By using LEKIDs the prospect rises to multi-pixel arrays with an extremely small footprint. Moreover, these arrays have the advantage of frequency-division multiplex readout method (FDM). However, the increase of packing density of LEKIDs leads to an increase of crosstalk between adjacent pixels and complicates the readout process significantly. In this paper, we present a newly developed LEKID structure with reduced crosstalk and present their improved performance in terms of equally spaced resonance frequencies within an array. Thus, the array can be realized with a large number of pixels in a small analog readout bandwidth. We present simulation and measurement results of various linear arrays with 5 pixels, which are constructed with the new LEKID structures. When designing the resonance frequencies importance was attached to an equal spacing between the resonance frequencies at around 6 GHz within our available analog readout bandwidth of 400 MHz. The samples were fabricated in niobium thin film technology on silicon and sapphire substrates and then measured at liquid helium temperatures.
By using LEKIDs the prospect rises to multi-pixel arrays with an extremely small footprint. Moreover, these arrays have the advantage of frequency-division multiplex readout method (FDM). However, the increase of packing density of LEKIDs leads to an increase of crosstalk between adjacent pixels and complicates the readout process significantly. In this paper, we present a newly developed LEKID structure with reduced crosstalk and present their improved performance in terms of equally spaced resonance frequencies within an array. Thus, the array can be realized with a large number of pixels in a small analog readout bandwidth. We present simulation and measurement results of various linear arrays with 5 pixels, which are constructed with the new LEKID structures. When designing the resonance frequencies importance was attached to an equal spacing between the resonance frequencies at around 6 GHz within our available analog readout bandwidth of 400 MHz. The samples were fabricated in niobium thin film technology on silicon and sapphire substrates and then measured at liquid helium temperatures.
The development of large multi-pixel detector applications with lumped element kinetic inductance detectors (LEKID) requires a detailed knowledge about the mode of operation in respect of the microwave regime. In this paper the basic properties of LEKID structures for applications at liquid helium temperatures were investigated. In consideration of the operation temperature, the different microwave behavior of LEKID structures coupled to microstrip and coplanar strip readout transmission lines, e.g. loaded and unloaded quality factors, have been examined. For this purpose several test structures, which are suitable for large arrays, in multi-pixel applications, were designed and characterized with frequency around 6 GHz. Loaded quality factorsQLup to 10,000 at 4.2 K were achieved. The devices were fabricated in niobium thin film technology on silicon substrates. LEKID arrays with 3 resonators were designed, fabricated and measured with our self-developed FDM readout system. The results of the measurements will be presented and discussed.
High-resolution applications demand a strong increase in number of pixels of detector arrays. In terms of the lumped-element kinetic inductance detector, this requires a detailed knowledge of the microwave properties of each single pixel; moreover, the unwanted interaction (cross-talk) between them has to be examined and eliminated. In this paper, the basic microwave properties of lumped-element kinetic inductance detector structures were investigated. Simulations demonstrate the influence of the embedding design parameters, e.g., orientation and coupling distances, in respect of the microwave behavior. In addition, different array arrangements to increase the number of pixels and to identify possible reasons causing unwanted cross-talk were examined. For this purposes several samples in niobium thin film technology on silicon substrates were designed, fabricated and characterized. Loaded quality factorsQLup to 3 000 at 4.2 K were achieved. The results of the simulations and measurements of small and large arrays will be presented and discussed.
Large-scale arrays of microwave kinetic inductance detectors (MKIDs) are attractive candidates for use in imaging instruments for next generation submillimeter-wave telescopes such as CCAT. We have designed and fabricated tightly packed ~250-pixel MKID arrays using lumped-element resonators etched from a thin layer of superconducting TiNxdeposited on a silicon substrate. The high pixel packing density in our initial design resulted in large microwave crosstalk due to electromagnetic coupling between the resonators. Our second design eliminates this problem by adding a grounding shield and using a double-wound geometry for the meander inductor to allow conductors with opposite polarity to be in close proximity. In addition, the resonator frequencies are distributed in a checkerboard pattern across the array. We present details for the two resonator and array designs and describe a circuit model for the full array that predicts the distribution of resonator frequencies and the crosstalk level. We also show results from a new experimental technique that conveniently measures crosstalk without the need for an optical setup. Our results reveal an improvement in crosstalk from 57% in the initial design down to ≤2% in the second design. The general procedure and design guidelines in this work are applicable to future large arrays employing microwave resonators.
The concept of the radio-frequency superconducting nanowire single-photon detector (RF-SNSPD) allows frequency-division multiplexing of the bias and readout lines of several SNSPDs. Using this method, a multi-pixel array can be operated by only one feed line. Consequently, the system complexity as well as the heat load is significantly reduced. To allocate many pixels into a small bandwidth the quality factor of each device is crucial. In this paper, we present an improved RF-SNSPD design. This new design enables a simple tuning of the quality factor as well as the resonant frequency. With a two-pixel device we have demonstrated the operation without crosstalk between the detectors and showed the time, spatial, and photon number resolution. Thereby a single pixel requires only a bandwidth of 14 MHz.
Abstract The 1-100 MHz frequency band remains relatively unexplored for Frequency Division Multiplexing (FDM) readout of TransitionEdge Sensor (TES) arrays, despite its potential to increase channel counts beyond the traditionally used 1-5 MHz range. Thisgap, however, is becoming increasingly addressable with advancements in the fabrication of high-resonance-frequency LCresonators and wide-band Superconducting Quantum Interference Devices (SQUIDs). It still partly stems from understudiedcrosstalk challenges in this higher-frequency regime, where mutual inductance, SQUID input inductance, and carrier leakageeffects tend to be more pronounced, which may limit the feasibility of high-multiplexing systems.In this work, we aim to addressthis less explored area by modeling crosstalk between the 1-100 MHz range under a lumped-parameter framework, with afocus on static crosstalk (ECTstatic)—a metric that has received less attention in lower-frequency studies. To mitigate mutualinductance, we propose a chessboard pixel arrangement that maximizes frequency differences between adjacent channels.Leveraging the 1-100 MHz bandwidth with a 0.1 MHz channel spacing, this configuration can suppress crosstalk by up to 59.4dB.Our simulations show that with this layout and through dynamic carrier frequency design, reducing mutual inductance to 3nH enables 186 channels under a 1% ECTstatic constraint when the SQUID input inductance is 1 nH; this count increases to 299channels when the input inductance is reduced to 0.5 nH. Relaxing the threshold to 3.2% or 10% further expands the channelcounts—for instance, reaching 612 or 2067 channels with 1 nH input inductance, respectively.This study preliminarily exploresthe crosstalk characteristics of 1-100 MHz FDM for TES arrays, offering a framework for low-crosstalk, high-channel-countreadout systems and highlighting key trade-offs in this relatively unexplored frequency range.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.