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M. Roos
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Low-Tc Superconducting Detector for Future CMB Missions
Designing Lens-Antenna coupled Microwave Kinetic Inductance Detectors applicable to 50-90 GHz using β-Ta
Master thesis
(2026)
-
M. Roos, J.J.A. Baselmans, H.S.J. van der Zant, S.O. Dabironezare, K. Karatsu
The next generation of Cosmic Microwave Background (CMB) missions require sensitive detectors to probe the processes that shaped the early Universe. The 50-90GHz frequency range is of particular interest, both for the measurement of the weak B-mode polarization patterns from primordial gravitational waves and for detection of spectral distortions in the CMB.
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.
The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.
The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.
Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.
We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.
Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.
...
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.
The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.
The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.
Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.
We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.
Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.
...
The next generation of Cosmic Microwave Background (CMB) missions require sensitive detectors to probe the processes that shaped the early Universe. The 50-90GHz frequency range is of particular interest, both for the measurement of the weak B-mode polarization patterns from primordial gravitational waves and for detection of spectral distortions in the CMB.
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.
The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.
The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.
Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.
We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.
Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.
Since Microwave Kinetic Inductance Detectors (MKIDs) offer great multiplexing capabilities, high sensitivity, easy fabrication, and reduced cost, they would be ideal for CMB missions. MKIDs are superconducting pair-breaking detectors, which makes them capable of detecting photons with a minimum photon energy of twice the superconducting gap energy. The superconducting gap energy is intrinsic to the superconductor embedded in the detector's hybrid microwave resonator structure, and thus poses a direct limit on the frequencies the detector can measure. For example, the conventionally used Aluminum (Tc=1.2-1.4K) would only be able to detect photons with frequencies larger than 90GHz. β-phase Tantalum (β-Ta), a disordered superconductor with Tc=0.6-1.0K, would enable detection of frequencies as low as 45GHz.
In this thesis, we therefore investigate the viability of using β-Ta in designing hybrid MKIDs for frequencies in the 50-90GHz range.
The effect of quasiparticle trapping due to disorder in superconductors is known to reduce the sensitivity of MKIDs. Consequently, the volume of β-Ta within the microwave resonator must be minimized to mitigate this effect and achieve photon-noise-limited sensitivity. We opt for a lens-antenna coupled hybrid MKID design to decouple the frequency sensitive antenna and the active volume of the resonator, such that both can be optimized independently.
The detectors presented in this thesis have been designed for 70GHz radiation, at the center of the 50-90GHz range. The lens antenna features an extended hemispherical lens coupling radiation from a black body source to a twin-slot antenna. The twin-slot antenna couples this pair-breaking radiation to the volume of β-Ta. We consider β-Ta/NbTiN hybrid MKIDs for the quarter-wave resonators. To approximate the properties of the superconducting materials at both readout and pair-breaking frequencies, Mattis-Bardeen theory is used.
Given the large normal state resistivity of β-Ta, the narrow coplanar waveguide in the hybrid MKID has a large characteristic impedance. This makes it difficult for the twin-slot antenna to match in impedance. Therefore, the dimensions of the narrow coplanar waveguide have to be optimized to minimize both its active volume and characteristic impedance, simultaneously. We obtain a trade-off in its dimensions, which is also limited by UV lithography fabrication limitations. The twin-slot antenna design is subsequently optimized for a sufficient impedance match.
We identify radiation losses at readout frequencies due to the addition of the twin-slot antenna structure. These losses are found to exceed the dissipation within the quasiparticle system predicted by Mattis-Bardeen theory at low operating temperatures, and would therefore dominate the internal quality factor of the MKIDs.
Conclusively, this thesis presents a viable methodology to design MKIDs in the range of 50-90GHz using β-Ta. The dimension optimizations of the narrow coplanar waveguide and the twin-slot antenna are not fully decoupled, and their impedance matching poses challenges to the MKID design. For future research, it is important to find an approach that would give more engineering freedom in their separate designs.
Pre-Flight Simulation of DESHIMA2.0
Observation of a High-Redshift Galaxy with the Current DESHIMA2.0 Instrument on the ASTE Telescope
To better understand the formation of stars in dusty star-forming galaxies (DSFGs), and the evolution of this type of submillimeter galaxies (SMGs), it is of high significance to perform submillimeter/far-infrared surveys. The Deep Spectroscopic High-redshift Mapper 2.0 (DESHIMA2.0) on the ASTE telescope in Chile has been designed to observe the redshifted emission lines of these DSFGs to measure both their (spectroscopic) redshifts and molecular compositions. DESHIMA2.0 is designed to observe across the 220-440 GHz frequency band using its 347 channel integrated superconducting spectrometer (ISS) chip.
The current DESHIMA2.0 instrument has been tested in the lab. Its 332 filter channels with center frequencies in the range of 204-391 GHz have a spectral resolution of f/δf ≈ 340. Their coupling efficiencies can be approximated by Lorentzian functions.
The gravitationally lensed ultraluminous high-redshift DSFG J1329+2243 with redshift z = 2.04 shows emission lines of molecular gases like CO and H2O in the frequency band of DESHIMA2.0. Using the Time-dependent End-to-end Model for Post-process Optimization (TiEMPO), an 8-hour observation of the J1329+2243 galaxy has been simulated for both the lab-measured chip and the designed chip. Two measures of sensitivity, the noise equivalent flux density (NEFD) and the minimum detectable line flux (MDLF), are theoretically derived and subsequently used to be compared with the results of the simulations. The results yielded from these simulations are ultimately used to report on the overall performance of the lab-measured chip.
To run a TiEMPO simulation using the lab-measured chip, adaptations had to be made to the model for it to accept customizable chip data. Within TiEMPO, variable spectral resolutions and coupling efficiencies had to be introduced as well as a crucial addition to enable the creation of a new filterbank. Given the results of the simulations, it can be stated that this implementation of the lab-measured chip has been successful.
After applying an ON-OFF (dual) sky chopping technique to the simulated observation to cancel fluctuations of the atmospheric transmission, the atmosphere-corrected antenna temperature of the J1329+ 2243 galaxy could be found. The standard deviation of the noise showed to be scaling inversely proportional to the square root of the integration time. For five separate emission lines of the galaxy within the range of 200-310 GHz, a signal-to-noise ratio (SNR) analysis was performed and compared to the estimated theoretical proportionality to the square root of the integration time. For lines that show overlap in the spectrum, the detection was proven to be more difficult. Optimizations in the strategy used to define these signal-to-noise ratios would improve this finding.
The performance of the current DESHIMA2.0 instrument is sufficient to detect (SNR≥5) the bright CO(7-6) line of an ultraluminous high-redshift galaxy like J1329+2243 after 5.5 minutes of observation time. After 50 minutes of observation time, it is also capable of identifying the CO(6-5), the H2O(211-202), the [CI](2-1), and the CO(8-7) emission line. ...
The current DESHIMA2.0 instrument has been tested in the lab. Its 332 filter channels with center frequencies in the range of 204-391 GHz have a spectral resolution of f/δf ≈ 340. Their coupling efficiencies can be approximated by Lorentzian functions.
The gravitationally lensed ultraluminous high-redshift DSFG J1329+2243 with redshift z = 2.04 shows emission lines of molecular gases like CO and H2O in the frequency band of DESHIMA2.0. Using the Time-dependent End-to-end Model for Post-process Optimization (TiEMPO), an 8-hour observation of the J1329+2243 galaxy has been simulated for both the lab-measured chip and the designed chip. Two measures of sensitivity, the noise equivalent flux density (NEFD) and the minimum detectable line flux (MDLF), are theoretically derived and subsequently used to be compared with the results of the simulations. The results yielded from these simulations are ultimately used to report on the overall performance of the lab-measured chip.
To run a TiEMPO simulation using the lab-measured chip, adaptations had to be made to the model for it to accept customizable chip data. Within TiEMPO, variable spectral resolutions and coupling efficiencies had to be introduced as well as a crucial addition to enable the creation of a new filterbank. Given the results of the simulations, it can be stated that this implementation of the lab-measured chip has been successful.
After applying an ON-OFF (dual) sky chopping technique to the simulated observation to cancel fluctuations of the atmospheric transmission, the atmosphere-corrected antenna temperature of the J1329+ 2243 galaxy could be found. The standard deviation of the noise showed to be scaling inversely proportional to the square root of the integration time. For five separate emission lines of the galaxy within the range of 200-310 GHz, a signal-to-noise ratio (SNR) analysis was performed and compared to the estimated theoretical proportionality to the square root of the integration time. For lines that show overlap in the spectrum, the detection was proven to be more difficult. Optimizations in the strategy used to define these signal-to-noise ratios would improve this finding.
The performance of the current DESHIMA2.0 instrument is sufficient to detect (SNR≥5) the bright CO(7-6) line of an ultraluminous high-redshift galaxy like J1329+2243 after 5.5 minutes of observation time. After 50 minutes of observation time, it is also capable of identifying the CO(6-5), the H2O(211-202), the [CI](2-1), and the CO(8-7) emission line. ...
To better understand the formation of stars in dusty star-forming galaxies (DSFGs), and the evolution of this type of submillimeter galaxies (SMGs), it is of high significance to perform submillimeter/far-infrared surveys. The Deep Spectroscopic High-redshift Mapper 2.0 (DESHIMA2.0) on the ASTE telescope in Chile has been designed to observe the redshifted emission lines of these DSFGs to measure both their (spectroscopic) redshifts and molecular compositions. DESHIMA2.0 is designed to observe across the 220-440 GHz frequency band using its 347 channel integrated superconducting spectrometer (ISS) chip.
The current DESHIMA2.0 instrument has been tested in the lab. Its 332 filter channels with center frequencies in the range of 204-391 GHz have a spectral resolution of f/δf ≈ 340. Their coupling efficiencies can be approximated by Lorentzian functions.
The gravitationally lensed ultraluminous high-redshift DSFG J1329+2243 with redshift z = 2.04 shows emission lines of molecular gases like CO and H2O in the frequency band of DESHIMA2.0. Using the Time-dependent End-to-end Model for Post-process Optimization (TiEMPO), an 8-hour observation of the J1329+2243 galaxy has been simulated for both the lab-measured chip and the designed chip. Two measures of sensitivity, the noise equivalent flux density (NEFD) and the minimum detectable line flux (MDLF), are theoretically derived and subsequently used to be compared with the results of the simulations. The results yielded from these simulations are ultimately used to report on the overall performance of the lab-measured chip.
To run a TiEMPO simulation using the lab-measured chip, adaptations had to be made to the model for it to accept customizable chip data. Within TiEMPO, variable spectral resolutions and coupling efficiencies had to be introduced as well as a crucial addition to enable the creation of a new filterbank. Given the results of the simulations, it can be stated that this implementation of the lab-measured chip has been successful.
After applying an ON-OFF (dual) sky chopping technique to the simulated observation to cancel fluctuations of the atmospheric transmission, the atmosphere-corrected antenna temperature of the J1329+ 2243 galaxy could be found. The standard deviation of the noise showed to be scaling inversely proportional to the square root of the integration time. For five separate emission lines of the galaxy within the range of 200-310 GHz, a signal-to-noise ratio (SNR) analysis was performed and compared to the estimated theoretical proportionality to the square root of the integration time. For lines that show overlap in the spectrum, the detection was proven to be more difficult. Optimizations in the strategy used to define these signal-to-noise ratios would improve this finding.
The performance of the current DESHIMA2.0 instrument is sufficient to detect (SNR≥5) the bright CO(7-6) line of an ultraluminous high-redshift galaxy like J1329+2243 after 5.5 minutes of observation time. After 50 minutes of observation time, it is also capable of identifying the CO(6-5), the H2O(211-202), the [CI](2-1), and the CO(8-7) emission line.
The current DESHIMA2.0 instrument has been tested in the lab. Its 332 filter channels with center frequencies in the range of 204-391 GHz have a spectral resolution of f/δf ≈ 340. Their coupling efficiencies can be approximated by Lorentzian functions.
The gravitationally lensed ultraluminous high-redshift DSFG J1329+2243 with redshift z = 2.04 shows emission lines of molecular gases like CO and H2O in the frequency band of DESHIMA2.0. Using the Time-dependent End-to-end Model for Post-process Optimization (TiEMPO), an 8-hour observation of the J1329+2243 galaxy has been simulated for both the lab-measured chip and the designed chip. Two measures of sensitivity, the noise equivalent flux density (NEFD) and the minimum detectable line flux (MDLF), are theoretically derived and subsequently used to be compared with the results of the simulations. The results yielded from these simulations are ultimately used to report on the overall performance of the lab-measured chip.
To run a TiEMPO simulation using the lab-measured chip, adaptations had to be made to the model for it to accept customizable chip data. Within TiEMPO, variable spectral resolutions and coupling efficiencies had to be introduced as well as a crucial addition to enable the creation of a new filterbank. Given the results of the simulations, it can be stated that this implementation of the lab-measured chip has been successful.
After applying an ON-OFF (dual) sky chopping technique to the simulated observation to cancel fluctuations of the atmospheric transmission, the atmosphere-corrected antenna temperature of the J1329+ 2243 galaxy could be found. The standard deviation of the noise showed to be scaling inversely proportional to the square root of the integration time. For five separate emission lines of the galaxy within the range of 200-310 GHz, a signal-to-noise ratio (SNR) analysis was performed and compared to the estimated theoretical proportionality to the square root of the integration time. For lines that show overlap in the spectrum, the detection was proven to be more difficult. Optimizations in the strategy used to define these signal-to-noise ratios would improve this finding.
The performance of the current DESHIMA2.0 instrument is sufficient to detect (SNR≥5) the bright CO(7-6) line of an ultraluminous high-redshift galaxy like J1329+2243 after 5.5 minutes of observation time. After 50 minutes of observation time, it is also capable of identifying the CO(6-5), the H2O(211-202), the [CI](2-1), and the CO(8-7) emission line.