We have measured the quasiparticle generation-recombination (GR) noise in aluminium lumped element kinetic inductors with a wide range of detector volumes at various temperatures. The basic detector consists of meandering inductor and interdigitated capacitor fingers. The inductor volume is varied from 2 to 153$\mu \mathrm{m}^{3}$by changing the inductor width and length to maintain a constant inductance. We started with measuring the power spectrum density (PSD) of the detectors frequency noise which is a function of GR noise and we clearly observed the spectrum roll off at 10 kHz which corresponds to the quasiparticle lifetime. Using data from a temperature sweep of the resonator frequency we convert the frequency fluctuation to quasiparticle fluctuation and observe its strong dependence on detector volume: detectors with smaller volume display less quasiparticle noise amplitude. Meanwhile we observe a saturated quasiparticle density at low temperature from all detectors as the quasiparticle life time$\boldsymbol{\tau}_{qp}$approaches a constant value at low temperature.
We have measured the quasiparticle generation-recombination (GR) noise in aluminium lumped element kinetic inductors with a wide range of detector volumes at various temperatures. The basic detector consists of meandering inductor and interdigitated capacitor fingers. The inductor volume is varied from 2 to 153μm3by changing the inductor width and length to maintain a constant inductance. We started with measuring the power spectrum density (PSD) of the detectors frequency noise which is a function of GR noise and we clearly observed the spectrum roll off at 10 kHz which corresponds to the quasiparticle lifetime. Using data from a temperature sweep of the resonator frequency we convert the frequency fluctuation to quasiparticle fluctuation and observe its strong dependence on detector volume: detectors with smaller volume display less quasiparticle noise amplitude. Meanwhile we observe a saturated quasiparticle density at low temperature from all detectors as the quasiparticle life timeτqpapproaches a constant value at low temperature.
The separation and optimization of noise components is critical to microwave-kinetic inductance detector (MKID) development. We analyze the effect of several changes to the lumped-element inductor and interdigitated capacitor geometry on the noise performance of a series of MKIDs intended for millimeter-wavelength experiments. We extract the contributions from two-level system noise in the dielectric layer, the generation-recombination noise intrinsic to the superconducting thin-film, and system white noise from each detector noise power spectrum and characterize how these noise components depend on detector geometry, material, and measurement conditions such as driving power and temperature. We observe a reduction in the amplitude of two-level system noise with both an elevated sample temperature and an increased gap between the fingers within the interdigitated capacitors for both aluminum and niobium detectors. We also verify the expected reduction of the generation-recombination noise and associated quasiparticle lifetime with reduced inductor volume. This study also iterates over different materials, including aluminum, niobium, and aluminum manganese, and compares the results with an underlying physical model.
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.
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.
We present measurements of quasiparticle generation-recombination noise in aluminium Microwave Kinetic Inductance Detectors, the fundamental noise source for these detectors. Both the quasiparticle lifetime and the number of quasiparticles can be determined from the noise spectra. The number of quasiparticles saturates to 10 μm−3 at temperatures below 160 mK, which is shown to limit the quasiparticle lifetime to 4 ms. These numbers lead to a generation-recombination noise limited noise equivalent power (NEP) of 1.5×10−19 W/Hz1/2. Since NEP∝N qp , lowering the number of remnant quasiparticles will be crucial to improve the sensitivity of these detectors. We show that the readout power now limits the number of quasiparticles and thereby the sensitivity.
Abstract The Cooper-pair breaking efficiency quantifies the effectiveness with which incident energy, such as that delivered by photons or phonons, is converted into quasiparticles—a process central to the response of superconducting pair-breaking detectors. We measure the pair-breaking efficiency in two far-infrared kinetic inductance detectors (KIDs) operating at 4.3 THz and 15 THz by comparing photon noise and thermal generation-recombination noise. Both KIDs are lumped-element devices consisting of a tantalum interdigitated capacitor in parallel with an aluminum inductor meandered line, which also acts as a low-volume absorber. The measured pair-breaking efficiencies are 0.42 and 0.07 for the 4.3 THz and 15 THz devices, respectively. The extremely low efficiency at 15 THz, a frequency well above the aluminum Debye frequency, is mainly attributed to rapid anharmonic decay of phonons, with additional phonon relaxation in the tantalum layer further reducing the available phonon population for pair-breaking. Furthermore, a relationship between the pair-breaking efficiency and detector resolving power is derived.
Kinetic inductance detectors (KIDs) have shown promising single-photon detection
capabilities. However, there has also been a consistent deviation from the limit
in energy resolution predicted by Poisson statistics. The consistent underperformance
suggests some additional mechanism has not been taken into account. The
work in this thesis has taken the novel approach of using aluminium (Al) lumped
element (LE) KIDs to model single-photon detection based on Mattis-Bardeen and
Kaplan theory.
This work has demonstrated generation-recombination (GR) noise limited performance
predicted by theory, when taking into account the quasiparticle density
and lifetime saturation at low temperatures. The model requires a single fitting
parameter that accounts for the quasiparticle generation efficiency . This work
shows = 0:4 is more appropriate for 30 nm Al lm.
It has also been recognised, through simulation and measurement, that there is
a position dependence in the response to photon absorption, which is dependent
upon LEKID architecture. An attempt was made to mitigate this effect by using a
hybrid device: an additional niobium (Nb) absorbing layer is used to cover the assumed
non-responsive regions of the LEKID structure. However, large amounts of
scatter in resonator position across the test array – thought to be due to processing
effects – have made it difficult to identify between Al and hybrid devices. Experimental
procedures were developed to assist device identification with some degree
of success but further development work is required. Nonetheless, the same test
array has shown single-photon detection in Al LEKIDs at visible and near-infrared
wavelengths.