Quasioptical Terahertz Spectrometer Based on a Josephson Oscillator and a Cold Electron Nanobolometer
We have developed a low temperature transmission spectrometer operating in a wide range of frequencies from 100 GHz to 1.7 THz. The spectrometer has utilized the unique properties of high-Tc superconducting Josephson junctions and the wideband response of sensitive Cold-Electron Bolometers (CEB). The voltage response of the CEB integrated with log-periodic and double-dipole antennas, has been measured using an oscillator consisting of high-Tc Josephson junction integrated on a separate substrate with a log-periodic antenna. Superconducting Josephson junctions with high characteristic voltages (IcRn larger than 4 mV at 4.2 K) are fabricated by depositing YBa2Cu3O7-x on miscut sapphire bi-crystal substrates, where the tilting axis is along the grain boundary. The cold electron bolometer having a superconductor-insulator-normal metal-insulatorsuperconductor (SINIS) structure was 200 nm wide, 10 μm long, and terminating tunnel junctions were 200x300 nm2 area. The response of the bolometer with a double dipole antenna has resonance shape with maximum corresponding to the designed central frequency of 300 GHz. A voltage response of the bolometer up to 4·108 V/W corresponds to a noise equivalent power of the bolometer of 1.2·10-17 W/Hz1/2. Our measurements demonstrate that the Josephson junction is overheated by the transport current up to 3 K at 1 mV bias when it is placed on a millikelvin stage. A high-Tc Josephson junction operated at temperatures below 2 K has the advantage of a high IcRn product that enhances the oscillation frequency to above 2 THz.
KeywordsMicrowave Sapphire Refrigeration
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