A new type of ultra-high-speed single-photon counter for visible and near-i
nfrared wavebands based on an ultrathin NbN hot-electron photodetector (HEP
) has been developed. The detector consists of a very narrow superconductin
g stripe, biased close to its critical current. An incoming photon absorbed
by the stripe produces a resistive hotspot and causes an increase in the f
ilm's supercurrent density above the critical value, leading to temporary f
ormation of a resistive barrier across the device and an easily measurable
voltage pulse. Our NbN HEP is an ultrafast (estimated response time is 30 p
s; registered time, due to apparatus limitations, is 150 ps), frequency uns
elective device with very large intrinsic gain and negligible dark counts.
We have observed sequences of output pulses, interpreted as single-photon e
vents for very weak laser beams with wavelengths ranging from 0.5 mum to 2.
1 mum and the signal-to-noise ratio of about 30 dB.