional band-pass filter unit has been designed and arrayed into a filterbank chip design. A test filterbank chip based on this design has been fabricated. Due to fabrication issues the detector yield was too poor to allow in-depth and statistically significant measurements. Despite this, the filterbank design is expected to outperform state-of-the-art superconducting filterbanks in terms of coupling efficiency. ... Read more

Today, one of the major goals of modern astronomy is the search for other habitable worlds and the presence of life on them. Crucial in this search is the atmospheric characterisation of small, rocky planets orbiting in the habitable zone around solar type stars. The LIFE initiative will be able to perform atmospheric characterisation of a sizeable subset of these planets in the mid-infrared (mid-IR) wavelength regime (5-20 𝜇m). The mid-IR is an important bandwidth as it contains some important atmospheric biosignatures. Extremely sensitive and highly efficient detectors are required to detect the faint signal from these small exoplanets. Current state-of-the-art detectors based on semiconductor technology are unable to meet these requirements. Microwave Kinetic Inductance Detectors (MKIDs) are superconducting pair-breaking detectors able of single-photon detection with no readout noise or
dark current. This makes MKIDs a promising candidate for mid-IR detectors for the LIFE initiative. In this thesis we investigate what development is necessary to meet the detector requirements set by the LIFE initiative. We also investigate how the performance of MKIDs can be reliably measured in the mid-IR.

Currently, there are no single-photon counting MKIDs designed for the mid-IR. Measurements are done with two MKID devices that originally have been designed for the near- and far-IR bandwidths. Prior to this work the near-IR detector has shown single-photon counting 1545 nm and the far-IR detector at 38 𝜇m. In this work we show the single-photon counting ability of MKIDs 3.8 and 8.5 𝜇m. This is the first time that single-photon counting has been shown at 8.5 𝜇m. The resolving power (𝐸/𝛿𝐸) at 8.5 𝜇m is found to be about 4. Experiments are planned at 18.5 𝜇m for which a setup has been designed with a cryogenic black-body radiator as the source. This is the longest wavelength required for the LIFE spectrometer. We also perform an optimisation of the near-IR detector geometry to see if a realistic device can be made that is sufficiently sensitive to 18.5 𝜇m radiation. The results show that a realistic design could in theory be made but this strongly depends on how the detector is limited by the noise.

Next steps are to design a dedicated MKID for the mid-IR to determine its efficiency and dark current. This will also require us to improve the current measurement setup as measurements show that we suffer from thermal background radiation which limits the detector performance. ... Read more

The mysteries of the early Universe are largely enshrouded in dust, product of the violent process of star formation. Due to the vast distances of our Universe, infrared light emitted by the heated dust back in those early stages can still be observed today, which has been observed to contribute to about half of the total cosmic background radiation. Gases fueling star-formation also radiate, but in the form of emission lines, which leave distinct spectral signatures that allow the study of the underlying physical processes. Given the expansion of the Universe, the evolutionary information is encoded in the cosmological redshift observed, making the far-infrared or terahertz (THz) regime specially suited for probing star-formation. Superconducting on-chip broadband THz imaging spectrometers with moderate spectral resolution coupled to large telescopes will allow the investigation the early Universe processes over large cosmological volumes. In this dissertation we propose two enabling technologies toward the advancement of this on-chip superconducting instruments: a broadband and moderate spectral resolution channelizing filter-bank, and a broadband phased array antenna as a reflector feed with beam-steering capabilities.

Octave-band THz channelizing filter-banks with moderate spectral resolution of the order R=500 are investigated in this work. These systems allow for a size reduction of several orders of magnitude compared to conventional spectrometers with similar spectral resolution. The proposed filters are half-wavelength resonators, which naturally provide a free-spectral range of an octave. The performance of those filters, both when in isolation and when embedded in a filter-bank, is analyzed using a newly-developed circuit model. This tool also provides design insights such as the required filter ordering and separation within the filter-bank to enable an efficient circuit. The actual implementation of the superconducting filter-bank on a chip is investigated for two of the main on-chip technologies: co-planar waveguide (CPW) and microstrip. Despite the easier manufacturing of co-planar circuitry, that technology is not suited for channelizing THz filter-banks as it suffers from radiation issues. Instead microstrip technology is non-radiative and, although it suffers from the moderate dissipation in deposited dielectrics such as a-Si, it provides a very reliable platform to build THz filter-banks. Half-wavelength I-shaped resonators are proposed as suitable filtering structures with which frequency-sparse filter-banks have been built to test their performance in semi-isolation. The measurements were based on both a frequency response characterization of the filters as well as their optical efficiency, showing good agreement between the two. The measured performance of these filters showed pass-bands with an average peak coupling efficiency of 27% and a spectral resolution R≈940. The coupling is significantly better than earlier results based upon planar technology.

The coupling between the quasi-optical reflector system of a telescope and the on-chip filter-bank requires of a broadband antenna. Currently, broadband integrated anti-reflection-coated lenses are being developed for this purpose, but their manufacturing is specially complicated for cryogenics and require mechanical actuators to perform beam scanning in the case of a multi-object spectrometer. In this dissertation, we propose a broadband phased-array antenna concept with electronic beam-steering that exploits two key properties of superconductors in its feeding network: the negligible conductor loss and the tunable kinetic inductance with a bias current. The focused connected array antenna concept proposed is based on the broadband impedance matching enabled by the connected arrays and the largely frequency-independent far fields of near-field focused apertures. To demonstrate this concept we designed, fabricated and tested two low frequency (3-6 GHz) prototypes in PCB technology: one pointing broadside and another one scanning. The measured fields met the predictions to a large degree and provided with a reflector aperture efficiency in excess of 60% over an octave of bandwidth and allowing to scan one half-power beamwidth at the lowest frequency with a frequency-averaged scan loss of 0.2 dB. Both the directivity and the gain were measured, allowing to report the losses, which chiefly originated from the tin-finished copper lines in the PCB. As a result, we can expect a highly-efficient reflector feed at THz frequencies with beam-steering capabilities in the near future.

The beam-steering concept proposed for the phased-array antenna relies on the current-dependent kinetic inductance of superconducting lines. With this effect, the phase velocity of biased superconducting lines may be modified, allowing thereby an electronic tuning of the phase-shift introduced. Prior to the integration of such phase-shifters with the phased-array antenna, we devised an on-chip platform based a tunable Fabry-Pérot resonator to quantify the phase-shifting capabilities at THz frequencies. In this concept, the dc bias currents are injected in the proximity of the edges of the resonator through 9th order Chebyshev stepped-impedance low-pass filters, whose high rejection mitigates any possible disturbance to the THz resonances. Using a circuit model including the resonator and the low-pass filters, as well as the simulated properties of the superconducting buried microstrip lines used in the designs, we anticipate an expected maximum tuning of dφ/φ=-df/f≈2%. With such tuning range millimeter-long tunable delay lines will be required for THz superconducting phased-array. ... Read more

Superconducting detectors such as Microwave Kinetic Inductance Detectors (MKIDs) have lead to the designs of THz on-chip spectrometers such as DESHIMA. With DESHIMA a wideband THz signal is fully sampled by several hundreds of channels of which the frequency is defined by an array of band-pass filters. Each band-pass filter which is connected to a separate MKID which can be read-out simultaneously by a microwave read-out signal. This single-pixel system can be expanded through the implementation of steerable antennas. Especially when several spectrometers are used to create a multi-pixel spectrometer, the process of measuring the Universe's background radiation could be sped up and also be used to calibrate the instrument. In this thesis an on-chip platform is designed which is able to quantify the achievable phase-shifting capabilities of a superconducting microstrip line at terahertz frequencies. This is done by exploiting the non-linear behavior of a superconductor's kinetic inductance to a dc current. DC-biased superconducting Fabry-Pérot (FP) resonators, replacing the role of the filter-bank mentioned above, have been investigated and designed to quantify the phase-shifting capabilities by probing the shift in the resonance frequency. The injection/extraction of DC biasing currents on the FP resonator need to be transparent to the THz frequencies. To do so, low-pass stepped-impedance filters have been designed. A Chebyshev filter has been implemented with a stepped-impedance filter which has a minimal rejection of 45 dB between 300 and 400 GHz. This is necessary to prevent leakage of the THz signal into the bias feeds. The two filters reactively load the FP and have an effect on the Q-factors. This change, without any bias current applied, is small enough to be neglected. The tuning of the FP with a dc bias current is limited by the critical current the superconductor can support. This is determined by the geometry and material characteristics of the superconductor. The sensitivity of the superconductor to a dc current is limited by its kinetic sheet inductance. The larger this value is, the larger the tuning range. The results of the simulations shown that with this design it is possible to obtain a phase shift of roughly 0.7% which coincides with phase shift values found in other studies. It is therefore possible to implement this design to create beam-steerable superconducting antennas. The design is being fabricated as this is written and will be measured in the coming months. ... Read more

Superconducting integrated circuits (SICs) represent a natural step forward for devices operating at frequencies from microwave up to sub-millimeter wavelengths. They offer massive miniaturization via compact design based on low-loss superconducting transmission lines. At sub-millimeter wavelengths, the development of SICs is driven by astronomical instruments where it could allow the realization of an imaging spectrometer, combining simultaneous imaging and spectroscopy capabilities into a single instrument analogous to integral field units in the infrared and optical regimes. Such an imaging spectrometer can be achieved with SICs by integrating the required elements, such as spectral filters and polarizers, with the detectors onto a single chip. Without this integration, the dispersive system for even a single spatial pixel at these wavelengths would be prohibitively large and could not be realistically scaled up to allow imaging. Astronomical signals are exceedingly weak, typically requiring many nights of exposure to get a good signal to noise ratio. It is therefore imperative that the instrument has minimal losses before its detectors. As a consequence, the losses of each element in the SIC needs to be minimized, which requires careful characterization of the individual elements, including antenna, filters, detectors and connecting transmission lines. The primary focus of this thesis lies on the experimental characterization of the wideband antenna and the low-loss superconducting transmission lines. ... Read more

Microwave Kinetic Inductance Detectors (MKIDs) are extremely sensitive radiation detectors based on superconducting resonators that can be combined in large arrays on a single readout line within a limited frequency bandwidth. This makes MKIDs ideal detectors for the ultimate far-infrared observatory: a future space-based actively cooled telescope with its performance solely limited by the low universe background radiation. However, to reach these detector requirements, state-of-the-art MKIDs still need a order of magnitude improvement in device sensitivity. In this work, the MKID sensitivity is improved by reducing the aluminium volume that absorbs pair-breaking radiation into quasiparticle excitations, while making sure all radiation is still absorbed. Furthermore, a key requirement is sufficient reduction of excess noise as to keep the device intrinsically limited by thermally driven random fluctuations in the number of quasiparticles in absence of radiation, or Generation-Recombination (G-R) noise. To this end, a model is developed that describes the noise contributions as function of device geometry, readout power, material properties and radiation power. Subsequently, a realistic MKID design is presented and tested that reduces excess noise and maximises the sensitivity, expressed as Noise Equivalent Power (NEP). At high temperatures, good overall agreement is found between the measured noise spectra and the model. At low temperature T = 120 mK, the measurement results give an optical NEP similar to current state-of-the-art MKIDs. The NEP is not as low as expected due to short quasiparticle lifetimes, an unexpected decrease in the G-R noise level and a very high excess noise attributed to Two-Level Systems (TLS) noise that starts to dominate the already low G-R noise spectrum at low temperatures. Possibly, the quick quasiparticle lifetime saturation and noise level drop are caused by a strong readout power effect, as the readout power is known to create excess quasiparticles and to cause a strongly non-thermal electron energy distribution in the aluminium strip of the MKID. However, the exact microscopic details of these effects are unknown and not studied in this project. Based on the current chip design, a straightforward way to improve device performance and study the readout power effect in more detail is a reduction of the high TLS noise levels, which is possibly fabrication related. This would allow an unobstructed view of the G-R noise spectrum at low temperatures, thereby allowing both a study of the readout power effect on the quasiparticle system, and ultimately achieving the improvement in NEP needed reach the detector requirements for the ultimate space-based far-infrared observatory. ... Read more

On-chip spectrometers, such as DESHIMA and SuperSpec, require transmission lines with very low loss of tanδ < 10^-4 to achieve sufficient system efficiency. Transmission lines with higher loss would introduce too much signal attenuation in the line from antenna to filter and in the filters themselves. Data regarding the losses of transmission lines at THz frequencies and sub-K temperatures is severely lacking. In this report an on-chip Fabry-Pérot resonator concept is demonstrated that can be used to measure the losses of a transmission line with high sensitivity at high frequencies. To create the in-line Fabry-Pérot resonator, a transmission line of certain length is coupled to a THz source via a twin-slot lens antenna on one side and to an Al-NbTiN hybrid MKID on the other side. The goal of this work is to measure the losses of microstrip lines at frequencies > 300 GHz, at a temperature of about 250 mK, with dielectric dominated loss in the range of 10^-3 > tanδ > 10^-5. There are several experimental challenges for measuring tanδ. The first challenge is the limited frequency resolution of the source, due to which resolving low tanδ can become impossible. Secondly it was experimentally found that there is stray light coupled to the detector which causes a spurious response with a level of −30dB with respect to the peak (unity) transmission of the Fabry-Pérot resonator. Taking these experimental challenges into account results in a Fabry-Pérot resonator design where the length, the mode number, and the coupler quality factor Q_c of the resonator are optimized. Furthermore multiple resonators on a single chip are used, each coupled to a separate antenna and detector, with different Q_c values. This design method is applicable for different dielectric materials and different transmission line configurations. Using this method a chip was designed and fabricated for a microstrip line based Fabry-Pérot resonator fabricated from sputter deposited superconducting NbTiN metal and a PECVD deposited a-Si layer. Using this chip a tanδ ≈ 10^-4 @ 350 GHz was measured, which represents the lowest loss values of a microstrip line at frequencies > 10 GHz ever measured. ... Read more

At the center of every galaxy there is a super-massive black hole of a million or more solar masses. In most galaxies the presence of this black hole can only be detected through its gravitational attraction, which affects the motion of nearby stars. However, in about 10% of the galaxies the super-massive black hole is the engine of one of the most luminous phenomena in the universe: an active galactic nucleus (AGN). In the local universe there are two types of AGN: ‘Radiative-mode’ and ‘Jet-mode’ AGN. In this thesis I show that these two AGN types are hosted by different galaxies and have different infrared properties. ‘Radiative-mode’ AGN are the ‘classical’ AGN which are bright emitters across the entire electromagnetic spectrum. They are thought to be powered by a super-massive black hole accreting matter at a high rate. I show that ‘radiative-mode’ AGN are predominantly found in intermediate mass galaxies with blue and green optical colors. These colors are indicative of active or recently terminated star formation and a young stellar population. Due to the presence of torus of hot dust near the black hole, galaxies with a ‘radiative-mode’ AGN typically show an excess of mid-infrared emission. ‘Jet-mode’ AGN lack the bright optical emission and excess infrared emission of ‘jet-mode’
AGN and can only be identified by means of their radio jet – a stream of relativistic particles that can reach far outside the AGN’s host galaxy. This absence of electromagnetic radiation and prominence of the radio jet is thought to be the result of the low accretion rate of the super-massive black hole driving this AGN type. I show that ‘jet-mode’ AGN have a strong preference for the most massive galaxies, which typically have little star formation. The presence or absence of a dusty torus and the resulting difference in broadband mid-infrared emission could be a powerful tool to separate ‘radiative-mode’ and ‘jet-mode’ AGN without using spectroscopy. Unfortunately, the inherent scatter in the mid-infrared emission of galaxies due to dust heated by stars is too large to separate the two populations reliably.
Far-infrared observations could help resolve this, by constraining the mid-infrared contribution of dust heated by stars. However, current far-infrared surveys do not have the depth or the area to give the number statistics required to calibrate this procedure. In this thesis I have investigated the properties of microwave kinetic inductance detectors. These detectors will enable the far-infrared instruments with 10.000 pixels as a result of their inherent potential for frequency domain multiplexing. This is a huge leap from the 100 pixel far-infrared instruments currently on telescopes. I have shown that microwave kinetic inductance detectors made from NbTiN and Al can satisfy all the requirements to enable a new generation of large format far-infrared cameras, which are required to constrain the far-infrared emission of many galaxies. ... Read more

Sub-mm astronomy in space calls for an array of photon noise limited detectors, both for imaging and broadband spectroscopy. Microwave Kinetic Inductance Detectors (MKIDs), superconducting resonance circuits, are a suitable candidate for this purpose due to its multiplexing potential, but in literature excess noise in phase readout is encountered and attributed to so-called two-level systems (TLSs). Reduction in TLS induced noise and loss will provide greater flexibility in design and a route towards background limited detector performance.
In this thesis, TLSs from surface and bulk sources are modelled, so that their behaviour can be predicted through numerical computations of the field distributions inside the resonators. These calculations not only provide a guide for sensible chip designs, but allow for interpretation of experimental data and determination of dominant TLS sources.
It is found that for Al CPW resonators on Si or SiN, the noise is surface dominated but with a non-negligible bulk contribution, while for microstrips on a SiN membrane, the noise is bulk dominated. As the loss in microstrips for narrow microstrips is dominated by the substrate-air interface, the dominant TLS loss and noise sources do not necessarily coincide and should be treated independently. This makes it impossible to determine the dominant CPW surface noise contribution. Additionally, microstrips and CPWs on the same dielectric perform similarly, while Si is better than SiN, both in terms of loss and noise, due to a combination of SiN interface and bulk effects. Finally, material dependent loss and noise parameters have been determined and the importance of thorough Si surface cleaning has been established, yielding the best Al CPW noise ever encountered.
For sub-mm astronomy in space, the logical path to improvement would be the use of thorough cleaned Si as a dielectric, overetching and the use of LEKIDS and hybrid resonators, where microstrips are still viable for use. Importantly, having located the important TLS locations for various cases, tackling these problems areas further could provide the step towards background limited performance in space. ... Read more