High-Tc Josephson Junctions as Quasiclassical THz Detectors

Abstract

[100]-tilt YBa 2 Cu 3 O 7-x bicrystal Josephson junctions (JJs) with resistances R n = (50-500) Ω and characteristic voltages I c R n (50 K) of 2.5 mV have been fabricated and their potential as classical terahertz detectors has been evaluated. The dc I-V curves of the JJs at temperatures T above 40 K have been found free of hysteresis and demonstrated high nonlinearity with the second derivatives d 2 V/dI 2 up to 10 7 -10 8 Ω/A at T = 50 K. The response ΔV(I) at the current I corresponding to the maximum of d 2 V/dI 2 has been observed in a power range of more than four orders of magnitude, and ΔV is directly proportional to the incident power P (square-law detection) at low power, then ΔV ~ P 1/2 (linear detection) at higher power. The responses ΔV(V) at low voltages V to external radiation with the frequencies from 0.05 to 3 THz have been experimentally found to be proportional to d 2 V/dI 2 (classical detection) at low frequencies and to dV/dI (Josephson detection) at high frequencies with a crossover near the low-frequency limit f l = (0.9 ± 0.3) THz of the ac Josephson effect in our JJs. Numerical simulations of such quasiclassical detector with R n = 300 Ω at T = 50 K predict the NEP-values of 5·10 -15 W/Hz 1/2 at the frequencies up to 1 THz.