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IEEE Transactions on Applied Superconductivity

Relationships Between Complex Impedance, Thermal Response, and Noise in TES Calorimeters and Bolometers

M. A. Lindeman, Bob P. F. Dirks, Jan van der Kuur, Piet A. J. de Korte, Roland H. den Hartog, Luciano G. Gottardi, R. A. Hijmering, Henk F. C. Hoevers, P. Khosropanah

Published Jan 7, 2011• 12 citations

Abstract

Complex impedance measurements are widely used to characterize superconducting transition edge sensors (TESs) in bolometers and microcalorimeters. Typically, models are fit to impedance data to find parameters which pertain to the performance of these detectors. After the parameters are determined, the models are then used to compute the response and noise of these devices. In this paper, we present general relationships between the measured impedance, the thermal response to power in the TES, and noise. We describe a method for measuring αIand βIof the superconducting phase transition, which does not require model fitting. We find bolometer response is determined from the impedance provided that the absorber is strongly coupled to the TES electron system. We also demonstrate how to calculate upper and lower limits on the noise directly from the impedance data without modeling. Additionally, the relations can be used to check the validity of the models and to understand what information can and cannot be obtained from measurements of impedance, response, and noise.

DOI: 10.1109/tasc.2010.2096173

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Complex impedance measurements are widely used to characterize superconducting transition edge sensors (TESs) in bolometers and microcalorimeters. Typically, models are fit to impedance data to find parameters which pertain to the performance of these detectors. After the parameters are determined, the models are then used to compute the response and noise of these devices. In this paper, we present general relationships between the measured impedance, the thermal response to power in the TES, and noise. We describe a method for measuring αIand βIof the superconducting phase transition, which does not require model fitting. We find bolometer response is determined from the impedance provided that the absorber is strongly coupled to the TES electron system. We also demonstrate how to calculate upper and lower limits on the noise directly from the impedance data without modeling. Additionally, the relations can be used to check the validity of the models and to understand what information can and cannot be obtained from measurements of impedance, response, and noise.

We present a model of the response of superconducting AC bolometers to signal and noise using parameters and matrix formalism that were previously adopted in modeling DC bolometers and microcalorimeters. The model was developed to investigate novel AC bolometer designs, which have properties not described by previous models. It is applicable to kinetic inductance devices, kinetic inductance thermometers, AC transition edge sensors, and other technologies based on AC read out and frequency mixing. The AC bolometer model includes the temperature and current dependence of both resistive and inductive bolometers and microcalorimeters. The bolometers may be thermal or athermal. The model compactly describes the complex relationships between electrical and thermal signals and it simplifies the associated calculations. This model is useful for characterizing bolometers, predicting the effects of various noise sources on performance and designing new detectors. Additionally, a measurement technique is developed for characterizing these devices.

Impedance measurements of superconducting transition-edge sensors (TESs) are a powerful tool to obtain information about the TES thermal and electrical properties. We apply this technique to a 20 µm × 20 µm Ti/Au TES, suitable for application in the optical and near-infrared range, and extend the measurements up to 250 kHz in order to obtain a complete frequency response in the complex plane. From these measurements we obtain important thermal and electrical device parameters such as heat capacity C , thermal conductance G and effective thermal time constant τ eff that will be compared with the corresponding values obtained from noise measurements.

The responsivity and noise of a voltage-biased superconducting transition-edge sensor depends strongly on the details of its thermal model, and the simplest theory for TES response assumes a single heat capacity connected to the heat bath. Here, analytical results are derived and discussed for the complex impedance, the responsivity and the noise of a transition-edge sensor, when the thermal model is not simple but consists of either two or three connected heat capacities. The implications of the differences of the models are discussed, as well.

A transition edge sensor series array (TES-SA) is an array of identical TESs that are connected in series by low-inductance superconducting wiring. The array elements are equally and well thermally coupled to the absorber and respond to changes in the absorber temperature in synchronization. The TES-SA total resistance increases compared to a single TES while the shape of the superconducting transition is preserved. We are developing a TES-SA with a large number, hundreds to thousands, of array elements with the goal of enabling the readout of a TES-based bolometer operated at 4.2 K with a semiconductor-based amplifier located at room temperature. The noise and dynamic performance of a TES-SA bolometer based on a niobium/aluminum bilayer is analyzed. It is shown that stable readout of the bolometer with a low-noise transimpedance amplifier is feasible.

Optical superconducting transition-edge sensor (TES) has been widely used in quantum information, biological imaging, and fluorescence microscopy owing to its high quantum efficiency, low dark count, and photon number resolving capability. The temperature sensitivity (αI ) and current sensitivity (βI ) are important parameters for optical TESs, which are generally extracted from the complex impedance. Here we present a method to extract αI and βI based on a two-fluid model and compare the calculated current–voltage curves, pulse response, and theoretical energy resolution with the measured ones. This method shows qualitative agreement that is suitable for further optimization of optical TESs.

Transition-edge sensor (TES) bolometers operate under strong electrothermal feedback, wherein power deposited on the bolometer is compensated by a corresponding change in electrical power dissipation. We present a comprehensive analysis of Johnson noise suppression that extends previous theoretical frameworks to the general case of AC-biased linear circuits with arbitrary parasitic impedances. Using a Thévenin-equivalent circuit formulation -- consisting of an ideal voltage source and complex series impedance external to the bolometer thermal island -- we derive analytical expressions for the noise-equivalent current in the presence of both bolometer and parasitic Johnson noise sources. Our analysis demonstrates that while the electrothermal feedback loop effectively suppresses Johnson noise originating from the bolometer resistance, it provides no suppression of Johnson noise from external series resistance. In the limit of high loop gain, the bolometer Johnson noise contribution vanishes while the parasitic contribution remains unsuppressed.