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IEEE Transactions on Terahertz Science and Technology

A Resonant Cold-Electron Bolometer With a Kinetic Inductance Nanofilter

L. S. Kuzmin

Published Mar 25, 2014• 14 citations
Publisher

Abstract

A novel type of resonant cold-electron bolometer (RCEB) is proposed. In this sensor, the internal resonance is organized by the kinetic inductance of an ultrasmall NbN superconducting nanostrip and the capacitance of the nanoscale superconductor-insulator-normal (SIN) tunnel junction. The size of kinetic inductance is about 300 times smaller than the size of geometrical inductance of the same value. This reduction of size gives an opportunity to create nanofilters with a total size considerably smaller than the wavelength. This internal resonance acts as a bandpass filter (instead of external filters) with a bandwidth of 3%-50%, something needed for radioastronomy applications. The use of NbN as Andreev contact material against escaping hot quasiparticles provides a unique opportunity to create an optimal CEB with only one SIN junction and an Andreev SN contact for thermal protection, in contrast to a classical CEB with two SIN junctions. The problem of hot quasiparticles escaping over the Andreev barrier (Δ) for Al electrode is overcome here by creating a barrier higher than quantum energy Δ > hf to have working kinetic inductance. Replacing two SIN junctions by one junction, we could use this junction of half the area to keep the same capacitive coupling. Therefore, a single-junction RCEB would have four times less area of junctions (and volume of absorber) in comparison with a two-junction RCEB and should show better noise performance due to the proportionally decreased e-ph (electron-phonon) noise component. The RCEB can be effectively used to create multiband elements that are actual tasks in submillimeter astronomy due to the benefit that comes from its ability to use co-located data, and problems with the dramatic increase of the size of the focal plane.

DOI
10.1109/tthz.2014.2311321

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A novel type of the resonant cold-electron bolometer (RCEB) has been realized for multichroic pixels. The internal resonance is organized by a kinetic inductance of the NbN superconducting nanostrip and a capacitance of the superconductor-insulator-normal (SIN) tunnel junctions. The basic idea of a multichroic system is to combine a wideband antenna and narrow-band RCEBs. We used a single Lambda slot with two RCEBs for 75 and 105 GHz, placed in the middle. Each RCEB included two SIN junctions with absorber connected to NbN kinetic inductance of 420 and 240 pH. SIN junctions had capacitances of 11 and 8 fF. Wave impedance of the slot antenna was near 15 Ω and resistance of the absorber was matched to this value. Matching of antenna with an external system was done by placing Si lens on the back side of a Si substrate. RF testing was done at 300 mK irradiating this chip by sweep generator from 60 to 120 GHz. The response curves showed clear resonances at 75 and 105 GHz with quality factor of 10 and 7. Kinetic inductance value was estimated at the level of 35 pH/sq. These experiments confirmed that the RCEB can be effectively used to create multiband elements.

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A novel concept of the optimal Cold-Electron Bolometer (CEB) with a Superconductor-Insulator-Normal (SIN) Tunnel Junction and Andreev SN contact has been proposed. This concept has been developed to improve noise properties of the CEB by increasing current responsivity for SQUID readout (in voltage-biased mode) in comparison with classical CEB concept comprising two SIN tunnel junctions. In this configuration the power of incoming signal is not split between two series junctions and the current response of a single junction is twice larger in comparison with double junction structure. The signal is concentrated from antenna to an absorber through capacitance of the tunnel junction from one side and Andreev contact from another side. HF matching is realized by resistance of a normal metal absorber that is independent on tunnel junction parameters. The volume of a normal metal is partly squeezed due to proximity effect of a superconducting electrode from the Andreev contact that also increases efficiency of the electron cooling. The concept is based on direct electron cooling of the absorber by SIN tunnel junction that serves as negative electrothermal feedback for the signal. Noise properties are considerably improved by decreasing the electron temperature. Ultimate performance of the CEB is determined by optical load converted to shot noise of the signal readout. The goal is to achieve noise-equivalent power (NEP) of the CEB with standard SQUID readout less than photon noise. Ultimate NEP better than photon noise can be achieved practically in wide range of power loads from 0.1 fW (SPICA) to 30 pW (OLIMPO, CLOVER, PILOT). Applicability of the CEB to post-Herschel space missions looks very promising.

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A novel type of resonant cold-electron bolometer (RCEB) is proposed. In this sensor, the internal resonance is organized by the kinetic inductance of an ultrasmall NbN superconducting nanostrip and the capacitance of the nanoscale superconductor-insulator-normal (SIN) tunnel junction. The size of kinetic inductance is about 300 times smaller than the size of geometrical inductance of the same value. This reduction of size gives an opportunity to create nanofilters with a total size considerably smaller than the wavelength. This internal resonance acts as a bandpass filter (instead of external filters) with a bandwidth of 3%-50%, something needed for radioastronomy applications. The use of NbN as Andreev contact material against escaping hot quasiparticles provides a unique opportunity to create an optimal CEB with only one SIN junction and an Andreev SN contact for thermal protection, in contrast to a classical CEB with two SIN junctions. The problem of hot quasiparticles escaping over the Andreev barrier (Δ) for Al electrode is overcome here by creating a barrier higher than quantum energy Δ > hf to have working kinetic inductance. Replacing two SIN junctions by one junction, we could use this junction of half the area to keep the same capacitive coupling. Therefore, a single-junction RCEB would have four times less area of junctions (and volume of absorber) in comparison with a two-junction RCEB and should show better noise performance due to the proportionally decreased e-ph (electron-phonon) noise component. The RCEB can be effectively used to create multiband elements that are actual tasks in submillimeter astronomy due to the benefit that comes from its ability to use co-located data, and problems with the dramatic increase of the size of the focal plane.
A novel type of the resonant cold-electron bolometer (RCEB) has been realized for multichroic pixels. The internal resonance is organized by a kinetic inductance of the NbN superconducting nanostrip and a capacitance of the superconductor-insulator-normal (SIN) tunnel junctions. The basic idea of a multichroic system is to combine a wideband antenna and narrow-band RCEBs. We used a single Lambda slot with two RCEBs for 75 and 105 GHz, placed in the middle. Each RCEB included two SIN junctions with absorber connected to NbN kinetic inductance of 420 and 240 pH. SIN junctions had capacitances of 11 and 8 fF. Wave impedance of the slot antenna was near 15 Ω and resistance of the absorber was matched to this value. Matching of antenna with an external system was done by placing Si lens on the back side of a Si substrate. RF testing was done at 300 mK irradiating this chip by sweep generator from 60 to 120 GHz. The response curves showed clear resonances at 75 and 105 GHz with quality factor of 10 and 7. Kinetic inductance value was estimated at the level of 35 pH/sq. These experiments confirmed that the RCEB can be effectively used to create multiband elements.
A novel concept of the optimal Cold-Electron Bolometer (CEB) with a Superconductor-Insulator-Normal (SIN) Tunnel Junction and Andreev SN contact has been proposed. This concept has been developed to improve noise properties of the CEB by increasing current responsivity for SQUID readout (in voltage-biased mode) in comparison with classical CEB concept comprising two SIN tunnel junctions. In this configuration the power of incoming signal is not split between two series junctions and the current response of a single junction is twice larger in comparison with double junction structure. The signal is concentrated from antenna to an absorber through capacitance of the tunnel junction from one side and Andreev contact from another side. HF matching is realized by resistance of a normal metal absorber that is independent on tunnel junction parameters. The volume of a normal metal is partly squeezed due to proximity effect of a superconducting electrode from the Andreev contact that also increases efficiency of the electron cooling. The concept is based on direct electron cooling of the absorber by SIN tunnel junction that serves as negative electrothermal feedback for the signal. Noise properties are considerably improved by decreasing the electron temperature. Ultimate performance of the CEB is determined by optical load converted to shot noise of the signal readout. The goal is to achieve noise-equivalent power (NEP) of the CEB with standard SQUID readout less than photon noise. Ultimate NEP better than photon noise can be achieved practically in wide range of power loads from 0.1 fW (SPICA) to 30 pW (OLIMPO, CLOVER, PILOT). Applicability of the CEB to post-Herschel space missions looks very promising.
A capacitively coupled hot-electron nanobolometer (CC-HEB) is the simplest and most effective antenna-coupled bolometer. The bolometer consists of a small absorber connected to the superconducting antenna by tunnel junctions. The tunnel junctions used for high-frequency coupling also give perfect thermal isolation of hot electrons in the small volume of the absorber. The same tunnel junctions are used for temperature measurements and electron cooling. This bolometer does not suffer from the frequency limitations in the submillimeter range due to the high potential barrier of the tunnel junctions as does the microbolometer with Andreev mirrors (A-HEB), which is limited by the superconducting gap. Theoretical analyses show that the two-junction configuration more than doubles the sensitivity of the bolometer in current-biased mode compared to the single-junction configuration used for A-HEB. Another important advantage of CC-HEB is its simple two-layer technology for sample fabrication. Samples were fabricated with an absorber made of a bilayer of Cr and Al to match the impedance of the antenna. Electrodes were made of Al and tunnel junctions were formed over the Al oxide layer. The coupling capacitances of the tunnel junctions, C ≈ 20 fF, in combination with the inductance of the 10 μm absorber create a bandpass filter with a central frequency around 300 GHz. Bolometers are integrated with log-periodic and double-dipole planar antennas made of Au. The temperature response of bolometer structures was measured at temperatures down to 256 mK. In our experiment we observed dV/dT=1.3 mV/K, corresponding to responsivity S=0.2.109 V/W. For amplifier noise Vna=3nV/Hz1/2 at 1 kHz the estimated total noise equivalent power is NEP=1.5.10-17 W/Hz1/2. The intrinsic bolometer self noise Vnbol=0.5 nV/Hz1/2 corresponds to NEP=3.10-18 W/Hz1/2. For microwave evaluation of bolometer sensitivity we used a black body radiation source comprising a thin NiCr stimulator placed on the cold plate of cryostat in front of a CC-HEB attached to an extended hemisphere sapphire lens. This measurements were consistent with estimates based on the dc responsivity of the bolometer.
An optimized concept of the Cold-Electron Bolometer (CEB) with Superconductor-Insulator-Normal (SIN) Tunnel Junction and Andreev SN contact for voltage-bias mode has been developed. The responsivity of this design is much better than the widespread concept of the CEB with two SIN tunnel junctions for the current-biased mode (SINIS structure) due to degradation of responsivity by a factor of two in voltage-bias (VB) mode. Besides that, for the realization of the CEB with the same capacitance, we need twice larger junctions that lead to four times larger area and volume of the absorber. As a result, the NEP of one SIN junction CEB is half that of two SIN junction CEB. A novel concept of the quasiparticle cascade amplifier (QCA) based on harvesting the waste energy of relaxed quasiparticles without any additional voltage bias has been developed. The distinguish feature of the cascade principle is utilizing the same dc voltage as used for CEB. The cascade principle of signal amplification proved to be possible due to the conversion of electrons to holes in the second absorber. The amplifier is based on the optimized CEB with one SIN junction and one SN contact. The coefficient of power amplification of the incoming signal to the potential energy of hot quasiparticles is huge and can achieve a level of 140 at the optimal bias point. The coefficient of power amplification of the QCA can reach a value of 30 and more to overcome the noise of the SQUID amplifier. For a power load of 6 pW, the total NEP of the CEB is decreased to the level of photon noise NEP, down to 0.7*NEPphot.
The Cold-Electron Bolometer (CEB) is an ultrasensitive device designed for the detection of cosmic microwave background radiation. The key to its sensitivity is the electron cooling of the absorber by the superconductor-insulator-normal metal (SIN) tunnel junction. At a voltage near and below the superconducting gap, the electrons in the absorber are cooled well below the phonon temperature of the normal metal. This translates to the enhanced sensitivity of the CEB. This thesis describes the work we have done on the optimization of electron cooling of the normal metal absorber, and our measurement of the sensitivity of the CEB. We have optimized the electron cooling of the absorber by SIN tunnel junctions. The best electron cooling was achieved when normal metal traps were added in proximity to the superconducting electrodes in addition to the advanced geometry of the superconducting electrodes. With these modifications, we have decreased the electron temperature by 198 mK. With just the advanced geometry, the electron temperature drop was 129 mK. With just a simple geometry, the drop in temperature was 56 mK.The noise equivalent power (NEP) of the CEB was also measured at 100 mK to be at the level of 10^-18 W/Hz^1/2 at 1 kHz. The NEP was obtained by measuring the noise of the CEB, and then dividing that by its power responsivity, dV/dP. The main limitation in our measurements was the noise component from the amplifier. Finally, we have made measurements on the temperature sensitivity of the SIN tunnel junctions. We have compared the sensitivity between single and ten SIN junctions in series and found that it increases proportionally to the number of junctions. The best temperature responsivity obtained for 10 junctions was approximately 15 μV/mK. Using such thermometer, we have been able to measure the temperature stability of the Oxford Instruments cryogenfree refrigerator to be plus/minus 250 μK for a period of 8 hours. The resolution of the thermometer was measured to be plus/minus100 μK, limited by the noise due to the amplifier. The current and future works are also discussed. These include: a CEB array designed to distribute the total power load, improving further the sensitivity; a one-thousand SIN tunnel junction series thermometer in order to realize and understand the fundamental limitation of such a device; a superconducting cold-electron bolometer (SCEB) for high background power loads; and different read-out systems including an on-chip quasiparticle amplifier (QPA).
A novel concept of the parallel/series array of Cold-Electron Bolometers (CEB) with Superconductor-Insulator-Normal (SIN) Tunnel Junctions has been proposed for matching with JFET readout. The current-biased CEBs are connected in series for DC and in parallel for HF signal. A signal is concentrated to the absorber through the capacitance of tunnel junctions and additional capacitance for coupling of superconducting islands. Due to dividing power between CEBs in the array and increasing responsivity, the noise matching could be effectively optimized and the photon Noise Equivalent Power could be easily achieved at 300 mK with a room temperature JFET readout.