Pii: S0964-1807(98)00110-0

ÐThe ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN ®lms deposited on sapphire by reactive magnetron sputtering into either a 5 10 mm microbridge or 25 1 mm wide, 5 mm long strips connected in parallel. Both structures were placed at the center 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 biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T= 2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating e€ect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity R=3.9 V/W. # 1999 Elsevier Science Ltd. All rights reserved