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We fabricated and experimentally studied a wideband metamaterial comprising a 2D array of annular antennas with Superconductor-Insulator-Normal metal-Insulator-Superconductor (SINIS) bolometers. The annular antenna array metamaterial was designed for a frequency range of 300–450 GHz and consists of periodically arranged electrically small rings, each containing two or four SINIS bolometers connected in series or parallel. These periodic structures with a unit cell size of about one-tenth of a wavelength act as a distributed absorber and receive two orthogonal polarizations. The unit’s small cell size provides a higher density of bolometers and therefore increases the bandwidth and the dynamic range of a single pixel. Theoretical estimates at a central frequency of 345 GHz yield absorption efficiencies of over 80% of the incident RF power within the 300–450 GHz frequency range. The average absorption by the metamaterial matrix in the given frequency band is at least twice as high in comparison to the half-wave circle matrix. We measured the optical response at sample temperatures as low as 100 mK using a quasi-optical setup that consisted of an immersion sapphire lens, bandpass mesh filters, and a variable temperature cryogenic blackbody radiation source. The measured series array voltage responsivity was 1.3 × 109 V/W for radiation temperatures ranging from 2 K to 7.5 K. The current responsivity for the parallel array was 2.4 × 104 A/W at a bath temperature of 100 mK. The spectral response was measured in a 240–370 GHz range with a Backward Wave Oscillator radiation source. The measured equivalent temperature sensitivity could be reduced to 100 μK/Hz1/2 at a 2.7 K radiation temperature level, a value that is suitable for anisotropy measurements in cosmic microwave background radiation.

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We have developed a bolometer with a suspended normal-metal absorber connected to superconducting leads via tunneling barriers. Such an absorber has reduced heat losses to the substrate, which greatly increases the responsivity of the bolometer to over 10 9 V/W at 75 mK when measured by dc Joule heating of the absorber. For high-frequency experiments, the bolometers have been integrated in planar twin-slot and log-periodic antennas. At 300 GHz and 100 mK, the bolometer demonstrates the voltage and current response of 3 Â 10 8 V/W and 1.1 Â 10 4 A/W, respectively, corresponding to the quantum efficiency of $15 electrons per photon. An effective thermalization of electrons in the absorber favors the high quantum efficiency. We also report on how the in-plane-and transverse magnetic fields influence the device characteristics. In superconducting bolometers, the electromagnetic radiation increases the temperature T e of the electron system in a normal-metal (N) absorber. Superconducting (S) leads connected to the absorber through insulating barriers (I) form tunneling junctions. The tunneling current across such a junction is a strong function of T e , which thereby offers a sensitive readout of the radiation power. At the low bath temperature T, the absorbed photon energy hf ) k B T is distributed among quasiparticles through their multiple interactions and is eventually lost into the cold bath through electrical contacts and a substrate. Here, h is the Planck constant, f is the frequency of incident radiation, and k B is the Boltzmann constant. The balance between the radiation-and cooling-powers determines T e . Decoupling the absorber from the heat sinks would increase T e and the detector response through the multiplication of excited electrons. It should be remembered that for the photon energy hf ) k B T e , the energy distribution of electrons can be substantially different from the Fermi distribution, depending on the quasiparticle interactions and tunneling rate of the excited electrons through the SIN junction. For estimations of bolometric sensitivity, it is usually assumed that the absorbed radiation is equivalent to dc Joule heating of the same power.Heat loss from an absorber can be reduced by fabricating the absorber on, e.g., a silicon nitride membrane supported by metallic thin-film beams having a small thermal conductance.1 Also, the effective thermal capacity of absorbers fabricated on a membrane is smaller than for absorbers on a bulk substrate.2 Further improvements can be achieved if an absorber is suspended without any supporting membrane. For an electron that absorbs a 300 GHz photon, the electron-phonon and electron-electron scattering times are about 0.2 ns and 1 ns, respectively. The excited electron creates a high-energy phonon that can easily escape from the absorber if the absorber sits on a substrate or is connected to electrodes of the same material.3 Using different materials for the absorber and electrodes can further improve thermal insulation due to a high acoustic-impedance mism...