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IEEE Transactions on Applied Superconductivity

Record-Low NEP in Hot-Electron Titanium Nanobolometers

Boris S. Karasik, David I. Olaya, Jian Wei, Sergey Pereverzev, M. E. Gershenson, Jonathan H. Kawamura, William R. McGrath, Andrei V. Sergeev

Published Jun 1, 2007• 40 citations
Publisher

Abstract

We are developing hot-electron superconducting transition-edge sensors (TES) capable of counting THz photons and operating at . We fabricated superconducting Ti nanosensors with Nb contacts with a volume of on planar Si substrates and have measured the thermal conductance in the material, G=4times10-3W/K at 0.3 K, caused predominantly by the weak electron-phonon coupling. The corresponding phonon-noise NEP=3times10-19W/Hz1/2. Detection of single optical photons (1550 nm and 670 nm wavelength) has been demonstrated for larger devices and yielded the thermal time constants of 30 mus at 145 mK and of 25 mus at 190 mK. This hot-electron direct detector (HEDD) is expected to have a small enough energy fluctuation noise for detecting individual photons with v>THz where NEP~3times10-20W/Hz1/2is needed for spectroscopy in space.

DOI
10.1109/tasc.2007.897167

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IEEE Transactions on Applied Superconductivity•Nov 20, 2014•20 citations
We are developing hot-electron superconducting transition-edge sensors (TES) capable of counting THz photons and operating at . We fabricated superconducting Ti nanosensors with Nb contacts with a volume of on planar Si substrates and have measured the thermal conductance in the material, G=4times10-3W/K at 0.3 K, caused predominantly by the weak electron-phonon coupling. The corresponding phonon-noise NEP=3times10-19W/Hz1/2. Detection of single optical photons (1550 nm and 670 nm wavelength) has been demonstrated for larger devices and yielded the thermal time constants of 30 mus at 145 mK and of 25 mus at 190 mK. This hot-electron direct detector (HEDD) is expected to have a small enough energy fluctuation noise for detecting individual photons with v>THz where NEP~3times10-20W/Hz1/2is needed for spectroscopy in space.
We discuss an implementation of a hot-electron transition-edge sensor (TES) capable of counting THz photons. The main need for such a THz calorimeter is spectroscopy on future space telescopes with a background limited NEP/spl sim/10/sup -20/ W/Hz/sup 1/2/. The micromachined bolometers will unlikely reach such sensitivity at temperatures above 10 mK. The hot-electron TES with sufficient sensitivity will still have a time constant /spl sim/0.1-1.0 ms that is too short for integrating a flux of THz background photons arriving at a rate of <100 s/sup -1/. The Hot-Electron Photon Counter based on a submicron-size superconducting Ti bridge with Nb Andreev contacts will be able to detect individual photons above 170 GHz due to its very low heat capacity. A discrimination of the low energy fluctuations with a threshold device would allow for realization of an NEP/spl sim/10/sup -20/ W/Hz/sup 1/2/ at /spl ges/1 THz while operating at 300 mK. With the sensor time constant of a few microseconds, the dynamic range is /spl sim/30-40 dB. A compact array of the antenna-coupled counters can be fabricated on a silicon wafer without membranes.
We are presenting experimental results on the electrical noise in small titanium hot-electron nanobolometers with the critical temperature above 300 mK. The noise data demonstrate good agreement with the conventional bolometer theory prediction. The noise is dominated by the thermal energy fluctuations (phonon noise) when the operating temperature is set just a few mK below the superconducting transition. The corresponding noise equivalent power (NEP) is about 3 times 10-18W/Hz1/2for the smallest measured device. The relative NEP's for the two devices measured scale roughly as the square root of the device volume as one would expect from the theory. Therefore an additional factor of 2-3 reduction of NEP may be feasible if the length and width of our device are further reduced. The demonstrated combination of the low NEP and the relatively high operating temperature is attractive for submillimeter low-background applications.
Superconducting transition-edge sensors (TESs), working as single-photon detectors with high quantum efficiency, low dark count rate, and photon number resolving capability, have been successfully used in quantum information experiments as well as astronomical transient object observations. Aiming to achieve high count rate, we are developing titanium-based superconducting TES single-photon detectors with a critical temperature of ~300 mK. Ti-based TESs with active area from 10 × 10 μm to 40 × 40 μm are first fabricated on silicon substrate coated with SiOx. The fabricated TES devices show a thermal conductance scaling with the active area and an effective time constant of ~4 μs from pulsed laser diode illumination. In order to get high quantum efficiency, we further design and fabricate a 1550-nm optical cavity based on dielectric mirror and antireflection layer, and study its absorption with an IR spectrometer. Ti TES devices on the dielectric mirror are then fabricated and the superconducting transition is measured.
We report on the measurement of a very low noise equivalent power of 3×10−19 W/Hz1/2 at 620 GHz in a superconducting antenna-coupled hot-electron bolometer. The sensing element is a micron-size titanium transition-edge sensor with NbTiN superconducting contacts fabricated on a sapphire substrate. The high sensitivity is due to the small device volume, low operating temperature, and weak electron-phonon coupling in titanium film. Measurements were done using a cryogenic black body emitter producing well-controlled femtowatt power levels. The achieved optical sensitivity is suitable for the low-background spectroscopy of molecular lines on next generation space telescopes.
We have demonstrated the multiplexed operation of titanium hot-electron nanobolometers (nano-HEB). Because of their low thermal conductance and small electron heat capacity nanobolometers are particularly interesting as sensors for far- infrared spectroscopy and mid- and near-IR calorimetry. However, the short time constant of these devices (~ µs at 300- 400 mK) makes time domain or audio-frequency domain multiplexing impractical. The Microwave SQUID (MSQUID) approach pursued in this work uses dc SQUIDs coupled to GHz-band microresonators which are, in turn, coupled to a transmission line. We used a 4-element array of Ti HEBs operated at 415 mK in a He3 dewar with an optical fiber access. The microwave signal exhibited 10-MHz wide resonances at individual MSQUD frequencies between 9 GHz and 10 GHz. The resonance depth is modulated by the current through the bolometer via a change of the SQUID flux state. The transmitted signal was amplified by a cryogenic amplifier and downconverted to baseband using an IQ mixer. A 1-dB per Φ 0 /2 responsivity was sufficient for keeping the system noise at the level of ~ 2 pA/Hz 1/2 . This is more than an order of magnitude smaller than phonon noise in the HEB. The devices were able to detect single near-IR photons (1550 nm) with a time constant of 3.5 µs. A digital transceiver for simultaneous generation of 16 probing tones and for processing of 16 downconverting signals has been demonstrated as well.
Transition edge sensors (TESs) with high energy resolution (ΔE) at telecommunication wavelengths have been realized in the last years for application to experiments of quantum optics and quantum information. In these fields, lasers with repetition rate of the order of 50 MHz are usually applied and hence TES with recovery times lower than 20 ns would be desirable. To this aim, Ti TESs with area ranging between 1 μm × 1 μm and 2 μm × 2 μm are fabricated and characterized, to study the effects of the different sizes of active areas and critical temperatures on the energy resolutions and on the response times. Titanium TESs have been fabricated by thermal evaporation and Electron Beam Lithography technique on Si/SiN substrates, and they have been electrically and optically characterized. The experimental data have been analyzed and correlated to the detector geometrical parameters. The superconducting transitions of TESs with areas down to 1 μm × 1 μm result to be affected by lateral proximity effects of Nb wirings. Ti TESs with critical temperature of 470 mK show response time of about 150 ns and energy resolution of 0.16 eV.