  A highly sensitive photodetection system with a detection limit of 1 photon/s
was developed. This system uses a commercially available 200-mm-diameter
silicon avalanche photodiode (APD) and an in-house-developed ultralow-noise
readout circuit, which are both cooled to 77 K. When the APD operates at a low
gain of about 10, it has a high-linearity response to the number of incident
photons and a low excess noise factor. The APD also has high quantum efficiency
and a dark current of less than 1 e/s at 77 K. This photodetection system will
shorten the measurement time and enable higher spatial and wavelength
resolution for near-field scanning optical microscopes.
