Picosecond response of a superconducting hot-electron NbN photodetector

The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrat hin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sp uttering into either a 5 x 10 mu m(2) microbridge or 25 1 mu m wide, 5 mu m long strips connected in parallel. Both structures were placed at the cent er of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples b iased in the resistive (switched) state and illuminated with 100 fs wide la ser pulses at 395 nm wavelength. At T = 2.15 K, we obtained an approximatel y 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electro n heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-s peed response of NbN devices makes them an excellent choice for an optoelec tronic interface for superconducting digital circuits, as well as mixers fo r the terahertz regime. The multiple-strip detector showed a linear depende nce on input optical power and a responsivity R = 3.9 V/W. (C) 1999 Elsevie r Science Ltd. All rights reserved.