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Superconductor Science and Technology

Optical response of a titanium-based cold-electron bolometer

Ernst Otto, Michael A. Tarasov, Paul Grimes, Artem M. Chekushkin, L. S. Kuzmin, G. Yassin

Published Jul 4, 2013• 4 citations
Publisher

Abstract

We present experimental results on the testing of cold-electron bolometer (CEB) detectors comprised of a thin Ti film absorber and two SIN junctions integrated with a planar antenna. The CEB performance was tested in a He-3 sorption cryostat HELIOX-AC-V at bath temperatures of 280-305 mK. The optical response was measured using the hot/cold load method by flipping a Cu reflector opposite a blackbody surface inside a 3 K shield and using a thermal source with variable temperature. In the first experiment, the detector chip was mounted in an optical sample-holder whose aperture was switched towards or away from a blackbody source changing the incident radiation temperature from 3 K to 270 mK. As a result, we measured the optical response to a 3 K/270 mK radiation temperature change. The measured voltage response value for the detector integrated in a double-dipole antenna was Delta V-out = 120 mu V. This corresponds to a noise equivalent power of NEP = V-n/(dV/dP) = 3.5 x 10(-17) W Hz(-1/2), where dV/dP is the voltage to power response obtained from the incoming power estimation based on the Planck formula.

DOI
10.1088/0953-2048/26/8/085020

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We present experimental results on the testing of cold-electron bolometer (CEB) detectors comprised of a thin Ti film absorber and two SIN junctions integrated with a planar antenna. The CEB performance was tested in a He-3 sorption cryostat HELIOX-AC-V at bath temperatures of 280-305 mK. The optical response was measured using the hot/cold load method by flipping a Cu reflector opposite a blackbody surface inside a 3 K shield and using a thermal source with variable temperature. In the first experiment, the detector chip was mounted in an optical sample-holder whose aperture was switched towards or away from a blackbody source changing the incident radiation temperature from 3 K to 270 mK. As a result, we measured the optical response to a 3 K/270 mK radiation temperature change. The measured voltage response value for the detector integrated in a double-dipole antenna was Delta V-out = 120 mu V. This corresponds to a noise equivalent power of NEP = V-n/(dV/dP) = 3.5 x 10(-17) W Hz(-1/2), where dV/dP is the voltage to power response obtained from the incoming power estimation based on the Planck formula.
Two series/parallel arrays of ten cold-electron bolometers with superconductor-insulator-normal tunnel junctions were integrated in orthogonal ports of a cross-slot antenna. To increase the dynamic range of the receiver, all single bolometers in an array are connected in parallel for the microwave signal by capacitive coupling. To increase the output response, bolometers are connected in series for dc bias. With the measured voltage-to-temperature response of 8.8 μV/mK, absorber volume of 0.08 μm3, and output noise of about 10 nV/Hz1/2, we estimated the dark electrical noise equivalent power (NEP) as NEP = 6 * 10-18W/Hz1/2. The optical response down to NEP = 2 * 10-17W/Hz1/2was measured using a hot/cold load as a radiation source and a sample temperature down to 100 mK. The fluctuation sensitivity to the radiation source temperature is 1.3 * 10-4K/Hz1/2. A dynamic range over 43 dB was measured using a backward-wave oscillator, a variable polarization grid attenuator, and cold filters/attenuators.
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).
Cold electron bolometers integrated with twin-slot antennas have been designed and fabricated. Optical response was measured in 0.06-0.6 K temperature range using black body radiation source at temperature 2-15 K. The responsivity of 0.3109 V/W was measured at 2.7 K radiation temperature. The estimated ultimate dark responsivity at 100 mK can approach Sv=1010 V/W and reduces down to 1.1108 V/W at 300 mK for the sample with absorber volume of 510-20 m3. At high power load levels and low temperatures the changes of tunneling current, dynamic resistance and voltage response have been explained by non-thermal energy distribution of excited electrons. Distribution of excited electrons in such system is of none-Fermi type, electrons with energies of the order of 1 K tunnel from normal metal absorber to superconductor instead of relaxing down to thermal energy kTe. This effect can reduce quantum efficiency of bolometer from hf/kTph in ideal case down to single electron per signal quantum in the high power case.
In this paper, we present the measurements of optical response dependence on power load of a Cold Electron Bolometer integrated in a twin slot antenna. These measurements are also compared to the models of the bolometer limit and the photon counter limit. The responsivity of 0.22*109V/W was measured at 0.22 pW radiation power from a black body at 3.5 K. According to our estimations, for optimized device the voltage responsivity at 100 mK electron temperature can approach S v = 1010V/W for power load below 0.1 pW and decreases down to 107V/W at 300 mK for 5 pW signal power in a sample with absorber volume of 5*10-20m3. In the case of low bath temperatures and high applied RF power the changes of tunneling current, dynamic resistance and voltage response are explained by non-thermal energy distribution of excited electrons. Distribution of excited electrons in such system at lower temperatures can be of non-Fermi type, hot electrons with energies of the order of 1 K tunnel from normal metal absorber to superconductor instead of relaxing down to thermal energy kTein absorber before tunneling. This effect can reduce quantum efficiency of the bolometer at 350 GHz from η = hf/kTph> 100 in ideal case down to single electron per absorbed photon ( η = 1) in the high power case. Methods of preserving high quantum efficiency are discussed.
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.
Abstract Cold-electron bolometers have shown suitability for use in modern fundamental physical experiments. In this study, the fabrication and measurements of the samples with cold-electron bolometers integrated into coplanar antennas are performed. The absorber layer was made using combined aluminum-hafnium technology to improve quality of aluminum oxide layer and decrease electron–phonon coupling. The samples of two types were measured in a dilution cryostat at various temperatures from 20 to 300 mK. The first sample with Ti/Au/Pd antenna shows response in the two frequency bands, at 7–9 GHz with bandwidth of about 20%, and also at 14 GHz with 10% bandwidth. The NEP below 10 aW Hz − 1 is reached at 300 mK for 7.7 GHz signal. The second sample with aluminum made antenna shows response in the frequency range 0.5–3 GHz due to the effect of kinetic inductance of superconducting aluminum.