High-Tc Josephson Junctions as Quasiclassical THz Detectors

[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.