Bandwidth and noise of submillimeter wave cuprate bicrystal Josephson junction detectors

Detectors made from superconducting cuprate YBa₂Cu₃O_x bicrystal Josephson junctions (BJJs) on sapphire and NdGaO₃ substrates have been fabricated and characterized in the frequency band 200-900 GHz. Junctions on sapphire substrates had a normal state resistance R_N≈15-60 Ω, and I_CR_N product up to 2.5 mV at T=4.2 K. Junctions on NdGaO₃ substrates had lower R_N=1-5 Ω and I_CR_N=0.4-0.9 mV at T=77 K. Three types of detecting devices have been investigated in both the broadband and the frequency-selective detection modes. One type was patterned with log-periodic antenna, and two others with Pt-metal double-slot antenna designed for a central frequency f=300 GHz and f=400 GHz, respectively. Measurements at f=320 GHz of the reception bandwidth Δf for a device with double-slot antenna gave a quality factor Q=f/Δf≈10. A low-noise cooled 1-2 GHz bandwidth amplifier enables a better sensitivity in the self-pumping frequency mixing mode, avoiding the 1/f noise. The dependence of the spectral density of noise on voltage was compared to the data for the Josephson emission linewidth obtained by the selective detector response method. Also discussed are measurements at 500 GHz of the NEP values carried out at different experimental conditions.