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.

Superconducting Nanowire Single-Photon Detectors (SNSPDs) are characterized by high detection efficiency, high counting rates, low dark count rates, and minimal timing jitter, making them indispensable in fields such as quantum information science, free-space optical communication, and fundamental physics. Conventional SNSPD architectures, however, require the superconducting nanowires to be biased with a constant current and necessitate continuously operating readout circuits to capture the output signals. This leads to efficiency issues under prolonged low-load conditions and poses significant challenges for the development of multi-pixel SNSPD arrays. This thesis begins by examining conventional SNSPD designs, critically analyzing the shortcomings of previous active quenching methods, and proposing improvements to the digital sub-circuit. The introduction of a differential amplifier in place of the original single-ended main amplifier successfully addresses output offset issues, pushing the count rate beyond 50MHz. Moreover, this work introduces a novel persistent current SNSPD that leverages the memory characteristic of persistent currents in a superconducting loop, enabling photon detection without the continuous drive from interface circuits, and allowing for unified readout. A design methodology for SNSPD loops is proposed based on various simulation techniques and the physical and electrical characteristics of SNSPDs. This methodology was applied to design multiple samples tailored to different loop coupling scenarios. Additionally, the thesis outlines the biasing and readout logic for the persistent current SNSPD, ensuring independent photon detection without external circuit interference. In the demonstration system, the energy consumption per detection event was measured at 7pJ, with the average power consumption dependent on the frequency of rebias operations.

For a long time, the resolution of light microscopy was restricted to approximately 200 nm, as described by Abbe’s diffraction limit. Single-Molecule Localization Microscopy (SMLM) overcomes this limit by capturing many frames of a sample labeled with blinking fluorophores, where each frame shows a different subset of molecules. The fluorophores are located by fitting a model of their point spread function (PSF) and these localizations are combined to create an image with 20-50 nm resolution. Localizing a fluorophore is only possible when no other fluorescent molecules are active in the surrounding diffraction-limited area so that its PSF appears as an isolated blob. However, high-density regions unavoidably contain overlapping PSFs, which lead to localization errors as the number of emitters in the region of interest is unknown. To prevent this, the density of active fluorophores is typically kept between 0.01-0.1 μm−2 and data acquisition can take up to days, making dynamic imaging impossible. This theoretical research explores the possibility of enabling high-density SMLM by using Single-Photon Avalanche Diode (SPAD) arrays, detecting every incident photon with picosecond timing precision, instead of capturing the total intensity in a 10-100 ms interval like the conventionally used sCMOS cameras. The photon arrival times from simulated SPAD measurements are used to determine the number of emitters in the field of view, which is directly related to the second-order quantum coherence of the signal. This coherence can be calculated by dividing the arrival times into discrete time bins and convoluting the signal with itself. To correct the number of emitters for the bias that is introduced by the discretization of the data, an analytical expression is derived and validated with simulations. Using this correction, the number of Alexa647 fluorophores can be determined from a 0.1 ms simulated measurement with a relative standard error of 1% independent of emitter count, using a laser intensity of 330 kWcm−2 and 100% detection efficiency. Considering an experimental setting in which 10 kWcm−2 intensity and 10% detection efficiency are more realistic, a 1.5 s measurement is needed to obtain the same accuracy, and a 15 ms interval is required to obtain a standard error of 10%. The newly acquired information about the number of emitters will make it possible to locate fluorophores with overlapping point spread functions for multi-emitter fitting. Consequently, SPAD arrays will provide the ability to image high emitter densities, which enables faster data acquisition and dynamic imaging.

Magnetic resonance imaging (MRI) is a clinical imaging technique that allows for non-invasive visualization inside the human body with excellent soft tissue contrast with a sub-millimeter resolution. Qualitative MRI is used to visually highlight normal or pathological components by exploiting the physical properties of different tissues. However, these acquisitions provide minimal consistency between scans, patients, and scanners. To address this issue, quantitative MRI (qMRI) provides absolute measures that give meaningful physical information about tissues, enabling objective comparisons. Relaxometry, a branch of qMRI that characterizes tissues through their magnetic relaxation properties, has been employed to quantitatively assess various diseases with different biomarkers in the past. However, certain radiofrequency (RF) pulses used to induce relaxation times weighting in the MRI signal are sensitive to field inhomogeneities, which makes consistent quantification of relaxation times difficult. In order to improve sensitivity and detect more diseases, better contrast mechanisms and biomarkers are crucial. One promising technique is Relaxation Along a Fictitious Field (RAFF), which may serve as a biomarker for a wide range of diseases due to its sensitivity to slow molecular motion in tissue. Currently, it has the downside of being sensitive to off-resonance and B₁⁺ artifacts, which hampers clinical application. This project aims to develop novel contrasts for quantitative MRI by investigating the performance of adapted RF pulses. Ultimately, the goal is to reduce the susceptibility to off-resonance and B₁⁺ artifacts for the RF pulses.

Single-photon detection is extremely important for a number of different applications such as quantum cryptography, CMOS testing and even biomedical research. Most of the applications of single-photon detector require high efficiency combined with high time resolution, high count rates and low dark counts. Superconducting nanowire single-photon detectors has emerged provides this combination unlike any other available single-photon detectors. Some of the applications of superconducting nanowire single-photon detectors (SNSPD's) require larger area SNSPD's without affecting it's performance. For this purpose, multi-pixel SNSPD's would provide a solution. To this end, prototype 4- and 16-pixel detectors have been characterised in terms of a number of performance parameters such as critical current, count rate behaviour and timing jitter. Aside from this, simulations have been performed on detection statistics and relative illumination of the individual pixels of a multi-pixel SNSPD. Results show that for 4-pixel detectors, all desired aspects have been achieved, but not yet combined in one detector. The 16-pixel detectors showed excellent critical currents and count rate behaviour but, most likely due to the fact that they were prototype chips with little to no protection, had several not functioning pixels.