normal metal strip at low temperatures (so called electron effect) to produce a large A novel concept of the normal-metal hotelectron bolometer using capacitive coupling of the absorber to antenna (NHEB-CC) has been proposed. The concept is based on the normal metal hot-electron microbolometer with Andreev mirrors (NHEB-A) using a SIN junction as a temperature sensor [1-3]. The NHEB-A technology brings a possibility to fabricate the sub-mm wave receiver for space applications with a noise equivalent power NEP about 3x1 0' 8 W/Hz at 100 mK. However, the Andreev mirrors could give certain frequency limitations for using this bolometer for higher frequencies with the energy quantum hf higher than the energy of a superconducting gap A. The NHEB-CC technology with capacitive coupling of the absorber to antenna avoids these problems. The effective high frequency coupling of the external signal in NHEB-CC is achieved by a low impedance of the coupling capacitances. At the same time, the capacitance gives reliable protection against escaping the hot electrons from the absorber. Usual tunnel junctions with proper thickness of the barrier can be used as capacitors. In such realization, we keep the main advantage of the NHEB technology very small thermal conductance between electrons and phonons due to small volume of an absorber (typical value of G is 2x10 W/K for T=100 mK) in combination with frequency independent protection of the absorber against thermal leak to the antenna.

Mar 1, 19981 citation

Capacitively Coupled Hot-Electron Nanobolometer with SIN Tunnel Junctions

L. S. Kuzmin, M. Yu. Fominsky, Alexei Kalabukhov +2 more

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.

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIEFeb 1, 20036 citations