Ultrasensitive hot-electron nanobolometers for terahertz astrophysics

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Abstract

The submillimetre or terahertz region of the electromagnetic spectrum contains approximately half of the total luminosity of the Universe and 98% of all the photons emitted since the Big Bang1. This radiation is strongly absorbed in the Earth's atmosphere, so space-based terahertz telescopes are crucial for exploring the evolution of the Universe2,3. Thermal emission from the primary mirrors in these telescopes can be reduced below the level of the cosmic background by active cooling, which expands the range of faint objects that can be observed. However, it will also be necessary to develop bolometers—devices for measuring the energy of electromagnetic radiation—with sensitivities that are at least two orders of magnitude better than the present state of the art. To achieve this sensitivity without sacrificing operating speed, two conditions are required. First, the bolometer should be exceptionally well thermally isolated from the environment; second, its heat capacity should be sufficiently small. Here we demonstrate that these goals can be achieved by building a superconducting hot-electron nanobolometer. Its design eliminates the energy exchange between hot electrons and the leads by blocking electron outdiffusion and photon emission. The thermal conductance between hot electrons and the thermal bath, controlled by electron–phonon interactions, becomes very small at low temperatures (1 × 10−16 W K−1 at 40 mK). These devices, with a heat capacity of 1 × 10−19 J K−1, are sufficiently sensitive to detect single terahertz photons in submillimetre astronomy and other applications based on quantum calorimetry and photon counting.

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Figure 1: Background-limited detector sensitivity for THz spectroscopy in space.
Figure 2: Fabrication of titanium nanobolometers.
Figure 3: Thermal conductance for hot-electron nanobolometers.
Figure 4: The time constants in hot-electron nanobolometers.

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Acknowledgements

We thank J.H. Kawamura at JPL/Caltech for help with the experiments. The work at Rutgers was supported in part by the National Aeronautics and Space Administration (NASA) grant NNG04GD55G, the Rutgers Academic Excellence Fund, and the NSF grant ECS-0608842. The research at the Jet Propulsion Laboratory, California Institute of Technology, was carried out under a contract with NASA. The work at SUNY at Buffalo was supported by NY STAR and NATO grants.

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Correspondence to Boris S. Karasik or Michael E. Gershenson.

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