Superconducting coplanar-waveguide (CPW) resonators made of NbTiN and Al have shown photon noise limited detection for optical loading equivalent to the typical sky signal at modern sub-mm telescopes. While the detection efficiency of these hybrid MKIDs is equal to the value expected based on antenna simulation and the known efficiency of quasiparticle generation, this is not the case for conventional CPW MKIDs made from pure Al. Given that the hybrid MKIDs use the large gap superconductor NbTiN to trap photo-excited quasiparticles in the sensitive area of the resonator made from Al, it is postulated that the origin of the lower detection efficiency of pure Al resonators is due to quasiparticle diffusion. During the conference we will report on the measurements comparing NbTiN/Al and pure Al resonators and the effect of quasiparticle diffusion on the detection efficiency of these MKIDs.
Superconducting coplanar-waveguide (CPW) resonators made of NbTiN and Al have shown photon noise limited detection for optical loading equivalent to the typical sky signal at modern sub-mm telescopes. While the detection efficiency of these hybrid MKIDs is equal to the value expected based on antenna simulation and the known efficiency of quasiparticle generation, this is not the case for conventional CPW MKIDs made from pure Al. Given that the hybrid MKIDs use the large gap superconductor NbTiN to trap photo-excited quasiparticles in the sensitive area of the resonator made from Al, it is postulated that the origin of the lower detection efficiency of pure Al resonators is due to quasiparticle diffusion. During the conference we will report on the measurements comparing NbTiN/Al and pure Al resonators and the effect of quasiparticle diffusion on the detection efficiency of these MKIDs.
We have measured and compared the response of hybrid NbTiN-Al Microwave Kinetic Inductance Detectors (MKIDs) to changes in bath temperature and illumination by sub-mm radiation. We show that these two stimulants have an equivalent effect on the resonance feature of hybrid MKIDs. We determine an electrical noise equivalent power (NEP) from the measured temperature responsivity, quasiparticle recombination time, superconducting transition temperature, and noise spectrum, all of which can be measured in a dark environment. For the two hybrid NbTiN-Al MKIDs studied in detail, the electrical NEP is within a factor of two of the optical NEP, which is measured directly using a blackbody source.
In the next decades millimeter and sub-mm astronomy requires large format imaging arrays and broad-band spectrometers to complement the high spatial and spectral resolution of the Atacama Large Millimeter/submillimeter Array. The desired sensors for these instruments should have a background limited sensitivity and a high optical efficiency and enable arrays thousands of pixels in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and Al have shown to satisfy these requirements. We present the second generation hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and amplitude readout for loading levels P850GHz < 10 fW. Thanks to the increased responsivity, the photon noise level achieved in phase allows us to simultaneously read out approximately 8000 pixels using state-of-the-art electronics. In addition, the choice of superconducting materials and the use of a Si lens in combination with a planar antenna gives these resonators the flexibility to operate within the frequency range 0:09 < v < 1:1 THz. Given these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.
Microwave Kinetic Inductance Detectors (MKIDs), with high sensitivity and relatively simple frequency multiplexing, are emerging as a kind of large-array detectors in the terahertz (THz) band. An MKID, like a superconducting resonator, is composed of a planar capacitor and inductor that are made from superconducting films. The energy gap of the superconducting film for the planar inductor (at least part of it) is lower than the energy of photons to be detected. The kinetic inductance of the superconducting film irradiated by the photons is increased, shifting the resonance frequency toward a lower frequency. The frequency shift is proportional to the kinetic inductance fraction, i.e., the ratio of the kinetic inductance to the total inductance of the planar superconducting inductor. In this paper, we thoroughly investigate the kinetic inductance fraction for lumped-element and antenna-coupled MKIDs, which are both made from NbTiN. The detailed simulation, calculation, and measurement results will be presented.
Spectroscopic observations of a large number of galaxies in the far-infrared are critical in the study of galaxy evolution. These observations are difficult at the moment, because of the lack of good far-infrared (FIR) detector technology. Microwave Kinetic Inductance Detectors (MKIDs) are showing promising results to fill this void. A common MKID design is the antenna-coupled coplanar waveguide (CPW) resonator patterned in a superconducting film. These resonators are extremely sensitive to changes in the Cooper pair density. FIR photons are energetic enough to break Cooper pairs. This means a superconducting resonator can be used as a pixel for a FIR camera. The inherent multiplexing advantage of MKIDs means they can easily be fabricated in arrays of up to 5000 pixels. By putting such an array in the SAFARI instrument on the cooled SPICA telescope a blind spectroscopic survey that can detect Milky Way sized galaxies out to redshift z = 2 will be possible. However, to achieve background limited performance on a cooled telescope like SPICA the sensivity of MKIDs needs to be improved by at least an order of magnitude. This project aims to investigate if this can be achieved by reducing the width of the CPW to much below what has typically been made using optical lithography (> 1 ?m). CPW resonators with a central line width as narrow as 300 nm were made in NbTiN using electron beam lithography and reactive ion etching. Using this fabrication method NbTiN resonators with a central line width varying between 0.3 ?m and 3 ?m were made. These resonators were used to do a systematic study of the width dependence of the kinetic inductance fraction, quality factor, responsivity, read-out power, noise and NEP. It is shown that the width dependence of all these properties does not change down to 300 nm. Encouraged by these results two prototype submicron pixel arrays were made in aluminum. These showed sensitivities comparable to resonators with a few micron wide CPW. The main reason for this lack of sensitivity improvement is the lower maximum read-out power of submicron wide aluminum resonators. Based on these results power handling seems a key issue that will determine if submicron resonators are a viable route to improve the MKIDs sensitivity. These results of this project indicate that aluminum has a different width dependence of its power handling compared to NbTiN. Therefore, a systematic study of submicron resonators in aluminum, which focuses on the power handling, is required.
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.
The sensitivity of microwave kinetic inductance detectors (MKIDs) based on coplanar waveguides (CPWs) needs to be improved by at least an order of magnitude to satisfy the requirements for space-based terahertz astronomy. Our aim is to investigate if this can be achieved by reducing the width of the CPW to much below what has typically been made using optical lithography (>1 μm). CPW resonators with a central line width as narrow as 300 nm were made in NbTiN using electron beam lithography and reactive ion etching. In a systematic study of quarter-wave CPW resonators with varying widths it is shown that the behavior of responsivity, noise and power handling as a function of width continues down to 300 nm. This encourages the development of narrow KIDs using Al in order to improve their sensitivity.