We describe the first phase of experimental study of the superconducting bridge RFTES detector at temperatures 20-300 mK including the measurement of its thermal conductance, which fits the model of electron gas heating. We discuss the idea of the RFTES scheme, which is based on the probing of microwave loss near superconducting transition of the bridge. The heat applied to the bridge is generated by the probing signal at the frequency of the high-Q resonator. Since the real part is dominating in the nonlinear impedance of the bridge, the applied heat provides merely amplitude modulation of Q suggesting the suppression of phase jitter of the resonator. The bridge was made from a 50-nm-thick hafnium film (TC≈ 380 mK) sized to 2.5 μm × 2.5 μm. The resonator and the rest of the circuit were made from 200-nm-thick film of niobium (TC≈ 9 K) demonstrating the loaded Q-factor up to and above 10 000 at 1.5 GHz. A cryogenic semiconductor amplifier was used in the readout circuit. The thermal conductance was measured using the steady Q regime of the resonator and found to follow T5down to and below G ≈ 1×10-13W/K. The NEP below 10-18W/√Hz is estimated for the electron temperature of the bridge about 300 mK.
We describe the first phase of experimental study of the superconducting bridge RFTES detector at temperatures 20-300 mK including the measurement of its thermal conductance, which fits the model of electron gas heating. We discuss the idea of the RFTES scheme, which is based on the probing of microwave loss near superconducting transition of the bridge. The heat applied to the bridge is generated by the probing signal at the frequency of the high-Q resonator. Since the real part is dominating in the nonlinear impedance of the bridge, the applied heat provides merely amplitude modulation of Q suggesting the suppression of phase jitter of the resonator. The bridge was made from a 50-nm-thick hafnium film (TC≈ 380 mK) sized to 2.5 μm × 2.5 μm. The resonator and the rest of the circuit were made from 200-nm-thick film of niobium (TC≈ 9 K) demonstrating the loaded Q-factor up to and above 10 000 at 1.5 GHz. A cryogenic semiconductor amplifier was used in the readout circuit. The thermal conductance was measured using the steady Q regime of the resonator and found to follow T5down to and below G ≈ 1×10-13W/K. The NEP below 10-18W/√Hz is estimated for the electron temperature of the bridge about 300 mK.
A high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ( T C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10 –13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10 –19 W/Hz 1/2 at a temperature of 150 mK.
We demonstrated and analyzed the smooth microwave-driven transition of a superconducting bridge made from Hf into its normal state at bath temperatures below the critical temperature of Hf (T c ≈ 380 mK). The bridge is integrated on a silicon chip with both the 600-700 GHz double-slot antenna and the 1.5-GHz CPW quarter-wave resonator (Q-factor ∼10 4 ) made from 100-nm Nb film. The experimental bridge was sized 2.5 um by 2.5 um by 50 nm and tested at temperatures down to 50 mK. Similar to the technique of MKID, we measured the dependence of transmission S 21 on microwave power at the bottom of the resonance curve. It was found that the microwave power absorbed in the bridge fits to the model of hot electron gas, P∼T e 6 -T ph 6 . The internal NEP down to ≈10 −18 W/⎷Hz is estimated due to thermal noise at the optimum electron gas temperature, T e ≈ 320 mK. The NEP can be scaled down below ≈10 −19 W/√Hz via reasonable reduction of the bridge volume. The new detector circuit is suitable for integration within a large imaging array exploiting the FDM-readout.
AbstractA high-quality superconducting resonator with a microbridge of hafnium film for use in a circuit for readout a terahertz-band imaging array with frequency division multiplexing is demonstrated experimentally. The variability of the impedance of the bridge at a frequency of 1.5 GHz, which is a key factor in the control of the quality of the resonator, is studied. The bridge, having a thickness of about 50 nm, a critical temperature T _ C ≈ 380 mK, and a plan size of 2.5 × 2.5 μm, was connected as a load of a resonator made of niobium film with a thickness of about 100 nm ( T _ C ~ 9 K). It is shown that the bridge smoothly changes its impedance proportionally to the bias power in the entire temperature range. The effective thermal insulation of the bridge was measured in a dilution cryostat at temperatures of 50–300 mK. Thermal conductivity G of the bridge was calculated and found to be ~4 × 10^–13 W/K, which gives an estimate of the sensitivity of the structure in the bolometric mode NEP ≈ 8 × 10^–19 W/Hz^1/2 at a temperature of 150 mK.
Our new detector and readout concept brings together TES (Transition Edge Sensor) and MKID (Microwave Kinetic Inductance Detector) technologies and exploits the idea of a microwave-induced superconducting transition in a small thin-film microbridge. The superconducting transition of the bridge manifests itself as variation in the Q -factor of niobium resonators at 5-8 GHz, somewhat similar to MKID operation. We present data showing the potential for developing this concept into multipixel detector arrays. Single-pixel sensitivity was measured at 4.5 K for an input band of 600-700 GHz using a prototype 10-nm-thick Nb bridge of size 1 μm × 500 nm. Radiation from human skin was detected with a resolution better than 1 K/rtHz, which is encouraging for terahertz imaging applications. To further improve device sensitivity, we are also developing Hf-based devices that operate near 0.35 K. Details about the physics and stability of these devices are discussed.
We propose a new detection technique based on radio-frequency (RF) bias and readout of an antenna-coupled superconducting nanobolometer. This approach is suitable for Frequency-Division-Multiplexing (FDM) readout of large arrays using broadband low-noise RF amplifier. We call this new detector RFTES. This feasibility study was made on demonstrator devices which are made in all-Nb technology and operate at 4.2 K. The studied RFTES devices consist of an antenna-coupled superconducting nanobolometer made of ultrathin niobium films with transition temperature Tc = 5.2 K. The 0.65-THz antenna and nanobolometer are embedded as a load into a GHz-range coplanar niobium resonator (Tc = 8.9 K, Q = 4000). To heat the superconducting Nb nanobolometer close to the Tc, the RF power at resonator frequency f = 5.8 GHz is applied via a transmission line which is weakly coupled (-11 dB) to the loaded resonator. The THz-antenna of RFTES was placed in the focus of a sapphire immersion lens inside a He4-cryostat equipped with an optical window and a semiconductor RF amplifier. We have demonstrated optical response of the RFTES to THz radiation. The demonstrator receiver system employing the RFTES device showed an optical Noise-Equivalent Power (NEP) 1e-14 W/sqrt(Hz) at 4.2 K.
Cryogenic systems are essential to many modern scientific and technological applications, including MRI superconducting magnets, particle accelerators, astrophysical detectors and quantum computing platforms, all of which operate at temperatures well below 120 K. In this framework, accurate temperature monitoring is, therefore, critical. However, conventional Resistance Temperature Detectors (RTDs) suffer from significant limitations in cryogenic environments, such as wiring complexity, power dissipation, limited low-temperature operation and large footprint, which restrict their integration in increasingly complex cryogenic infrastructures. Superconducting devices offer a promising alternative due to their high sensitivity at low temperatures, low power dissipation and compatibility with integrated quantum technologies. Inspired by Microwave Kinetic Inductance Detectors (MKIDs), this work investigates superconducting microwave resonators as minimally invasive, frequency-multiplexed cryogenic thermometers operating below 12 K. In particular, strategies to tune and enhance temperature sensitivity are explored by studying the effects of the superconducting thin-film thickness and surface nanostructuring. The first part of the study examines NbTi(N) planar resonators with thicknesses ranging from 20 nm to 150 nm, where temperature sensing is based on the temperature dependence of the kinetic inductance. S-shaped split-ring resonators resonating near 1.1 GHz are inductively coupled to a coplanar transmission line and characterized under cryogenic conditions. A clear trade-off between temperature sensitivity and quality factor is observed: thinner films achieve higher sensitivity, up to approximately 19 MHz/K for 20 nm thick devices, while the quality factor, typically exceeding 10^4, is found to degrade when the thickness is reduced. This trade-off leads to an optimal temperature resolution, with sub-microkelvin noise-equivalent temperature (NET) values. In particular, a minimum NET of (0.53±0.09) µK/Hz^(1/2) at 4.2 K is achieved for a 100 nm thick resonator, which is also exploited to study low-frequency temperature fluctuations in liquid helium. In the second part, a possible approach to enhance the temperature sensitivity, through surface nanostructuring, is investigated. Arrays of nanogaps are patterned in high-current regions of a 1.3 GHz resonator, in order to locally suppress superconductivity, reduce the critical temperature of the film and, therefore, accentuate the slope of the temperature-sensitive curve. These weak-links are realized by partially etching a 100 nm NbTi film, to leave 20-30 nm thick remaining nanogaps. Measurements reveal a significant increase in temperature sensitivity, reaching values as high as up to (62±1) MHz/K, which correspond to one order-of-magnitude improvement over non-nanostructured devices. Although this enhancement comes with increased noise-equivalent temperature, the results demonstrate the feasibility of engineering superconducting weak links to improve RF cryogenic thermometry and provide a solid foundation for future optimization. In reason of the achieved results, such an approach could find successful applications for a sub-µK high-resolution cryogenic temperature monitoring, specifically for integrated quantum architectures, further benefiting from the frequency-multiplexing that naturally arises from the multiple allocation of different superconducting microwave resonators.